ORGANIC REACTION MECHAN I SMS I 97 I

An annual survey covering the literature dated December I 970 through November I 97 I

Edited by

B. CAPON University of Glasgow

C. W. REES University of Liverpool

Interscience Publishers

A division of JOHN WILEY & SONS

London * New York - Sydney - Toronto

ORGANIC REACTION MECHANISMS 1971

ORGANIC REACTION MECHAN I SMS I 97 I

An annual survey covering the literature dated December I 970 through November I 97 I

Edited by

B. CAPON University of Glasgow

C. W. REES University of Liverpool

Interscience Publishers

A division of JOHN WILEY & SONS

London * New York - Sydney - Toronto

Copyright 0 1972 by John Wiley & Sons Ltd.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, elec- tronic, mechanical photocopying, recording or otherwise, without the prior written permission of the Copyright owner. Library of Congress Catalog Card Number 66-23143 ISBN o 471 13472 4

Printed in Great Britain by William Clowes & Sons Limited London, Colchester and Beccles

Contributors

D. C. AYRES

R. BAKER

A. R. BUTLER

B. CAPON

R. S. DAVIDSON

T. L. GILCHRIST

M. J. P. HARGER

A. C. KNIPE

A. LEDWITH

P. J. RUSSELL

I. D. R. STEVENS

R. J. STOODLEY

R. C. STORR

Department of Chemistry, Westfield College,

Department of Chemistry, The University, South-

Department of Chemistry, St. Salvator’s College,

Department of Chemistry, The University, Glasgow

Department of Chemistry, The University, Leicester

Department of Chemistry, The University, Liverpool

Department of Chemistry, The University, Leicester

Department of Chemistry, The New University,

Donnan Laboratories, The University, Liverpool

Donnan Laboratories, The University, Liverpool

Department of Chemistry, The University, South-

Department of Organic Chemistry, The University,

Department of Organic Chemistry, The University,

University of London

ampton

University of St. Andrews

Ulster

ampton

new castle-upon-Tyne

Liverpool

Preface

This seventh volume of the series is a survey of the work on organic reaction mechanisms published in 1971. For convenience, the literature dated from December 1970 to November 1971, inclusive, was actually covered. The principal aim has again been to scan all the chemical literature and to summarize the progress of work on organic reaction mechanism generally and fairly uniformly, and not just on selected topics. Therefore, certain of the sections are somewhat fragmentary and all are concise. Nearly 5000 papers have been reported, and those which seemed a t the time to be more significant are normally described and discussed, and the remainder are listed.

Our other major aim, second only to the comprehensive coverage, has been early publication since we felt that the immediate value of such a survey as this, that of current awareness, would diminish rapidly with time. In this we have been fortunate to have the expert cooperation of the English office of John Wiley and Sons.

July 1972 B.C. C.W.R.

Contents

1. Carbonium Ions by R. BAKER . . 1 Bicyclic and Polycyclic Systems . . 1

Participation by Double and Triple Bonds . . 20 Participation by Aryl Groups. . . 16

Reactions of Small-ring Compounds . . 25 Metallocenylmethyl Cations and Other Derivatives . . 36 Stable Carbonium Ions and their Reactions . . 38 Other Reactions . . 43

2. Nucleophilic Aliphatic Substitution by I. D. R. STEVENS . 49 Ion-pair Phenomena and Borderline Mechanisms . . 49 Solvent and Medium Effects . . 55 Isotope Effects . . 58 Neighbouring Group Participation . . 60 Deamination and Related Reactions . . 79 Reactions of Aliphatic Diazo-compounds . . . 81 Fragmentation Reactions . . 82 Displacement Reactions at Elements Other than Carbon . . 83

.

Arnbident Nucleophiles . . . 95 Substitution at Vinylic Carbon . . 97 Reactions of a-Halogenocarbonyl Compounds . . 101 S N 2 Processes and Other Reactions. . . 103

3. Carbanions and Electrophilic Aliphatic Substitution by D. C. AYRES . . 111 Carbanion Structure . . 111 Reactions of Carbanions . . 113 Proton Transfer, Hydrogen Isotope Exchange and Related Reactions . . 119 Electrophilic Reactions of Hydrocarbons . . . 125 Organometallics: Groups Ia, IIa, I11 . . 127 Organometallics : Other Elements . . 130 Miscellaneous Reactions . . 133

4. Elimination Reactions by A. C. KNIPE . Stereochemistry and Orientation in E2 Reactions The ElcB Mechanism . The E2C Mechanism . Gas-phase Elimination Reactions . Other Topics .

5. Addition Reactions by R. C. STORR . Electrophilic Additions . . Nucleophilic Additions . Cycloadditions .

. 135

. 135

. 137

. 140

. 142

. 145

. 151

. 152

. 161

. 164

contents ... Vll l

6. Nucleophilic Aromatic Substitution by A. R. BUTLER . The SNAr Mechanism . Heterocyclic Systems . Meisenheimer and Related Complexes . substitution in Polyhaloaromatic Compounds . Benzyne and Related Intermediates . Other Reactions . .

7. Electrophilic Aromatic Substitution by A. R. BUTLER Sulphonation . Nitration . Nitrosation . Azo Coupling . Halogenation . Metal Cleavage . Decarboxylation . FriedelLCrafts and Related Reactions . Hydrogen Exchange . Miscellaneous Reactions .

8. Molecular Rearrangements by R. J. STOODLEY . Aromatic Rearrangements . Sigmatropic Rearrangements . . Electrocyclic Reactions. . Rearrangements Involving Cycloreversions and Cycloadditic Anionic Rearrangements . Cationic Rearrangements . Metal-catalysed Rearrangements . Rearrangements Involving Electron-deficient Heteroatoms Isomerizations . Rearrangements Involving Ring Openings and Closures .

m s . .

9. Radical Reactions by A. LEDWITH and P. J. RUSSELL Introduction . Structure and Stereochemistry . Decomposition of Peroxides . . Decomposition of Azo-compounds . . Diradicals . Atom-transfer Processes . Additions . Aromatic Substitution . . Rearrangements . . S,2 Reactions . Reactions Involving Oxidation or Reduction by Metal Salts Radical Ions and Electron-transfer Processes Nitroxides .

.

. .

179 179 181 184 188 188 192

195 197 198 200 200 201 204 204 205 206 208

211 212 216 228 235 239 243 253 257 261 264

275 275 277 281 287 290 296 304 313 320 326 331 336 348

contents ix

Autoxidation . Pyrolysis and Other Gas-phase Processes. Radiolysis, ESR Spectroscopy and Miscellaneous

. .

10. Carbenes and Nitrenes by T. L. GILCHRIST . Structure . Methods of Generation . Cycloadditions ,

Insertions and Abstractions . . Rearrangements and Fragmentations . Reactions with Nucleophiles and Electrophiles . Carbenoids . Transition-metal Complexes .

.

11. Reactions of Aldehydes and Ketones and their Derivatives b y B . C ~ p o ~ . .

Formation and Reactions of Acetals and Ketals Hydrolysis and Formation of Glycosides . . Hydration of Aldehydes and Ketones and Related Reactions Reactions with Nitrogen Bases . Hydrolysis of Enol Ethers and Esters Enolization and Related Reactions . Aldol and Related Reacti ms . Other Reactions .

.

.

12. Reactions of Acids and their Derivatives by B. CAPON . Carboxylic Acids . Non-carboxylic Acids .

13. Photochemistry by R. S. DAVIDSON . Physical Aspects . Carbonyl Compounds . Carboxylic Acids and Related Compounds Olefins . Aromatic Hydrocarbons . Heterocyclic Compounds . Nitrogen-containing Compounds . Halogen-containing Compounds . Carbonium Ions and Carbanions . Miscellaneous Compounds . Other Photoreactions .

.

14. Oxidation and Reduction by M. J. P. HARCER. . Ozonation and Ozonolysis . Oxidation by Metallic Ions .

. 353

. 355

. 359

. 367

. 367

. 368

. 373

. 376

. 381

. 384

. 387

. 389

. 393

. 393

. 397

. 402

. 404

. 408

. 409

. 413

. 414

. 419

. 419

. 457

. 467

. 469

. 475

. 490

. 493

. 501

. 504

. 508

. 516

. 517

. 518

. 520

. 527

. 527

. 529

contents X

Oxidation by Molecular Oxygen . . 537

Hydrogenation and Hydrogenolysis . . 555

Other Oxidations . . 541 Reductions . . . 546

Author Index, 1971 . . 561

Subject Index, Cumulative, 1970-1971. . . 611

CHAPTER 1

Carbonium Ions’

R. BAKER

Chemistry Department, The University, Southampton

Bicyclic and Polycyclic Systems . . 1 Derivatives of Norbornane and Related Compounds . . 1 Other Bicyclic Systems . . 5 Polycyclic Systems . 13

Participation by Aryl Groups . . 16 Arylalkyl Compounds . . 16 Benzonorbonene Derivatives . . 18 Other Reactions involving Phenyl Participation . . 19

Participation by Double and Triple Bonds . . 20 Double-bond Participation . . 20 Triple-bond Participation . . 25

Reactions of Small-ring Compounds . . 25 Cyclopropylmethyl Derivatives . . 25 Participation by More Remote Cyclopropyl Rings . . 29 Reactions of Cyclopropyl Derivatives . . 32 Reactions of Cyclobutyl Derivatives . . 34 Protonated Cyclopropane Intermediates . . 34

Metallocenylmethyl Cations and Other Derivatives . . 36 Stable Carbonium Ions and their Reactions . . 38 Other Reactions . . 43

Bicyclic and Polycyclic Systems

Derivatives of Norbornane and Related Compounds 13C and 1H NMR and Raman spectroscopic investigations of the 1,2-dimethylnorbornyl cation have been reported.2 In SbF5-SOz and FS03H-XbF5-SO2, the amount of 6,2 u delocalization is nearly identical with that in the 2-methylnorbornyl cation, but in the former the barrier to C1,C2 Wagner-Meerwein shift is lowered so far that it cannot be frozen out on the NMR time scale even a t -140°C. This low barrier results from a de- generate tertiary-tertiary rearrangement. The C-HG ezo bond is involved to a major extent in the delocalization.

1 S. Winstein and M. Sakai, “Non-Classical Ions and Homoaromaticity”, Kagaku No. Ryoiki, 25, 127 (1971); R. E. Leone and P. von R. Schleyer, “Degenerate Carbonium Ions”, Angew. Chem. Int. Ed., 9, 860 (1970) ; W. R. Dolbier, “Mechanisms of Solvolytic Spirane Rearrangements”, Mech. MoZ. Migr. 3, 1 (1971); R. C. Bingham and P. von R. Schleyer, Fortortschr. Chem. Forsch., 18, 1 (1971); M. J. Goldstein and R. Hoffman, “Symmetry, Topology and Aromaticity”, J. Am. Chem. Soc., 93, 6193 (1971).

2 G. A. Olah, J. R. DeMember, C. Y. Lui, and R. D. Porter, J. Am. Chem. Soc., 93, 1442 (1971); see Org. R a t i o n Mech., 1970, 2.

2 Organic Reaction Mechanisms 1971

The relative rates of 6,2- and 3,2-hydride shifts (k6:k3) are <lo0 and t200 in the acetolysis of d3-[1-14C]cyclopentenylethyl toluene-p-sulphonate and 2-exo-[4-l4C]- norbornyl toluene-p-sulphonate, respectively.3 14C studies have also shown that in the solvolysis of exo-norbornyl p-bromobenzenesulphonate, isotopic scrambling occurs in the starting ester owing to internal return and also in the initial product as well as in the reaction intermediates.4

Deuterium tracer studies have added further evidence that classical ions generated in deaminations can survive several Wagner-Meerwein rearrangements or hydride shifts.5 An endo-endo hydride shift to a secondary carbonium ion has been observed in the deamination of 3-exo-aminobornane-2-exo-o1,6 and a 3,2-endo,endo-methyl migration in the decomposition of a triazoline.7 The results were interpreted in terms of open classical intermediates. /3-Deuterium isotope effects have been reported to be inconsistent with anchimeric assistance in the solvolysis of exo-norbornyl derivatives. Differences in the force-constant change associated with the hydrogens a t C-2, C-3 and C-6 in formation of the transition states for the exo- and endo-derivatives are suggested as the origin of the observed isotope effects.8 The observation that /3-deuterium isotope effects in the solvolysis of 1,2-dimethyl-exo-2-norbornyl p-nitrobenzoate in solvents varying from 50 to 70 vol. % ethanol are directly related to the amount of elimination product is suggested as being due to rate-determining elimination in competition with substitution and internal return.9

Acetolysis of the epimeric exo-6-methoxycarbonyl-2-norbornyl p-bromobenzene- sulphonates proceeds through classical carbonium ions with kezolkendo = 4.4, although some Wagner-Meerwein rearrangement occurs.10 Little positive charge was believed to reside on the migrating carbon atom in the transition state for migration. endo-6- Methoxycarbonyl-exo-2-norbornyl p-bromobenzenesulphonate solvolyses with anchi- meric assistance and lactone formation.

In the solvolysis of a-fenchyl toluene-p-sulphonate (1) in acetic acid, a mixture of products is obtained, but the main component is 4-methylsantenyl acetate (2) resulting from methyl migration, Wagner-Meerwein rearrangement and hydride shift.11 (3) is also produced by a process involving solvent assistance and hydride shift.

Rates of displacement reactions on (4)-( 7) have been measured, the relative rates being found to be very similar to the reactivities observed for solvolyses. Steric inter- actions between the leaving group and other groups are again the determining factor.12 Rearrangements of a series of dihydrodicyclopentadiene derivatives in orthophosphoric acid have also been studied.13

Low exo:endo rate ratios have been observed in the solvolysis of a number of exo- 2,3-o-arylene-5-norbornyl toluene-p-sulphonates (8) and (9).14 A small increase in

3 C. J. Collins and C. E. Harding, Ann. Chem., 745, 124 (1971).

5 V. F. Raaen, B. M. Benjamin, and C. J. Collins, Tetrahedron Letters, 1971, 2613; see Org. Reaction C. C. Lee, B.-S. Hahn, L. K. M. Lam, and D. J. Woodcock, Can. J . Chem., 48, 3831 (1970).

Mech., 1970, 6. P. Wilder, Jr., and W. C. Hsieh, J . Org. Chem., 36,2552 (1971).

7 S. Rengaraju and K. D. Berlin, Tetrahedron, 27,2399 (1971). E. Scheppele, Chem. Comm, 1971,592.

K. Humski, Croat. Chem. Acta, 42, 501 (1971). 10 G. W. Oxer and D. Wege, Tetrahedron Letters, 1971, 457. l 1 A. Coulombeau, C. Coulombeau, and A. Rassat, Bull. SOC. Chim. France, 1970, 4389. 12 I. Rothberg and R. V. Russo, J. Chem. SOC. (B), 1971, 1214. 13 P. Wilder, D. J. Cash, R. C. Wheland, and G. W. Wright, J. Am. Chem. Soc., 93,791 (1971). 14 R. Baker and T. J. Mason, J . Chem. Soc. (B), 1971, 1144.

Carbonium Ions 3

OTs

(3)

exolendo rate ratio with change in solvent from acetic acid to formic acid was interpreted as resultling from an increasing carbon-carbon bond participation. The reactions were discussed in terms of kd and ks pathways but it was suggested that, in these systems, some “leakage” could occur.

In the addition of bromine and chlorine to exo,exo- and endo,endo-5,6-dideuterionor- bornene, proton loss to form a tricyclic product occurs from C-6 with exo and endo stereoselectivity, respectively.15 It was suggested that elimination could occur from (10) in bromination and from (1 1) in chlorination. Elimination from unsymmetrical bridged ions was also considered. Greater than 50% of exo-cis addition was found for hydrogen chloride and bromide to 2,3-dideuterionorbornene, and a classical norbornyl carbonium ion was suggested.16 In formic acid, methanol and hydrogen fluoride, Wagner-Meerwein rearrangement became equally important.

A detailed re-investigation has been made of the bromination of norbornene and the products have been found to consist of a mixture of five dibromides, bromonortricyclene and 2-exo-bromonorbornane.~7 From 14C studies, 2-exo,3-endo-dibromonorbornane was

15 N. H. Werstiuk and I. Vancas, Can. J . Chem., 48, 3963 (1970). 16 J. K. Stille and R. D. Hughes, J. Org. Chem., 36, 340 (1971). 17 D. R. Marshall, P. Reynolds-Warnhoff, E. W. Warnhoff, and J. R. Robinson, Can. J . Ch,em., 49,

885 (1971).

4 Organic Reaction Mechanisms 1971

OTs H I I

H OTs ( 8 ) (9)

R1 =Rz =H R1 =RZ=OMe R1 or Rz =NOz, R1 or R Z =H

I -H+

D

shown to arise from bromonium ion (12) and also carbonium ion (13). endo-Attack has been observed in the reaction of bromine with anti-7-bromo-5-phenylbenzonorborn- adiene and syn-7-bromo-2-phenylnorbornene.18 It was suggested that the reversal from normally observed ezo attack is due to the reduction in the u delocalization in the transition state for this pathway.

Palladium chloride-copper chloride catalysed addition reactions to tricyclo[4.2.1 .O2,5]

derivatives have been studied.Lg Products from two competitive reactions involving a copper( 11) complex and palladium( 11) complex were obtained, but neither process appeared to proceed through free carbonium ions. Reaction of camphene, norbornene, benzonorbornene and dibenzobarrelene with Pb( OAC)~-~(N& appears to proceed through carbonium ion intermediates.20

1* R. Caple, G. M. S. Chen, and J. D. Nelson, J . Org. Chem., 36, 2870 (1971). 19 C. J. R. Adderley, J. W. Nebzydoski, M. A. Battiste, R. Baker, and D. E. Halliday, Tetrahedron

20 E. Zbiral and A. Stutz, Tetrahedron, 27,4953 (1971). Letters, 1971, 3545.

Carbonium Ions 5

The 7-norbornyl cation has been generated from ~z-deuterio-2-bicyclo[3.2.0]heptyl p-bromobenzenesulphonate in acetic acid and products have been studied.21 Observed 13C chemical shifts in norbornyl derivatives have been interpreted in terms of bond lengths and inductive and steric effects.22 Special kinetic salt effects have been measured in the solvolysis of t-butyl bromide, exo-2-chloro-1- and -2-methylnorbornane, isobornyl chloride and camphene hydrochloride.23

An intermediate diazotic acid (14) is formed in the reaction of 1,l-disubstituted hydrazines with nitrous acid, which decomposes by two alternative routes.24 One involves formation of a nitrenium ion (15) which undergoes rearrangement and reaction with solvent and, in the other, nitrous oxide is lost with hydrogen transfer to the nitrogen.

Other studies include assessments of torsional effects in the [2.2.1] system;25 equilibra- tion reactions of norborneols and 1-methylnorborneols ;26 dependence of rates on sodium hydroxide concentration in the hydrolysis of optically active exo- and endo-norbornyl toluene-p-sulphonates;27 sulphur-oxygen bond cleavage of secondary and tertiary toluene-p-sulphonates under nucleophilic solvolytic conditions ;2* properties of 2-aryl- norbornene oxides and the dimer formed by dehydration of the 2-p-anisyl derivative ;29 reactions of endo-tricyclo[3.2.1.0~~4]oct-6-ene exo-oxide;30 formation of norbornylene- mercurinium ions;31 sulphuric acid and formic acid catalysed hydration of endo- and exo-norbornene-5-carboxylic acids;32 lactone formation from 2-endo-cyano-, 2-endo- cyanomethyl- and 2-endo-carboxymethyl-5-norbornenes;33 and deamination of endo- and exo-bornylamines.34

Other Bicyclic Systems

Further studies on “memory effects” have appeared. Magnification of a memory effect by substitution of an alkyl group for hydrogen of the non-migrating group R3 in (16) y to the original cationic charge, providing a more stable doubly rearranged ion, is reported.35 In deamination reactions of (17) and (18) the effect of the y-methyl substitu- tion on the multiplicative memory effect was a factor of 6 for the C-1 (“near”) versus C-4 (“far”) ring expansion comparison and about 13 in the comparison with the parent system. However, limits ranging from 1450 to 64,000 for the multiplicative memory effects, indicating very large enhancements of selectivity by methyl substitution, were

21 B. Funke and S. Winstein, Tetrahedron Letters, 1971, 1477. 22 J. B. Grutzner, M. Jautelat, J. B. Dence, R. A. Smith, and J. D. Roberts, J . Am. Chem. SOC., 92,

23 C. A. Bunton, T. W. Del Pesco, A. M. Dunlop, and K. U. Yang, J . Org. Chem., 36,887 (1971). 24 P. G. Gassman and K. Shudo, J . Am. Chem. SOC., 93, 5899 (1971). 25 S. P. Jindall, S. S. Sohoni, and T. T. Tidwell, Tetrahedron Letters, 1971, 779; S. P. Jindall and

T. T. Tidwell, Tetrahedron Letters, 1971, 787; J . M. Mellor and C. F. Webb, Tetrahedron Letters, 1971,4025.

7107 (1970).

28 A. Coulombeau and A. Rassat, Bull. SOC. Chim. France, 1970,4393. 27 P. Hirsjarvi, T. Kiutamo, J . Korvenranta, E. Tenhunen, and M. Vilen, Suom. Kemistilehti, 43,

28 P. G. Gassman, J. M. Hornback, and J. M. Piescone, Tetrahedron Letters, 1971, 1425. 29 T. J. Gerteisen, D. C. Kleinfelter, G. C. Rrophy, and S. Sternhell, Tetrahedron, 27, 3013 (1971). 80 B. C. Henshaw, D. W. Rome, and B. L. Johnson, Tetrahedron, 27,2255 (1971). 31 G. A. Olah and P. R. Clifford, J . Am. Chem. SOC., 93, 1261 (1971). 82 H. Geiger, Tetrahedron, 27, 165 (1971). 33 T. Sasaki, S. Eguchi, and M. Sugimoto, Bull. Chem. SOC. Japan, 44,1382 (1971). 34 D. V. Banthorpe, D. G. Morris, and C. A. Bunton, J . Chem. SOC. (B), 1971,687. 86 J. A. Berson, J. M. McKenna, and H. Junge, J . Am. Chem. SOC., 93, 1296 (1971).

519 (1970).

6 Organic Reaction Mechanisms 1971

(14)

1 -NaO

AX R

(17) R =Me, H X=NH2

CHzX (19)

R2 + I I

I l l - R1-C-4-C-

R3 R

“far branch”

- c-1 -

R R R

Carboniurn I o n s 7

found in deamination and solvolysis of the 1 -methyl-2-norbornylcarbinyl systems (19and20).

Evidence has been obtained that the free-energy benefit (Ad P”) of replacing hydrogen by methyl monotonically approaches zero a t zero overall dF*.36 Thus, in reactions of I-methyl-7-norbornylcarbinyl derivatives (21), despite the extra stability of tertiary cation (22) over secondary cation (23), the ratio of products from “far” and “near” branches is close to unity. The data suggest that ddP* of methyl for hydrogen replacement a t the site of a developing carbonium ion (C-j3) in these rearrangements (24) is about 0.4-1.1 kcal mole-’ compared to 6 kcal mole-1 for overall dF*.

Whilst the relatively unselective nature of deamination reactions compared with solvolytic processes has been accepted for some time, in some deaminations the reverse has been demonstrated.37 Competing paths used to examine selectivity are migration of a /3-substituent (R1) and migration of a /3 ring member (G, J) in carbonyl derivatives of general formula (25) ; the series of structures (26)-(31) was examined. Deamination invariably produces an increase in preference for ring expansion over the solvolytic reactions; in some cases a selectivity ratio as large as 100 is found. The results were rationalized in terms of ground-state conformations, which in the bicyclic systems places the leaving group as far as possible from the bulky bicyclic system. In this situation migration is not favoured in the solvolytic processes, so that the migration in deamination has enhanced relative importance. Normally, in systems such as 3-phenyl-2-butyl the energies of transition states for deamination do not lie far above the barriers for internal rotation and the choice of phenyl or methyl migration depends on the distribution in the diazonium ion ground state.

“Memory effects” in formation of (33) and (34) resulting from hydride shift, and rearrangement in carbonium ion reactions of (32), have been shown not to result from ion-pairing effects.38 The hydride shift process results in ca. 20% preservation of optical purity in p-bromobenzenesulphonate hydrolysis, but in deamination the hydroxyl group in the ring-expanded product replaces the migrating carbon with inversion of configuration. The results are consistent with either a slow interconversion of conforma- tional isomers or non-classical bonding.

A number of studies on trans-fused cyclopropanes have been published. In the solvo- lysis of the trans-bicyclo[6.1 .O]non-2-y1 derivatives (35 and 36), the trans,trans isomer was found to be greater than l o 4 times as reactive as the trans,& isomer.39 This difference arises because in (36) the leaving group lies over the cyclopropane ring so that only a

36 J. A. Berson and J. W. Foley, J. Am. Chem. Soc., 93, 1297 (1971). 57 J. A. Berson, J. W. Foley, J. M. McKenna, H. Junge, D. S. Donald, R. T. Luibrand, N. G. Kundu,

W. J. Libbey, M. S. Poonian, J. J. Gajewski, and J. B. E. Allen, J . Am. Chem. SOC., 93, 1299 (1971). 38 J. A. Berson, R. T. Luibrand, N. G. Kundu, and D. G . Morris, J. Am. Chem. SOC., 93,3075 (1971). 39 K. B. Wiberg and T. Nakahira, J . Am. Chem. SOC., 93,5193 (1971); P. G. Gassman, E. A. Williams,

and F. J. Williams, J. Am. Chem. SOC., 93, 5199 (1971).

8 Organic Reaction Mechanisms 1971

xc26 Rz

(29) xx I R1 (30)

6HzX

(31)

poor interaction exists between the developing p-orbital and the cyclopropane ring. I n reaction of (35) the developing p-orbital has its lobe directly over the bridging C-C bond. No “cross-over’’ in products was found for the epimeric derivatives, and the stereochemistry was the same for the two reactions. Only the tranqtrans epimer showed scrambling in the solvolysis when the material was labelled a-D (37).

No evidence for cyclopropane participation was found in the solvolysis of 4-trans- bicyclo[5.1 .O]octane p-bromobenzenesulphonate.40 It was concluded that requirements for participation, which involve either the interaction of a single bent cyclopropyl bond with an orthogonal p-orbital, or the interaction of the orbitals of two different cyclo- propyl carbon-carbon single bonds with two ends of the p-orbital, are not satisfied with this system. Acid-catalysed addition to trans-bicyclo[5.1 .O]oct-3-ene occurs with product- determining protonation a t the bridgehead.41 Products indicate that protonation of the cyclopropane ring is preferred over that of the double bond, which is consistent with the reactive nature of the former, as shown in solvolytic studies of analogous systems.

A bisected bishomoallylic intermediate has been suggested on the basis of similar rates and products found in the solvolysis of endo- and ezo-2-bicyclo[3.1 .O]hexyl 3,5- dinitrobenzoates; substitution of a 5-methyl group had a similar effect on the solvolysis of the two epimers.42

40 P. G. Gassman, J. Seter, and F. J. Williams, J . Am. Chem. SOC., 93, 1673 (1971). 41 P. G. Gassman and F. J. Williams, J . Am. Chem. SOC., 93, 2704 (1971). 42 E. C. Friedrich and M. A. Saleh, Tetrahedron Letters, 1971, 1373.

Carboniuln Ions 9

0 0 ODNB ODNB

Studies of the bicyclo[3.l.0]hex-3-en-l-y1 cation are reported. On the basis of the solvolysis of 2-ezo-chloro-exo-bicyclo[3.1.0]hex-3-ene (38) which gives predominantly 2-ex0 derivatives, and photolysis of benzene in deuteriophosphoric acid, benzovalene (39) is suggested as the key intermediate in the photolytic hydration of benzene.43 All the deuterium is incorporated into the 6-endo position (see 40) in the photolysis, which excludes the benzenonium ion (41) as a possible intermediate.

OH

(40)

The bicyclo[3.1 .0]hex-3-en-2-yl cation has also been generated in SbFS-SO&lF at -100” and in FS03H-SO2ClF at -120O.44 NMR results indicate that the carbonium ion has a “closed” cyclopropane ring with conjugation from the C-1-C-6 and C-5-C-6 bonds to the ally1 system. A slow sigmatropic rearrangement in the ion was observed

43 J. A. Berson and N. M. Hasty, Jr., J. Am. Chem. Soc., 93, 1549 (1971). 44 P. Vogel, M. Saunders, N. M. Hasty, Jr., and J. A. Berson, J. Am. Chem. SOC., 93, 1551 (1971).

10 Organic Reaction Mechanisms 1971

with AF' = 15 i 1 kcal mole-1 at -90": this compares with ring opening to benzen- onium ion with AP' = 19.8 kcal mole-1. The activation for the sigmatropic rearrange- ment is substantially higher than that found for the analogous heptamethyl ion (AT' =

9 kcal mole-1 at -89'). In line with the probable antihomoaromatic character of the four-electron cationic

intermediate, solvolysis of the bicyclo[3.2.l]octa-2,6-dienyl p-nitrobenzoates (42) is 235 times slower in aqueous acetone at 100" than that of the monoene anal0gue.~5 No evidence for the intermediacy of a 1,4-bishomotropylium ion was found in the solvolysis of (43) and (44), and both epimers reacted more slowly than cyclopent-2-enyl p-nitrobenzoate, an allylic model compound.46 The results are consistent with interven- tidn of the allylic intermediate (45). Solvolysis of bicyclo[4.3.l]deca-2,4,8-trien-ezo-7-y1 p-nitrobenzoate (46) in aqueous acetone is, however, losfaster than that of a comparable allylic system, this being consistent with the intervention of the bishomotropylium ion (47).47

4-Acetoxy- and 4-hydroxy-l-methylbicyclo[2.2.2]oct-2-yl toluene-p-sulphonates rearrange on reaction with methyl-lithium or methylmagnesium iodide in ether, to yield bicyclo[3.2.l]octene derivatives.@ It has been suggested that cis- and trans-2- pinanyl p-nitrobenzoates (48 and 49) solvolyse through delocalized ions that can be interconverted.49

Common intermediates involving equilibrating ions are suggested for acid-catalysed hydration of a- and p-pinene.50 With sulphuric acid in anhydrous acetic acid, X - and t3-pinene appear to give an intimate ion pair that is stabilized by the counter-ion against attack by external nucleophiles ;51 olefin is formed by proton loss to the counter-ion. Support for the hypothesis was found by use of 10% aqueous acetic acid, in which the differences in product formation from the two olefins are reduced owing to greater dissociation of ion pairs, and also in the easier attack by external nucleophiles.

*j A. F. Diaz, M. Sakai, and S. Winstein, J . Am. Chem. SOC., 92, 7477 (1970). 4O D. Cook, A. Diaz, J. P. Dirlam, D. L. Harris, M. Sakai, J. C. Barborak, P. yon R. Schleyer, and

47 P. Seidl, M. Roberts, and S. Winstein, J . Am. Chem. SOC., 93, 4089 (1971). 48 W. Kraus and C. Chassin, Ann. Chem., 747, 98 (1971). 49 J. R. Salmon and D. Whittaker, J . Chern. SOC. (B), 1971, 1249. 50 C. M. Williams and D. Whittaker, J . Chem. SOC. (B), 1971,668. 51 C. M. Williams and D. Whittaker, J . Chem. SOC. (B), 1971, 672.

S. Winstein, Tetrahedron Letters, 1971, 1405.

Carboniurn Ions 11

I OPNB

(48)

x

ion-pair

ion-pair

The stereochemistry of the addition of acid to bicycle[ 1.1 .O]butane has been examined.52 In addition of chlorosulphonyl isocyanate to a number of bicyclo[l.l .O]- butanes and bicyclo[2.1 .O]pentanes, the mechanism appears to involve initial S~2- l ike attack a t the least hindered bridgehead carbon atom followed by either ring inversion or rearrangement of a cyclobutyl to a cyclopropylcarbinyl carbonium ion.53 Choice of the latter pathway is determined by the extent of substitution.

A number of reports have appeared of Ag- and other metal-catalysed rearrangement of bicyclobutanes although the mechanism still appears unclear. Thus, with tricyclo- [4.1.0.0297]heptane (50) and l-methyltricyclo[li.l.O.O2~7]heptane stepwise bond cleavage is suggested, with the various metal catalysts acting as specific Lewis acids.54 I n this case the mechanism is considered to involve hybrid (51) of a metal-complexed carbene and a metal-bonded carbonium ion, and the products obtained from reaction of (50) with a number of metals can be envisaged as arising through (52a) and (52b). Although the way in which the different catalysts control the nature of the products remains to be elucidated, species such as (52) were trapped with formation of methyl ethers when the reaction was performed in the presence of the dicarbonylchlororhodium dimer in methanol. In similar studies it has been suggested that isomerizations of bicyclo[l.l.O]- butanes55 and tricyclo[4.1.0.02~7]heptanes56 in the presence of silver catalysts involve rupture of two strained bonds to produce a cationic species with Ag+ bonded to an sp2- hybridized electron pair (argentocarbonium ion). An experiment with tricyclo[4.1.0.02,7]- heptane has, however, indicated that initial attack of Ag(1) on bicyclobutane involves a one-bond rupture since again methyl ethers were obtained that were envisaged as arising from an intermediate similar to (52).57 The intermediacy of carbenoid-Ag(1) or argento- carbonium ions in all Ag(1)-catalysed reactions has, however, been questioned. In comparison of Pd(I1)- and Ag(1)-catalysed reactions of bicyclobutanes somewhat different

52 G. Szeimies and A. Schlober, Tetrahedron Letters, 1971, 3631. 53 L. A. Paquette, G. R. Allen, Jr., and M. J. Broadhurst, J . Am. Chem. SOC., 93,4603 (1971). 54 P. G. Gassman and T. J. Atkins, J . Am. Chem. SOC., 93, 4597 (1971); see also P. G . Gassman and

55 L. A. Paquette, R. P. Henzel, and S. E. Wilson, J . Am. Chem. SOC., 93, 2336 (1971). 56 L. A. Paquette and S. E. Wilson, J . Am. Chem. Soe., 93,5934 (1971). 57 M. Sakai, H. H. Westberg, H. Yamaguchi, and S. Masamune, J . Am. Chem. SOC., 93, 4611 (1971).

T. Nakai, J . Am. Chem. Xoc., 93, 5897 (1971).

12 Organic Reaction Mechanisms 1971

I I

products were obtained.58 In the Pd(r1) reactions, products resembled those obtained from the corresponding diazo compounds, but products from the Ag(1) reactions re- sembled those of carbonium ion rearrangements. A suggested hypothesis for these processes was heterolytic cleavage of the C-1 -C-2 bond followed by a rearrangement of cyclopropylcarbinyl-allylcarbinyl type.

Conformationally isomeric carbonium ions are formed in solvolysis of cis- and tram- bridgehead p-nitrobenzoates of the bicyclo[4.4.0]decane, bicyclo[4.3.0]nonane and bicyclo[3.3.0]octane series.59 Rates of reaction of the epimeric pairs differed substantially and this, together with product variations, requires different intermediates. Ion pairs do not appear to play an important role but the carbonium ions are suggested as retaining some of the ring geometry of their precursors. 6-0xabicyclo[3.2.l]oct-l-ylmethyl p-bromobenzenesulphonate has been synthesized

and allowed to react in acetic acid; the large amount of unrearranged product was suggested as due to dipole-induced resistance to C-C bond migration.60 The reactivities of 9-substituted 9-chlorobicyclo[3.3.l]nonanes have been examined.61 Other studies

58 M. Sakai and S. Masamune, J . Am. Chem. Soe., 93,4610 (1971). 59 R. C. Fort, Jr., R. E. Hornish, and G. A. Liang, J. Am. Chem. SOC., 92, 7558 (1970). 6o E. J. Grubbs, R. A. Froehlich, and H. Lathrop, J . Org. Chem., 36, 504 (1971).

L. Baiocchi, M. Giannangeli, and G. Palazzo, Tetrahedron Letters, 1970,5025; 1971, 2077.

Carbonium Ions 13

include acid-catalysed rearrangements of tetrafluoro-l-methoxy-3,5-dimethylbenzo- barrelene ;62 4-hydroxycyclohexa-2,5-dienones~3 and e m - and endo-l,2,4,4-tetramethyl- bicyclo[3.2.0]hept-6-en-2-ols;~4 thermal rearrangement of bicyclo[6.1.0]nonatrienyl chloride;65 and solvolysis of 7-methylenebicyclo[3.2.l]oct-l-yl toluene-p-sulphonate.66

Polycyclic Systems Calculations of bridgehead reactivities have been extended and improved.67 Slight modifications have been made to the computer program of Westheimer's classical treatment. New "ideal" bond angles (112.4' for CCHzC, 111.3" for CCHC, 106.1" for HCH, 110.7" for HCHC and 107.8' for HCC) have been used to conform with those of simple alkanes and "harder" non-bonded potentials have been adopted. Results obtained were less satisfactory for halides than for sulphonate esters, possibly owing t o the greater conformational flexibility of the former. Some of the variations in kOTslkBr in tertiary systems are, it is suggested, the result of ground-state interactions in the esters which would be unimportant with halides; this might also be a factor with secondary systems. The extremely low reactivity of tricyclo[5.2.1.04~10]dec-lO-yl toluene-p- sulphonate was interpreted as resulting from the absence of anti-periplanar C-C or C-H bonds to afford hyperconjugative stabilization of the transition state.

Solvolyses of 2-adamantyl toluene-p-sulphonate and 1 -adamantyl bromide in 80% ethanol and 75% dioxan show only minor rate enhancement on addition of sodium azide. No correlation is found between relative rate and product data, indicating the limiting character of the reactions.68 The Sneen, Carter and Kay linear free-energy relationship between carbonium ion stability and azide incorporation was extended to include a number of less reactive substrates.

The non-variance of the a-D effects in solvolysis of 2-adamantyl 2,2,2-trifluoro- ethanesulphonate in a number of solvents indicates the lack of solvent participation in the rate-determining step.69 Solvolysis rates of 1 -adamantyl toluene-p-sulphonate have been correlated against Y values,70 and silver-ion assisted reactions of 1-adamantyl halides in ethanol have been studied.71

Rates of solvolysis of (53)-(55) have been compared.72 (54) and (55) react ca. 103 times slower than (53) as a result of non-bonded interactions: no rearrangement was observed.

The [3,5,7-2Hl]-l-adamantyl carbonium has been generated in SbP5-SOzClF.73 NO proton scrambling was seen after heating a t 105" for 90 min, setting a lower limit for the

62 H. Heaney and S. V. Ley, Chem. Comm., 1971,1342. 63 G. F. Burkinshaw, B. R. Davis, E. G. Hutchinson, P. D. Woodgate, and R. Hodges, J . Chem. Soc.

64 L. B. Jones and V. K. Jones, J . Org. Chem., 36,1017 (1971). 135 J. C. Barborak, T. M. Su, P. von R. Schleyer, G . Boche, and G . Schneider, J . Am. Chem. SOC., 93,

(C), 1971,3002.

279 (1971). 86 F. E. Ziegler and J. A. Kloek, Tetrahedron Letters, 1971,2201. 67 R. C. Bingham and P. von R. Schleyer,J. Am. Chem. SOC., 93,3189 (1971); Tetrahedron Letters, 1971,

23. 68 D. J. Raber, J. M. Harris, R. E. Hall, and P. von R. Schleyer, J . Am. Chem. Soc.,93,4821 (1971). 69 J. M. Harris, R. E. Hall, and P. von R. Schleyer, J . Am. Chem. Soc., 93, 2551 (1971); V. J. Shiner,

70 D. N. Kevill, K. C. Kolwyck, and F. L. Weitl, J . Am. Chem. Soc., 92, 7300 (1970). 71 D. N. Kevill and V. M. Horvath, Tetrahedron Letters, 1971, 711. 72 R. C. Bingham, P. van R. Schleyer, Y. Lambert, and P. Deslongchamps, Can. J . Chem., 48, 3739

Jr., and R. D. Fischer, J . Am. Chem. SOC., 93,2553 (1971).

I

(1970). 79 P. Vogel, M. Saunders, W. Thielecke, and P. von R. Schleyer, Tetrahedron Letters, 1971, 1429.

14 Organic Reaction Mechanisms 1971

Br I

/6 Q rate of 2 x 10-5 sec-1 and leading to the minimum energy barrier of EA > 29 or 30.5 kcal mole-1. This compares with EA = 15 kcal mole-1 found for 1,2 hydride shifts in acyclic ions.

Reaction of bromine with protoadamantene has led to the formation of 2,4-diaxial- dibromoadamantane,74 and 1,2- and 2,4-disubstituted adamantanes have been prepared by a reaction sequence beginning with 4-protoadamantanone.75 Protoadamantane has been shown to undergo ionic brominationin the 6-p~sitionaspredicted byconformational- analysis calculations ;76 the solvolysis rate of 6-bromoprotoadamantane in 80% ethanol was also shown to agree to within a factor of three with the value predicted on the basis of strain calculations.

Under Koch-Haaf carboxylation conditions 2-(l-adamantyl)propan-2-01 yields 3-isopropyladamantane-1 -carboxylic acid as a consequence of rearrangement by inter- molecular hydride shifts.77 Use of dilute conditions in the Koch-Haaf reaction has been demonstrated to provide a synthetic route to 2-( 1-adamantyl)-2-methylpropionic acid. Rearrangements also take place in the bromination of 2-methyladamantane; 4-eq- and 4-ax-bromo-2-(dibromomethylene)adamantane are formed together with 1-bromo- adamantane and cis- and trans-1-bromo-4-methyladamantane.78 Homoadamantyl methyl ketone is a rearrangement product from addition of 1-adamantyl cation to prop-2-ynyl alcohol.79

78% of the total scrambling possible on a statistical basis is found after heating [2-W]adamantane (56) with aluminium bromide in CS2 a t 110” for 8 hr; suggested rearrangements are indicated in the attached formulae.80 Rearrangement of 2-methyl- adamantane to 1-methyladamantane proceeds by a similar skeletal rearrangement but is facilitated by the methyl group. Oligomerization of 2,4-dehydroadamantane by aluminium chloride in carbon disulphide appears to proceed by initial hydride abstraction from the tertiary position.81 Evidence has been presented for formation of a small amount of 1,2‘-biadamantane in this reaction.82

Reaction of spiro[adamantane-2,4’-homoadamantan-5’-ol] in concentrated HzSO4 gave adamantylideneadamantane and 2,2’-biadamantane which arise from the same cation by proton extrusion and intermolecular hydride abstraction from the tertiary

74 D. Lenoir, P. von R. Schleyer, C. A. Cupas, and W. E. Heyd, Chem. Comm., 1971,26. 75 B. D. Cuddy, D. Grant, and M. A. McKervey, Chem. Comm., 1971,27. 76 A. Karim, M. A. McKervey, E. M. Engler, and P. von R. Schleyer, Tetrahedron Letters, 1971,3987. 77 D. J. Raber, R. C. Fort, E. Wiskott, C. W. Woodworth, P. von R. Schleyer, J. Weber, and H.

78 J. R. Alford, D. Grant, and M. A. McKervey, J . Chem. SOC. (C), 1971,880. 79 J. K. Chakrabarti and A. Todd, Chem. Comm., 1971, 556. 80 2. Majerski, S. H. Liggero, P. von R. Schleyer, and A. P. Wolf, Chem. Comm., 1971, 1596. 81 H. J. Storesund, Tetrahedron Letters, 1971, 3911. 82 H. J. Storesund, Tetrahedron Letters, 1971,4353.

Stetter, Tetrahedron, 27,3 (1971).

Carbonium Ions 15

R+

RH -

position of another molecule.83 Oxidation of adamantane in fuming sulphuric acid is considered to involve hydride transfer from the adamantane nucleus to the acid.84 A pinacol-pinacolone rearrangement and other reactions of highly hindered spiro- adamantane85 and pyrolysis of 2-adamantyl methanesulphonate have been reported.86

In the acetolysis of ezo-tetracyclo[3.3.0.0.~~~.0~~~]oct-3-yl~-nitrobenzoate (57), deuter- ium labelling implicates stepwise, stereospecific cleavage of first one, followed by the second cyclopropane to yield ultimately the allylic cation (58) ; degeneracy arising from equilibration of the C-3, C-2 and C-1 bridges was excluded.87 Products arising from the three distinct carbonium ions were identified and an approximate energy diagram for the reaction was constructed.

Product studies on the acetolysis of (59) and (60) have indicated that steric or strain effects could not have been responsible for the high degree (>95y0) of stereochemical

CH3 OTs

WoTs WCH3 83 E. Boelema, H. Wynberg, and J. Strating, Tetrahedron Letters, 1971,4029. 84 H. W. Geluk and J. L. M. A. Schlatmann, Rec. Traw. Chim., 90,516 (1971). 85 G. B. Gill and D. Hands, Tetrahedron Letters, 1971, 181. 88 J. Boyd and K. H. Overton, Chem. Comm., 1971,211. 87 W. Lotsoh and A. S. Kende, Angew. Chem. Int. Ed., 10, 559 (1971).

16 Organic Reaction Mechanisms 1971

retention observed in the solvolysis of the secondary toluene-p-sulphonates. Nearly equal acetate distribution was found in the reactions of (59) and (60).88

Treatment of 1-(N,N-dich1oroamino)adamantane (61) with AlC13-CH2C12 gave a rearrangement product that was converted into endo-7-(aminomethyl)bicyclo[3.3.1]- nonan-3-one by aqueous acid. The mechanism is considered to involve electron-deficient nitrogen (62) rearrangement and collapse of the carbonium ion with nucleophiles.89

DNHz +!?- Q-cl

Sativene has been isomerized by treatment with Cu(0Ac)z in refluxing acetic acid to a mixture of isomeric sesquiterpenes consisting of recovered sativene (7%), cyclosativene (32%) and isosativene (61 Y0).9O Neoclovene has been prepared.91

Participation by Aryl Groups

Arylalkyl Compouds

The effects of sodium azide on the solvolysis of a series of 1-aryl-2-propyl and l-aryl-l- propyl toluene-p-sulphonates has been investigated.92 Reactions involving higher amounts of the ks component show greater rate enhancement due t o added azide and higher amounts of azide product. Solvolysis of the p-aryl derivatives were dissected into their component k, and k , pathways. A plot of log ks against log (azide ion-water com- petition ratios) shows a marked deviation and no correlation is found against the carbonium ion-selectivity relationship.93 Although this has been previously suggested as

8* W. L. Dilling and J. A. Alford, Tetrahedron Letters, 1971, 761; see Org. Reaction Mech., 1969, 18. 89 P. Kovacic, J. H. Liu, E. M. Levi, and P. D. Roskos, J . Am. Chem. Soc., 93,5801 (1971).

91 T. F. W. McKillop, J. Martin, W. Parker, J. S. Roberts, and J. R. Stevenson, J . Chem. SOC. (C),

92 D. J. Raber, J. M. Harris, and P. von R. Schleyer, J . Am. Chem. Soc., 93, 4829 (1971); see Org.

93 R. A. Sneen, J. V. Carter, and P. S. Kay, J . Am. Chem. Soc., 88, 2594 (1966); see also ref. 68.

J. E. McMurry, J . Org. Chem., 36, 2826 (1971).

1971,3375.

Reaction Mech., 1969, 21.

Carbonium Ions 17

evidence for the intermediacy of ion pairs, direct displacement on the neutral substrate was favoured in the present case.

Results from acefolysis of threo-3-aryl-2-butyl p-bromobenzenesulphonates have been discussed in terms of k , and k, processes. A suggestion was made that the ks pathway involves initial ionization to an ion-pair intermediate.94

The ethylenephenonium ion and ethylene-p-toluenium ions have been prepared in SbFS-SO2ClE' at -78" : styryl carbonium ions are also formed.95

14C isotope effects for the solvolysis of phenethyl and p-methoxyphenethyl p-nitro- benzenesulphonate in formic and trifluoroacetic acid have provided further evidence for aryl participation.96 Deuterium isotope effects have been measured in the formolysis of threo-I-methyl-2-p-tolylpropyl toluene-p-sulphonate and the results are considered to be consistent with an unsymmetrical transition state.97

Neighbouring aryl group participation has been observed in the deamination of 3-arylalanine ethyl esters in trifluoroacetic acid and is more important when a p- methoxy rather than a nitro group is a substitutent in the aromatic ring.98 In the solvolysis of 2,2,2-triphenylethyl toluene-p-sulphonate in alcohol-dioxan solution, substitution is more favourable than elimination under high pressure, owing to the decrease in volume of the system.99

2-( I-Azuleny1)ethyl toluene-p-sulphonates (63) underg9 acetolysis 68,000 times faster than phenethyl toluene-p-sulphonate. A series of compounds substituted in the

3-position was used and, from the p = 3.745 and the absence of substantial deuterium scrambling (0 when X = H and 12% when X = NOz), ion-pair return from the inter- mediate was discounted. The processes, it was concluded, proceed totally by the k A pathway.100 Neighbouring-group participation has also been observed in mass-spectral fragmentation of some azulenes.101 4-(p-Methoxyphenyl)[2,2-2Hl]b~tyl p-bromobenzenesulphonate; in formic acid,

yielded dideuterated 6-methoxytetralin (36%) and 4-(p-methoxyphenyl)butyl alcohol after LiAlH4 reduction; 74% of the tetralin was shown to arise by an Arl-5 pathway. The relative rate of An-5 to Arl-6 pathway was calculated as 5.6 and the selectivity was noted as low for an electrophilic aromatic substitution by a poor nucleophile.102

94 H. C. Brown and C. J. Kim, J . Am. Chem. Soc., 93,5765 (1971). 95 G. A. Olah and R. D. Porter, J . Am. Chem. Soc., 92, 7627 (1970). 96 Y. Yukawa, T. Ando, M. Kawada, K. Token, and S. G. Kim, Tetrahedron Letters, 1971, 847. 97 S. L. Loukas, F. S. Varveri, M. R. Velkou, and G. A. Gregoriou, Tetrahedron Letters, 1971, 1803. 98 K. Koga, C. C. Wu, and S. Yamada, Tetrahedron Letters, 1971, 2283. 99 Y. Okamoto and T. Yano, Tetrahedron Letters, 1971, 919. 100 R. N. McDonald and J. R. Curtis, J. Am. Chem. Soc., 93, 2530 (1971). 101 R. G. Cooks, N. L. Wolfe, J. R. Curtis, H. E. Petty, and R. N. McDonald, J . Org. Chem., 35, 4048

102 V. R. Haddon and L. M. Jackman, J . Am. Chem. Soc., 93, 3832 (1971). (1970).

18 Organic Reaction Mechanisms 1971

It is suggested that cyclized products from acid-catalysed dehydration of 3-methyl-3- (p-toly1)butan-1-01 arise through Arl-4 and Ar1-5 participation.103

No participation was observed in the base-promoted reaction of 3-(p-hydroxypheny1)- propyl p-bromobenzenesulphonate.104 MO calculations have been made on the phenethyl carbonium ion to phenonium ion transformation.105

Benzonorbornene Derivatives

p-Deuterium isotope effects have been measured for solvolysis of ezo- and endo-2- benzonorbornenyl p-bromobenzenesulphonates.106 Small isotope effects were found for for exo-derivatives in acetic and formic acid but the value obtained after substitution of two nitro groups into the 6- and the 7-position of the benzene ring was slightly greater than for the parent compound. The small deuterium isotope effects were ascribed to aryl participation but, as this decreased with the dinitro derivative, hyperconjugation with neighbouring C-H bonds could increase with a corresponding increase in isotope effect. The larger isotope effect with the dinitro derivative was also discussed in terms of greater release of steric compression in the transition state than in the parent compound. Larger isotope effects were found in solvolysis of the endo-derivatives, involving no aryl participation. A larger effect of the 3-exo-proton in solvolysis of ezo-derivatives was considered to result from the favourable alignment with the vacant p orbital on C-2, compared to the 60" dihedral angle between the 3-endo-proton and the p orbital. No geometric dependence of 3-exo- or 3-endo-protons was found in solvolysis of the endo- isomers, owing to an identical alignment of both protons with the vacant p orbital of the classical ion.

Effects of substituents on solvolysis of benzobicyclo[2.2.2]octen-2(exo and endo)-yl toluenesulphonates have also been studied, and many of the features of the analogous [2.2.l]system were exhibited.107

Solvolysis of a series of endo- and exo-chromium tricarbonyl complexes of benzo- norbornenyl methanesulphonates (64-66) have been examined.lO* Acetolysis of (66)

~ o ~ s ~ o M s ~

H

H Cr(C0)a H OMS

(64) (65) (66)

(67) (68)

103 A. A. Khalaf and R. M. Rbberts, J . Org. Chem., 36, 1040 (1971). 104 W. le Noble and B. Gabrielsen, Tetrahedron Letters, 1971,3417. 105 E. I. Snyder, J . Am. Chem. Soc., 92, 7529 (1970). 106 H. Tanida and T. Tsushima, J . Am. Chem. SOC., 93,3011 (1971). 107 H. Tanida and S. Miyazaki, J . Org. Chem., 36,425 (1971). lo8 R. S. Bly and R. C . Strickland, J . Am. Chem. SOC., 92, 7459 (1970); D. K. Wells and W. S.

Trahanovsky, J . Am. Chem. SOC., 92, 7461 (1970).