Dynamic NMR Spectroscopy on Rotation about the C=C Bondin Enamines Containing the JV-if Bond

V. I. Bakhmutov and E. I. Fedin

Institute of Organo-Element CompoundsAcademy of Sciences of the USSR

Moscow 117312, USSR

I. Introduction

II. The Mechanisms of Z,E-isomerization of Enamines Containing the N-H BondA. The Thermal Mechanism of Rotation about the C=C BondB. The Proton-transfer Induced Z,E-isomerization of EnaminesC. Catalysis of Z,E-isomerization in Enamines Containing the N-H Bond

III. Z,E-isomerization and Proton Exchange

IV. Conclusions

References

142

143144146149

150

152

152

I. INTRODUCTION

Dynamic NMR spectroscopy provides a con-venient means for studying the kinetics of chem-ical processes falling within the time scale of themethod (1). Depending on the choice of indicatornuclei and on the technique applied, NMR spec-troscopy allows one to measure the lifetimes ofmagnetically non-equivalent positions, rangingfrom 10* s [double resonance with transfer ofsaturation from one position to another (2)] to10" * s [pulse methods including T^p measure-ments (3)]. The rate constants of transitionsaffecting the lineshapes of NMR signals can bereadily calculated, assuming two-centerexchange, by the Gutowsky-Holm equation

= 1/T- = TTAVO//2 (1)

at the coalescence temperature of the signals,where AvQ is the difference between the reso-nance frequencies of the signals in the absence ofexchange (4). The complete analysis of NMRlineshapes with the use of a computer (1,5)yields the rate constants of the transitions atvarious temperatures and hence permits one todetermine, from the Arrhenius and Eyring equa-tions, activation parameters of the transitions.The dynamic NMR method has found wide use in

studying the simplest transformation of polarethylenes - rotation about the double bond ortheir thermal Z,E-isomerization (the symbols Zand E correspond, respectively, to the cis andtrans arrangement of the 'principal' substituentsrelative to the double bond). In NMR studies ofmainly enamines containing a tertiary aminogroup, Sandstrom, Kessler, Shvo et al. (6-17)treated the Z,E-isomerization as a rotationthrough a dipolar transition state:

I1C = i -

K2 R2\

B*c=c

R2

The magnitude of &GT for rotation in such mol-ecules is rather low (from 8 to 30 Kcal/mol) dueto delocalization of the electron pair of the nitro-gen atom. The principles of rotation about theC=C bond through the dipolar transition state

142 Bulletin of Magnetic Resonance

(the thermal mechanism of theZ,E-isomerization) have been discussed in previ-ous reviews (15, 16) and may be summarized asfollows:

1) An increase in electron-donating power ofR,, R2 and in acceptor capacity of X, Ypromotes polarization of the C = C bond inthe molecular ground state and stabilizesthe charges in the transition state, therebydecreasing the energy barrier to rotation.2) An increase in size of the substituents atthe double bond increases steric strain inthe molecular ground state, which alsodecreases the rotation barrier.3) An increase in dielectric constant of themedium favors polarization of the C=Cbond in the ground state and separation ofcharges in the transition state, whichdecreases the barrier to rotation about theC=Cbond.4) The Z,E-isomerization is a unimolecularreaction. It is characterized by a 'normal'magnitude for the pre-exponential factor(logA = 12-13) and a negative entropychange.

Thus, the rotation barrier is a very usefulquantitative characteristic of the structure oftertiary enamines. Compared to enamines con-taining a tertiary amino group, those with anN-H bond are more complicated systems. Indeed,the latter can undergo tautomeric transforma-tions (18-22) with tautomeric equilibrium fullyshifted, as a rule, towards the enamine form.Such compounds are protonated and deproto-nated to give cations and anions where the orderof the C=_=-=C bond seems to be lower than thatin the initial state (18). As a result, enaminescontaining the N-H bond may exhibit a variety ofreaction pathways by which the geometricalisomerization can proceed. These pathways areanalyzed in the present review on the basis ofthe available literature. It is to be noted that thedata on enamine isomerization, discussed in thiswork, have all been obtained from NMR spec-troscopy. This unique method permits a simulta-neous investigation both of the isomerizationsand of proton transfer reactions.

II. THE MECHANISM OFZ,E-ISOMERIZATION

OF ENAMINES CONTAININGTHE N-H BOND

The kinetics of isomerization of enaminescontaining a primary or secondary amino groupwere studied mainly by the proton resonancemethod with enamino ketones and enamino

ethers of the general formula:

c=c- N H C(O)X

where X = Alk or OAlk. Determination of thespatial configuration of these compounds by 1HNMR is no problem when R, = R2 = H,because the observed magnitude of the spin-spincoupling constant J-CH=CH- c'ear"ly reveals thespatial arrangement of the coupled protons (23).In the case of derivatives of tri-substituted ethy-lenes (for instance, R, = H, but R, * R2) theconfiguration can be ascertained by comparingthe observed value of the olefinic proton chemicalshift with that calculated by the increment equa-tion (24)

5CH ~ 5 - 2 5 gem cis trans- (2)It should be noted, however, that with unsatu-rated compounds containing bulky substituents,the difference between the experimental and cal-culated

Table 1. The Kinetic Parameters of rotation about the C=C Bondof Enamines 1 in Nitrobenzene (30).*

R

CH3p-CH3O-Php-CH3-Ph

Php-Cl-Ph

202221.25

]sactKcal/mol

Z

_

.k

.0 7.0

20,22,2425.

E

,k.0 7.0

19212k2k

AKcal/mol

Z

.7

.6. 0.2

E

19.721.62k. 021.. 2

Z

-11..- 1 2 .

-6 .-9 .

2000

AS're.u.

-11.-12

-6-9

E

.2

.0.0.0

20232526

AG^ 2 J SKcal/mol

Z

.9

.2.0

27.0

E

' 20.23.25.26.27-

03*79200

*The estimated errors are 0.4Xcal/mol for EaAGr and 1.5 - 2.0 e.u. for AST.

A. The Thermal Mechanism of Rotationabout the C=C Bond

Recently, comprehensive XH NMR studieswere carried out on the kinetics of isomerictransformations in a series of enamines 1 con-taining the N-H bond (28-32).

E-1 Z-1Table 1 lists the activation parameters for theZ,E-transitions, as calculated from the tempera-ture dependence of the COOCH3 signals throughthe full lineshape analysis (28-30). As seen fromTable 1, the isomerization parameters AGT,A H T , and EaC of enamines 1 in nitrobenzenedecreases with increasing electron-donatingpower of R. At the same time, in the series ofN-aromatic enamines, a satisfactory correlationis observed between In h^ E a n ( i a n f the sub-stituent in the para position of the aromatic ring(30). These data, together with the fact that thekinetic order of the process is equal to unity,point to a thermal, unimolecular mechanism forthe isomerizations. The same thermal

mechanism for rotation about the .C=C bond,involving formation of a dipolar transition state,has been suggested by Knippel et al. (31) instudies on the temperature dependence of the x HNMR spectra of ethyl derivatives of enamines 1in monitoring the CH= signals. As shown inTable 2, the rotation barrier in dimethylsulfox-ide, as determined from the coalescence temper-atures, diminishes with increasing electron-do-nating power of the para substituent of thearomatic ring. Thus, the Z,E-isomerization ofenamines 1 containing an NH proton is similarin these cases to the isomerization of enaminescontaining a tertiary amino group. There are,however, a few differences. As seen from Table3, an increase in electron-accepting capacity ofsubstitutent X results in simultaneously increas-ing, rather than decreasing, the energy barrier torotation about the C=C bond in enamines 2:

,x

P h N

Hc=c

/ x

c., R =

Frank et al. (32) explain this phenomenon interms of different effects exerted by substituent

144 Bulletin of Magnetic Resonance

Table 2. Barriers to rotation about the C = Cbond of Enamines 1 in DMSO*.

R

p-(CH3)2N-Php-CH3OPhp-CH3PhPhp-Cl-Php-CH3CO-Ph

Z

1 5 - 6 3 2

17.2'51 7 . 6 7 218 . I " 11 9 - 2 "19.8 1 1 7

y, Kcal/mol

E

151617171819

. I 3 2

.9 ' s 2 7 2

78 1. 6 "

*Reference 31 dealt with ethyl ethersof a-nitro^-aminoacrylic acid.

Table 3. Barriers to Rotationabout the C=C Bond of Enamines

2 in Nitrobenzene for VariousSubstituents X.

l C

Kcal/molReference

NO2O NCOOEt

2 6 . 6 1 ' 2 , 2 6 . 2 " 3023.fc1*2 321 9 . 8 " 32

X on the energy of the ground and transitionstates. In our opinion, the observed regularitycan be attributed to the formation of an intramo-lecular hydrogen bond, whose strength rises asthe acceptor power of substituent X is increased.The formation of intramolecular hydrogen bondsin enamines 1 and 2 and related compounds hasbeen convincingly demonstrated by XH NMRstudies of the chemical shifts of the amino protonin various solvents (29-36). Further evidence infavor of hydrogen-bond formation is provided bythe magnitude of the spin-spin coupling constantin the =CH-NH fragment, which ranges from 13

to 16 Hz, depending on the nature of R, and isindicative of a trans orientation of the interactingnuclei. It should be noted that for enamine 1with R = H, the constants JcH-NH1S an

with R = CH3 in nitrobenzene, the spin-spincoupling constant of the fragment NH-CH3remains unchanged after coalescence of the Z-and E-isomer signals (42).

B. The Proton-transfer InducedZ,E-isomerization ofEnamines

1. Associative Mechanism

When studying the 1 H NMR spectra of ena-mines of the general formula

/ NH-

vCO

C-CH2-C=io

tB /c==c\

CO NDabrowski et al. (43, 44) observed that in D2 Oand ND3 these compounds could undergo isom-erization and, simultaneously, isotopic exchangeof the olefinic proton, proceeding, as commonlyaccepted, via a tautomeric transition Com-pounds containing a tertiary amino group exhib-ited no isomerization under the same conditions.This fact allowed the authors to conclude thatthe isomerization proceeded through a tautomerictransition to yield an unstable but 'freely rotat-ing' imino form B. (The relation between isom-eric transformations and isotopic exchange of theolefinic proton was also discussed in references45, 46). The rate of the isomerization proved,however, to be well above the rate of isotopicexchange (qualitative estimates), which providedevidence for competing mechanisms for the pro-cess. Dabrowski and Terpinski (44) believe thatone enamine molecule can act as a base onanother molecule to eliminate a proton from thenitrogen atom. The double bonds of the resultinganion A" and cation A*

20 CC C

A O-C-C-C-N-0

+ HO-G-C-C-NH-

A+lose partly their double bond character, whichfacilitates rotation about the C=-=-=C bond. Theisomerization mechanism involving formation ofthe active (in the sense of rotation) forms due tointeraction of two enamine molecules should bereferred to as the 'associative' mechanism. Thismechanism was supported kinetically bydynamic NMR spectroscopy in studies of isomer-izations of enamine 1 with R = H in pyridine(29). It should be noted that the structure of thiscompound has one amino proton not involved information of the intramolecular hydrogen bond.As seen from Figure 1, the lineshape ofCOOCH, signals depends on enamine concentra-

2 Hz

Figure 1. The lineshapes of COOCH3 signals ofenamine 1 with R = H in pyridine at 70 C andvarious concentrations (1.0 - 0.1 M, 2.0 - 0.3 M,3.0 - 0.4 M).

146 Bulletin of Magnetic Resonance

Table 4. Activation Parameters for Z,E-isomerization ofExamines 1 in Pyridine.

R

CH,H

E

Kcal/mol

Z

12,10,

E

.2 12.

.2 8.it5

AH+Kcal/mol

Z

11.69-5

E

11.7.78

z

-20-26

AS+e.u.

.2

E

-18.-30.

h8

AG+:I 9 8Kcal/mol

Z

17-717.4

E

17-17.

20

tion. The kinetic order of the Z,E-isomerization,n, determined from equation 3 (47)

1/rZ(E) = dZ(E)/dt[Z(E)] = (3)

by calculating the lifetimes T ^ J ; at various con-centrations of enamine 1 with R = H was foundto be, at the indicated temperature, between 1.7(100 C) and 2.0 (50 C). This corresponds to twoenamine molecules being involved in the kineti-cally controlled stage of the process. It should benoted that the decrease in the isomerizationparameters E a c t , AHT, AST (Table 4) observedon going from the thermally isomerizing enamine1 with R = CH3 to enamine 1 with R = H is atvariance with the effect of the substituent at thenitrogen atom. This could not be explained by thethermal rotation mechanism and also suggeststhat the isomerization mechanism has changed.

2. Dissociative Mechanism

The anion A" in the above scheme can begenerated by interaction of an enamine contain-ing an N-H proton with a base whose role maybe played by the solvent. Such a mechanism ofZ,E-isomerization of enamines should be referredto as the 'dissociative' mechanism. The existenceof the dissociative mechanism was confirmed bydynamic NMR studies of enamines 1 with N-a-romatic substituents in dichloromethane-pyridinemixtures (30, 37). The lifetimes of the Z- andE-isomers were found by full analysis of thelineshapes of the COOCH3-signals. The effect ofsubstituent R on the Z,E-isomerization barrier

height in enamines 1 with R = Ar (Table 5), thepartial first order kinetics of the reaction in ena-mine and pyridine (37), as well as the kineticisotopic effect found from the lineshape of theCOOCH3 groups of enamine 1 with R = Ph,deuterated at the nitrogen atom, all pointed to amechanism involving cleavage of the N-H bond,followed by rapid rotation in the anion

Az~ * A E 'AE" + H* * AE.

The dissociative mechanism was also supportedby the kinetics of isomerization of enamine 3with R = H (48).

C=CNCO 2CH,

CO,CH,

NR

Z3In acetonitrile and pyridine the compoundshowed isotopic effects (as established by ana-lyzing the lineshapes of COOCH3 -signals, seeTable 6 and Figure 2). Substitution of the radi-cals CH3 and CH3CO for the nitrogen protonresulted in a sharp increase of the rotation bar-rier (AGZ E 2 ' 8 = 17.5 Kcal/mol for R = H in

Vol. 6, No. 3 147

Table 5. The Effect of Substituent R on Barriers of Z,E-isomerizationof Enamines 1 in a CH2 Cl2 :Pyridine Solution (30).

R

Kcal/mol

Kcal/mol

p-N02-Ph

14.2O.2

p-

15.

15.

Cl-Ph

60,

k0.

.2

.2

15

15

Ph

.9+0

7o

.2

.2

P-CH3-Ph

16.5O.

16.3O.

,2

.2

p-OH-Ph

16.8O.2

l6.50.2

acetonitrile and pyridine and AGg jj4 2 3 > 24Kcal/mol for R = CH3, CH3CO in'dimethylsul-foxide). Finally, it is only the dissociative mech-anism that can explain the inhibiting effects ofan acid on the rate of isomerizations of enamine3 with R = H in acetonitrile (Figure 3), wherethe acid reduces the lifetime of the anionic form(48).

The conformational lability of the anionicforms of enamines can be accounted for by thedecreased order of the f! C. bond. As demon-strated by MO LCAO calculations, the dissocia-tion of the N-H bond in enamine 1 (R = CH3)leads to CC double bond nonbonding interactionand to a threefold decrease of the rotation barrierabout the C = C bond in the anionic form (49a). Itshould be noted that the decreased order of theC: C bond in the anionic form is not sufficientperhaps for the isomerization of the enolic sys-tems. As demonstrated by the dynamic NMRspectroscopy in reference (49b,c) the fast isom-erization of the enolic systems is achieved by theprotonation-deprotonation reaction of the enolicanion with the participation of the keto form:

HO C = C - H +G

H

o=cc=i.e., by the transformation from a double bond toa single bond.

Table 6. The Lifetimes of Z,E-isomers .of Enamine 1 with R = Ph

in a CH2 Cl2 :PyridineSolution and of Enamine 3 with R = H

in Pyridine and their N-deutero-derivativesunder the Same Conditions.

Compound r

1 with R=Ph3 with R=H

eff1s

0.0.

C(H)*

Oil.4003'

00

s

.0284'

.09'

:(D) Ref

301.8

reff rZTE / ( r E ) .

Figure 2. COOCH3 signals of enamine 3 with R= H (1) and its deutero-analog (la) in pyridine(a) and acetonitrile (b).

148 Bulletin of Maenetic Resonance

Huisgen et al. (50) showed that the rate of itsisomerizations in benzene is extremely small

(CH3COOH)-10

Figure 3. Isomerization of enamine 3 with R =H in CH3CN at 20 C in the presence of aceticacid: 1.0 - 0.09 M solution, 2.0 - 0.18 M solution.

C. Catalysis\of Z,E-isomerization in Ena-mines Containing the N-H Bond

Inasmuch as enamines containing a primaryor secondary amino group can undergo isomeri-zation via migration of the N-H proton, a rela-tionship should be expected to exist between therate of Z,E-isomerization of these systems andacidic-basic properties of the solvent. In studiesof enamine 4

H ' X CO 2 CH 3

C6H1nNHN /CO2CH3

(k,25 = 8.X10- = 1.7X10" ).Addition of acetic acid in catalytic quantities pro-duced a dramatic increase in the reaction rates(the above constants become equal to 1.1X10"4

and 0.24X10" respectively). Theauthors rationalized the observed phenomenon interms of isomerization proceeding via a short-lived imino form:

RNH=CCH,CO,CH,

The acid catalysis of the Z,E-isomerization ofenamines was also reported by Frank et al. (32).In the cited work the addition of trifluoroaceticacid to chloroform and nitrobenzene solutions ofenamines 2 was found to decrease the coales-cence temperature of the COOCH3 -group signalsin the * H NMR spectra. The effect of the acid onthe rate of isomerizations of enamines was alsoinvestigated in reference 51. Table 7 lists theisomer lifetimes calculated by full analysis of thelineshapes of the COOCH3 signals. It is seenthat an increase in acid concentration speeds upthe isomeric transitions. Thus these results, too,can be accounted for by protonation of enaminesto give the above imino form. It should be notedthat the deceleration of Z,E-isomerizationobserved with enamine 1 with R = Ph atlow acid concentrations implies suppression ofthe dissociative mechanism discussed above.

Unlike acids, triethylamine does not catalyzethe isomerization of enamine 4 in benzene (50).The addition of triethylamine does not, however,have a catalytic effect in the case of enamines 3with R = H and of enamines 1 with R = Alk orAr which contain two powerful electron-acceptinggroups increasing the acidity of the N-H proton(29, 48). This catalytic effect can be explained interms of the dissociative mechanism ofZ,E-isomerization, as was observed in the pres-ence of pyridine. Thus, from the above data itmay be concluded that the effect exerted by anacid or a base on the rate of isomerizations of aparticular enamine containing the N-H bond isnot simple and depends on the structure of theenamine.

rfVol. 6, No. 3 149

Table 7. The Lifetimes of Z- and E-isomers of 1 withR = CH3 sad Ph in Acetonitrile at Various Concentrations of CF3COOH (51).

0d0025

[CF3C00H]Mol/L

.0

.027

.135

.27

.7

1

000000

R

.E-c

S

.1653

.17"

. 1453

.073"

.127"25

.037"25

s

0.0.0.0.0.0.

r zt O (

30533253

27"13"39"2II"2

R =

: TEtc

s

0.82"-

0.82 5 3-

5 0.035"

5 0.09553

Ph

Tzfc

s

1.0"-

1.0s3-

0.053"0.019s3

III. Z,E-ISOMERIZATION AND PROTONEXCHANGE

As mentioned above, NMR spectroscopyoffers an unique means for revealing the mecha-nisms of isomerizations in enamines containingthe N-H bond. In particular, by using thismethod a relationship was found between theisomerizations and intermolecular protonexchange reactions. Kozerski et al. (52) studiedthe temperature behavior of the x H NMR spec-tra of enamines 5

H /NHCH,

CH,C

in the region of the CH3C0 signals, which areresponsive to the isomerization, and the NCH3signals, which are indicators for intermolecularproton exchange due to the splitting on the NHproton. It was demonstrated that in aproticsolvents the activation energy parameters forboth processes (AG^) were practically the same(Table 8). This coincidence allowed the authorsto conclude that the isomerization occurs withparticipation of the NH proton, i.e., proceeds viaformation of ionic forms. Similar results werereported in reference 42 for enamine 1 with R =Ar, where the full analysis of the lineshapes ofthe COOCH3 and CH3 group signals showed therate of the isomerization in a dichloromethane-pyridine mixture to be very close to the rate ofthe N-H bond cleavage. The partial first order

Table 8. The values of AG^ forRotation about the C=C

Bond and for Proton Exchange inEnamine 5 in Various Solvents (52).

Solvent

in bulk

C2C 1 6

C 2 C 1 e :pyridine

H20

Process

isomer izationNH exchange in

cis formNH exchange in

trans formisomerizationNH exchange in

trans formNH exchange in

cis formisomer izationNH exchange in

trans formNH exchange incis form

isomerizationNH exchange in

both forms

AG+ tCKcal/mol

19.6102

19.5'4

19.6"20.611*

ig.gioj

18.9"419.8"

19.0*5

18.27018.63*

IV. 7 s

kinetics in enamine and pyridine, observed forboth Z,E-isomerization and proton exchange, aswell as the similar effect exerted by the substit-uent in the para position of the aromatic ring,suggested that the transfer of the NH protonbetween molecules of enamine 1 with R = Arwas catalyzed by pyridine and followed by theloss of conformational stability of the compounds.Thus, with aprotic solvents simultaneity may be

150 Bulletin of Magnetic Resonance

expected for the isomerizations and NH protonexchange in enamines 1 with R = Ar and 5. Avery different situation is observed with proton-donating solvents. As seen from Table 8, inaqueous solution the isomerization of enamine 5necessitates much greater energy than does theproton exchange reaction. Dynamic NMR studiesof the isomerization and proton exchange kineticsfor compounds 1 ( R = Ar) and 3 (R = H) inCH3OH (53), GH3OH-CHC13 (54), and pyridine-water mixtures (55) demonstrated that enamineswith N-H bonds are isomerized in the proton-do-nating solvents through a unimolecular transitionstate (thermal mechanism, Section II.A). On theother hand the proton transfer reactions gothrough a bimolecular transition state, contain-ing one molecule of proton-donating solvent:

/>

l

X = H, CH3

Perhaps the difference in the rates of the dis-cussed processes can be accounted for by thisfact. The relation between Z,E-isomerization andproton exchange in enamines conaining the N-Hproton depends not only on the nature of the sol-vent but also on the structure of the enamine.Thus in studies of the temperature behavior ofthe *H NMR spectra of enamine 1 with R =CH3 in pyridine (29), it turned out that thelineshape of the COOCH3 signals - indicators forisomerizations - was independent of concentra-tion of the isomerizing compound. However, forNCH3 signals whose lineshape depends on therate of proton exchange, such concentrationdependence was clearly observed (Figure 4).Analysis of the spectra showed thatZ,E-isomerization and proton exchange werecharacterized by first and second kinetic orders,respectively. In other words, the intermolecularNH exchange in enamine 1 with R = CH3 seemsto occur through a closed transition state

;N N;H

while the isomerizations proceed by the mecha-nism of thermal rotation about the C=C bond,discussed in Section II. Thus kinetic studies ofNH exchange in isomerizing enamines may pro-vide a better insight into the mechanism of their

5 Hz

Figure 4. The lineshapes of CH3N signals ofenamine / with R = CH3 in pyridine at 110 Cand various concentrations (1.0 - 0.1 M, 2.0 - 0.2M, 3.0 - 0.5 M, 4.0 - 0.8 M).

Z,E-transitions.

IV. CONCLUSIONS

The data reviewed in this paper suggest thatZ,E-isomerization of enamines containing a N-Hbond do not follow a single mechanism. Depend-ing on a number of factors, isomerizations of theenamines may proceed through the thermalmechanism and through the proton exchangereaction. Among the factors affecting the isom-erization rate and pathway are the nature ofN-substituents and those at the double bond aswell as the properties of the solvent (first of all,its acidic-basic properties). It seems that in thegeneral case, solutions of the enamines mayexhibit several simultaneous pathways of

Vol. 6, No. 3 151

Z,E-isomerization. An increase in the ability ofN-substituents and those at the double bond to beinvolved in delocalization of negative charge, aswell as an increase in basic properties of the sol-vent, should raise the probability of the dissocia-tive mechanism of the Z,E-isomerizations. Anincrease in enamine concentration should in turnfavor the associative mechanism. The diversityof the mechanisms of 'rotation' about the C=Cbond in enamines containing the N-H bondimplies that the rotation barriers calculated fromthe temperature dependence of the NMR spectracannot be used as a characteristic of the doublebond if the 'rotation' mechanism is unknown.Thus in the case of enamines 1 with R = Arthese barriers characterize the cleavage of theN-H bond.

Considering enamines containing the N-Hbond as model compounds, we believe that thedata reviewed in this paper will aid in recogniz-ing the nature of isomerizatiori reactions of tau-tomeric molecules which are extensively investi-gated by NMR spectroscopy.

REFERENCESXJ. M. Jackmsin and F. A. Cotton, Dynamic

Nuclear Magnetic Resonance Spectroscopy, Aca-demic Press, New York, 1975.

2W. Philipsborn, Angew. Chem. Intern. Ed.10, 472 (1971).

3T. C. Farrar and E. D. Becker, Pulse andFourier Transform NMR. Introduction to Theoryand Methods, Academic Press, New York, 1971.

4H. S. Gutowsky and C. H. Holm, J. Chem.Phys. 25, 1228 (1956).

5N. M. Sergeev, Uspekhi Khimii 42, 769(1973).

*I. Wennerbeck and J. Sandstrom, Org.Magn. Reson. 4, 783 (1972).

7J . Sandstrom, U. Sjostrand, I. Wennerbeck,J. Am. Chem. Soc. 99, 4526 (1977).

' J . Sandstrom and U. Sjostrand, Tetrahedron34, 371 (1978).

' U . Sjostrand and J. Sandstrom, Tetrahedron34, 3305 (1978).

1 0 H. O. Kalinowski, H. Kessler, and A. Wal-ter, Tetrahedron 30, 1137 (1974).

X1H. Kessler, Chem. Ber. 103, 973 (1970).1 2

J. Shvo, E. C. Taylor, and J. Bartulin, Tet-rahedron Lett. 3259 (1967).

1 3 J. Shvo and I. Belsky, Tetrahedron 25, 4649

(1969).1 4R. Osman and J. Shvo, Tetrahedron 34,

2321 (1978).1 5 H. O. Kalinowski and H. Kessler, in Topics

in Stereochemistry, N. L. Allinger and E. L. Eliel,

Eds., Interscience, New York, Vol. 7, 1972, p.296.

1' See p. 218 in reference 1.

17 E. P. Prokofev, Z. A. Krasnaya, arid V. F.

Kucherov, Org. Magn. Reson. 6, 240 (1974).1

' Ya. F. Freimanis, Khimia Enaminoketonov,Enaminoiminov, Enaminotionov, Zinatne, Riga,1974.

X*S. I. Yakimovich and L. N. Ignatyuk, in

Reaktsionnaya Sposobnost i Mekhanism ReakisiyOrganicheskikh Soedineniy, Leningrad, LGUPress, 1971, p. 137.

2 0 H. Albrecht, W. Funk, and M. Th. Reiner,Tetrahedron 32, 479 (1976).

2 1 S . I. Yakimovich, I. V. Zerova, and N. N.Gavrilova, Zh. Org. Khim, 14, 266 (1978).

2 2 J . V. Greenhill, Chem. Soc. Rev. 6, 277(1977).

2 3 J . M. Emsley, J. Feeney, and L. H. Sut-cliffe, High Resolution Nuclear Magnetic Reso-nance Spectroscopy, Pergamon Press, Oxford,Vol. 2, 1966, p. 4.

2 * U. Matter, C. Pascual, E. Pretsch, A. Pross,

W. Simon, and S. Sternhell, Tetrahedron 25, 691(1969).

2 5 E. Knippel, M. Knippel, M. Michalik, H.Kelling, and H. Kristen, Tetrahedron 33, 231(1977).

2V. I. Bakhmutov, K. K. Babievsky, and E.

I. Fedin, Izv. Akad. Nauk SSSR, Ser. Khim. 647(1979).

2 7V. I. Bakhmutov, K. K. Babievsky, K. A.Kochetkov, and E. I. Fedin, Izv. Akad. NaukSSSR, Ser. Khim. 1161 (1980).

2 8V. I. Bakhmutov, V. A. Burmistrov, K. K.Babievsky, V. M. Belikov, and E. I. Fedin, Izv.Akad. Nauk SSSR, Ser. Khim., 2820 (1977).

2' V . I. Bakhmutov, K. K. Babievsky, V. A.

Burmistrov, E. I. Fedin, and V. M. Belikov, Izv.Akad. Nauk SSSR, Ser. Khim., 2719 (1978).

3 V. I. Bakhmutov and V. A. Burmistrov,

Org. Magn. Reson. 12, 185 (1979).3 i E . Knippel, M. Knippel, M. Michalik, H.

Kelling, and H. Kristen, J. Prakt Chemie 320,457 (1978).

3 2 J. Frank, P. Dvortsak, G. Horvath, Z. Mer-

zaros, and G. Toth, Acta Chim. Hungaricae 89,91 (1976).

3 3 See p. 9 in reference 18.

34R. Huisgen, K. Herbig, A. Siegl, and H.Huber, Chem. Ber. 99, 2526 (1966).

3 5 E. Knippel, M. Knippel, M. Michalik, H.Kelling, and H. Kristen, Z. Chemie 446 (1975).

3' E . Knippel, M. Knippel, M. Michalik, H.

Kelling, and H. Kristen, Z. Chemie, 402 (1975).3 7

V. I. Bakhmutov, V. A. Burmistrov, K. A.Kochetkov, Izv. Akad. Nauk SSSR, Ser. Khim.

152 Bulletin of Masmetic Resonance

4, 829 (1980).3 B

G. Levy and G. Nelson, Carbon-13 NuclearMagnetic Resonance for Organic Chemists, Inters-cience, New York, 1972, p. 93.

3' M. G. Ahmed and P. W. Hickmott, J. Chem.

Soc:, Perkin Trans. II, 838 (1977).* W. Schwotzer and W. Philipsborn, Helv.

Chim. Acta 60, 1501 (1977).4 1 J . Dabrowski, A. Skup, and M. Sanelski,

Org. Magn. Reson. 1, 341 (1969). -4 2

V. I. Bakhmutov, K. A. Kochetkov, and E.I. Fedin, Izv. Akad. Nauk SSSR, Ser. Khim. 6,1295 (1980).

4 3 J . Dabrowski, J. Terpinski, and K. Kam-ienska, Roczniki Chem. 39, 769 (1965).

4 4 J . Dabrowski and J. Terpinski, RocznikiChem. 41, 697 (1967).

45R. Wehrli, H. Heimgartner, H. Schmidd,and H.-J. Hansen, Helv. Chim. Acta 60, 2034(1977).

4 'M. Kondo, Bull. Chem. Soc. Japan 51, 2973

(1978).4 7 B . I. Ionin and B. A. Ershov, YaMR Spek-

troskopia v Organicheskoy Khimii, Leningrad,"Khimia," 1967, p. 287.

4 8V. I. Bakhmutov, V. A. Burmistrov, K. K.Babievsky, K. A. Kochetkov, B. A. Kvasov, V.

M. Belikov, and E. I. Fedin, Org. Magn. Reson.11,308(1978).

4'a Yu. A. Borisov, K. K. Babievsky, V. I.

Bakhmutov, Yu. T. Struchkov, and E. I. Fedin,Izv. Akad. Nauk SSSR, Ser. Khim. 1, 123(1982).

4'

b S . M. Esakov, A. N. Petrov, and B. A.Ershov, Zh. Org. Khimii 11, 680 (1975).

4'C V. I. Bakhmutov, K. K. Babievsky, V. M.

Belikov, and E. I. Fedin, Izv. Akad. Nauk SSSR,Ser. Khim. 4, 894 (1981).

50K. Herbig, R. Huisgen, and H. Huber,Chem. Ber. 99, 2546 (1966).

5 i a V . I. Bakhmutov and K. K. Babievsky,Izv. Akad. Nauk SSSR, Ser. Khim. 9, 2166(1978).

5 l b V . I. Bakhmutov, K. K. Babievsky, and E.I. Fedin, ibid. 9, 2029 (1982).

5 2 E. Gzerwinska, L. Kozerski, and J. Boksa,

Org. Magn. Reson. 8, 345 (1976).5 3V. I. Bakhmutov and K. A. Kochetkov, Izv.

Akad. Nauk SSSR, Ser. Khim. 5, 1157 (1980).5 4

V. I. Bakhmutov and E. I. Fedin, Izv. Akad.Nauk SSSR, Ser. Khim., 8, 1776 (1981).

5 5 V. I. Bakhmutov and E. I. Fedin, Izv. Akad.

Nauk SSSR, Ser. Khim,, 1, 119 (1982).

Vol. 6, No. 3 153