JOURNAL OF MASS SPECTROMETRY, VOL. 31, 761-766 (1996)

Electron Ionization Mass Spectra of 1-(1-Naphthy1)ethyl Phenylacetates : a Study of Radical Cation Rearrangementst

J. Stuart GrossertJ G. Bradley Ybard and James A. Pincock Department of Chemistry, Dalhousie University, Halifax, NS, Canada, B3H453

Jonathan M. Curtisf National Research Council of Canada, Institute for Marine Biosciences, 1411 Oxford Street, Halifax, NS, Canada, B3H 321

A series of esters, including three specifically deuterated esters, was prepared from 1-(1-naphthy1)ethanol and phenylacetic acids having different phenyl substituents. The electron ionization (EL) mass spectra revealed the formation of the expected cleavage ions, plus two types of rearrangement ions. The formation of one type of rearrangement ion from M+' is shown to involve a process in which a benzylic hydrogen atom migrates to the naphthyl side of the ester. Subsequent cleavage results in a radical cation, a, whose structure was investigated by tandem mass spectrometric (MS/MS) experiments. The other type of rearrangement ion, b, is apparently formed by a regular McLafferty-type process with hydrogen transfer from the ethyl chain to the acid side of the ester. An MS/MS experiment on molecular ions from 1-(1-naphthyl)ethanol, which are isobaric with a, showed the presence of two fragmentation pathways, one pathway being similar to that seen for the esterderived ion u and the other a simple a-cleavage process. The mechanistic study was supported by accurate mass measurements on all relevant ions.

KEYWORDS: electron ionization mass spectrometry; tandem mass spectrometry; phenylacetate esters; benzyl esters; 1-(1-naphthy1)ethyl esters

INTRODUCTION

As part of a continuing study on the photochemistry of arylmethyl esters,' the need arose to measure the oxygen-18 content of esters which had been selectively isotopically enriched at the alcohol oxygen atom. During the course of this work, various rearrangement processes became evident from electron ionization mass spectrometry (EIMS) of the esters. EIMS of esters has been studied in some detail.' Major fragmen- tation patterns include the characteristic McLafferty rearrangements, together with a-cleavages at the car- bony1 group. In our experiments, a series of phenylace- tate esters (1-8) were prepared from the alcohols 9 and 10. Examination of the EI mass spectra of these esters (summarized in Table 1) revealed the presence of peaks at m/z values corresponding to the rearrangement ions, (I and b, and also to the cleavage ions, c and d (Fig. 1). Here we describe an investigation of these rearrange- ment processes which resulted in evidence to suggest that the radical cations, a, apparently have the same

t NRCC No. 39696. # Authors to whom correspondence should be addressed.

structure as the molecular radical cations of the corre- sponding alcohols, 9 and 10. The proposed structure of ion a is in accord with recent EIMS studies of 1- phenylalkan-1-01s.~

EXPERIMENTAL ~

Synthesis and characterization of compounds

The esters in this series, 1-8, are all formed from 1-(1- naphthy1)ethanol (a-methyl-1-naphthalenemethanol) (9),

esterified with phenylacetic acid or with certain of its derivatives. The carboxylic acids used were all com- mercially available and were purified by re- crystallization to give colourless, dry crystals with melting points corresponding to standard literature values. The isotopic content of 2,2-dideutero-2-phenyl- acetic acid (11) (PhCD'COOH) (Aldrich Chemical), was found to be >99% dideuterated by proton NMR spec- troscopy (in CDC1, solution, referenced to internal TMS and recorded on a Bruker AC250F NMR spectrometer). The alcohol 9 was prepared by reduction

CCC 1076-5174/96/070761-06 0 1996 by John Wiley & Sons, Ltd.

Received 27 September 1995 Accepted 18 March 1996

162 J. S. GROSSERT ET AL.

R X Y z l H H H 2 D H H 3 H D H 4 H H CH, 5 H H H B D H H 7 H H H 8 H H H

H H H H

OCH, CI

OCH,

NO,

9 R = H n 10 R = D 8-y-

of 1'-acetonaphthone (Aldrich Chemical) with sodium borohydride in aqueous ethanol and recrystallized from ligroin (b.p. 40-60°C) to give fine, long, silky needles, m.p. 65-66 "C (W." m.p. 65-66 "C). Preparation of 1- deuterio-1-(1-naphthy1)ethanol (10) (m.p. 64.0-64.5 "C) was carried out similarly, but using sodium boro- deuteride. The proton NMR spctrum of 10 showed that the methine carbon was -99% deuterated, which was confirmed by mass spectrometry and infrared spectros- copy (recorded as a melt between two NaCl plates on a Nicolet Model 205 Fourier transform IR spectrophotometer).

The acids were converted into their acid chlorides by stirring a solution in benzene with an excess of oxalyl chloride for 4-5 h, after which the liquids were removed by rotary evaporation. A cooled, stirred, benzene solu- tion of the acid chloride was treated with slightly less than the stoichiometric amount of alcohol 9 or 10 in benzene, followed by a slight excess of pyridine, which resulted in the formation of a white precipitate. The reaction mixtures were allowed to stand overnight and were worked up by partitioning between dichloro- methane and aqueous phases, followed by appropriate washing and drying. The esters obtained invariably appeared to be clean by thin-layer chromatography (TLC) on silica gel (dichloromethane), and were obtained as colourless oils (TLC pure) after purification by chromatography over silica gel using ligroin-

Figure 1. Major acetates.

d

fragment ions of 1 -(1 -naphthyl)ethyl phenyl-

dichloromethane (3: 1) as eluent. The purity of each ester was checked by mass spectrometry and by proton and carbon-13 NMR and infrared spectroscopy, details of which follow.

NMR and IR spectra were in agreement with liter- ature values.4

'H NMR; 6 = 1.66 (s, 3H), 3.68 (s, 2H), 7.2-8.1 (m, 12H). 13C NMR; 6 = 21.6 (CH,), 41.76 (CH,), 123.23, 125.36, 125.68, 126.30, 127.12, 128.59, 128.90, 129.38(Ar CH), 133.82, 134.04, 137.25 (Ar C), 170.87 (C=O). IR: v m m = 3062, 3033, 2981, 1733, 1511, 1262,1245,1109,797,777 cm-'.

'H NMR: b = 1.65(d, J = 6.5 Hz, 3H), 6.64 (4, 6.5 Hz, lH), 7.2-8.1 (m, 12H). 13C NMR: 6 = 21.70 (CH,), 70.03 (CH), 123.20, 125.35, 125.67, 126.29, 127.12, 128.46, 128.59, 128.90, 129.34 (Ar CH), 130.23, 133.81, 134.0, 137.28 (Ar C), 170.87 (C=O). IR: v,,, = 3060, 3030, 2983, 2933, 1733, 1250, 1210, 1069,1044,800,778 cm-'.

'H NMR: 6 = 1.67 (d, J = 6.4 Hz, 3H), 2.26 (s, 3H), 3.69 (s, 2H), 6.64 (9, 6.4 Hz, lH), 7.1-8.1 (m, 11H). 13C NMR: 6 = 19.63, 21.73 (CH,), 39.59 (CH,), 70.02 (CH), 123.18, 125.32, 125.65, 126.14, 126.26, 127.40, 128.43, 128.88, 130.28, 130.33 (Ar CH), 130.20, 132.81, 133.80, 136.86, 137.33 (Ar C), 170.75, (C=O). IR: vmax = 3051, 3020, 2981, 2932, 1733, 1255,1240,1156,1068,800,777,747 cm-l.

'H NMR: 6 = 1.67 (d, J = 6.7 Hz, 3H), 3.62 (s, 2H), 3.77 (s, 3H), 6.63 (9, 6.7 Hz, lH), 6.84, 7.18 (Ar AAXX', J = 8.55 Hz, 4H), 7.3-8.1 (m, 7H). 13C NMR: 6 = 21.68 (CH,), 40.81 (CH,), 55.27 (OCH,), 69.96 (CH), 113.98, 123.21, 125.33, 125.65, 126.26, 128.44, 128.87, 130.37 (Ar CH), 126.09, 130.23, 133.79, 137.31, 158.71 (Ar C), 171.15 (C=O). IR: vmX= 3058. 3049. 2984, 2959, 2933, 2910, 2836, 1732,1513, 1248,1156,1037,800,778 cn-'.

'H NMR: 6 = 1.66 (s, 3H), 3.61 (s, 2H), 3.77 (s, 3H), 6.75-7.25 (Ar AAXX, 4H), 7.3-8.1 (m, 7H). 13C NMR: 6 = 21.56 (CH,), 40.80 (CH,), 55.27 (OCH,), 69.7 (CD), 113.96, 123.21, 125.32, 125.64, 126.24, 128.43, 128.87, 130.35 (Ar CH), 126.09, 133.79, 137.25, 158.69 (Ar C), 171.15 (C=O). IR: v,,, = 3047,2980, 2957, 2934, 2910, 2836, 1731, 1513, 1301, 1248,1177,1164,1034,777 cm-'.

6.63 (q, 6.6 Hz, lH), 7.1-7.3 (Ar AABB', 4H), 7.3-8.1 (m, 7H). 13C NMR: 6 = 21.62 (CH,), 41.01 (CH,), 70.32 (CH), 123.13, 123.27, 125.32, 125.73, 126.32, 128.59, 128.71, 128.93, 130.73 (Ar CH), 130.22, 132.44, 133.08, 133.84, 137.08 (Ar C), 170.41 (C=O).

1159,1091,1068,801,777 cm-'.

'H NMR: 6 = 1.71 (4 J = 6.7 Hz, 3H), 3.79 (s, 2H), 6.66 (9, 6.6 Hz, lH), 7.3-8.2 (m, 11H). 13C NMR: 6 = 21.57 (CH,), 41.41 (CH,), 70.82 (CH), 123.01, 123.36, 123.77, 125.30, 125.80, 126.39, 128.79, 129.02, 130.37, (Ar CH), 130.22, 133.87, 136.77, 141.30 (Ar C), 169.46 (C=O). IR: v,, = 3074,3053,2984,2935,

'H NMR: 6 = 1.68 (d, J = 6.7 Hz, 3H), 3.63 (s, 2H),

IR: v,,, = 3050, 2983, 2934, 1736, 1492, 1253, 1218,

STUDIES ON THE EIMS OF ARYL PHENYLACETATES 163

1729, 1603, 1516, 1346, 1258, 1220, 1163, 1068, 1043, 778,732 cm-l.

Mass spectrometry

Most mass spectra were obtained using a VG ZAB-EQ mass spectrometer by electron impact ionization at a source temperature of 200-210 "C (sample decomposi- tion was observed above 210 "C), an accelerating voltage of 8 kV and an ionizing electron energy of 70 or 12 eV (nominal). Accurate mass measurements were made at a mass resolution of 8000 (10% valley definition) by voltage scanning using PFK reference ions. Linked scans used collisional activation by helium in the first field-free region, causing a beam attenuation of - 50%. Mass-analysed ion kinetic energy scans of mass-selected ions, collisionally activated using helium in the second field-free region, used similar collision conditions. The tandem mass spectra of the molecular ion from alcohol 9, its deuterated analog 10 and ions from ester 2 were obtained usin a VG AutoSpec oa-TOF mass spectrometer, with time-of-flight (TQF) analysis of the fragment ions formed from collisional activation using Xe collision gas at 800 eV (laboratory frame).

RESULTS AND DISCUSSION

Examination of the 70 eV EIMS of esters 1-8 (Table 1) reveals that the base peak is almost always due to bond cleavage to give ions c (Fig. l), presumably because the predominant radical charge site in the ester molecular ions is on the naphthyl moiety. However, in the EIMS of the ester 5, which contains a 4-methoxyphenyl moiety, and its deuterated analogue 6, the benzylic ions d are of abundance approximately equal to that of ions c. This suggests that for 5 and 6, a significant fraction of the radical charge resides on the methoxyphenyl ring. It should be noted that accurate mass measurements per- formed on all the ions listed in Table 1 were in agree- ment to within 0.5 mDa of the m/z value predicted for the assigned structures.

The observation of ions a is intriguing. These are odd-electron ions which may be formed by the same rearrangement process which leads to the base peak in the EI mass spectrum of benzyl acetate (acetic acid phenylmethyl ester) at m/z 108, or [M - 42]+'. '-' The same types of ions are also observed in the EI mass spectra of phenyl acetate and related esters.'~~ However, the importance of the process which leads to the forma- tion of ions a diminishes rapidly as the structures of the esters become more complex and other reaction path- ways become accessible, This is readily apparent in the EI spectrum of the next homologous ester, 2- phenylethyl acetate, in which the abundance of the [M - 42]+' ion is only 0.7% of that of the base peak." In this case the base peak, m/z 104, is also a radical cation, but one which arises from an energetically favourable

McLafferty rearrangement. In the case of 1, the ratio of the abundance of rearrangement to cleavage ions (a/d) changed from 0.09 at 70 eV to 2.0 at 12 eV, which is consistent with Brown's observations on competing rearrangement and cleavage processes, such as occur in benzyl a ~ e t a t e . ~

We wished to confirm that the source of the proton in the formation of the rearrangement ions a was indeed from the benzylic position adjacent to the phenyl ring. This was achieved by the preparation of ester 3, which was dideuterated at the benzylic position. It was found that the fragment ions b and d in the EI mass spectra of 3 are two m/z units higher than those from the nonde- uterated equivalent 1, while the fragment ion c (the naphthyl moiety) had the same m/z value in 3 as in 1. Furthermore, the rearrangement ion a in the EI mass spectrum of 3 is at m/z 173, which is one m/z unit higher than for the non-deuterated equivalent 1. The abun- dance of m/z 172 (a - 1) in ester 3 was 20% of that of m/z 173 (a); for comparison in 1, the absorbance of m/z 171 (a - I) was 30% of that of m/z 172 (a). These data show clearly that one of the deuterium atoms at the benzylic position migrates across the carbonyl group to give the radical cation a.

When the phenyl ring is substituted with a methyl group at the ortho position (ester 4), fragment ions a could also in principle arise by hydrogen transfer from this methyl group, although this transfer would require a transition state involving at least a seven-membered ring. As seen in Table 1, the abundances of ion a in 1 and 4 were comparably low. This suggests that there is little if any, hydrogen migration from an ortho-methyl group on the phenyl ring to the alcohol oxygen atom of ester 4. When substituents on the benzene ring were made more electron-donating or -withdrawing than a hydrogen atom (5, 7 and 8), relatively minor changes in the relative abundances of the rearranged ions a were observed. However, dramatic changes in the abun- dances of the cleavage ions d correlated appropriately with the electron-donating or -withdrawing effects of the functionalities on the phenyl ring, in accord with the Hammett concept.

Linked scans at constant B/E, using collisional acti- vation with helium, were performed for molecular ions and ion source-derived fragment ions of type a. The major fragment ions observed in these experiments are listed in Table 2. In all cases, with the exception of the methoxy esters 5 and 6, ions a were observed as frag- ments of the molecular ions M'' in these tandem mass spectra. The failure to observe ions a from 5 and 6 in these experiments is consistent with their low abun- dances in the corresponding EI mass spectrum (Table 1).

The fragment ions observed in MS/MS experiments on ion source-derived type a ions from esters 1, 7 and 8 (Table 2) were similar to those observed in the EI mass spectrum of the parent alcohol 1-( l-naphthyl)ethanol(9) and its deuterated analog (10). The 70 eV EI mass spec- trum of 10 shows three dominant fragment ions, at [M - 15]+, [M - 431' and m/z 43 (Fig. 2(a)). These are also the major fragments in the EI mass spectrum of the benzylic alcohol, l-phenylethanol (a-methyl- phenylmethan~l).~.' EI mass spectra of alcohol 9 and its l-deuterated derivative (10) have not been published4

764 J. S. GROSSERT ET AL.

Table 1. The m/z values and normalized abundance (%, in parentheses) of the major ions in the EI mass spectra of 141-naphthy1)ethyl phenylacetates (1-8)

Compound M *. 8 b c d

1 (70 eV) 290 (25) 172 (2) 136 (1) 155 (100) 91 (22)

2 (70 eV) 291 (7) 173 (1) 136 ( t l ) 156 (100) 91 (60)

3 (70 eV) 292 (14) 173 (3) 138 (2) 155 (100) 93 (28) 3 (12 eV) 292 (65) 173 (4) 138 (3) 155 (100) 93 (3)

4 (70 eV) 304 (6) 172 (1) 150 (2) 155 (100) 105 (16)

1 (12eV) 290 (100) 172 (6) 136 (6) 155 (84) 9'1 (3)

2 (12eV) 291 (100) 173 (2) 136 (2) 156 (50) 91 (1)

5 (70 eV) 320 (4) 172 (< I ) 166 (1) 155 (loo) 121 (94) 5 (12 eV) 320 (70) 172 (6) 166 (12) 155 (100) 121 (75) 6 (70 eV) 321 (3) 173 (<1) 166 (1) 156 (95) 121 (100) 6 (12eV) 321 (100) 173 (<1) 166 (8) 156 (100) 121 (82)

7 (70 eV) 324 (54) 172 (5) 170 (2) 155 (100) 125 (22)

8 (70eV) 335 (18) 172 (5) 181 ( ~ 1 ) 155 (100) 136 (2) 8 (12 eV) 335 (100) 172 (5) 181 (<1) 154 (60) 137 (2)

but have been studied in some detail (see Fig. 2(a)).12 The tandem mass spectra (Table 2) of ester-derived ions a were indistinguishable in all but one important point from the tandem mass spectra obtained by collisional activation of the molecular ions of the alcohols. As exemplified by the tandem mass spectra of the molecu- lar ion from alcohol 10 and of ion a from ester 2 (Fig. 2(b) and (c), respectively, obtained using an AutoSpec oa-TOF spectrometer), the alcohols show an additional fragment ion at m/z 45 (m/z 46 for lo), which must arise from a simple a-cleavage process in which the largest radical, the naphthyl ring, is lost.13 This process was not seen in the tandem mass spectra of ions u derived from the esters and is helpful in understanding the rearrangement processes of these ions. A proposal for the rearrangement-fragmentation process of the mono- deuterated ester 2 is outlined in Scheme 1. Ions a can be formed by sequential migrations of a hydride ion (a 1,2-

shift to a cationic centre) and a hydrogen atom. The latter migration is through a seven-membered transition state in the case of the esters, but is a five-membered transition state in the case of the alcohols. It is pro- posed that ions u may reasonably have the structure of ketone radical cations, and can fragment by a-cleavage in either direction to give the observed fragment ions at m/z 43 and 130 (Scheme 1 and Fig. 2). The difference between the esters and the alcohols lies in the fact that the latter also show an a-cleavage process, q (Scheme l), not available to the esters, which results in the forma- tion of a fragment ion at m/z 46 (m/z 45 for non- deuterated compounds).

The fact that fragment ions a were not observed for 5 and 6 in the tandem mass spectra of their molecular ions is consistent with the low abundance of these ions in their mass spectra, and probably reflects the com- petition from the enhanced stability of fragment ion d

Table 2. Fragment ions from linked scans at constant B/E

Fragment ions (mp with relmive

Ion tyP Compound PTncunor ion (Da) ebundancs (%) in premhese9)

1 M +. 2 M" 3 M +. 4 M +-

6 M +*

6 M+' 7 M+' 8 M +.

172 (lo), 155 (100) 173 (4). 156 (100) 173 (9). 155 (1 00) 257 (2), 172 (2). 155 (100) 273 (4). 167 (7), 155 (100) 274 (3,168 (5). 156 (100) 172 (7). 155 (100) 172 (25), 155 (100)

a, c a. c a. c [M - 471'. a, c [M-47]+.[b+H],c [M -47]+, [b +HI, c a. c a. c

1 a (172)' 157 (60). 129 (100) [a - 151. [a -431

3 c (155) 153 (100). 127 (40) [C - 23, [C - 281 3 a (173) 158 (100). 1 3 0 (82) [a - 1 51, [a - 431

4 a (172) 157 (75). 129 (100) [a-151. [a-431 7 a (172) 157 (85). 129 (100) [a - 151, [a - 431

[a - 151. [a - 151 8 B (172) 157 (100).(129 (78)

' Similar MS/MS results were obtained by collisional activation of the molecular ion derived from 9 (see Experimental).

STUDIES ON THE EIMS OF ARYL PHENYLACETATES 765

50 90 130 d z

170

[m - W O . ] +

50 I30 170 nv'r

U 50 90 130

d Z 170

Figure 2. (a) 70 eV El mass spectrum of deuterated alcohol 10; (b) tandem mass spectrum of the M+' ion from alcohol 10; (c) tandem mass spectrum of ion a from ester 2.

due to the presence of the para-methoxy group. The for- mation of fragment ion a (m/z 173) from the molecular ion of 2 (m/z 291) was further confirmed by a precursor ion scan (linked scan at constant P I E ) , as was the for- mation of b (m/z 166) from the molecular ion of the methoxy ester 6 (m/z 321). Unfortunately, the MIKE spectra for the fragmentation M"+a (1, 5 and 8) showed unresolved peaks, which prevented the evalu- ation of the kinetic energy released during the rearrangement process. Therefore, no correlation could be made between the energy release values for this frag- mentation and the functionalities on the phenyl ring as described by a possible Hammett-type relationship.

Deuteration c1 to the naphthalene ring (esters 2 and 6) enabled some information to be obtained about the

rearrangement process forming ions b. These would be expected to arise from a McLafferty-type process via a six-membered transition state involving the transfer of a hydrogen atom from the methyl group, rather than of the proton on the carbon atom a to the naphthyl ring. Since no incorporation of deuterium into b occurred in the spectra of 2 and 6, it is reasonable to conclude that the usual McLafferty-type process involving a six- membered transition state must have been operational. However, in these esters, even this normally favoured process is overshadowed by competing cleavage pro- cesses (giving ions c and d).

In conclusion, it should be noted that there has been significant interest in rearrangement processes of simple formate, acetate and propionate esters.14-17 These

766 J. S. GROSSERT ET AL.

2+‘

Me D )=< m/r 46 mh43 d z 130

both benzylic esters and alcohols can lead to the forma- tion of ions a, presumably via a 1,Zhydride shift and a hydrogen atom migration, as shown by pathway p in Scheme 1. These ions a probably have the structures of ketone radical cations and can fragment by rr-cleavage in either direction. However, molecular ions of benzylic alcohols can also fragment directly via a-cleavage prior to isomerization (pathway q in Scheme l), resulting in an additional fragment ion not observed in the tandem mass spectra of the ester fragment ions a. It may be that the proposed pathways in Scheme 1 will apply in general to the EIMS of all benzylic species with both a heteroatom and a second hydrogen atom, which is accessible to abstraction through a transition state of appropriate geometry. Formation of ions a is dis- favoured by the introduction of a strongly electron- releasing group on the phenyl ring, which in turn facilitates the formation of ions d and also the antici- pated McLafferty-type process to give ions b.

Scheme 1.

studies attest to the fact that even in the cases of simple esters, understanding the actual structures of the ions observed in their EI mass spectra is a complex process. Undoubtedly, this applies equally to the compounds NMR were measured Dr D. L. Hoop at the

Atlantic Region Magnetic Resonance Center, Dalhousie University. described in this work* we suggest that the Thanks are also due to the IMB laboratory of the NRC for facilities results reported, together with the literature results on (J.s.G. and G.B.Y.) and to Dr R. K. Boyd (IMB) for helpful dis- some phenylalkanols, offer evidence that the EIMS of cussions.

Acknowledgements

REFERENCES

1.

2.

3.

4.

5. 6.

7.

8.

J. W. Hilborn, E. Macknight, J. A. Pincock and P. J. Wedge, J. Am. Chem. SOC. 116, 3337 (1994). and references cited therein; J. M. Kim and J. A. Pincock, Can. J. Chem. 73, 885 (1995). F. W. McLafferty and F. TureEek, hterpretation of Mass Spectra, 4th edn, pp. 251-258. University Science Books, Mill Valley, CA (1 993). H. Budzikiewicz, G. Drabner and Ch. Hammes, Org. Mass Spectrom. 28,1326 (1 993). P. D. Theisen and C. J. Heathcock, J. Org. Chem. 53, 2374 (1 988). P. Brown, Org. Mass Spectrom. 3,1175 (1 970). 0. L. Corina, J. N. Wright and K. E. Ballard, Org. Mass Spectrom. 18,60 (1 983). H. Nakata, K. Iwata, T. Kondo, H. Yoshizumi and A. Tate- matsu, Shitsuryo Bunseki 32, 297 (1 984) ; Chem. Abstr. 102. 1 48552 (1 984). A. A. Gamble, J. R. Gilbert and J. G. Tillett, Org. Mass Spectrom. 5,1093 (1971).

9. V. J. Feil and J. M. Sugihara, Org. Mass Spectrom. 6, 265

10. E. M. Emery, Anal. Chem. 32,1495 (1 960). 11. N. M. M. Nibbering and Th. J. de Boer, Org. Mass Spectrom.

12. J. S. Grossert, P. K. Dubey and J. M. Curtis, to be published. 13. F. W. McLafferty and F. TureEek, hterpretation of Mass

Spectra, 4th edn., p. 60. University Science Books, Mill Valley, CA (1 993).

14. B. J. Smith, M. T. Nguyen and L. Radom. J. Am. Chem. SOC. 114,1151 (1992).

15. N. Heinrich, T. Drewello, P. C. Burgers, J. C. Morrow, J. Schmidt, W. Kulik, J. K. Terlouw and H. Schwarz, J. Am. Chem.Soc. 114,3776 (1992).

16 D. T. Leeck, K. M. Stirk, L. C. Zeller, L. K. M. Kiminkinen, L. M. Castro, P. Vainiotalo and H. 1. Kenttamaa, J. Am. Chem. SOC. 11 6,3028 (1 994).

17. C. A. Kingsmill, D. Suh, J. K. Terlouw and J. L. Holmes, J. Am. Chem. SOC. 116,7807 (1994).

(1972).

1,365 (1 968).