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1712St ruc tur al Effects in Solvolytic Reactions. 18. T heRelative Electron Releasing Properties of Methyl,Phenyl, and Cyclopropyl Groups to the ElectronDeficient Ce nte r as Indica ted by Solvolysis. ACritical Examination of 1 3 C Chem ical Shifts as a Basisfor Establishing the Electron Densities in C arboniu m IonsHerbert C. Brown* and Edward N. Peters'**Contribution from the Richa rd B . Wetherill Lab oratory, Purdue University,West Lafayette, Indiana 47907. Received December 22 , 1975

Abstract: Rate constants (25 "C) for the solvolysis of the p-nitrobenzoates, RMe2COPNB, in 80% aqueous acetone exhibitmajor increases as the g roup R is varied from methyl (1.00), isopropyl (2.9), or tert-butyl (4.4) to phenyl (969) and to cyclo-propyl (503 000). The small difference in the rates for R = methyl, isopropyl, and tert-butyl indicates that relief of B straincannot be a significant factor in the large rate enhancements observed for R = phenyl and cyclopropyl. These large rate en-hancements mu st arise from stabilization of the respective transition sta tes (and the cationi c intermediates subsequently pro-duced) by major electron supply from the phenyl and cyclopropyl groups to the electron deficient center. Yet these rate s failto correlate with the I3C chemical shifts for the carbonium carbon of the respective ca tions (R = methyl, 329.2; R = phenyl,254.4; R = cyclopropyl, 280.6 ppm), although it has been proposed t ha t such shifts do provide a m easure of the electron densi-ties at the ca rbonium carbo n atoms. Consequently, there exists here a m ajor discrepancy between conclusions based on solvoly-tic dat a and conclusions based on 13C shifts as to the relative magnitu des of the charge delocalization from the ca tionic centerinduced by phenyl and cyclopropyl groups. It is pointed out th at ra te dat a for a number of systems are internally consistent inpointing to electron release toward ca tionic centers from sub stituents increasing from alkyl to phenyl to cyclopropyl, whereasthe data for I3C shifts are not SO consistent. The results argue for a reconsideration of the reliability of I3C shifts as a m easureof the electron densities in carbonium ions and of the stabilities of these cations, as well as of the application of such shifts asa diagnostic probe for nonclassical structu res.

Ever since the pioneering work of Ingold and his co-work-e r ~ , ~ , ~he solvolysis of organic ha lides an d relat ed derivativeshas been a majo r tool for studying the effects of structu re onchemical reactivity. In this way, a large body of consistentinformation has been accum ulated on the effects of s tructureand of substi tuents on t h e r a te s of s o l v ~ l y s i s . ~ - ~uch s tudiessuffer from the weakness th at they give us primarily th e dif-ference in energy between the initial and t he transition states.However, it is now relatively easy to co rrect for the energy ofthe ground state ' and the H amm ond postu late jus tif ies con-sidering the transition stat e for representative SN 1 solvolysesto be quite close to the actu al carbonium ion in termediates .8In recent years, it has become possible to prepa re the ca r-bonium ions under sta ble ion conditions and to observe themspe ctro s~o pic ally .~ h ile i t is to be hoped that th is new toolfor the s tudy of carboni um ions will lead us to a better u nder-standing of the relationship between struc ture and stability,lOearly attempts, based on the idea of a simple relation betweenI 'C chemical shifts and electron density, led to conclusionswhich run directly counter t o positions previously arrived a ton the basis of solvolytic data."

For example, Olah a nd h is co-workers examined the ter t-butyl cation (1) and the phenyldimethyl- (2) and the cyclo-propyldimethylcarbonium (3) ions and had reported I3Cchemical shifts of 329.2, 254.4, and 280.6 ppm respectively,'C h a r t I. They proposed f rom these results th at the phenylgroup must be considerably more electron releasing than acyclopropyl gro up, ' I a conclusion which runs count er to thatindicated by num erous other results,I2 especially to IH N M Rdata for a secondary system.I3This pape r desc ribes a study of the solvolysis of the thre eter t iary systems d irect ly related to the three ter t iary cat ionsexamined in the 13C study of Cha rt I2 of which we made apreliminary report in 1973.2 The s tudy was l ater ex tended t oinclude the related tertiary 1-R-I-phenyl-1-ethyl series.

Char t I

I3C M R shifts,ppm from TMSResults

1 2 3329.2 254.4 280.6

Synthesis. The tertiary alcohols were prepared by the ad-dition of Grignard reagents to the appropriate ketones. Thetertiary alcohols were converted into p-nitrob enzoa tes bytreating their lithium salts with p-nitrobenzoyl ch10ride.I~Kinetic Studies. The rate constants and thermodynamicparam eters of the deriva tives selected for solvolysis in 80:20acetone-water are summarized in Tables I an d 11.Discussion

Relative Effectivenessof Phenyl and Cyclopropyl Groups inFacilitating Solvolysis. 1, I-Diphenyl- 1-ethyl p-nitrobenzoate(5) reacts 72 t imes fas ter than 2-phenyl-3-methyl-2-butylp-nitrobenzoate (4). Thus, both phenyl groups in 5 are helpingto s tabil ize the ca t ion . However, l-phenyl-l-cyclopropyl-l-ethyl p-n itrobenzoate (6) eacts over IO4 times as fast as 2-phenyl-3-methyl-2-butyl p-nitrobenzoate (4). Clear ly , thecyclopropyl group in 6 s stabilizing the adjacent carboniumion center mo re than t he additiona l phenyl ring in 5. In fact ,the cyclopropyl ring is 353 times more effective than the ad -ditional phenyl in stabilizing the transition state.However , it might be argued tha t these data for the morestabilized phenyl derivatives in Cha rt I1 do not provide a fai rtest of the proposal. Perhaps structures should be examinedJ o u r n a l of the American Chem ical Society / 99.6 / M a r c h 16, 1977

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1713Table 11. Rate D ata for the Solvolysis of the Tertiary I-P henyl-I-ethyl p-Nitrobenzoates, R(Ph)CH $OPN B, in 80% Acetone

A H + ,Substituent k1 ' 5" x 106, Re1 rat e, kcal AS*,R S-I 25 "C mol eu

Table I. Rate Data for the Solvolysis of the Tertiary 2-Propyl p-Nitrobenzoates, R(CH&COPNB, in 80% AcetoneAH*,Substituent k I25' x 106, Re1 rate, kcal LIS*,R S-I 25 'C mol-' eu

Methyl" 7.45 x I O - S b 1.00 29.2 -7.1Isopropyl 2.15 x 1 0 - 4 b . r 2.9 28.5 -7.3t e r t -Bu ty ld 3.25 X 4.4 29.0 -4.8Phenyl 7.22 X b , e 969 24.8 -8.2Cyclopropyl 37.5bJ,g 503 000 20.8 -9.0H . C. Brown and W. C. Dickason, J . Am . Chem. SOC., 1, 1226(1969). Calculated from dat a at other temperatures. k I I50" = 4 19X s-' , kl 1 2 5 0= 47.9 X s-l. E. N. Peters and H . C. Brown,J .Am.Chem.Soc. , 97,2892(1975). ek17So=33.6X 0-6s-',k1'00"= 39 1 X 1 0 - 6 s - 1 . ~ k 1 5 0 0 = 6 1 4 X0-6s- ' .gTheproductofsol -volysis is unrearranged cyclopropyldimethylcarbinol: D. C. Poulterand S. Winstein, J . A m . Chem. SOC., 1,3650 (1969).

Chart I1

4 5 6Re1 rat e 1.00 71.6 25,300which are directly related to the trimethyl, phenyldimethyl,and cyclopropyldimethylcarbonium ions util ized in th e I3Cstudies of Chart I. Accordingly, we undertook to obtain kineticdat a for th e solvolysis of the three p-nitrobenzoates (C har t 111)Ch art I11

fH3cn,IH,II

H3C--C 4 P N B H,C- Fe B ,C-Y-OPNB

7 8 9Log k,, -1 -10.13 -7.14 -4.43I3C NMR shifts, 329.2 254.4 280.6ppm f rom TMSrelated to these three cationic intermediates under essentiallyidentical conditions.I t was observed th at th e replacement of a methyl group inthe ter t -butyl system (7 ) by a phenyl group (8) increases ther a t e a t 2 5 O C by a factor of l o 3 . Replacement of the phenylgro up by a cyclopropyl group (9) i ncreases t he ra t e a t 25 OCby another comp arable factor of 102.7.Before we can proceed to interpret these results, i t is nec-essary to establish that differences in B strain7 ar e not a sig-nificant factor in the observed r ates. An isopropyl group h assteric requirements similar to those of a phenyl or a cyclopropylgroup,I5 and a tert-butyl grou p must h ave considerably larg ersteric requirements than a methyl, phenyl, or cyclopropylg r o u p t 6 C h a r t IV). Solvolysis of isopropyldimethylcarbinyl(10) and tert-butyldimethylcarbinyl p-ni t robenzoate (11)yields rates of 2.9 a nd 4.417 larger tha n t hat of 7. Clearly, reliefof B strain cannot represent a m ajor contribution in the factorsof 969 and 503 000 observed for the respective effects of aphenyl and cyclopropyl group in the corresponding deriva-tives.

Isopropyl 9.51 x 10-3 0 . 6 1.00 26.7 -5.7Phenyl 0.68 1b. c 71.6 26.0 -0.3Cyclopropyl' 24 1 25 300 20.8 - 5 .5'I E. N. Peters and H. C. Brown, J . A m . Chem. Soc., 95, 2397(1973). Calculated from data at other temperatures. k 7 5 0 = 42 4X S - I , k I S 0 " 21.8 X S - ' .

Chart IVFI

I II

CHI H3C--C-CHI HF-CC-CH3Hf-C> methyl,20 quite dif-

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1714ferent from that arrived at in the earlier studies.2is22 la h nowsuggests that this modified order is the result of significantneighboring-group deshielding of the carbo nium ion center bythe cyclopropyl substitue nt.Som e of Olahs own data illustrate th e same difficulties. Forexample, cycloprop ylcarbonium ions, Cp,Me3-,C+ (x = 0to 3) , exhibi t much smal ler changes in th e I3Cshi f t as com-pared to the correspond ing phenyl series, PhXMe3-J? (x =0 to 3). The da ta would appear to be in better agreement witha sw amping effect resulting fro m larger electron supply fromthe cyclopropyl substituents th an from the phenyl groups.Furthermore, if the I3C shift is simply related to charged e l o ~ a l i z a t i o n , ~ ~he observed 13Cshifts would be expected tovary in the sam e order m ethyl , cyclopropyl , and phenyl , ac-cording to Olahs own proposed interpretation, irrespective ofwhether the comparison is based on th e +C or p-C va lues(Chart VI). However, this is not observed in the dimethyl-Chart VI

t t t13 14 15C NMR shif ts (+C ) 254.4 229.3 246.313C NMR shifts @-C), 156.0 148.2 145.6ppm f rom TMSLog h ( R O P N B ) -7.14 -6.17 -4.62phenyl- (13), diphenylmethyl- (14), and the cyclopropyl-methylphenylcarbonium ions (15). Indeed, the trend of the I3Cshifts observed for th e para carb ons parallels that for the sol-volysis of the corresponding p-nitrobenzoates, as reported here(Le., 8,5, and 6, respectively), whe reas the tren d of the valuesfor the + C shi f ts i s di fferent .Similarly, the sam e trend of the I3C hift of the para carbonatom s is observed in the series in Ch art VI 1 of tert-cumyl- (13),Chart VI1

H$ ,g/CH 1 Ph \ +/Ph-L + ?

t t t13 16 17

C NMR shifts (+C) 254.4 211.9 261.1NM R shifts (p-C), 156.0 144.1 136.6ppm f rom TMStriphenylmethyl- (16), and dicyclopropylphenylcarbonium ons(17),whereas the trend for the carbo nium carbon s differs. Theresults can be rationalized in terms of Olahs latest proposalof a deshielding of the carbon ium carbon by neighboring cy-cIopropyI.20Fur ther s upport is provided by a recent study of Volz andhis co-workers of the I9Fchem ical shifts of cations 18,19,and2024 Ch art VII I). These autho rs concluded th at their resultsare in agreement with solvolytic, equilibrium, and polaro-graphic studies with the cyclopropyl group stabilizing thecarbonium ion bet ter than th e phenyl group.It is of interest that O lahs own earlier IH N M R resul ts l edhim to a conclusion opposite to that he reached on the basis ofhi s I3C esults, namely, that th e cyclopropyl grou p stabilizes

Chart VI11

F F F18 19 20

19F shift , -59.7 -78.7 -81.99ppm from CC1,Fthe carbonium ion bet ter than the phenyl group in a re la tedseries of secondary ions (21-23)13 (C ha rt IX) .Chart IX + + +H - C C H , H -CCH , +C--Cn,I I I6 A

21 22 23H NMR shifts, 13.5 9.8 8.14ppm f rom TMS

Finally, in a study of the relative stab ilizing effects of phenyland cyclopropyl substituents, Taft, H ehre , and co-workers havenoted that N M R chem ical shi f t-energy corre lat ions are notgeneral.25Usin g pulsed ion cyclotron resonance spectroscopyand a b init io molecular orbital theory to provide informationon the ionic species in the ga s phase (eq l ) , hey concluded tha t

neither the calculated H charge densities nor the I3C shiftsproperly reflect th e relative stabilizin g effects of the phenyl an dcyclopropyl group on the tertiary ions in the gas phase. Th eyconcluded tha t the phenyl grou p is superior to cyclopropyl instabilizing a secondary carbonium center, but that the reverseis tru e for tertiary carbo nium centers. H owever, these calcu-lations are for the gas phase a nd their applicabili ty to solutionphenomena is uncertain.A Critical Examination of I3C Shif ts as a Basis for Estab-lishing Electron Densities in Carbocations. Th e quest ion wenow face is whether the phenyl-cyclopropyl question we haveexplored in the present study represents a n exception, so tha twe can rely on such interpretation of I3Cshifts in other sys-tems, or ar e there reasons to exercise caution in utilizing suc hinterpretations of 13C shifts in other systems.Examination of the 13C hemical shifts of tert-c umy l cationsdoes reveal the existence of a linear relation ship with u+ con-Howev er, many reactions involving changes in sub -sti tuents in the meta an d para positions of the arom atic ringare correlated by the H am me tt relationship, but th e same kindof quantitative treatm ent ca nnot be carried over to other sys-tems where st ructura l var iat ions are made a t or near the re-action center.Certain data already available suggest the hazards involvedin applying this procedu re to cations involving such structuralmodifications at the cationic center. Th e isopropyl cation is farless stable than th e tert-bu tyl cation and m ust possess a muchhigher concentration ?f c harg e at the cationic carbon, buttheir I3Cshifts are 318.8 and 329.2 pp m, respectively.This apparent anomaly might be a consequence of the ten-dency fo r such highly active secon dary species to exist as t ightion pairs. If so , the anomaly m ight vanish wi th m ore stablecationic species. Both thermodynamic and kinetic data revealthe t r iphenylmethyl ca t ion to be more stable than the di -

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1715Table 111. Preparation of Tertiary Alcohols

Bp or Lit. bp or Lit.Compd mp,"C n Z 0 D mp, "C nZ0D2,3-Dimethyl- 120-121 1.4158 117-118 1.413 I "2-butanol (740 mm)'

propanol ( I O mm) mm)?2-Cyclopropyl- 123 1.4331 123.4-123.6 1.432 57d2-propanol (7521,l-Diphenyl- 80.5-81.2 81e1-ethanol

2-Phenyl-2- 90 93-94 ( 13

J . Lindner, Monatsh. Chem., 32, 419 (1911). V. J. Shiner, J .Am. Chem.Soc., 76,1603 (1954). A. Kloges,Ber., 35,2633 (1902).P. Bruylants, Bull. SOC. him. Belg., 36, 153 (1927). e A. Jung andF. Jung, Bull . Soc. Chim. Fr., 269 (1966).phenylmethyl cation.28However, t he observed chemical shiftsare 211.9 and 199.4 ppm, respectively.21 Thus, even theserelatively stable cations d o not ob ey the proposed relationship( C h a r t X ) .C h a r t X

p K R + l H a -13.3 -6.63Log h , (RCI in -4.27 -1.90EtOH)2sbppm from TM SIt might be argued that a st ructura l change from a secon-dary to a tertiary carbo nium ion is so la rge that the I3Cshiftsshould not be expected to obey the proposed relationship.Perh aps th e relationship should be applied only to a closelyrelated family of secondary or of tertiary cations.For example , is i t safe to conclude that th e 2-methylnor-bornyl cation is 50% n o n c l a ~ s i c a l , ~ ~n spite of the mass ofchemica l evidence favoring the classical structure, ' merelybecause the 2-methylnorbornyl cation exhibits a I3Cchemical

shi f t of 269.9, as compared to 329.2 for ter t -butyl? Is i t safeto conclude that th e 2-bicyclo[2.1. l lhex yl an d 2-methylbicy-cl0[2.1 .l]hexy l cations are classical, merely on th e basis of 13Cshifts,30 n spite of the argum ents th at this system is a far bettercandidate for u bridging than th e related 2-norbornyl deriva-t i v e ~ ? ~,3 2Difficulties ar e evident even when w e restrict ourselves tovery closely related secondary or tertiary derivatives. For ex -ample , the I3Cchemical shifts of the 7-norbornenyl and 7-norbornadienyl cations and the logarithms of their rates ofsolvolysis for the corresponding chlorides do not follow theproposed r e l a t i o n ~ h i p ~ ~ ? ~ ~Char t XI). Finally, it is qui te clearC h a r t XI

"C NM R shi fts, 199.4 211.9

Lo g k, -6.09 -3.22I3C NM R sh i f t s 31.6 41.1(C7) fo r cat ion ,ppm from TMS

that the relationship is not operative, even for the closely re-lated group of three tertiary derivatives that was the mainobjective of this s tudy (C harts I and 111).It is to be hoped th at fu rther w ork will place on a quant i ta-tive basis the fac tors influencing t he 13Cshift in carbocations.Wi th a sound theoretical fou ndation, i t should be possible toutilize such shifts to determ ine the c harg e densities in su ch ions.

Table IV. Preparation of p-Nitrobenzoatesp-Nitrobenzoate Mp, "C Lit. mp, "C Anal.

5 135 dec C, H , N9 86.5-87.5 89-90b8 133-134 136-1 37'10 78.8-79.5 82?

Ll L. F. King, J . Am . Chem. Soc., 61,2383 (1939). M . Hanackand K. Gcerler, Ber., 96,2121 (1963). M. D. Cameron et al.,J . Org.Chem., 19, 1215 (1954).

This is not possible now .lo An empir ical approach t o utilizingI3C shifts for estimating charg e densities is of value only so longas majo r exceptions ar e not encounter ed. Several such excep-tions have been pointed ou t here.10%34Conclusion

In view of these developments, it is highly question able if I3Cshifts can be used in their present state of understan ding o oprove unequivocally the str ucture of th e 2-norbornyl ca tion,lgbthe 2-bicyclo[2.l .l]hexyl cation,30or of other proposed clas-sical and nonclassical ions.35,36Experimental SectionPreparation of Tertiary Alcohols and p-Nitrobenzoates. 2-Iso-propyl-, 2-phenyl-, and 2-cyclopropyl-2-propanols nd 1,l -diphe-nyl- 1-ethanol were prepared by the addition of methylmagnesiumchloride to commercially available 3-methyl-2-butanone, acetophe-none, cyclopropyl methyl ketone, and benzophenone, respectively. Thephysical data are listed in Table 111. Thep-nitrobenzoates were pre-pared by treating the lithium salt of the alcohols with p-nitrobenzoylch10 ride.l~ he physical and analytical data ar e listed i n Table IV .Kinetic Procedure. The procedure employed in determining th e rateconstants of p-nitrobenzoates followed that described earlier.References and Notes(1) Postdoctoral research associate on a grant (GP 31385) provided by the(2) For a preliminary report, see H. C. Brown and E. N. Peters, J. Am. Chem.National Science Foundation.

Soc., 95 , 2400 (1973).C. K. lngold et al., J. Chem. Soc.. 901-1029 (1940).C. K. Ingold, "Structure and Mechanism in Organic Chemistry", 2nd ed,Cornell University Press, Ithaca, N.Y., 1969.A. Streitwieser, Jr., "Solvolytic Displacement Reactions", McGraw-Hill,New York. N.Y.. 1962.(6) W. J. le Noble, "Highlights of Organic Chemistry", Marcel Dekker, New(7) H. C. Brown, "Boranes in Oraanic Chemistry". Cornell University Press,York, N.Y., 1974.Ithaca, N.Y., 1972.(8 ) G S.Hammond, J.Am. Chem. Soc., 86 , 1957(1964).(9) G. A. Olah, "Carbocations and Electrophilic Reactions", Wiley, New York,N.Y., 1974.10) D. G. Farnum. Adv. Phys. Org.Chem., 11, 123 (1975).11) G. A. Olahand A. M. White, J. Am. Chem. Soc., 1, 5801 (1969).12) H. G. Richey. Jr., "Carbonium Ions", Vol. 111, G. A. Olah and P. v. R. Schleyer.Ed., Wiley-Interscience, New York, N.Y., 1972, Chapter 25 .13) C. U. Pittman, Jr., and G. A. Olah, J. Am. Chem. Soc.,87 , 5123 (1965).14) H. C. Brown and E. N. Peters, J. Am. Chem. SOC.,7, 1927 (1975).15 ) H. Hart and P. A. Law, J. Am. Chem. Soc., 6, 1957 (1964).16) For a recent discussion on the steric effects of a tert-butyl group, see E.

N. Peters and H. C. Brown, J. Am. Chem. Soc., 97 , 2892 (1975).(17) E. N. Peters and H. C. Brown, J. Am. Chem. Soc.,96 , 263 (1974).(18) (a) H. C. Brown, J. D. Brady, M. Grayson, and W. H. Bonner, J.Am. Chem.Soc., 79 , 1897 (1957); (b) Y . Okamoto and H. C. Brown, ibid., 79 , 1909(1957).(19) P. v. R. Schleyer andG. W. Van Dine, J.Am. Chem. Soc., 88,2321 (1966).The data are for 100 "C, rather than the 25 "C values used otherwise inthis paper.(20) G. A. Olah and R. J. Spear, J.Am. Chem. Soc., 97 , 1539 (1975).(21) G. A. Olah and P. W. Westerman, J. Am. Chem. Soc., 5, 7530 (1973).(22) G. A. Olah. P. W. Westerman. and J. Nishimura,J. Am. Chem. SOC.,96 ,. 3548 (1974).(23) (a) H. Spieseke and W. G. Schneider, J.Chem. Phys., 35, 731 (1961); (b)G. L. Nelson, G. C. Levy, and J. D. Cargioli, J. Am. Chem. Soc.. 4, 308911972)- -,(24) H. Volz, J.-H. Shin, and H.-J. Streicher, Tetrahedron Len., 1297 (1975).(25) J. F. Wolf, P G. Harch, R. W. E. Taft, and W. J. Hehre, J.Am. Chem. Soc.,(26) G. A. Olah, C. L. Jeuell, and A. M. White, J. Am. Chem. Soc.. 91 , 396197 , 2902 (1975).( 1969).

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1716(27) (a)M. . Bursey and F.W. M clafferty, "Carbonium Ions", Vol. I,G. A. Olahand P. v . R. Schleyer, Ed., Wlley-lnterscience, New York, N.Y.. 1967, p288: (b) F . P. Lossing and G. P. Semeluk, Can. J. Chem., 48, 955 (1970);(c)L. Radom, J. A. Pople, and P. v . R. Schleyer, J. Am . Chem. SOC.,94 ,5593 (1972).(28) (a )N . C . Deno and A. Schriesheim, J.Am. Chem.Soc., 77,3051 (1955);(b) A. C. Dixon and G. E. K. Branch, ibid, 58, 492 (1936);S.Nishida, J. Org.Chem.. 32, 2692 (1967).(29) (a) G. A. Olah, A. M. W hite, J. R. DeMember,A. Commeyras, and C. Y. Lui,J. Am . Chem. SOC.,92, 4627 (1970); (b) For a recent restatement of thedata and interpretation, see G. A. Olah, Acc. Chem. Res., 9, 41 (1976).

(30) G. A. Olah, G. Liang, and S. P. Jindal, J. Am . Chem. SOC., 98, 2508(31) G. Seybold, P. Vogel, M. Saunders, and K. B. Wiberg, J. Am . Chem. Soc.,(32) M. J. S. ewar, Chem. Br., 11, 97 (1975).(33) S. Winstein and C. Ordronneau, J. Am . Chem.Soc., 82, 2084 (1960).(34) G. M. Kramer. Adv. Phys. Org. Chem., 11, 177 (1975).(35) (a) G. A. Olah, D.P. Kelly, C. L . Jeuell and R. D.Porter, J. Am . Chem. SOC.,92, 2544(1970): (b)G. A. Olah, C . LCJeuell,D. P. Kelly , and R. D. Porter,ibid., 94, 147 (1972).(36) D. P. Kelly and H. C. Brown, J. Am . Chem. SOC.,97, 3897 (1975).

(1976).95, 2045 (1973).

Ring Opening Reactions in Cycloalkane MolecularIons. A Collisional Activation a nd FieldIonization Kinetic Study 'Friedrich Borchers,ts Karsten Levsen,*Za Helmut Schwarz,2bChrysostomos W esdemiotis,2b and Roland WolfschutzZ bContribution from the Department of Physical Chemistry, University of Bonn,D 5300 Bonn, and the Department of Organic Chemistry, T echnical University,D 1000Berlin, Germany. Received August 3, 1976

Abstract: Collisional activation and field ionization kinetic measure ments demonstra te tha t alkyl substituted cycloalkane mo-lecular ions with three-, four-, and five-membered rings (n-pentylcyclopropane, n-butylcyc lobutane, l ,2-diethylcyclobutane,and n-propylcyclopentane) undergo ring opening within ca . s to form initially the 1-alkene molecular ions which isomer-ize to some extent t o double bond isomers. However, six-, seven-, and eight-membered rings (ethylcyclohexane, 1,2-dimethyl-cyclohexane, 1,3-dimethylcyclohexane,methylcycloheptane, a nd cyclooctane) remain i ntact prior to decomposition.

The occurrence of ring-opening reactions in alkylcycloal-kane molecular ions prior to fragmentation has been postulatedrepeatedly . Thus, Stevenson3 demonstrated tha t i f methylcy-~ l o p e n t a n e - a - ' ~ Cndergoes loss of a m ethyl radical about 50%of the label is retained in th e remaining [CsH 9]+ ion suggestingan isom erization to a linear ion prior to decomposition. Moredetailed studies by Meye rson et al.495 using compou nds labeledwith 2 H and I3C in the a! position showed that the label re-tention in the [C sHg]+ ion is nearer to 40% and depends on th eionizing voltage; with increasing electron energy the loss of theoriginal side chain becomes more pronounced. From theseresults the authors concluded that at leas t two competingmechanism s a re operatin g, Le., loss of the original side chainfrom the in tact r ing and methyl loss af ter r ing opening to alinear intermediate. Similar results were obtained for ethyl-cyclopentane," although a shift toward simple cleavage hasbeen postulated with increasing length of the side chain. Incontr ast to these alkylcyclopentane ions the methyl radical lostfrom methylcyclohexane includes only the original sidechain .4An extensive and elegant field ionization kinetic study byFalick and Burlingam e6 fully supported th e earlier conclusionsan d shed further light on the details of these processes. Thelifetime m easurements of methylcyc1opentane-a-l3Cevealedtha t a t t imes