SolvolysisRateStudies1 Chloro 1 Alkylcycloalkanes

7/28/2019 SolvolysisRateStudies1 Chloro 1 Alkylcycloalkanes

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Some studies on the solvolysis of 1-ch loro-1-alk ylcycloalkanes

a n dDepnrtrnerlt of Clzernistry, Utliuersity ofcn lgn ry , Cnlgclry, Altn., C an ad a T6G 2G2

Received March 20, 1979'K . R A N G A N A Y A K U L U ,. V A S U M A T H IE V I ,R . B A L A JI AO , a n d K. R A JF S W A R I .a n . J . C h e m . 58, 1484(1980) .Th e effect of the bulk of the sidechain on the rate of solvolysis of I -alkyl cyclope ntyl , cycloh exyl , and cycloheptyl chlor ibeen studie d. With the excep t ion of the t -butyl sy stems , the rat io of solvolysis rates for the three r ing system s fal ls in agive nTh e slower rate of solvolysis in the six-membered r ing system may be due to an extra act ivat ion energy contr ibut ion caus edconversion of a neutral chair form to the twist boat o r half chair confo rmat ion , pr ior to th e actual solvolysis. In the ca se of fiseven-mem bered r ing system s the format ion of an intermediate carbon ium ion is ster ical ly favoured ( I -st rain orecl ipsing intercons istent wi th ear l ier f indings. Th e faster rate of solvolysisof I - t -butylcycloalkyl chlor id es isl ikely due to arearrang emen t rwhere alkyl par ticipation enha nces th e rate of solvolysis.K. R A N G A N A Y A K U L U ,. V A S U M A T H I E V I ,R. BALAJI AOe t K. R A J E S W A R I .a n . J . C h e m . 58, 1484 (1980).On a etudik I 'ef fet de I 'encombrement de la chaine laterale sur la vi tesse de solvolyse des chlorures d'alkyl- l cyclopcyclohexyle e t cycloheptyl e . A I 'exception d es sys t em es t er t -butyl e , l e r appor t des v i t esses d e so lvolyse des t ro is sycycl iques cor r espon d 2 une ser i e donnee . L e f a it que l a v i tesse de solvolyse es t p lus l ent e dans l es cycles a 6 c h a i n o n s pe u t i tune contr ibut ion supplementaire de I 'energie d'act ivat ion provo quee par I ' interconversion d 'un e forme chaise neutre en unbateau-croi see ou demi -chai se avant l a so lvolyse . Dans l e c as de s sys tkmes cycl iques a 5 e t 7 chain ons, la format ion d'un ca rbin t ermedia i re es t f avor i see pa r de s f ac t eur s s t er iques ( t ens ion- I ou in t er ac tion kcl ipsee) en accord avec l es r esul t a t s antkr ieurement . L e fai t que la vi tesse d e solvolyse des ch lorures de ter t -butyl- I cycloalkyle soit plus grande est probableme nune react ion d e t ransposi t ion oG la par t icipat ion du gro upe alkyle augmen te la vi tesse d e solvolyse. [Tradu it pa r le j

IntroductionThe solvolyses rates of various ring halides havebeen extensiv ely studied (1-5). A 125-fold in creas ein the rate of solvolysis of l a relative to 2a has beenattributed to relief in steric strain in the transitionstate of the chloride l a and an increase in stericstrain in the transition state of the chloride 2 0 .Since strain effects could originate from the ring aswell as the sidechain, we have studied the effect ofthe bulk of the sidechain on th e rate of solvolysis of1-alkyl cyclopentyl l a - d , cyclohexyl 2 a - d , andcycloheptyl chlorides 3 a - d .

la-d 2a-d 3a-da , R = M e; c , R = i-Prb, R = E t ; d , R = t-Bu

ExperimentalAll the compounds used in this study were prepared usingprocedures repor te d in the l i terature (5-10). Al l solvolysis ex-per iments w ere carr ied out in 80% aqu eou s ethanol at 30C. Th era t e const ant s for compound s la-c ,-2a-c, 2nd 3n-c were mea-sured us ing the procedu re adopted by Brown e t a / . ( 1 I) . The r a t e

const ant s for compounds Id , 2d , and 3 d were measured uautomatic capac i tance br idge with a digital control un1673A, Genera l Radio Compa ny. Co ncord . MA, U.S.A.Product analys i s i n each case was done by car ry ingsolvolysis react ion in water . T he chlor ide (500 mg) was a10 cml of water and a l lowed to s t i r a t r oom t empera tudays . The r eac t ion mixture was ext r ac t ed wi th pentan30mL) and dr i ed over anhydrous magnesium sulphasolvent was remo ved using a f ract ionat ing colum n (24the r es idue was analysed by nmr spect roscopy and gres idue in t he ca se of Me, Et , and i -Pr subst it u t ed comwas found t o be a mixture of unrearr anged a lkene and a l cnmr analysis.In the cas e of t - butyl sys t ems the r es idue in each cpeared to conta in more than two product s and these werated by pre parat ive glc on a Var ian Aerograph model 90-318 in. x 6f t , 10% SE-30 on 60180 chromosorb P colcolumn t empera ture of 125C was used to separa t e proId and 2d and a t empera ture of 135C was used in casEach gl c separa t ed f r ac t ion was analysed by i t s nmr sp(obtained on a Var ian XL-200 spectrometer) in CDCtet ramethyl si l ane a s t he in t ernal s t andard . Th e nmr were consi s t ent wi th the s t ruc tures proposed a nd the cshif ts of the character ist ic peaks are repor ted (on the st ructu res) in ppm (6) downfie ld f rom TMS . Th e peaks rar e s harp s ingle t s unless o therwi se ment ioned and in t egfor the number of protons. The resul ts are given in detsequent ly .

Results and Discussion- - The results of the solvolyses experimen

'To whom al l cor r espondence should be addressed. given in Ta ble 1. Th e solvolysis data for 3-c2Revision received M arch 10, 1980. 3-alkylpentanes have also been included for0008-4042180114148 4-06 $01 OO/O

@ 1980 Nat ional Research Counci l of CanadaIConsei l na t ional de r echerches du Can ada


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R A N G A N A Y A K U L U ET AL.T A B L E . Rate constants for the solvolysis of I-chloro-1-alkyl cycloalkanes in 80%aqueous ethanol --Rate constant (lo sk , 30"C), s-I

Comp ound Me Et i-Pr I-B UI-Chloro- I-alkyl cyclo penta ne 88.05 81.38 40.83 246I-Chloro-I-alkyl cyclohexan e 1.30 1.22 1.0 4.271-Chloro-1-alkyl cycloheptane 81.94 76.66 35.55 542.73-Chloro-3-alkylpentaneD 2.38 2 .75 1.83 44.44"From ref. 11 .

TA BL E . Rate ratios for the solvolyses of 1-chloro-I-alkyl cy-cloalkanesValue

Ratio Methyl Ethyl i-Propyl t-Butyl

parison purposes (1I). The kinetics show first orderbehaviour in all the cases studied. The rate ratiosfor the solvolyses of five-, six-, and seven-membered ring compounds are given in Table 2.The rate of solvolysis in any given ring systemwhere the R sidechain is changed could be affectedby one or more of the following factors:(i) In going from the reactant to the transitionstate leading to the carbonium ion intermediate,there should be a decrease in the eclipsing interac-tion in the case of the five-membered ring System,4+5, whereas in the case of the six-membered ringsystem, 6+ , there should be an increase (I-strain)(12). This will have the effect of increasing the ratewith increase in the bulk of the 1-alkyl group in thefive-membered ring system, and correspondinglydecreasing the rate in the six-membered ring sys-tem. However, the near constancy of the relativerates of the five- and the six-membered ring sys-tems in going from the methyl to the t-butyl side-chain suggests that this is not an important con-tributing factor in causing the basic differences insolvolysis rates between these two ring systems.(ii) In the case of the five-membered ring system,there are four a-hydrogens (ring) in a favourableorientation for hyperconjugative stabilization, see5, while in the case of the six-membered ring sys-

tem there a re only two such hydrogens, 7 (13). Thislatter statement assumes that the solvolysis reac-tion in the six-membered ring system proceeds via achair conformation in the transition state. Brown eta l . (14) have made adetailed study of the solvolysesof 1-aryl-1-cycloalkyl and 1-aryl-1-alkylcarbiny3,5-dinitrobenzoates. Their results reveal a closesimilarity in the p+ values for methyl, isopropyl,cyclobutyl, and cyclohexyl groups indicating nosignificant difference in the electronic contribu-tions to an electron deficient centre. However, thep+ values for cyclopentyl and cycloheptyl groupsare considerably less negative and this has beenattributed to a combination of I-strain and superiorhyperconjugative contributions by the cyclopentylgroup, and to I-strain alone in the case of the cyclo-heptyl group. p-Deuterium isotope effects involv-ing ring hydrogens are known for the solvolysis ofboth cyclopentyl (2,2,5,5-d,) tosylate (15a) andcyclohexyl(2,2,6,6-d,) tosylate (15b),k$k,,= 2.06and 2.34, respectively. This isotope effect impliesthat hyperconjugative stabilization is more impor-tant in the cyclohexyl case so that this hypercon-jugative argument can hardly explain the faster rateof solvolysis found for the cyclopentyl system.(iii) Conformational preference for hypercon-jugative participation by the C-H hydrogens ofthealkyl group in the five- and seven-membered ringsystems. From electron spin resonance results andfrom the observation of secondary P-deuteriumisotope effects in bridgehead systems (16), one canshow that hyperconjugation is dependent on theangle (0) between the "p" orbital on the trigonalcarbon and the neighboring C-H bond (Fig. I). Ifindeed this is the reason for the observed differencein solvolysis rates of five- and six-membered ringcompounds, then there should be a large difference

X

4 5 6 7 FIG. . View alon g the C-C bon d (z).


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C A N . J . CHEM . VOL. 58 , 1980TABLE . Rate ratios for the solvolysis of 1-methyl- and I-methyl-d, cycloalkylchlorides in 80% aqueous ethanol

Compound T e c ) ~ c H , / ~ c D , ~ c H , / ~ H Reference

in the isotope effect of the 1-CD3-cycloalkyl sub-stituted compounds. The Me-d3 isotope effects forthe s o i ~ o l ~ s i ~f methylcyclopentyl chloride andmethylcyclohexyl chloride are given in Table 3.Since the isotope effects are temperature depen-dent, one can assign a rough value of 1.3 forkCH,IkCD,t 25C for methyl cyclohexyl chloride.Thus, it can be concluded that the 1-alkyl groupstabilises the intermediate carbonium ion better in asix-membered ring system than in the five-membered ring. Therefore, this factor also seemsnot to be responsible for the observed rate differ-ences in the solvolysis of five- and six-memberedring compounds.(iu) An increase in Baeyer strain in the formationof a five-membered ring cation (109" to 115") com-pared to a six-membered ring cation (109" to 120").This effect is likely to reverse the rates of solvolysisof five- and six-membered ring systems from theobserved results, and therefore may not be thesignificant factor in changing the rates of solvolysisin different ring systems.(u) Decreasing solvation in all the ring systemswith increase in the bulk of the alkyl group.To summarize, the regular decrease in the sol-volysis rates of all the three ring systems with theincrease in the bulk of the alkyl group suggestsrelief of eclipsing interactions in the chloride, dif-ferential hyperconjugative participation of the P-hydrogens of the ring or of the 1-alkyl sidechain,and Baeyer strain are not the major contributingfactors accounting for the differences in solvolysisrates of the different ring systems. However, in agiven series la-c, 2a-c, and 3a-c the results couldvery well be explained by considering hypercon-jugative participation of P-hydrogens of the 1-alkylgroup, in addition to solvation effects.

Solvolysis of t-Butyl SystemsThe t-butyl chlorides Id, 2d, and 3d all solvolysemuchfaster than the methyl systems la , 2a, and 3a.

Obviously some new factor must be involveand as a first step we have carried out a panalysis. For the product study, the solvolId, 26, 3d was carried out in water in orsimplify the nature of the products, specificavoid the formation of ethers which will onlyplicate the analysis. In each case both th(200 MHz) and glc analyses showed the preserearranged products.The products from the chloride Id could separated in glc under the conditions used. Thfraction contained the unrearranged alkene bsecond fraction consisted of a mixture of thcohols, l-t-butylcyclopentanol, l ,2,2-trimc yclohexanol, and 2-( l-methylc yclopenpropanol. The identity of these alcoholconfirmed by synthesizing these, l-t-butylpentanol and 1,2,2-trimethylcyclohexanoling to the literature procedures (10, 18a) anmethylcyclopenty1)-2-propanol y adding mlithium to 1-acetyl-1-methylcyclopentanesupplied by Dr. T. S . Sorensen). The nmr speof the mixture of these authentic alcohols mwell with that of the second fraction.In the case of the chlorides 2d and 3d thvolysis products could be separated well undglc conditions used and the products could beidentified.The ratios of each of the products , the chshifts of the characteristic proton signals, aretention times are given in Table 4. Recoveperiments have also been carried out for thvolysis of the Me, Et , and i-propyl compounthese show no rearranged products. The fastof solvolysis in the I-butyl compounds canfore be correlated with the appearance ofrangement products. Brown et al . (10) have sthe rate of solvolysis of 1-t-butylcycloalkylcp-nitrobenzoates and the results are as focyclopentyl, k , = 0.236 s-l, cyclohexyl, k ,x s-l, cycloheptyl, k , = 1.15 s-l. The


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R A N G A N A Y A K U L U ET A L .TABLE. S O I V O I Y S ~ Sf Id, 2d . an d 3d

gj"- 5 . 2 8b, w/ Z = 5 Hz) 0 . 9 3 8 "1.13Id Rat io 78 7 10 5Retent ion t ime 2 .6 -.0(min)

ZdRat io 54 32 14

Retent ion t ime 3.5 6 .8 9 .0(min)'"'H14.70

&H5,59 "'87 8 83 (I.J = 3.5Hz)M

Ratio 52Retent ion t ime 5 .0(min)

sponding ratios, kt.butyJk,e,hy,,are 112, 134, and273. Using the chlorides the corresponding ratiosare kt-butyJkmethyl 2.8, 3.28, and 6.62. In the p-nitrobenzoates there is a large rate difference be-tween the methyl and t-butyl substituents and thishas been atrributed by Brown et al. (10) to thereliefof steric strain in going to the transition state. Onfirst glance these results seem to be different fromthose obtained by us using 1-t-butylcycloalkylchlorides. However, the F-strain involving ionisa-tion ofp-nitrobenzoates is larger than for the corre-sponding chlorides (19) and therefore the chloridesappear to be better for studies of structural effectsin which large F-strain factors should be avoided(10). Also, introduction of two or three bulkygroups at a tertiary center can result in far largerrate enhancements (20). However, the significantfactor is that the relative reactivities of five-, six-,and seven-membered ring compounds are not verydifferent, irrespective of the leaving group. Thus:

It seems to us that the dramatic rate increase in

t-butyl substituted cycloalkyl chlorides is noprimarily of F-strain origin because: (a) thechlorides are much less susceptible to this factor asshown above and (b ) there should be a steady progression from R = Me to R = t-butyl, if this phenomenon is involved. It has been suggested thamethyl participation in t-butyl substituted systemenhances the rate of solvolysis (21). Our results areconsistent with this argument although it is somewhat peculiar that in the cyclopentyl and cyclohexyl series the rearranged products only accounfor 15% and 14% respectively of the total. Thus theparticipation does not involve the complete breaking of the methyl carbon bond in the solvolysitransition state.Solvolysis Rate Com parisons Between the ThreeRing SystemsUp to now we have discussed two phenomena: aslight rate decrease in all the three rings in goingfrom Me to i-Pr; secondly, the fairly large increasin all cases with the t-butyl substitutent. Neither othese factors relates directly to the question of whythe six-membered ring system solvolyses slowethan its counterparts. We have previously suggested that eclipsing interactions are not of primarimportance in causing the rapid rate in the five


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1488 C A N . J . C H E M . VOL. 58 , 1980

membered ring system, since a rate increase wouldhave been expected with increasing bulk. This re-lief of eclipsing interactions is the usual argumentgiven, e.g. work of Brown (5, lo), for this rateincrease and this could still be possible if compen-sating factors were at work, i.e. a large rate de-crease on going from methyl to isopropyl due tosolvation and/or hyperconjugation effects, ba-lanced by a smaller increase caused by the eclipsingrelief. The rate increase in the seven-memberedcase seems to fit Brown's I-strain concept. Onepossibility for the cyclohexyl system could be thatthe solvolysis might not involve the chair form andin this case the large rate decrease predicted ongoing from Me to i-Pr would not necessarily apply,i.e. development of eclipsing interactions in a chaircation intermediate 8. In the case of the six-membered cation several conformations, 8-13,could be considered. Taking steric factors aloneinto consideration, the carbonium ion intermediatein the six-membered ring system is more crowdedthan the neutral molecule. Recently Sorensen e t al.(22) have shown that the twist boat form of methyl-cyclohexyl cation 10 is more stable (ca. 500cal/mol) than the chair form 8, although there is arapid equilibration of the two conformers even atlow temperatures. With an increase in the bulk ofthe 1-alkyl group it is likely that the equilibriumwould be shifted even more towards the half chairor the twist boat form since there is a smallereclipsing interaction of the alkyl group with C,-hydrogens. In the ground state chlorides, the dif-ference in energy between the chair form and thehalf chair or twist boat form will be even higher (ca.3-4 kcallmol). Although the chair conformation isundoubtedly the ground state for the chlorides, theeclipsing interaction in the chair cation may make itmore economical for the molecule to solvolyse via atwist boat ground state as long as the energy gainthrough forming a better cation transition state isenough to counter-balance the energy differencesbetween the neutral chair and the twist boatchlorides. In any case, the solvolysis rate would beslower than in the other two ring systems since it is

a matter of deciding between the best of twsituations.In some ways the conformational argumvolving a twist boat transition state seems since this fits with the small rate decrease obsin going from methyl to isopropyl, and it shonoted that some solvolysis data on secondaclohexyl systems have already been rationaliinvolving a non-chair conformation transition(15a, 23).

AcknowledgementsWe wish to thank the University Grantsmission, India, for providing financial assistaone of us (M.V.D.) and to Professors T.S. sen and R. E. Robertson, Department of Chtry, University of Calgary, for providing somoratory facilities and for useful discussions.

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R A N GA N A Y A KU LU ET A L. 14817. K. L . SERV IS, . BoREIE, and D. E . SU NK O. e trahedron,

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SolvolysisRateStudies1 Chloro 1 Alkylcycloalkanes