|
1. |
Rearrangements of pinane derivatives. Part IV. Solvolysis of the myrthanyl toluene-p-sulphonates |
|
Journal of the Chemical Society, Perkin Transactions 2,
Volume 1,
Issue 1,
1972,
Page 23-27
P. I. Meikle,
Preview
|
PDF (594KB)
|
|
摘要:
1972 23Rearrangements of Pinane Derivatives. Part 1V.l Solvolysis of theMyrtanyl Toluene-p-sulphonatesBy P. I. Meikle, J. R. Salmon, and D. Whittaker," The Robert Robinson Laboratories, University of Liverpool,P.O. Box 147, Liverpool L69 3BXMethanolyses of the cis- and trans-myrtanyl toluene-p-sulphonates both proceed by part unimolecular and partbimolecular mechanisms. The unimolecular reactions proceed with synchronous shift of the C-2 hydrogen to givethe pinan-2-yl cation. Formation of this ion by an indirect route removes the counterion from the vicinity of thecarbonium ion centre, thus diminishing ion-pair effects on the reactions of the ion. Both isotope effects and productstudies indicate that the bimolecular reaction proceeds by direct attack of the nucleophile on the substrate, ratherthan by attack on an ion-pair.IN Part I11 of this series, we considered the solvolysesof the cis- and trans-pinan-2-yl 9-nitrobenzoates inmethanol containing sodium methoxide.The reactionshowed a bimolecular component, but at low baseconcentrations proceeded mainly via a unimolecularprocess. Internal return from an intimate ion-pairgave some bornyl P-nitrobenzoate from cis-pinan-2-yl9-nitrobenzoate, and a-fenchyl P-nitrobenzoate fromtrans-pinan-2-yl 9-nitrobenzoate. Solvolysis productswere also obtained, but we were unable to discoverwhether they arose from a solvent-separated ion, inwhich case the P-nitrobenzoate ion continued to affectthe course of reaction, or from a ' free ' (i.e. solvatedonly by solvent molecules) ion, the P-nitrobenzoate ionbeing too far away to influence behaviour.Evidence of ion pairing has also been observed byWinstein in the study of nopinyl bromobenzene-sulphonates, in which reaction extensive return froman intimate ion-pair to rearranged bromobenzene-sulphonates was observed.Solvolysis products againoffered no clue as to whether they arose from a solvent-separated ion pair or a free ion.An obvious approach to the question of influence ofcounter-ion on solvolysis products would be variation ofthe leaving group. However, in the pinanyl andnopinyl systems, which are unusually prone to ion-paireffects, the lability of the systems makes synthesis ofa series of esters with widely differing leaving groupsimpractical. To date, only one pair of esters has beenstudied in each of the pinany1,l nopinyl,2 and norpinyl3Part 111, J.R. Salmon and D. Whittaker, J . Chem. Soc.,S. Winstein and E. C . Friedrich, J . Amer. Chem. Soc., 1964,(B), 1971, 1249.86, 2721.systems, and skeletal changes between these systemsmake comparisons difficult. We therefore sought tostudy the problem by reaction of a pinanyl ester inwhich the ester group is on the neighbouring carbon atomto the C-2 position which we wish to study. Generationof a carbonium ion by hydride shift should give an ionin which the counter-ion is already remote, and shouldthen readily separate from the carbonium ion. Wechose the myrtanyl esters, since solvolysis of a primaryester, provided it does not involve a bimolecular reaction,should give the maximum driving force to a hydrideshift to yield a tertiary carbonium ion.In a prelimin-ary communication,* we have shown that reaction doesinvolve generation of a carbonium ion at C-2, and haveshown that the ion is probably delocalised.EXPERIMENTALMaterials.-The P-pinene used in preparation of materialswas purified by distillation through a helix-packed columnat 165-166" at 760 mm.cis-Myrtanyl Toluene-p-suZphonate.-cis-Myrtanol, pre-pared by the method of Brown,6 had b.p. 81-83" at 0.6 mm.The n.m.r. spectrum showed a doublet, J = 7.5 Hz atT 6-40 due to the -CH,-OH protons. Reaction withtoluene-p-sulphonyl chloride in pyridine for 24 hr.6 gavecis-myrtanyl toluene-p-sulphonate, as white crystals, re-crystallised from n-hexane to m.p.75.5-76" (Found:C, 66.4; H, 7-8; S, 10.6. C1,H2,0,S requires C, 66-2;H, 7.8; S, 10.4%).3 W. Kirmse and R. Siegfried, J . Amer. Chem. SOC., 1968, 90,4 J. R. Salmon and D. Whittaker, Chem. Comm., 1967, 491.5 H. C. Brown and G. Zweifel, J . Amer. Chem. Soc., 1964, 86,6604.393.R. Tipson, J. Org. Chem., 1944, 9, 23524 J.C.S. Perkin I1Repetition of the above preparation using sodium boro-deuteride in the alcohol synthesis gave an alcohol similarexcept that the -CH2-OH protons now gave a single peakdue to replacement of the C-2 hydrogen by deuterium ; thispart of the n.m.r. spectrum was not affected by esterification.trans-Myrtanyl Toluene-p-su1phonate.-trans-Myrtanol,prepared by the method of Brown 7 had b.p. 81-83'at 0.6 mm.The n.m.r. spectrum showed a doublet,J = 7-5 Hz at T 6.58 due to the -CH2-OH protons. Reac-tion as above yielded trans-myrtanyl toluene-p-sulphonate,m.p. 50-50-5" (Found: C, 66.5; H, 7.7; S, 10.7. Cl,-H,,O,S requires C, 66.2; H, 7.8; S, 10.4%).Repetition of the preparation using sodium borodeuterideinstead of sodium borohydride again gave a similar alcohol,except that the -CH,-OH protons gave a single peak due todeuterium on C-2. The position of the label was notaffected by conversion into the toluene-psulphonate.Samples of the methyl ethers of cis- and trans-myrtanolwere prepared by reaction of the sodium compound of thealcohol with methyl iodide. Both were liquids, andwere purified by preparative scale g.1.c.Authentic samplesof other products were prepared as described previous1y.lKinetic Procedure.-The solvent used throughout thiswork was methanol, purified by the method of Vogel.8All kinetic measurements were carried out at 85-0 "C.Solutions of the ester, 0.032~, in methanol plus the ap-propriate amount of sodium methoxide were sealed inPyrex tubes at room temperature. The toluene-p-sulphonicacid liberated in neutral reactions was titrated with sodiumhydroxide solution, using Bromothymol Blue as indicator ;for reactions in basic solution the excess base was titratedwith hydrochloric acid, using Bromothymol Blue asindicator.Product studies were carried out under similar conditionsand then extracted with pentane; the extract was washed,dried, and the pentane removed; the product was analysedby g.1.c.using a 50-ft. capillary column coated with S.E.33 at 74 "C. Products were identified by i.r. spectra ofsamples obtained by preparative scale g.l.c., using a 6-ft. x$ in. column, packed with 60-80 mesh Celite coated with20% Carbowax 400 at 130 "C.RESULTS AND DISCUSSIONKinetic data on the cis- and trans-myrtanyl toluene-$-sulphonates in methanol at 85" showed the reactionsto be first order; in the presence of base, the rateincreased and the reactions deviated from first-orderkinetics, becoming intermediate in behaviour betweenfirst order and second order. The first-order rateconstants, recorded over the initial part of the reactionwhere necessary, are recorded in Table 1.A plot ofthe rates at low base concentration against base con-centration gave straight lines, whose intercepts werethe rate constants for the unimolecular reaction, andfrom whose slopes the bimolecular rates could beobtained.These were as follows: for the cis-ester, k, = 1.50 xsec.-l and k , = 5.7 x 1. mole-l sec.-l; for thetrans-ester, k, = 0.64 x sec.-l and k, = 36.8 x1. mole-l sec.-l. At higher base concentrations, therate of reaction of the cis-ester increases faster thanH. C. Brown and G. Zweifel, J . Amer. Chem. SOC., 1961, 83,2 544.would be expected on this scheme; that of the trans-esterdoes not.The rate constants show that the cis-ester, in whichthere is some steric interaction between the gem-dimethylbridge and the toluene-fi-sulphonate group, reacts moreslowly by the bimolecular route and more rapidly byTABLE 1Rates of solvolyses of cis- and trans-myrtanyl toluene-p-sulphonates in methanol at 85"[NaOMe] k , x lo5Ester (0.032~) (4 sec.-lcis-Myrtanyl toluene-psulphonate 0 1.500.033 1.660.05 1.770.10 2.070.88 17.3~is-[2-~H,]Myrtanyl toluene-p-sulphonate 0 0.780.033 0.93trans-Myrtanyl toluene-p-sulphonate 0 0.580.05 1-860.10 4.260.88 34tr~ns-[2-~H,]Myrtanyl toluene-p- 0 0-46sulphonate 0.10 4-29the unimolecular route than does the less-hinderedtrans-ester, as would be expected.The solvolysis products, summarised in Table 2, areconsistent with the mechanisms outlined above.Reac-tion of the cis-ester gives, with increasing base con-centration, increasing amounts of p-pinene, the ex-pected product of E2 elimination.This reactionalmost certainly causes the rapid increase of rate ofreaction of the ester at high base concentration. Thisreaction does not take place with the trans-ester. Wesuggest that this is because of steric hindrance by thegem-dimethyl group to approach of base to the hydrogenon the (3-carbon atom. The p-pinene is almost certainlynot entirely a product of E2 elimination, as it couldwell arise in low yields from the unimolecular reaction.Both reactions yield the corresponding myrtanylmethyl ethers, which would be the expected productsof bimolecular reaction. On the basis of the datagiven above, we calculate that the composition ofproducts from the cis-ester in 0.94~-base would be 30%from SN2 reaction, 7% from S x l and 53% from E2,corresponding well with the measured values of 30, 5,and 55%.A similar calculation for the trans-estersuggests 98% of SN2 product, as observed.The rest of the reaction products suggest extensivereaction by generation of a carbonium ion a t C-2. Thiscould arise by a hydride shift either during or followingionisation, the former being the more probable as itdoes not involve preliminary formation of a primarycarbonium ion. We sought to differentiate between thepossibilities by replacing the hydrogen on C-2 with adeuterium; participation of the C-2 hydrogen in ionis-ation of the toluene-$-sulphonate should be detectedby an unusually large p-deuterium isotope effect.* A.I. Vogel, ' A Text-book of Practical Organic Chemistry,'Longmans, London, 1951, 2nd edn., p. 1681972Typical examples of p-deuterium isotope effects forreactions in which a lJ2-hydride shift is believed toaccompany departure of a toluene-@-sulphonate groupare kH/kD of 1.85 for the erythro-form and 1.72 for thethreo-form of 3-cyclohexyl-2-butyltoluene-@-sulphon-ate and, for 3-methyl-2-butyltoluene-~-sulphonate,10K H / k D values of 2-14 in aqueous 80% ethanol, 2-26 inacetic acid, and 2.24 in formic acid. These are in goodagreement with the value of &/kD = 1.9 which has beencalculated on the basis of a three-centre symmetricaltransition-state.11We found that in neutral methanol, the p-deuteriumisotope effect on the solvolysis of cis-myrtanyl toluene-$-sulphonate was kH/kD = 1.92; in the presence ofO.O33~-sodium methoxide, it fell to 1.79.Comparisonof calculated values of k, and k , with those given earlier25We thus have two isomers whose substitution reactionsshow simultaneous unimolecular and bimolecular com-ponents, the former proceeding with a synchronoushydride transfer. The system is particularly wellsuited to test the suggestion of Sneen l2 that bimolecularreactions may proceed by rate-determining attack of anucleophile on an intimate ion-pair, rather than on anun-ionised substrate. This suggestion would have theeffect of unifying mechanisms &1 and SN2, since bothreactions would consist of preliminary ionisation to anion-pair, which could spontaneously decompose (SN 1)or be attacked by a nucleophile (SN2).We have alreadyshown that ionisation of both cis- and trans-myrtanyltoluene-@-sulphonates is accompanied by a shift of theC-2 hydrogen, generating a carbonium ion centre at C-2.There is no evidence of this hydride shift being reversible,TABLE 2Products of alkaline methanolysis of cis- and trans-myrtanyl toluene-p-sulphonatesEstercis-Myrtanyl toluene-p-sulphonate ( 0 . 0 3 2 ~ )tram-Xlyrtanyl toluene-p-sulphonate ( 0 . 0 3 2 ~ )cis-Pinan-2-yl p-nitrobenzo-trans-Pinan-2-yl p-nitro-ate (0.032~)benzoate (0.032~)for the hydrogen[NaOMe]0.0330.300.670.943.00.0330.0490.0990-300.670.943.00.033(WProducts ofbimolecular reaction(moles yo) 7Myrtanyl MyrtanylB- methyl methyl Un-Pinene ether ether known19 6 139 20 248 30 255 30 352 46rT6 382 782 90I 951 971 981 99Composition of products arising from carbonium ion at C-2(moles %)IU-Terpinylmethylether35312823363635333013a-Pinene4696J;I833 *trans-U- Pinan-Fenchyl Bornyl 2-ylu- Cam- Limo- Terpino- methyl methyl methylFenchene phene nene lene ether ether ether7 1 8 6 6 27 2 11 6 5 28 1 10 5 5 9 51 2 8 I ) 7 41 8 7 3 7 42 7 9 6 J 32 4 10 5 r 3 67 1 10 6 6 2 ;1 ; 8 6 ) 7 5r1 4 3 3 2 acis-Pinan-2-ylmethylether252526282727272627252 1 1 5 J 7 2 15 30 23 0.033* Formed mainly from elimination within an intimate ion-pair.compounds indicates that for theunimolecular reaction &/kD = 1-92 while for the bi-molecular reaction kH/kD = 1.0.The isotope effecton the unimolecular reaction is clearly within the regionto be expected for participation of hydrogen in theionisation. The isotope effect on the trans-ester inneutral solutions where the reaction is dominantly s N 1is 1-26, which, though much less than that observed inthe cis-ester, is still consistent with hydride transferduring ionisation, being much greater than the normalP-deuterium isotope effects. The lower effect may bea result of steric hindrance from the gem-dimethylgroup to the hydride transfer. However, reaction inthe presence of 0-lM-sodium methoxide, which productstudies indicate to be 90% bimolecular reaction showsthe isotope effect to be 1.0.This agrees well with theresult obtained on the cis-ester, and has the advantageof being measured directly. The results clearly showthat the unimolecular reaction involves hydride transferduring ionisation, and is not consistent with formationof a primary carbonium ion.* D. J . Cram and J . Tadaniev, J. Amer. Chem. SOC., 1959, 81,lo S. Winstein and J . Takahashi, Tetrahedron, 1958, 2, 316.2737.as the stereospecificity of the bimolecular reactionsprecludes any isomerisation of the esters. If the bi-molecular reaction involved nucleophilic attack on anion-pair, it would have to proceed at C-2 giving productsof reaction at that atom, and would show an isotopeeffect similar to that observed in the unimolecularreaction.The data given above contradict both predictions.Reactions of cis- and trans-myrtanyl toluene-@-sulphonates give the appropriate myrtanyl methylethers by reaction at C-10 in yields which correspondto calculated rate constants for the SN2 reaction.Theisotope effects observed on the unimolecular reactionsare not paralleled in the bimolecular reactions. Clearlyour data are not consistent with the sN2 reactionsinvolving nucleophilic attack on an ion-pair.Alternatively, it could be suggested that the rate-determining step of ionisation was separation of anintimate ion-pair. Formation of an ion-pair at C-10could then be followed by spontaneous ionisation,l1 R.A. More O’Ferrall, J. Chem. SOC. ( B ) , 1970, 785.l2 R. A. Sneen and J . W. Larsen, J. Amer. Chem. SOC., 1969,91, 362; R. A. Sneen and J . W. Larsen, J. Amer. Chem. SOC.,1969, 91, 603126 J.C.S. Perkin I1involving hydride transfer, or nucleophilic attack.However, this appears unlikely in the light of knownreactions in which ion-pair formation involves neighbour-ing-group participation,13 and the lack of solventinfluence on the isotope effect .loWe conclude that in this instance bimolecular sub-stitution involves attack on the un-ionised substrate,although it should be pointed out that methanolysis ofa primary alkyl toluene-9-sulphonate is a particularlyunf avourable system for observation of the ion-paircis-myrtan ylmethyl ether + B-pinenecc-fenchene + 8-pinene (?)counterion to C-2, we should not expect any pinanylester, but would expect bornyl and a-fenchyl esters.We prepared one of the main products of bornyl toluene-+-sulphonate methanolysis,l* camphene hydrate methylether, and showed it to be absent from our solvolysisproducts.Clearly, we have no products of estersarising from internal return from an ion-pair at C-2.The products reveal some differences in yields ofcamphene, a-fenchene, and a-pinene between the twosystems, which we suggest may result from elimination+C H ~ O S O ~ C ~ H G C H ~ 6H2' * S 0 2 C 6 H 4 CH 3trans-m yrtan yl Camphenemethyl ether +p-pinene + + a-pinene/3-pinene (3)SCHEME3,9-pinene + cc-pinene + cc-fenchene + a-fenchylmethyl ether + trans-pinan-2-ylmethy ether/3-pinenea-pinene +camphene + bornyl methylether +CZS-pinan-2- ylmethylether + limonene + terpinolenemethyl ethera-terpinyl +mechanism, so that this could prove to be an exceptionalrather than a general case.The unirnolecular reaction, like the biinolecularreaction, shows no evidence of being influenced by ion-pair formation.The products of solvolysis of the myrtanyl esters,given in Table 2, show that rearrangement resultingfrom ion-pair return to a rearranged ester is absent.In Part I11 we showed that pinan-2-yl esters solvolysedvia an intimate ion-pair, return from which gave bornyland a-fenchyl esters.If hydride shift during ionisationof the myrtanyl esters were accompanied by shift of thel3 S.Winstein and D. Trifan, J . Amer. Chem. SOC., 1951, 74,1147, 1154.l4 W. Huckel, C. M. Jennewein, H. J. Kern, and 0. Vogt,Annalen, 1968, 719, 157.by removal of a proton from the ion by the departingtoluene-p-sulphonate group. We have previouslysuggested that the first step in formation of camphene,a-fenchene, and p-pinene from the pinan-2-yl ions isremoval of a proton from C-10. Extension of thistheory to reactions of the myrtanyl esters suggests thatremoval of a proton from C-10 by the departing toluene-$-sulphonate group, either the C-2 hydrogen duringtransfer or a C-10 hydrogen (yields of olefins are toosmall to permit isolation of sufficient material from thedeuteriated ester solvolysis to check this point) wouldgive rise to a-fenchene from the cis-ester and camphenefrom the trans-ester, as observed, the mechanism being16 C.M. Williams and D. Whittaker, J . Chem. SOC. (B), 1971,6681972 27essentially an a-elimination. It is probable l5 thatelimination via this route also gives some @-pinenefrom both myrtanyl esters, but this is obscured byformation of p-pinene by other routes. A furtherelimination product can be formed in this way fromthe trans-ester (11) since the departing anion mustpass close to C-3, and can hence remove a proton fromthis carbon atom to initiate an elimination processleading to a-pinene.15 Steric hindrance of the gem-dimethyl group and unfavourable substrate conform-ation make this process less likely in the cis-ester, sothat the yield of a-pinene from the trans-ester exceedsthat from the cis.Other solvolysis products from the two myrtanylesters are almost identical, and are consistent withformation of an equilibrating pair of delocalised ions,(111) and (IV), which we discussed in Part 111.Evid-ence from the eliminations discussed above, however,shows that it is the cis-ester which gives rise to (111)and the trans- to (IV); clearly the direction of ringexpansion is controlled by steric factors, as this cis-ester would have been expected to give (IV) if the ringexpansion had assisted the hydride transfer reaction.It probably does not do so because the C-2 hydridel6 C. )I. Williams and D. Whittaker, J . Chern. SOG. ( B ) , 1971,672.moves towards C-10 at a shallow angle which is un-suitably aligned for assistance from ring expansion.Ring expansion does however, occur more rapidly thanthe attainment of planarity by C-2.The data in Table 2 contrast with results obtainedon the pinan-2-yl P-nitrobenzoates, in that the ratio ofcis- to trans-pinan-2-yl methyl ether is almost constantwith increasing base concentration. Clearly, theequilibration of (111) and (IV) is more rapid in thiscase, which is consistent with the absence of the stabilis-ing factor of a solvent separated counterion. It isprobably the difference between a solvent separatedion-pair and a solvent-solvated ion which is also respon-sible for the large increase, by a factor of almost 2, inthe yield of ring-opened products. This is consistentwith the absence of ring-opened products from intimateion-pairs formed during solvolysis,l although ringopening can occur when the ion-pair has a long life.16We conclude that we have obtained a solvent-solvatedion, and that it shows no qualitative differences fromthe solvent separated ion-pair obtained earlier. Thequantitative differences observed are considerable,however, and can be considered in terms of a speedingup of rearrangements of the ion relative to the rate ofcapture by an external nucleophile.[1/1243 Received, July 20th, 1971
ISSN:1472-779X
DOI:10.1039/P29720000023
出版商:RSC
年代:1972
数据来源: RSC
|
|