DEUTERIUM TRACERS IN KORBORNENE ADDITIONS
July 5 , 1964
260 1
ORGANIC AND BIOLOGICAL CHEMISTRY [CONTRIBUTIOS FROM
THE
DEPARTMENT O F CHEMISTRY
O F THE
UNIVERSITY
OF
DELAWARE, NEWARK,DEL.]
Deuterium Tracer Studies on the Course of Addition to Norbornene.
IX
BY HAROLD KWARTA N D J . L. KYCE RECEIVED DECEMBER 11. 1963
X comparison of the course of addition of DBr to norbornene under polar and free radical reaction conditions has been carried out. T h e circumstances of these experiments have presumably involved no extraneous or endemic directive effects. The results of polar addition can be reconciled (only) with a single path consistent with a cationic intermediate, in contrast to reports of a multiplicity of pathways for addition t o substituted norbornenes. The failure t o find significant amounts of trans addition product with DBr where other unsymmetrical addition reagents (i.e., Br2, ArSCI, etc.) are known to follow this reaction route points clearly t o the existence of directive substituent influences and t h e intermediacy of onium ion (or stable T ) complexes in the latter reagents. T h e free radical course, as has been previously suspected, was found t o be strikingly different. The major product is the result of cis-exo addition unuccompanied by any rearrangement t h a t could be traced v i a the deuterium label. About one-fifth of the total product arises from the unusual process of attack by bromine radicals from the endo side of the double bond. The possible occurrence of a bromine bridged radical in the course of forming this ( e n d o ) material as well as the other minor product, exo-trans, has been given some consideration, b u t the dominant pathway of the radical addition appears t o be subject to steric control of product formation from the carbon radical intermediate.
The additions to norbornene and substituted norbornenes have recently been studied and summarized.' In general, the results of the polar additions considered were interpreted on the assumption of s-complexes and bridged cation intermediates. Since then, however, the occurrence of complexes2 and nonclassical carbonium ions3 in polar addition reactions has been brought into question and the evidence upon which the earlier conclusions were based was re-examined. This has been made possible in some cases by the availability of newer analytical techniques. A particular case in point is the study by Cristol and ~ o - w o r k e r sof~ ~ the ~ course of addition of polar reagents to trimethylenenorbornene. I n contrast to the ionic mechanisms of addition in which bridged ions, or their equivalent equilibrium of ionic structures, have been invoked to explain the kinetic and stereochemical results, i t has been repeatedly suggested t h a t analogous evidence for bridging in free radical addition mechanisms is not to be found in the bicyclic However, the very recent report by Skell, Tuleen, and Readio8 of stereochemical evidence for radicals involving bromine bridging urges restudy of these earlier conclusions. In most of the reactions studied, whether ionic or free radical, a commonly suspected2 influence has been the ability of either an already existing substituent, or a group, having entered in the attack stage of the reaction, directing the course (cis,trans, or rearranged) of the second stage. For instance, in the reaction of X+'Ywith norbornene (the question may be asked) what is the influence of X in the intermediate carbonium ion or radical controlling the position and ease of bond formation with Y in the product-forming step? U7hen the entering group X is hydrogen, no directive influence should be operating, but the problem of analyzing the stereo( 1 ) 1. K a p l a n , H . Kwai-t, a n d P. von R Schleyei-, J . A m . ChPm. Soc., 82, 2341 (1960), a n d references cited the)-ein. ( 2 ) R I J S. 1)ewar a n d R . C F a h e y , ibrd., 85, 224.5, 2248 (1963). (3) See discussion p e r t i n e n t to ref. 16. ( 4 ) S J Cristol, W. K . Leifert, D . W. Johnson, a n d J B . J u r a l e , J A m . C h ~ mSoc., 84, 3 9 1 8 (1962) (.5j S J . Cristol. 1, K G a s t o n , a n d D W. J o h n s t o n , T p t r a h p d r o n Letters, 4, 185 (1963). ( 6 ) S J Cristol, G. D Brindell, a n d J. A R e e d e r , J A m Chem. SOL, 80, 6 3 5 (1958), a n d m a n y references cited therein ( 7 ) S. J. Cristol a n d G D Brindell, i b i d . , 76, 5699 (1954) ( 8 ) P. S Skell, D. I, T u l e e n , a n d P D R e a d i o , i b i d . , 80, 2849 (1963).
chemical course of the addition reaction arises. If deuterium is substituted for the hydrogen, the products formed via the various possible routes would no longer be identical and could be distinguished by an appropriate degradation scheme. Again, the possible intrusion of deuterium isotope effects (in particular) on the course and facility of the addition reactions has to be confronted, but, in general, such effects are of secondary nature. Our selection of DBr as the preferred reagent for investigation was dictated b y several considerations. A study of the competitive elimination of HBr and DBr with strong base from appropriate norbornyl bromide preparation (as well as analogous norbornyl tosylate) was carried out for the purpose of providing a basis for interpreting the results of the suggested degradative scheme. This study is reported e l ~ e w h e r e . ~I n addition, the availability of some very fine background in the literature of deuterium isotope effects in general elimination reactionsloB1land in related systerns12presented a basis for estimating the proper magnitudes of deuterium isotope effect corrections of the analytical data. Finally, the (DBr) reagent of choice, being capable of undergoing both free radical and ionic additions to norbornene, afforded us thereby a direct comparison of these two mechanisms of addition to bicyclic olefins which has not been previously accessible. Results The polar addition of DBr to norbornene was carried out with a 4'i.5'3G DBr solution in D 2 0 , using suitable protection against the incursion of a photocatalyzed reaction. The mixture was stirred vigorously a t G O o for 3 hr. and afforded a better than 8OYGyield of pure exo-norbornyl bromide. KO contamination of the product with the endo epimer could'be detected b y vapor phase chromatographic techniques. The free radical addition of DRr to norbornene was performed in hexane solution using a quartz flask reactor and irradiating with an ultraviolet source to (9) H. K w a r t , T . T a k e s h i t a , a n d J . 1,. h-yce, % b i d . , 8 6 , 2GOG (lY6.i ). (10) V. J . S h i n e r , i b i d . , 76, 1603 (1934). (11) W. H . Snundei-s, J r , a n d D H E d i s o n , i b i d . , 82, 138 ( 1 9 6 0 ) . (12) N A. LeBel. P. r). Beirne, E R Kai-ger, J C Powet-s, a n d P M . S u h r a m a n i a n , ibid.,8 8 , 3199 (1963)
HAROLD KWARTAND J. L. NYCE
2602
Vol. S(i
substituent influence, the exclusive product to be observed is the norbornyl derivative with l ' in t h e ( ' v u configuration Our experimental results (above) ~ p plying the newer analytical techniques which were not available to the earlier workers, confirm this conclusion (only) for the polar addition Any cwIo product ~vould have been readily detected. as will be shown later When X is hydrogen attack by U a t the C 4 -or C4-positions in cation I cannot be equivalent Our use of
-HBr and/or -DBr
3
f
A&
Cz=51%
= 49%
Fig. 1.-Flow
diagram of polar addition of DBr
nearly 807, completion. The product obtained here, in distinction to the polar addition, consisted of a mixture of the epimers in the approximate ratio of 20% endo- to 807c of the exo-norbornyl bromide, as determined both by infrared spectrophotometry and vapor phase chromatography. As noted in our equilibration studies, the occurrence of any thermodynamic control of product formation is to be correlated with formation of some endo epimer. We therefore deduce t h a t our product is the result of control by kinetic factors alone. The products of both procedures (respectively) were subjected to a degradative routine, involving dehydrohalogenation with strong base as the first step with isolation of the norbornene in good yield, followed by oxidation to the cis-1,3-cyclopentanedicarboxylicacid. The product a t each juncture was analyzed for its deuterium content to determine how the results could be correlated with the mode of addition. The results are summarized in Table I. TABLE I Polar addition"
Sorbornyl bromide Sorbornene cis- 1,3-CyclopentanedicarboxyIicacid
F r e e radical additionb
100
100 54 38.6 51" Od a The base used for elimination was 3-methyl-3-pentoxide in 3-metli~l-3-pentanola t 120". T h e base used for elimination was 3-methyl-3-pentoxide in p-cymene a t 130'. c In other runs of this reaction a maximum of 58% of the deuterium was found in the acid. The only circumstances under which any deuterium was retained here were those occurring in solvents in which some concomitant polar addition \ \ a s suspected.
Discussion The Polar Addition of DBr to Norbornene.-It has earlier been proposed that when X f Y - is added to the n ~ r b o r n e n e ' ~ -skeleton ~j in the absence of any directive
Cation 1 ; X = H or D Cation 11, X = polar group; a i z . Br, Sir, etc.
DBr, however, does permit a distinction (that is not possible with HBr) in that, by means of the degradative procedures mentioned above, the alternative modes of attack (by Y - or its equivalent) are correlatable with the deuterium distribution. The over-all loss of 49c/, deuterium (Table I) in the sequence leading to the diacid end product (hzBz)is in good agreement with expectation on the basis of either a nonclassical structure or the equilibrium of classical structures depicted for cation I . In the nonclassical ion the Ca- and C4-positions are exactly equivalent since a plane of symmetry exists as defined by C1,6,5 and the midpoint of the C Z ~ , ~ bond. In the classical ion equilibrium, a steric argument called the "windshield wiper effect" has been advancedI6 to anticipate this result. Of course, 30 tack should be expected a t each of the C3-and C4-atoms only if we ignore any possible directive effect of a deuterium a t C2. In the light of repeated demonstration^'^^ 1 7 , 1 8 * ' q of favored cis elimination of H Y from norbornane derivatives by means of strong base, the reactions outlined in Fig. 1 are very readily interpreted. I t will be seen t h a t three structures can result from the addition of DBr to norbornene, the cis-exo product (X), trans (B), and a rearranged product (C) in which the deuterium is bonded anywhere except on the same ethylene bridge as the bromine atom. I t is very evident from the degradative sequence outlined in Fig. 1 and the data in Table I that C must comprise 31yc of the total addition product and .I B the remaining lSc/c, since C is the only one of the three products which would retain deuterium in the dibasic acid terminus. Of the deuterium content (-297,)that is not accounted for by the structures C, C1, and C2, fully 947, is lost during the dehydrohalogenation step ( A B + ;Il B,) and the residue, or about OYc,is retained in €31. Several alternative explanations of these data may be considered: (1) a 6 7 , proportion of De endo attack completed by trans addition to form structure B and
+
+
+
(13) J D Roberts. E I< T r u m b u l l , J r , W B e n n e t t , a n d I< Armstrrmg, Soc., 72, : < I 1 6 (1950) (14) I> Schmerling, ibrd., 68, 19.5 (1946) ( 1 5 ) J D . Roberts, I, U r b a n e k , a n d K .4rmstrony, r b i i i , 71, :304$1(111119). (16) H . C Brown, Abstracts, 139th S a t i o n a l hleeting of t h e American Chemical Society, S t . Louis, 110 , M a r c h 21, 1961. p 2 ~ 0 ,"Srin-Classical Intermediates," Organic Reaction >techanisms Conference, Brmrkhaven, S Y , S e p t . 3 , 1962. (17) S J Cristol a n d E F Hriegger. J A m ( h r m S O L , 79, ?A18 ( 1 9 5 7 ) . (18) S J . Cristol a n d H I. Hanse. ibid , 74, 219:i (1952) (19) C H I l e P u y , R I). T h u r n , G F Morris. I ) K n'edegaertner. a n d J A. B e c k m a n , Abstracts. 142nd S a t i o n a l Meeting of t h e American C h e m i cal Society, Los Angeles, C a l i f , April. 1963 p OR1
J A m Chem
July 5, 1964
DEUTERIUM TRACERS IN NORBORNENE ADDITIONS
all-cis elimination; ( 2 ) no endo addition of Dej (B = O), but 6% of trans elimination occurring in A with removal of the endo proton ; or (3) a combination of circumstances 1 and 2 . The very considerable body of evidence demonstrating a preference for exo attack215 in the norbornyl system does not per se rule out explanation 1 since we have found that in the free radical addition (see later section) about 2UY0 of the reaction product may be formed through initial attack from the endo bonding direction. This reservation, however, is weakened by the knowledge that in the polar reaction the analogous attacking reagents are considerably enlarged b y solvation and tend to be more discriminating with regard to the steric environment of the locus of attack. Cristol and co-workers5 have expressed the view that initial endo attack in polar addition is quite unlikely on the bicyclo [2.2.1]heptane ring. T o be taken into consideration here also are the results of LeBel and co-workers'? who have reported that in the elimination reaction under similar conditions of base strength and temperature, kcis/klrans K 15 and k H / k D 22 3.4. Furthermore, they have found that an exo is abstracted about three times more readily than a corresponding endo proton, which implies t h a t the k H l k D preference is just about cancelled out by the kexo,'kendo preference. It seems reasonable, then, that the observed 6y0 formation of B1 (of the total of A1 B1) is in good accord with the occurrence of a minor amount of trans elimination accompanying a predominant course of cis elimination, as suggested by explanation 2. Admittedly, it cannot be claimed t h a t an exclusive path has been compellingly demonstrated for the formation of component B which comprises (only) less than 3y0of the polar addition product. It seems very clear, bowever, t h a t the great majority of the product consists of just two components, A and C, developing in nearly equal amounts, whose formation path may be conceived without resort to equivocation. Our data, in fact, are consonant with several interesting conclusions which may be summarized as follows. Since the polar addition of DBr is unbiased with respect to forming rearranged and nonrearranged products, this result stands in marked contrast to the analogous case of endo-trimethylenenorbornane, which has been reported b y Cristol and co-worker$ to undergo polar addition of the elements of DOCHBwith production preponderantly (>€#I%) of rearranged materials. Evidently, the remote trimethylene substitution has exerted some directive influence in the product-forming step, an influence which does not exist in the unsubstituted norbornene. Furthermore, unlike the substituted case presenied by Cristol, et ~ l . our , ~ data do not compel the requirement of two alternative paths or intermediates in the polar addition reaction. -4 single structure such as cation I may be reconciled with the common intermediate which, uninfluenced by remote substituent effects, leads with equal probability to both rearranged and unrearranged products. This identification of cation I is further enhanced by the observation of an exclusive preference for cis addition in formation of the unrearranged product. Trans product ~ ~ ex~~~ formation via a protonated n - c ~ m p l e xis ~here cluded.
+
( 2 0 ) G . S. H a m m o n d a n d T 11 N e v i t t , J A m Chem S O L ,76, 4121 ( 1 9 5 5 ) ; C . H Collins a n d G. S.H a m m o n d , J Orp. Chem , 36, 911 (1960)
2603
I t must be emphasized that both the occurrence of trans and assorted nortricyclenic products2,21and t h e proportions of rearranged and unrearranged product found in a given case are the consequences of the operation of factors which are not endemic to the polar mechanism of addition to the bicyclic double bond. Thus, as has been suggested earlier,z the formation of trans addition product from bicyclic olefins can be anticipated where the addition reagent is capable of forming a relatively stable n-complex or onium ion in the attack phase of the reaction as, for example, is characteristic of such reagents as Brz or ArSC1. The product composition will be deviated, also, where substituent effects are evident-either resulting from the influence of a remote substituent already resident on the ring, or one introduced via attack of the electron deficient moiety of X+6Y-s with formation of a cationic intermediate like 11, possessing (now) a substituent capable of directing the subsequent product-forming phase of the reaction. Free Radical Addition of DBr to Norbornene.-The intervention of a nonclassical, bridged free radical structure in free radical additions to acyclic and monocyclic olefins has never been substantiated.6 Convincing demonstrations of the classical character of additions of p-thiocresol to norbornene6,?' and other bicyclic c o r n p ~ u n d shave ~ ~ ~been ~ presented. This conclusion has usually been inferred from the failure to find any rearrangement product in the reaction mixture. While the nonstereospecific nature of radical addition mechanisms has been widely r e p ~ r t e d , ? ~ a .num?~ ber of exceptions involving stereospecific addition of hydrogen bromide may be cited.26 The latter may be regarded as special cases of steric control where the product-forming step is completed a t the side of the intermediate radical which is least hindered by the bulky bromine atom attached at the adjacent carbon. The product obtained by working up the photo- and peroxide-catalyzed reaction of dry HBr and norbornene (in the cold) was found to be similar in infrared spectrum to that of pure em-norbornyl bromide except for six extra bands of moderate intensity. By admixing various proportions of the pure exo isomer with this product, all of these six bands decreased in intensity while the other bands remained substantially constant. Finally, some pure endo-bromide was prepared by the Diels-Alder reaction of vinyl bromide and cyclopentadienez7followed by hydrogenation over platinum. The exo-bromide was separated by exhaustive solvolysis making use of the large difference in rates of the c x o and endo epimers in this reaction, as previously noted." (21) See ref 2 as well a s H K w a r t a n d K K Miller. J A m ( ' h e i i i Sor 78, .it378 (1956). (22) S . J Cristol a n d I< P . Arganbright. ibid . 79, 60:19 (1957) ( 2 3 ) J A. Berson a n d W 11 Jones, ibid , 78, 6045 (18.3B) ( 2 4 ) See C Walling, "Free Radicals in Solutii,n," J o h n 1Viley a n d Sons. Inc , N e w York, N . Y , 1957, for a general review of t h e early literature (In this subject (2.5) ( a ) F G Bordwell a n d U'. A H e w e t t . J A m Cherii .SOL , 7 9 , :14R:1 ( 1 9 5 7 ) , ( b ) H I, Goering, I ) I Relyea, a n d I ) W I.arsen. t b l t i . 78, ,348 (19.x) (26) ( a ) H. L . Goering, P I Abell, a n d R I: Aycock (19.>2), ( b ) H 1, Goering a n d 1, I. S i m s . r b i c i , 77, :346.5 (I! Goering a n d I) W I,arsen, tbiri , 79, 2 t i i : i f l ! J 5 7 ) , 81, .->!I: S Skell a n d I< C Allen, i b i d , 80, >$)!I7[ 1!1581, 81. .7:38:i [ I ~P(1960) (27) K Alder a n d H F I
