2315 always less than 0.05, and coupling constants were accurate to f0.05 Hz.~‘ (44)Edgell, et a / . , have recently published an infrared study of a series of inorganic salts in THF which provide additional insight into the structure of ion pairs in solution: W. F. Edgell, J. Lyford, R.
Acknowledgment. J. B. G . thanks the Commonwealth Scien1,ific and Industrial Research Organization for the award of a Senior Studentship. Wright, W. Risen, and A. Watts, J . Amer. Chem. Soc., 92, 2240 (1970).
Chloride-Induced Elimination from 2-Phenylcyclopentyl, 2-Phenylcyclohexyl, and 2-Norbornyl Brosylates in Acetone P. Beltrame,* * A. Ceccon,2 and S. Winstein Contribution from the Department of Chemistry, University of California, Los Angeles, California 90024. Received June 24, 1971 Abstract: cis- and trans-2-phenylcyclopentyl and 2-phenylcyclohexyl p-bromobenzenesulfonates by reaction with n-Bu4NC1in acetone give variable amounts of olefinic products. The cis isomers (fraction of elimination FE = 0.81-0.99) show a preference for Saytzeff elimination to the extent of 99.7 % I-phenylcyclopentene and 99.5 % 1-phenylcyclohexene. The trans isomers (FE= 0.55-0.22) have a >90% preference for Hofmann products, 3of second-order rate constants for elimination phenylcyclopentene and 3-phenylcyclohexene. The ratio kciS/ktrans at 50” is 48 for cyclopentyl derivatives and 1140 for cyclohexyl derivatives. exo-2-Norbornyl p-bromobenzenesulfonate under the same conditions gives, besides endo- and exo-norbornylchlorides, nortricyclene and norbornene (FE = 0.70) with nortricyclene largely predominant. Kinetic measurements show that, while nortricylene and norbornene are formed in a 9 :1 ratio in ionization processes, the halide-induced elimination gives them in the ratio 99.4 :0.6. endo-2-Norbornylp-bromobenzenesulfonatereacts with n-Bu4NC1to give mainly exo-2-norbornyl chloride with no endo chloride and a small amount of eliminationproducts (FE= 0.05).
S
everal years ago it was discovered that halide ions in acetone are effective in promoting elimination reaction^.^ Since then, a number of papers on the subject have been published. The substrates investigated were alkyl tosylates, brosylates, and halides; while primary derivatives give little or no e l i m i n a t i ~ n , ~ and tertiary derivatives present fractions of elimination -~ substrates are close or equal t o ~ n i t y , ~secondary more flexible in giving a blend of substitution and elimination products. 3,4$7 The most useful solvent-salt system for the study of these reactions seems to be acetone containing tetra-n-butylammonium halides, although other salts and dipolar aprotic solvents have been sed.^^^ An interpretation of the halide-induced elimination has been advanced that requires an interaction of the halide ion with both a-carbon and 6-hydrogen atoms at the transition state (E2C-like transition state).?^^ However, this suggestion has not been generally accepted,5~6~1C,11 and the whole question seems to be far from being solved. (1) NATO Fellow, 1962; address correspondence to this author at: Instituto Chimico, Universita, Cagliari, Italy. (2) CNR (Rome) Fellow, 1966-1967. (3) S. Winstein, D. Darwish, and N. J. Holness, J . Amer. Chem. Soc., 78,2915 (1956).
(4)S. Winstein, “Chimica teorica,” Accademia naz. Lincei, Rome, 1965, p 327. ( 5 ) J. F. Bunnett and E. Baciocchi, J. Org. Chem., 35,76 (1970). (6) D. J. McLennan and R. J.Wong, TetrahedronLett., 881 (1970). (7) G. Biale, A. J. Parker, S. G. Smith, I. D. R. Stevens, and S. Winstein, J . Amer. Chem. Soc., 92, 115 (1970). (8) R. A. Bartsch, J. Org. Chem., 35, 1023 (1970). (9) A. J. Parker, M. Ruane, G. Biale, and S . Winstein, Tetrahedron Lett., 2113 (1968). (10) D. Eck and J. F. Bunnett, J . Amer. Chem. Soc., 91,3099 (1969). (11) See, however, D. Cook and A. J. Parker, Tetrahedron Lett., 4901 (1969).
As a contribution t o the understanding of the mechanism of this reaction, we present results obtained on 2-phenylcyclopentyl brosylates (I), 2-phenylcyclohexyl brosylates (11), and 2-norbornyl brosylates (111) reacting with n-Bu4NC1in acetone.
Results Rate coefficients kE+S for the total reaction were obtained by adding the observed rates of chloride ion consumption and acid production. The ratio of acid produced t o reacted substrate gave the fraction of elimination FE. In some cases F E was directly determined from product glc analysis. Rate coefficients kE for elimination and ks for substitution were calculated from the values of kE+s and F E . Results are given in Table I. Product analyses, by glc, gave the results summarized in Tables I1 and 111. Solvolyses, in the absence of chloride ion, were also studied, usually both in terms of rates and products. The results are gathered in Tables IV and V. An upward drift of first-order coefficients kl was observed during the kinetic runs of compounds cis-I, cis-11, and trans-11, which was attributed t o salt effects. A positive salt effect was ascertained for exo-111, by working at different concentrations of n-Bu4NC10,. The intervention of solvolysis in the reaction with chloride ion was estimated by comparing kl values, corrected for salt effect, with the rates of bimolecular reactions, expressed as pseudo-first-order coefficients k’ = kE+S X average [Cl-1. Considerable intervention of solvolysis was found only in the case of exo-2-norbornyl brosylate (see later). cis- and trans-2-Phenylcyclopentyl Brosylates. The cis isomer (cis-I) is the most reactive with n-Bu4NC1of Beltrame, Ceccon, Winstein / Chloride-Induced Elimination
2316 Table I. Rates of Elimination ( k ~and ) Substitution (ks) Reactions with n-Bu4NC1in Acetone 1o4ks, M - 1 Compounds cis-2-Phenylcyclopent yl-OBs
(cis-I) trans-2-PhenylcyclopentylOBs (rrans-I) cis-2-Phen ylcyclohexyl-OBs (cis-11) trans-2-Phenylcyclohex yl-OBs (trans-11) exo-2-Norborn yl-OBs (exo-111) endo-2-Norbornyl-OBs (endo-I1I)
10'k~+s,M-'
~O'FE
Temp, O C
1 0 2 Msalt
15.0 30.0
3.34 3.36
8 0 . 6 f 0.5 8 1 . 4 1 1.1
28.8 Z!Z 0 . 4 165 f 3
5.6 31
49.6 20.0 35.0
3.41 3.55 3.70
55.2 f 0.2* 96 f 1 99.4 f0 . 3
40.8 f 0 . 6 19.6 f 0 . 4 112 f 2
18.3 0.8 1
50.0 50.0
3.14 2.34 6.47
21.7 f 0.3* 74.6 f 1 . p 69.4 ZIZ 0 . 7
50.0
4
5c
Sa-'
SeC-'
Compd" cis-I tram-I
cis-11 rrairs-I1
10zM Temp, n-Bu4"C NCl
No. of half-
Elim products,
z
lives
50.0 4 . 5 0 99.7(IV); 0 . 3 ( V ) 50.0 3.92 17.8 (IV); 82.2 (V) 19.7; 80.2 23.9; 76.1 32.7; 6 7 . 3 44.6; 55.4 20.0 3.55 99.5 (VI); 0.5(VII) 35.0 3.70 99.5; 0.5 50.0 3.14 59.7(VI); 40.3 (VII) 70.2; 29.8
17, 650, 2000 34 49 66 170 560 20 15, 35 20 42
a Substrates are 0.016-0.021 M. Acetone contains 0.03 M 2,6lutidine.
23.2 134 22.5 18.8 111
2.18 =k 0 . 0 4 6.52 6.77
1.71 1.66 2.07
0.47 4.86 4.70
1.63d
1.55
0.08
OBs is p-bromobenzenesulfonate. Substrates are 0.016-0.020 M . Acetone contains 0.025-0.031 M 2,6-lutidine. zero time. c Determined by glc. G. Biale, Ph.D. Thesis, University of California, Los Angeles, 1963.
Table 11. Sgytzeff cs. Hofmann Products from Bimolecular Elimination Reactions of I and I1
104kE, M-I sec-I
Extrapolated to
seems to be very little dependent on temperature. Calculated values of 104ks and 104kEat 50" are 250 and 1070, respectively. Glc analysis showed that l-phenylcyclopentene (IV) is mainly produced together with a small amount of 3-phenylcyclopentene (V); IV and V were in the ratio 99.7:0.3 (Table 11). A third peak, whose area was 2 1 S z of the total area, was also revealed. Retention times, at a column temperature of 175" and at a flow rate around 100 ml/min, were: V, 3.6 min; IV, 6.5 min; third peak, 12.2 min. While IV and V were identified by the use of authentic samples, the third peak was assumed t o be the product of S N ~ substitution, i.e., trans-2-phenylcyclopentyl chloride. The nature of the products and their relative yields were independent of the length of the experiment (Table
Table 111. Products of Reactions of exo-I11 and endo-111 in the Presence of n-BuaNCl in Acetone a t 50.0"
---Compda
10'M /I-BudNCl
exo-111
2.34 3.41 6.47 11.5 3.15 10.5
endo-111 a
IO'FE
of total reaction-----
VI11
IX
exo-X
endo-X
(PlC)
2.5 1.7 1.2 1 .o 1.7 1.8
72.1 67.2 68.3 67.2 3.8 2.2
2.2 3.1 2.6 2.7 94.5 96.0
23.2 28.0 27.9 29.1
74.6 68.9 69.5 68.2
No. of half-lives 10 15, 22 6, 14 20 6 , 12 6
5.5 4.0
Substrates are 0.02 M. Acetone contains 0.02-0.03 M 2,6-lutidine.
Table 1V. Rates and Olefinic Products from Solvolysis of I and I1 in Acetone, in the Presence of rt-Bu4NC104
Compda cis-I trans-I cis-I1 rrarrs-11
10aM Temp, n-Bu4"C NCIOl
50.0 75.0 50.0 75.0 50.0 75.0
3.80 3.26 3.82 3.76 3.55 3.31
11). Solvolysis of cis-I accounts for less than 0.1 % of the products.
Olefinic products:
10%, sec-' 1.88~ d 0.937 f 0.002 1.70 f 0 . 2 0.181~ 0.1840
z
d 91.0 (IV); 9 . 0 (V) d 93.3 (IV); 6 . 7 ( V ) d d
Substrates are 0.017-0.020 M. Acetone contains 0.027-0.030 M 2,6-lutidine. * The only products, apart from a few per cent of unknown impurities. Initial rate. d Not determined.
all the compounds examined. From rate coefficients k ~ + at s 15 and 30" (Table I) an apparent activation energy of 20.2 kcal mol-' was obtained, and the rate coefficient at 50" was calculated to be 104kE+s = 1320 M-' sec-l. The fraction of elimination is 0.81 and Journal of the American Chemical Society
1 94:7 I April 5, I972
n.Bu,NCI acetone
QPh
OBs I
mphmph + -
Iv
+
substitution product
V
The trans-I isomer is less reactive than cis-I both in substitution and in elimination, according t o ks and k E values (Table I). The value of 0.552 for the fraction of elimination was estimated by extrapolation to time zero, because F E varied with time due t o a secondary reaction, probably elimination from the substitution product, Disregarding the correction for the secondary reaction and averaging FE values along the
2317 Table V. Rates and Products from Solvolysis of exo-I11 and of endo-I11 in Acetone at 50.0" Salt
102 M salt
n-Bu4NC104
4.55 8.15
Compda
exo-I11 endo-111
106 kt, SX-'
VI11
3.0 jZ 0.3b
7.4 7.2 7.3 1.9d
4.4 +z 0.3 5.3 jZ 0 . 2 0.13+z 0.01
Acetone contains 0.02-0.03M 2,6-lutidine. 5 Substrates are 0.01-0.02M. coefficient, k,, is 27 X 10-6 sec-1. c Not determined. d At 100".
run, a value of 0.558 was obtained. Olefinic products were IV and V, only the former markedly increasing with time. A third peak with a retention time of 17.2 min b a s observed; its area amounted to 34.2% of the total area at 34 half-lives and decreased t o 3.1% at 560 half-lives. At this time, a fourth peak (1.4% of the total area) with the retention time previously attributed to trans-2-phenylcyclopentyl chloride was observed. The third peak was assumed to correspond to cis-2-phenylcyclopentyl chloride ; its disappearance roughly accounts for the increasing amount of olefin IV, which changes the ratio 1V:V as shown in Table 11. Calculations based on the corrected F E value gave an approximate ratio 1V:V = 9:91 for the original olefinic product from trans-I. Solvolysis of trans-I accounts for
