J.Org. Chem., Vol.41,No.19,1976
The E2C Mechanism in Elimination Reactions S. K. Dayai, S. Ehrenson, and R. W. Taft, J. Am. Chem. SOC., 94,9114 (1972), and references clted therein. (a) F. De Sarlo, G. Grynkiewiz, A. Ricci, and J. H. Ridd, J. Chem. SOC.6, 719 (1971); (b) M. C. R. Symons. Tetrahedron Lett., 4919 (1971). 0. Exner and J. Jones, Collect. Czech. Chem. Commun., 27, 2296 (1961). (a) H. Schmidt and A. Schwerg, Tetrahedron Lett, 981 (1973); (b) H. Schmidt and A. Schwerg, Angew. Chem., 12,307 (1973). P. A. Scherr, M. D. Giick, J. H. Siefert, and R. D. Bach, J. Am. Chem. SOC., 97, 1783 (1975). interactions at the site of substitutionand at the wtho positions may wntain considerable steric and compressional components. (a) R. W. Tafl and i. W. Lewis, J. Am. Chem. SOC.,60,2436 (1958); (b) C. G. Swain and E. C. Lupton, J. Am. Chem. SOC., 90, 4328 (1968); (c) F. Hruska, H. M. Hutton, and T. Schaefer, Can. J. Chem., 43, 2392 (1985); (d) M. J. S.Dewar, R. Golden, and J. M. Harris, J. Am. Chem. Soc.,93,4187
,.-. .,.
119711
tercept allows addltlonal check of the goodness of the data. (13) A. R. Tarpley, Jr., and J. H. Goldstein, J. Phys Chem., 78, 515 (1972). (14) (a) T. Yonemoto, J. Magn. Reson., 13, 153 (1974); (b)G. B. Savitski, P.D. Ellis, K. Namikawa, and G. E. Maciei, J. Chem. Phys., 49, 2395 (1968). (15) (a) J. B. Stothers, “Carbon-13 NMR Spectroscopy”, Academic Press, New York, N.Y., 1972; (b) G. C. Levy and 0. L. Nelson, “Carbon-13 Nuclear Magnetic Resonance for Organic Chemists”, Wiley-interscience, New York, N.f., 1972. J. C. Mulier, Bull. SOC.Chim. Fr., 3, 1815 (1964). G. E. Maciel, J. Phys. Chem., 69, 1947 (1965). (a) J. A. Pople and M. Gordon, J. Am. Chem. SOC.,89,4253 (1967); (b) J. M. Sichel and M. A. Whltehead, Theor. Chim. Acta, 5, 35, (1966); (c) W. F. Reynolds, I. R. Peat, M. H. Freedman, and J. R. Lyerla, Jr., Can. J. Chem., 51. 1857 11973). (19) E. T. McBee, L’Serfaty, and T. Hodgins, J. Am. Chem. SOC., 93, 5711 (1971). (20) M. Bulioitt. W. Kitchina. D. Doddreii, and W. Adcock, J. Org. Chem., 41, 760 (1976). (21) D. M. Grant and B. V. Cheney, J. Am. Chem. Soc., 83, 5315 (1967). (22) F. Hruska, H. M. Hutton, and T. Schaefer, Can. J. Chem., 43, 2392 (1965). .
(12) Valid arguments have been proposed concernin the use of an intercept in the evaluation of free-energy however, it is believed that for this system the data obtained for X = Hare subject to as much error as when X # H, and thus an intercept should be used. The use of an in-
B
3201
I
-
The E2C Mechanism in Elimination Reactions. 8. Interaction of Conjugating Si-Sstituents with E2C- and EBH-Like Transition States D. M. Muir* and A. J. Parker* Research School of Chemistry, Australian National University, Canberra, A.C.T., Australia Received December 9,1975 Rates and olefinic products o f dehydrotosylation of secondary tosylates under conditions suitable for E2C, E2H, and solvolysis (El) reactions, respectively, have been measured. T h e kinetic products are compared w i t h those from equilibration. Quite different proportions of olefins are obtained according to the reaction conditions and this has obvious value for synthetic work. T h e tosylates studied contain groups, e.g., phenyl, acyl, viny1,capable o f conjugati n g w i t h t h e developing double b o n d in the transition state leading t o olefins. T h e product distribution f r o m E2Clike reactions i s n o t entirely consistent w i t h t h e concept of a very product- (olefin-) like E2C transition state.
It is generally agreed that the olefin-forming elimination from secondary and tertiary alkyl halides and arenesulfonates induced by halide ions in aprotic solvents proceeds through a product-like transition state which has a large degree of carbon-carbon double bond character I.1-3 There is little
I charge at C, or Cp and the leaving group is only loosely bonded to C,. Winstein and Parker suggested that the base B is bound to both /3 hydrogen and C, in I and describe the mechanism as E2C but there is less agreement on this p 0 i n t . l ~A~puzzling feature in terms of the product-like E2C transition state has been the similar substituent effect on rate of @-aryland 0methyl g r o ~ p s ,which ~ ~ ~both , ~ strongly ~ ~ enhance the rate of E2C-like eliminations relative to hydrogen. Where there is a choice of elimination pathways, e.g., dehydrotosylation of 11, P-phenyl substituents do not appear to dictate the direction of elimination to form an extended conjugated styrene system in preference to the methyl hyperconjugated system. The olefinic products are not close to their equilibrium proportions when phenyl substituents are involved.6
PhCH2CH(OTs)CH(CHs)z
I1 * Address correspondence t o Murdoch University, Murdoch, Western Australia.
To establish whether these difficulties with our mechanistic interpretation of E2C reactions1 were general for substituents capable of conjugation with developing double bonds, or were a peculiarity of aryl groups, e.g., steric factors inhibiting coplanarity of the phenyl ring with the developing double bond, we have studied the products of elimination from substrates having 0-methyl, &vinyl, 0-acyl, and P-phenyl substituents.
Results and Discussion We have difficulty in developing a consistent mechanistic description of the rates and proportions of olefinic products from the reactiobs of NBusBr in acetone containing 2,6-lutidine, the reactions of KOBu-t in tert- butyl alcohol, and the solvolysis in acetone-water of the tosylates shown in Table I. However, very small differences in the energy of transition states or of products can lead to what might at first appear to be rather different proportions of trans to cis olefin or of conjugated to unconjugated olefin. It may not be profitable to try to extend too far our E2C-E2H mechanistic thinking to explain differences in such small effects. Nevertheless, the results in Table I, together with some broad generalizations covering related compounds, could be of value to the organic chemist, anxious to decide between equilibration of olefins with KOBu-t/MezSO, reactions of tosylates with KOBu-t/ t-BuOH or with NBudBr/acetone/lutidine, or solvolysis as a means of obtaining a desired proportion of olefins. For this reason we present the results and make a few very brief generalizations. The tosylates 111,V, and VI in Table I can be dehydrotosylated in two directions as well as giving trans and cis isomers,
3202 J.Org. Chem., Vol. 41,No.19,1976
Muir and Parker
Table I. Products of Dehydrotosylation of Secondary Alkyl Tosylates RICH(OTS)R~ and Equilibration of Olefins RlCH(OTs)R2 No.
Ri
Rz
Mech
Log k e
I11
PhCHz
CH(CH&
E2Cg Eqh Sol. i E2Hj E2C Eq So1.k ..~ E2H E2C Eq Sol. E2H E2C Eq Sol. E2H E2C Eq Sol. E2H
-2.9
IV
V
CH3CH2
CHz=CH\
CH(CH3)z
CH(CH&
CHdCH VI
VI1
CH3COCHz
PhCH2
+
CH(CH&
CHzCH3
Conjugated" Otherb hyperconjugated olefins, % 0.10 1.8 0.16 37 0.043" 0.092" 0.20" 0.96"
-3.3f -2.8 -2.8 >-4f
-2.6 -2.9
1.0
4.0 0.53 3.5 35 110 20.3 13 1.6 15.6 0.50 41
-2.3f -3.45 -2.5 -2.7f
21.3 -4.35 -3.8f -2.6
Transcam cis
0.8
18
3.4 13.3 0.2 2.7 35k 2 1.6
1.7 2.2
1.2 1.0
5.4 0.2 0.7 10.4 0.3
2.6 100
200 33 130 14.5 52 5.2 15
1
