6077
protons. The comparable absorption for the trans,truns-2,4-
hexadiene-maleic anhydride adduct is a singlet at T 4.22. It was found that the ratio of hexadiene adduct to butadiene adduct was 2 :3. The approximate relative rate constants of trans,trans-2,4hexadiene and butadiene with maleic anhydride (kHlkB) can then be calculated from the following equation if CH and CB are taken to be the areas under the vinyl proton peaks of the adducts of
hexadiene and butadiene, respectively (MB, millimoles of butadiene; ME, millimoles of hexadiene). The same procedure was followed, this time with 730 mg (8.9 mmol) of hexadiene. CH/CBwas found to be 112.7and kalka is equal to 11.
Acknowledgment. We thank the National Science Foundation, the National Institutes of Health, and the Petroleum Research Fund, administered by the American Chemical Society, for grants in support of this work. The junior author gratefully acknowledges fellowship support at different times from the Woodrow Wilson Foundation, the National Science Foundation, and the National Institutes of Health.
Cycloaddition. IX. a-Acetoxyacrylonitrile as a Dienophile. Its Concerted 1,4 Cycloaddition to 2,4-Hexadiene Paul D. Bartlett and Kathleen E. Schueller Contribution f r o m the Converse Memorial Laboratory of Harvard University, Cambridge, Massachusetts 02138. Received April 18, 1968
Abstract: The thermal addition of a-acetoxyacrylonitrile to trans,trans-2,4-hexadiene yields a 36 : 64 mixture of the two stereoisomers of 4-cyano-4-acetoxy-cis-3,6-dimethylcyclohexene (7and 8). The absence of the two isomers with the methyl groups lrans shows that this 1,4 cycloaddition occurs by a concerted mechanism. In contrast to the case of butadiene, only 0 . 4 x of mixed cyclobutanes was present in the cycloadduct. It is concluded that in this case there is competition between a concerted Diels-Alder reaction and a less favorable stepwise mechanism leading to 1,2 cycloaddition.
I
n the preceding paper' we reported observing a minute amount of vinylcyclobutane in the product of thermal cycloaddition of ethylene to butadiene. The occurrence of any 1,2 cycloaddition, which implies a stepwise mechanism, raised the question whether such a weak dienophile as ethylene necessarily reacts concertedly in its Diels-Alder reactions, or whether some 1,4 addition might result from a branching of a biradical (stepwise) mechanism. This question could be answered for ethylene and a substituted butadiene having geometrical configuration: it was shown that the sole detectable adduct of ethylene and trans,trans-2,4hexadiene was the thermodynamically unfavored cis3,6-dimethylcyclohexene,establishing the mechanism as stereospecific and concerted. A closer competition between thermal 1,2 and 1,4 cycloaddition to butadiene has been reported in the case of a-acetoxyacrylonitrile (2), * which yields from 9 to 23 % of regiospecific3 but stereorandom 1,2 adduct (4 and 5). The dependence of the reaction rate on solvent polarity is so small as to exclude a common dipolar ion intermediate for the two modes of addition. Both the small magnitude and the direction of the polar solvent effect on product composition exclude this possibility for the minor (1,2) mode of addition. On the assumption that distinctly different mechanisms, one concerted and the other by way of a biradical, should show markedly different dependence on solvent (1) Part VIII: P. D. Bartlett and K. E. Schueller, J . Amer. Chem. Soc., 90, 6071 (1968). (2) J. C. Little, ibid., 87, 4020 (1965). (3) A . Hassner, J . Org. Chem., 33, 2684 (1968).
and temperature, Little2 advocated a common mechanism for the 1,2 and 1,4 addition, involving a single intermediate with apparent allylic 7r bonding at the site where the eventual ring closure was to be effected.
1+ 1
+cN
OAc
2
1 3
4
5
Our recent studies have led to the view that a fairly sharp experimental distinction can be drawn between concerted reactions, which, are stereospecific, and stepwise reactions by way of a biradical, which are n0t.~,5 We have applied this test to the reaction of a-acetoxyacrylonitrile with 2,4-hexadiene as the simplest homolog of butadiene that possesses configuration at both double bonds. Results Under conditions enabling us to duplicate one of Little's additions of a-acetoxyacrylonitrile to butadiene (4) L. K. Montgomery, I
