Abstract
THE Diels–Alder reaction, in which a stable six-membered ring is formed by 1,4-addition of an unsaturated linkage to a diene, has wide synthetic application. The role of the olefinic group as a dienophile is well known1. More recently, the addition of the cyano group to dienes with the formation of pyridinic products has been reported2. Acrylonitrile presents an interesting example in which the groups (C = C) and (C ≡ N), both dienophilic, are present. The present communication reports the results of a study on the reactions of acrylonitrile with butadiene. The relative reactivity of these two links of acrylonitrile in this reaction can be predicted from thermo-dynamic and kinetic considerations. The free energy changes at 700° K. for the formation of cyanocyclo-hexene and vinylpyridine are −21 and −14 kcal./mol. respectively. These results are qualitative, since the data for the cyclic products were estimated by the method of group increments3; but they show that the two reactions are both thermodynamically feasible in this temperature-range. The ultimate yields are a concern of reaction kinetics. The relative rates of these two reactions, assuming the cyclization to be the rate-controlling step, were calculated from the expression: using the kinetic data of related high-temperature Diels–Alder associations. From a consideration of the activation energies of the reactions of acrolein4 and cyanogen5 respectively with butadiene, a minimum value of − 11.9 kcal. was obtained for (Ea–Eb). The magnitude of for the activated complexes was taken as approximately − 1.1 e.u. from the difference in entropy of 3-cyanocyclohexene and 2-vinyldihydropyridine. Accordingly, at 400° C. in the homogeneous gas phase, it is predicted from these data that the formation of cyanocyclohexene is 5,800 times more rapid than the formation of vinylpyridine.
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JANZ, G., DUNCAN, N. Relative Reactivities of the Olefinic and Cyano Links of Acrylonitrile in the Diels–Alder Reaction. Nature 171, 933 (1953). https://doi.org/10.1038/171933a0
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DOI: https://doi.org/10.1038/171933a0
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