Retro-Diels−Alder Reaction of 4H-1,2-Benzoxazines to Generate o-Quinone Methides: Involvement of Highly Polarized Transition States

Abstract

Here, we describe mechanistic studies of the retro-Diels-Alder reaction of 4H-1,2-benzoxazines bearing various substituents on the benzene ring. 4H-1,2-Benzoxazines are very simple, but quite new, heterocyclic compounds that afford substituted o-quinone methides (o-QMs) through retro-Diels-Alder reaction under mild thermal conditions. The resultant o-QMs undergo Diels-Alder reaction in situ with dienophiles to give phenol and chroman derivatives. The mechanism of the generation of o-QMs has been little studied. Our experimental and density functional theory (DFT) studies have yielded the following results. (1) The generation of o-QMs, i.e., the retro-hetero-Diels-Alder reaction of 4H-1,2-benzoxazines, is rate determining, rather than the subsequent Diels-Alder reaction of the resultant o-QM with dienophiles. (2) The reaction rate is strongly influenced by the electronic features of substituents and the polarity of the solvent. The reaction proceeds faster in a polar solvent such as dimethyl sulfoxide, probably because of stabilization of the electronically polarized TS structure. (3) The reactions show characteristic positional effects of substitution on the benzene ring. While an electron-withdrawing group such as CF3 at C5, C6, or C7 positions decelerates the reaction, the same substituent at C8 accelerates the reaction, compared with the reaction of unsubstituted 4H-1,2-benzoxazine. In particular, substitution at C5 significantly decelerates the reaction as compared with the unsubstituted case. This is due to the difference in the inductive effect of CF3 at the different positions. Similar positional effects occur with a halogen (Cl) and a nitro group. All these data support the involvement of polarized TS structures, in which the O-N bond cleavage precedes the C-C bond cleavage.