Benzyne-Oxazole Cycloadducts: Isolation and Retro-Diels-Alder Reactions

Article abstract of DOI:10.1021/jo00290a025

A literature procedure for generating benzyne from 1-aminobenzotriazole and lead tetraacetate was modified by the use of two syringe pumps to effect simultaneous addition through opposing ports of a three-neck flask.Applied to the reaction of benzyne with 4-phenyloxazole at 0 deg C, this modification resulted in essentially quantitative formation of cycloadduct.Retro-Diels-Alder expulsion of benzonitrile with concurrent formation of isobenzofuran occurred when the adduct was heated.The sequence constitutes a useful method of preparation of isobenzofuran, which may be utilized in situ or isolated as a solution for subsequent application.Benzyne cycloadducts were also prepared from 4-(4-nitrophenyl)- and 4-(4-methoxyphenyl)oxazoles, in order to assess the effect of remote substituents on the retro-Diels-Alder reaction.First-order rate constants were determined at three temperatures in the range of 40-70 deg C.The order of reactivity was p-NO2 > p-H > p-OMe; the substituent effects were small, with a factor <4 separating the p-NO2 from p-OMe rate.All three substrates exhibited small ΔS(excit.) values.Expulsion of the nitrile is believed to occur as a concerted reaction with little charge development.The modified benzyne procedure was also applied to 2,5-diphenyloxazole, at 0 and -78 deg C.Oxazole was recovered (59percent and 43percent at the higher and lower temperature, respectively).No bis(benzyne) adduct was formed, showing that 1,3-diphenylisobenzofuran is not generated during the course of the reaction.The 1:1 benzyne-diphenyloxazole cycloadduct is not observed in the crude product of the0 deg C reaction, but an absorption at 6.09 ppm in the NMR spectrum of crude product from the -78 deg C reaction is attributed to this material.The latter crude product on standing at room temperature forms the isobenzofuran.Both reaction mixtures form 1,3-diphenylisobenzofuran by another mechanism involving a second intermediate.This more polar intermediate, which appears to be the major product at 0 deg C, slowly gives 1,3-diphenylisobenzofuran.The overall more efficient cycloaddition at - 78 deg C is attributed to entropy effects, which tend to favor Diels-Alder over other second-order reactions as the temperature is lowered.