Maleic anhydride–hexamethylbenzene mixtures in methylcyclohexane solution and in the solid state. Part II. Photochemical and thermal reactions

Abstract

A methylcyclohexane solution of maleic anhydride and hexamethylbenzene was irradiated with ultraviolet light in the absence and in the presence of filters that confined absorption, either nearly or completely, to the maleic anhydride–hexamethylbenzene charge-transfer complex. In the latter case the reaction mixture also contained toluene in a 6- and 25-fold molar excess over hexamethylbenzene. Pentamethylbenzylsuccinic anhydride and resinous substances were isolated in all experiments but no adducts of toluene with maleic anhydride were found. Evidence was obtained for formation of CO₂ during the irradiation. Ultraviolet irradiation of a mixture of maleic anhydride and hexamethylbenzene in the solid state produced 1,2, : 3,4-cyclobutanetetracarboxylic acid dianhydride. No adducts of hexamethylbenzene with maleic anhydride were detected. From a partly carbonized mixture obtained by heating equimolar quantities of maleic anhydride and hexamethylbenzene to 250° for 15·5 hr. and followed by hydrolysis were isolated pentamethylbenzylsuccinic acid, 4,5,6,7-tetramethylindane-1,2-dicarboxylic acid, and resinous materials. Only 4,5,6,7-tetramethylindane-1,2-dicarboxylic acid and the resins were isolated in a similar experiment but with the heating time extended to 17 hr. No detectable reaction occurred at 200° over the period of 14 hours.

It is believed that large contribution of the dative structure to the electronically excited complex (ca. 90%) resulted in the proton transfer within the complex to give a geminate pair of free radicals that combined yielding pentamethyl-benzylsuccinic anhydride. The course of the photochemical reaction in the solid state reflected the absence of the charge-transfer complex and the limited mobility of the components of the solid matrix. Crystalline products obtained in the thermal reactions probably originated from the addition of the pentamethylbenzyl radical, formed by cleavage of the benzylic C–H bond in hexamethylbenzene, to maleic anhydride.