Intramolecular migration of bulky substituents in the solid state: Vinylogous pinacol rearrangements induced thermally and by acid catalysis

A vinylogous pinacol rearrangement, involving an intramolecular 1,4-migrati on of the bulky thienothienyl substituent and a marked change of the crysta l structure, was thermally induced in the solid state for thienothienyl-sub stituted 9,10-dihydroxy-9,10-dihydroanthracenes, 1 and 3 similar to8. When heated over 180 to similar to 240 degreesC, these crystals were transformed quantitatively, accompanying dehydration, to crystals of the anthrone deri vatives without wetting or melting. Diol 8, which bears two isomeric thieno thiophene rings, afforded only a single anthrone product. Compounds 1 and 8 formed a mixed crystal and its thermally induced reaction yielded two anth rone products that were attributable to intramolecular migration but afford ed no anthrone derivative having the two substituents possible for the inte rmolecular combination. These results, along with the X-ray crystal structu re of the mixed crystal, demonstrate that the rearrangement in the solid st ate proceeds intramolecularly. The 1,4-pinacol type rearrangements were als o induced, along with the cis-trans isomerization, by cogrinding the diols with p-toluenesulfonic acid in a mortar at room temperature. The intramolec ularity of the rearrangement was again proved by the use of the mixed cryst al. The intervention of the carbocation in the solid-state grinding was asc ertained by the UV/vis spectra. The time courses of the product distributio ns showed discrete profiles between the trans and cis diols, 1 and 3, respe ctively; for the trans diol 1, fast isomerization to the cis isomer and con secutive formation of the rearranged product via the cis isomer were observ ed. On the basis of these observations, the solid-state 1,4-rearrangement w as deduced to occur preferentially via the cis configuration of the dihydro xy groups.