POLYCYCLIC THIOPHENES FROM THE DIRECT INSERTION OF HETEROSULFUR BRIDGES INTO VINYLARENES, BIARYLS, AND ANGULARLY CONDENSED ARENES

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Abstract
There are three synthetic methods for direct introduction of a heterosulfur bridging atom into a vinylarene, biaryl, or angularly condensed arene to form a polycyclic thiophene. Two methods employ elemental sulfur, with or without a catalyst (aluminum chloride). The third method, which uses hydrogen sulfide and a heterogeneous catalyst (usually a metallic oxide) at 450–630°, is reviewed in detail. Thus, results on 20 different substrates (including a number of compounds bearing pyridine rings) are presented. Experimental observations which concern mechanistic aspects of the reaction are summarized and interpreted. For most substrates it is suggested that an ECEC mechanism prevails, whereby an adsorbed sulfide ion attacks a π-electron-deficient, adsorbed substrate to form a dihydro intermediate, which then undergoes aromatization. The substrate is adsorbed onto the catalyst either via the π-electrons or n-electrons (for some pyridine compounds). Type of adsorption and/or relative reactivities of ring positions may control isomeric ratios of products. For 2- and 4-vinylpyridines as substrates it is proposed that 1,2-addition leads to β-pyridylethanethiols and bis-(β-pyridylethyl) sulfides, which undergo thermal cyclization. The sulfur bridging reaction as a laboratory and, also possibly, a natural route to the formation of polycyclic condensed thiophenes found in the environment is discussed.

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