Peculiar Reactivity of Isothiocyanates with Pentaphenylborole

The reactions of isothiocyanates with the antiaromatic pentaphenylborole were investigated, revealing significantly different outcomes than the analogous reactions with isocyanates. The 1:1 stoichiometric reaction products isolated include a seven-membered BNC5 heterocycle and a fused bicyclic 4/5-ring system. Studies suggest that the seven-membered ring undergoes an intramolecular [2 + 2] electrocyclic ring closure to produce the bicyclic system. The only derivative for which stoichiometry influenced the reaction outcome was 4-methoxyphenylisothiocyanate. The reaction of borole with an excess of 4-methoxyphenylisothiocyanate resulted in the formation of a fused tetracyclic species with two equivalents of isothiocyanate incorporated into the product. Rational pathways for these unusual transformations are presented.