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.