Ring Expansion Reactions of Pentaphenylborole with Dipolar Molecules as a Route to Seven-Membered Boron Heterocycles

Reactions of pentaphenylborole with
isocyanates, benzophenone,
and benzaldehyde produced new seven-membered heterocycles in high
yields. For 1-adamantyl isocyanate, a BNC5 heterocycle
was obtained from the insertion of the C–N moiety into the
five-membered borole, whereas for 4-methoxyphenyl isocyanate, a BOC5 heterocycle was generated from the insertion of the C–O
unit. These reactions are believed to occur via a mechanism wherein
coordination of the nucleophile to the borole (1-adamantyl, N-coordination
or O-coordination for 4-methoxyphenyl) is followed by ring expansion
to afford the observed seven-membered heterocycles. The selectivity
to form B–O- or B–N-containing heterocycles is based
on the polarization of the isocyanate implying tunable reactivity
for the system. Having observed that isocyanates react as 1,2-dipoles
with pentaphenylborole, we examined benzophenone and benzaldehyde,
which both reacted to insert C–O units into the ring. This
represents a new efficient method for preparing rare seven-membered
boracycles.