Dithienophosphinine

Dithienophosphinine (1) was obtained in three steps
starting with the ring expansion of dithienophosphole (3) by
reaction with benzoyl chloride, triethylamine, and water. The
three annellated rings of (1) are coplanar. Both the PC(Ph)
double bond (1.7281(17) Å) and the bridging C(Th)−C(Th)
bond (1.426(2) Å) are indicative of electronic delocalization in
the central ring. By comparison with parent phosphinine, (1)
is less aromatic (NICS(1) −8.8 vs −10.8) and more easily
reduced (at −1.94 V vs SCE in acetonitrile vs −2.27 V for the
parent). Both the highest occupied molecular orbital and the
lowest unoccupied molecular orbital are highly localized at the
PC(Ph) double bond and resemble the π and π* orbitals of a
phosphaalkene.