2‑Aryl-1,3-Benzoxaphospholes as Unwilling Participants for Catalytic Suzuki–Miyaura CC Coupling Reactions

Under
conditions typically effective to achieve a catalytic aryl–aryl
bond during palladium Suzuki–Miyaura (SM) coupling reactions
of ArB­(OH)2 and ArBr, reactions of p-tolylboronic
acid and 2-(BrC6H4)-1,3-benzoxaphosphole (BrC6H4-BOP, 1) failed to provide the anticipated
product of CC bond coupling, 2-(p-CH3C6H4-C6H4)-1,3-benzoxaphosphole.
An analysis of reaction mixtures by 1H and 31P­{1H} NMR spectroscopic methods showed the presence of
numerous broad resonances, suggesting the formation of labile metal
complexes with 1. This possibility was confirmed by determinations
of the solid-state structures of [(tBu3P)­(Ph-BOP)­M]2 (M = Pt, 5; M = Pd, 6) from the
reaction of Ph-BOP and [M­(tBu3P)2] (M = Pt, Pd). The structures of 5 and 6 are isomorphous and reveal unusual bridging μ2-P-BOP bonding modes. Computational studies have explored
the structures and relative energies and indicate that dimeric complexes
of this form can dissociate easily. Surprisingly, even when PC
double bonds of Ar-BOPs are sheltered by coordination to tungsten
pentacarbonyl, the Ar-BOP units resist yielding products of CC coupling
under similar SM conditions but instead yield unusual products whereby
CsOH adds across the PC bonds in [(CO)5W­{2-Ar-BOP}].
Computational studies are consistent with ready addition of hydroxide
to the tungsten-protected PC bond.