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.