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 H-1 and P-31{H-1} 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 [((Bu3P)-Bu-t)(Ph-BOP)M](2) (M = Pt, 5; M = Pd, 6) from the reaction of Ph-BOP and [M((Bu3P)-Bu-t)(2)] (M = Pt, Pd). The structures of 5 and 6 are isomorphous and reveal unusual bridging mu(2)-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)(5)W{2-Ar-BOP}]. Computational studies are consistent with ready addition of hydroxide to the tungsten-protected P=C bond.