Phosphorus ligands with a large cavity: Synthesis of triethynylphosphines with bulky end caps and application to the rhodium-catalyzed hydrosilylation of ketones

Trialkynylphosphines substituted with bulky triarylsilyl groups at the alkyne termini were synthesized. The new phosphines P(C equivalent to CSiAr3,)3 (Ar = 3,5-tBu(2)-4-MeOC6H2, 3,5-(Me3Si)(2)C6H3) are uncrowded near the phosphorus atom but bulky in the distal region. As a result, they contain a large cavity, at the bottom of which the phosphine lone-pair electrons are located. The compounds are stable to oxidation by air and hydrolysis. DFT calculations suggested that the triethynylphosphines are good pi-acceptor ligands, comparable with P(OAr)(3). ne trialkynylphosphines reacted with [{RhCl(cod)}(2)] (P/Rh = 1.1:1) to give selectively the monophosphine-rhodium complex [RhCl(cod)P(C CSiAr3)(3)]. X-ray crystal-structure analysis revealed that the {RhCl(cod)} fragment is fully accommodated by the cavity of the phosphine ligand. Compared to the effect of analogues with smaller end caps and PPh3, the trialkynylphosphines accelerated markedly the rhodium-catalyzed hydrosilylation of ketones with a trioganosilane. It is proposed that the higher catalytic activity observed with the holey phosphines is a result of the preferential formation of a monophosphine-rhodium species.