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CSiAr3)3 (Ar=3,5‐tBu2‐4‐MeOC6H2, 3,5‐(Me3Si)2C6H3) 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 π‐acceptor ligands, comparable with P(OAr)3. The 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 triorganosilane. 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. Holey phosphines! The end capping of triethynylphosphine with bulky groups results in the creation of ligands with a large cavity in which the phosphorus lone‐pair electrons are located. The novel coordination properties of these ligands lead to a rate‐accelerating effect in the rhodium‐catalyzed hydrosilylation of ketones. cod=1,5‐cyclooctadiene.