Using para hydrogen induced polarization to study steps in the hydroformylation reaction

Abstract

A range of iridium complexes, Ir(η³-C₃H₅)(CO)(PR₂R′)₂ (1a–1e) [where 1a, PR₂R′ = PPh₃, 1b P(p-tol)₃, 1c PMePh₂, 1d PMe₂Ph and 1e PMe₃] were synthesized and their reactivity as stoichiometric hydroformylation precursors studied. Para-hydrogen assisted NMR spectroscopy detected the following intermediates: Ir(H)₂(η³-C₃H₅)(CO)(PR₂R′) (2a–e), Ir(H)₂(η¹-C₃H₅)(CO)(PR₂R′)₂ (4d–e), Ir(H)₂(η¹-C₃H₅)(CO)₂(PR₂R′) (10a–e), Ir(H)₂(CO–C₃H₅)(CO)₂(PR₂R′) (11a–c), Ir(H)₂(CO–C₃H₇)(CO)₂(PR₂R′) (12a–c) and Ir(H)₂(CO–C₃H₅)(CO)(PR₂R′)₂ (13d–e). Some of these species exist as two geometric isomers according to their multinuclear NMR characteristics. The NMR studies suggest a role for the following 16 electron species in these reactions: Ir(η³-C₃H₅)(CO)(PR₂R′), Ir(η¹-C₃H₅)(CO)(PR₂R′)₂, Ir(η¹-C₃H₅)(CO)₂(PR₂R′), Ir(CO–C₃H₅)(CO)₂(PR₂R′), Ir(CO–C₃H₇)(CO)₂(PR₂R′) and Ir(CO–C₃H₅)(CO)(PR₂R′)₂. Their role is linked to several 18 electron species in order to confirm the route by which hydroformylation and hydrogenation proceeds.