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

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.