Rhodium(III) Acyl Hydrido, Acyl Hydroxyalkyl, Diacyl, Acyl Hydrido Aldehyde, and Acyl Hydrido Alcohol Complexes. Reduction of Aldehyde to Alcohol through Rhodium Hydroxyalkyl Complexes

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Abstract

[RhCl(COD)]2 (COD = 1,5-cyclooctadiene) reacts with o-(diphenylphosphino)benzaldehyde (PPh2(o-C6H4CHO)) (Rh/P = 1:1) in the presence of pyridine to give an acyl hydrido species, [RhHCl(PPh2(o-C6H4CO))(py)2] (1). In chlorinated solvents exchange of hydride by chloride gives [RhCl2(PPh2(o-C6H4CO))(py)2] (2). The reactions of 1 with PPh3 and of 2 with biacetyl dihydrazone (bdh) gives the pyridine substitution products [RhHCl(PPh2(o-C6H4CO))(PPh3)(py)] (4) and [RhCl2(PPh2(o-C6H4CO))(bdh)] (3), respectively. By using a 1:2 ratio of Rh to PPh2(o-C6H4CHO) [RhHCl(PPh2(o-C6H4CO))(κ1-PPh2(o-C6H4CHO))(py)] (5) with trans phosphorus atoms is formed. The aldehyde group may undergo two different reactions. In benzene 5 affords the acyl hydroxyalkyl species [RhCl(PPh2(o-C6H4CO))(PPh2(o-C6H4CHOH))(py)] (6) with cis phosphorus atoms, via a pyridine dissociation path. 6 undergoes dehydrogenation, with H2 evolution, to afford the diacyl derivative [RhCl(PPh2(o-C6H4CO))2(py)] (8), which shows fluxional behavior in solution, with the values ΔH⧧ = 8.8 ± 0.4 kcal mol-1 and ΔS⧧ = −16.7 ± 1 eu. Opening of the acylphosphine chelate appears to be responsible for the fluxionality. In methanol 5 undergoes displacement of chloride by the aldehyde to afford the cationic acyl hydrido aldehyde [RhH(PPh2(o-C6H4CO))(κ2-PPh2(o-C6H4CHO))(py)]+ (10), which can be isolated if precipitated immediately with an appropriate counterion. Longer reaction periods of 5 in methanol solution lead to a mixture of the diacyl 8 and the cationic acyl hydrido alcohol [RhH(PPh2(o-C6H4CO))(κ2-PPh2(o-C6H4CH2OH))(py)]+ (11). The spectroscopic characterization of some intermediates in this reaction evidence a bimolecular ionic mechanism as being responsible for the hydrogenation of the aldehyde with the hydroxyalkyl 6 being the source of both proton and hydride. Complex 11 can also be obtained by the reaction of 5 with NaBH4 in methanol solution.

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