This perspective focuses on the aldehyde C-H activation promoted by late transition metals, including the chelation-assisted reactions. The mechanisms currently accepted for different metal complexes, the reactivity of acyl hydrido species formed, and the reactions promoted on further aldehyde when using excess reagent are discussed. Homogeneous catalytic aldehyde decarbonylation or dimerization reactions are also reviewed.

[{Ir(Cod)Cl}2] (Cod = 1,5-cyclooctadiene) reacts with o-(diphenylphosphino)benzaldehyde (PPh2(o-C6H4CHO)) (Ir:P = 1:2) in methanol to give the hydridoirida-β-diketone complex [IrH{(PPh2(o-C6H4CO))2H}Cl] (1). The reactivity of 1 to afford neutral and cationic hydridoirida-β-diketones is also discussed

The activation of the C-H bond promoted by transition metal complexes is an active area of research. 1 The cleavage of the C-H bond in aldehydes is involved in many stoichiometric and catalytic reactions such as decarbonylation or hydroacylation reactions, where subsequent C-C bond rupture or new C-C bond formation also occurs. 1,2 Metal complexes containing olefins may activate the aldehyde C-H bond to undergo oxidative addition and olefin insertion into the M-H bond. Rearrangements and new C-C bond formation may also occur to afford a variety of acylallyl or acylalkyl derivatives and even unsaturated or saturated ketones. 3 Norbornadiene rhodium(I) complexes have been reported to react with o-(diphenylphosphine)benzaldehyde to undergo the chelate-assisted oxidative addition of aldehyde followed by hydrogen transfer to norbornadiene, giving selectively, by using the appropriate reaction conditions, nortricyclylrhodium(III) complexes or σ-norbornenylrhodium(III) derivatives with a noncoordinating

The dihydridoirida-β-diketone [IrH2{(PPh2(o-C6H4CO))2H}] (2) has been used as a homogeneous catalyst for the hydrolysis of ammonia- or amine-boranes to generate up to 3 equivalents of hydrogen in the presence of air. When using 0.5 mol% loading of 2, dimethylamine-borane is hydrolysed completely within 8 min at 30 °C and maintains its activity in consecutive runs. Ammonia-borane or tert-butylamine-borane is hydrolysed completely within 32 or 25 min respectively. Triethylamine-borane fails to be hydrolysed. Kinetic studies suggest a sequence of two consecutive first-order reactions, in which an intermediate builds up and finally falls, with the first step being the rate controlling step. ΔH1(‡) are in the range 65-85 kJ mol(-1) and negative values of ΔS1(‡) are obtained. A multinuclear NMR study of the catalyzed reaction shows the formation of a resting state (A) of the active catalyst proposed to be of the hydridodiacyl type [IrH(PPh2(o-C6H4CO))2(solvent)] with a hydride trans to the acyl group. In the absence of substrate a dormant species (B) is formed. By the reaction of hydridoirida-β-diketones with ammonia, the hydridoirida-β-ketoimine [IrHCl{(PPh2(o-C6H4CO))(PPh2(o-C6H4CNH))H}] (3) and the hydridobis(acylphosphane)aminoiridium(III) complex [IrH(PPh2(o-C6H4CO))2(NH3)] (4), with a hydride trans to phosphane, are formed. Aromatic amines such as aniline or anisidines afford cationic [IrH{(PPh2(o-C6H4CO))2H}(C6H4RNH2)]ClO4 (R = H (6); p-MeO (7); o-MeO (8)) hydridoirida-β-diketones with a coordinated amine group trans to the hydride. The dormant species B is proposed to be of the hydridobis(acylphosphine)aminoiridium(III) type with a hydride trans to the amine group.

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