Abstract
In this paper, several Mn(I) complexes were applied as catalysts for the homogeneous hydrogenation of ketones. The most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe) (CO)3(CH2CH2CH3)]. The reaction proceeds at room temperature under base-free conditions with a catalyst loading of 3 mol % and a hydrogen pressure of 10 bar. A temperature-dependent selectivity for the reduction of α,β-unsaturated carbonyls was observed. At room temperature, the carbonyl group was selectively hydrogenated, while the C=C bond stayed intact. At 60 °C, fully saturated systems were obtained. A plausible mechanism based on DFT calculations which involves an inner-sphere hydride transfer is proposed.
Highlights
The catalytic reduction of polar multiple bonds via molecular hydrogen plays a significant role in modern synthetic organic chemistry
It is interesting to note that many of these transition-metalcatalyzed hydrogenations rely on metal−ligand bifunctional catalysis, where complexes contain electronically coupled hydride and acidic hydrogen atoms
This has resulted in the development of novel and unprecedented iron and manganese catalysis, where this type of cooperation plays a key role in the heterolytic cleavage of H2
Summary
The catalytic reduction of polar multiple bonds via molecular hydrogen plays a significant role in modern synthetic organic chemistry. Mn(I) alkyl carbonyl complexes are known to undergo insertions to form highly reactive acyl intermediates (a wellknown reaction in organometallic chemistry12) which are able to activate dihydrogen, thereby forming the 16e− Mn(I) hydride catalysts (Scheme 2). We describe an additive-free hydrogenation of ketones at room temperature, utilizing Mn(I) alkyl carbonyl complexes fac-[Mn(dpre) (CO)3(CH3)] (dpre = 1,2-bis(di-npropylphosphino)ethane, fac-[Mn(dpre) (CO)3(CH2CH2CH3)] (2) and fac-[Mn(dippe) (CO)3(CH2CH2CH3) (dippe = 1,2-bis(di-isopropylphosphino)ethane) (3).
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