β-Amino Phosphine Mn Catalysts for 1,4-Transfer Hydrogenation of Chalcones and Allylic Alcohol Isomerization

Abstract

Mn complexes with amino acid derived PN ligands were used in the catalytic transfer hydrogenation (TH) of ketone and chalcone substrates in 2-propanol with mild heating. Moreover, chalcones are reduced selectively to the saturated ketone at short times and can be fully converted to the alcohol when reactions are prolonged. The mechanism of chalcone reduction was briefly considered. Allylic alcohols are not reactive in 2-propanol, but quantitative isomerization occurs in toluene. Thus, we suspect that the allylic alcohols are dehydrogenated and the resulting ketone is formed through a direct 1,4-hydrogenation of the chalcone. Finally, several other related ligands that have been used in Mn-based TH reactions were explored to test the viability of ligand design in favoring chemoselectivity. The β-amino phosphine ligands proved most effective in this regard.

Manganese—New prominent actor in transfer hydrogenation catalysis

“…Most of these systems relies on bidentate tricarbonyl Mn-NN complexes and are selective for C=O bond reduction. However, TH of chalcones catalyzed by PN complex developed by Lacy [23] proceeds selectively at the conjugated C=C bond, being consistent with the results of hydrogenation of these substrates using related Mn-PN complex [24]. Although demonstrating reasonable activities, there was still a gap to be bridged to approach the activity recorded with catalytic systems involving noble metals.…”

Towards ligand simplification in manganese-catalyzed hydrogenation and hydrosilylation processes

“…A formal change of pyridine moiety in picolylamine system by phosphine was reported by the group of Lacy in 2019 (Scheme 55) [88]. It was shown that complexes 124 bearing PN ligands easily available from optically pure amino-acids were competent catalyst for transfer hydrogenation of various ketones typically at 60 °C using 0.5 mol% catalyst loading albeit with negligibly low enantiomeric induction (11% ee max).…”

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

“…Later, Lacy reported that bidentate aminophosphine Mn( i ) complex Mn59 showed a comparable TON for the TH of ketones under similar reaction conditions (Table 8, entry 10). 164 The catalyst showed exceptional behavior towards α,β-unsaturated ketones. The reduction was thought to proceed via 1,4-hydride addition followed by isomerization, resulting in saturated ketones.…”