Catalytic redox isomerization of allylic alcohols with rhodium and iridium complexes with ferrocene phosphine-thioether ligands

Abstract

Complexes [M(P,SR)(diene)X] where (P,SR) = CpFe[1,2-C₅H₃(PPh₂)(CH₂SR)] (M = Ir, R = tBu or Bn diene = cod, X = Cl; M = Rh, diene = cod or nbd; X = BF₄ or Cl) were used as precatalysts for the redox isomerization of various allylic alcohols (7a–e) to the corresponding saturated ketones (8a–e) and or hydrogenation to the saturated alcohol (9a–e). In optimization studies using 1-phenyl-2-propen-1-ol (7a) in THF and in iPrOH/MeONa, the only observed product was the saturated alcohol 1-phenyl-1-propanol (9a) when working under a 30 bar H₂ pressure, but activation for only 1 min under H₂ pressure and then continuation under 1 bar of H₂ or Ar led to increasing amounts of the allylic isomerization product propiophenone (8a). Continued reaction under H₂ converted (8a) into (9a). The Rh precatalysts were more active than the Ir analogues. For the rhodium precatalysts (3) and (4), the redox isomerization reaction could be carried out after precatalyst activation in iPrOH/MeONa under Ar at 82 °C (without H₂) with complete conversion in 1 h (1% catalyst loading). However, longer reaction times resulted in slow transfer hydrogenation of (8a) leading to (9a) with low enantiomeric excess. Extension of the H₂-free activation of the Rh precatalysts in iPrOH to other allylic alcohol substrates (7b–d) yielded the corresponding ketones with good to excellent yields and excellent chemoselectivities under appropriate conditions.