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 30bar H2 pressure, but activation for only 1min under H2 pressure and then continuation under 1bar 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 1h (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.