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

International audience; Complexes [M(P,SR)(diene)X] where (P,SR) = CpFe[1,2-C 5 H 3 (PPh 2)(CH 2 SR)] (M = Ir, R = tBu or Bn diene = cod, X = Cl; M = Rh, diene = cod or nbd; X = BF 4 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 H 2 pressure, but activation for only 1 min under H 2 pressure and then continuation under 1 bar of H 2 or Ar led to increasing amounts of the allylic isomerization product propiophenone (8a). Continued reaction under H 2 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 H 2) 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 H 2-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.