Rhodium(III) complexes containing the ligand 2,6-bis[4′-(S)-isopropyloxazolin-2′-yl]pyridine ((S,S)-iPr-pybox): Efficient catalysts for asymmetric hydrosilylation of acetophenone

The dinuclear complexes [Rh2(μ-Cl)2(R)2{(S,S)-iPr-pybox}2][BF4]2 (R = (E)-C(Me)CH(Ph), C(Ph)CH2) have been synthesized by the reaction of the complexes trans-[RhCl2(R){(S,S)-iPr-pybox}] (R = (E)-C(Me)CH(Ph), C(Ph)CH2) with AgBF4. The rhodium(III) complexes cis-[RhCl2(R){(S,S)-iPr-pybox}] (R = COPh, CH2C(Me)CH2, CH2CHCH2, CH2CHCH(Ph), CHCCH2) and trans-[RhCl2{(E)-C(Me)CH(Ph)}{(S,S)-iPr-pybox}], recently reported, and the novel dinuclear complex [Rh2(μ-Cl)2{(E)-C(Me)CH(Ph)}2{(S,S)-iPr-pybox}2][BF4]2, exhibit high enantioselectivity in the hydrosilylation of acetophenone affording (S)-1-phenylethanol with high conversions and up to 89% ee. The species “RhCl{(S,S)-iPr-pybox}” is proposed as the real active rhodium catalyst and a pathway for its formation is also tentatively provided.

The novel dinuclear complex [Rh2(μ-Cl)2{(E)-C(Me)CH(Ph)}2(iPr-pybox)2][BF4]2 and several mononuclear rhodium(III) complexes [RhCl2(R)(iPr-pybox)] (R = acyl, allyl, allenyl, alkenyl), containing the enantiopure ligand (S,S)-iPr-pybox, have been examined as catalysts in the asymmetric reduction of acetophenone via the hydrosilylation reaction with diphenylsilane. Figure optionsDownload as PowerPoint slide