Asymmetric Hydrosilylation of Alkenes with Alkoxyhydrosilanes Catalyzed by Chiral Bis(oxazolinyl)phenyl-Rhodium Complex

Abstract

Asymmetric addition of alkoxyhydrosilanes to styrene derivatives was examined with chiral bis(oxazolinyl)phenyl-rhodium complex to give moderate ratios (up to 77:23) of α- and β-adducts and high enantioselectivity (up to 95% for the α-adduct).

rhodium - bisoxazoline - styrene - hydrosilylation - asymmetric catalysis

References

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6

Typical Procedure for the Hydrosilylation of Styrene and Diethoxymethylsilane with Rh(Phebox- ip )Cl _{2 (} H ₂ O) (Run 2, Table 1): To a solution of the rhodium complex 1 (4.9 mg, 0.01 mmol) and styrene (104 mg, 1.00 mmol) in toluene (1 mL) was added diethoxymethylsilane (134 mg, 0.12 mmol) at r.t. The mixture was stirred at 50 °C for 72 h and was concentrated to give an oily residue, which was purified by silica-gel column chromatography to give a mixture of α- and β-silylated products (3a and 3b, 224 mg, 94%). The ratio of 54:46 was determined by ¹H NMR. The mixture (224 mg, 0.94 mmol) was treated with excess of KF, K₂CO₃, and H₂O₂ (30%, 1 mL) in THF-MeOH (1:1, 2 mL) at 0 °C. The mixture was stirred for 12 h and was treated with sat. aq solution of Na₂S₂O₃ (6 mL) for 1 h. After usual work-up of extraction and concentration, the residue was purified by column chromatography to give a mixture 1- and 2-phenylethanol (110 mg). The ee of 1-phenylethanol was determined by HPLC with DAICEL CHIRALCEL OB (i-PrOH-hexane = 1:9, 0.5 mL/min); 96:4, 11.4 min. for S and 14.6 min for R. For run 4 with AgBF₄, the procedure is as follows: a suspension of the rhodium complex 1 (0.01 mmol) and AgBF₄ (0.02 mmol) in THF (1 mL) was stirred for 1 h. Then the solvent was removed under reduced pressure to give the corresponding cationic complex as solids followed by addition of solvent, alkene, and the silane.

7

We did not observe formation of simple reduction products of alkenes or vinylsilanes derived from ‘silylrhodation-β-hydride elimination’ in most cases.