Catalytic Enantioselective Epoxidation of Unfunctionalized Olefins: Utility of a Ti(Oi-Pr)₄-Salan-H₂O₂ System

Abstract

Salan ligands bearing an ortho-substituted phenyl group at C3 and C3′ were found to serve as an effective auxiliary of a Ti(Oi-Pr)₄-salan-H₂O₂ system for enantioselective epoxidation. Simple olefins were converted into the corresponding epoxides in good to excellent enantioselectivity.

Key words

asymmetric catalysis - epoxidations - titanium - hydrogen peroxide - salan ligand

References and Notes

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• 7 General Experimental Procedure. To a CH₂Cl₂ solution of salan ligand (6 mol%, 0.50 mL) was added a CH₂Cl₂ solution of Ti(Oi-Pr)₄ (5 mol%, 0.50 mL) under nitrogen atmosphere and the resultant solution was stirred at r.t. After 1 h, olefin (0.10 mmol)and subsequently 30% H₂O₂ (0.11 mmol) were added and the mixture was stirred at r.t. for 9 h. The solvent was removed under reduced pressure and the residue was purified by silica gel chromatography to give an epoxide. An ee was determined by HPLC analysis using a chiral stationary phase described in the footnote to Table 2

5

We have already reported that the in situ prepared titanium-salan complex was equally efficient to the isolated di-µ-oxo titanium-salan complex (1, ref. 4).

6

Compound 2e: 47%, 92% ee; compound 2n: 23%, 87% ee.