Abstract
We report a new generation of highly active and readily available homogeneous titanium catalysts for the epoxidation of nonactivated olefins with aqueous hydrogen peroxide. Key feature is the introduction of pentafluorophenyl substituents into a salan ligand derived from cis-1,2-diaminocyclohexane (cis-DACH). Our novel salan ligand is accessible in one single step by reductive alkylation of cis-DACH with 3-(pentafluorophenyl)salicylic aldehyde. In situ complexation with Ti(OiPr)(4) of the cis-DACH salan provides the titanium catalyst, which, in the presence of aqueous hydrogen peroxide, smoothly epoxidizes a broad spectrum of olefins with up to 95% yield at a catalyst loading of 0.5 mol-% only. The achiral cis-DACH salan catalyst showed syn-selectivity (4.7:1) in the epoxidation of a chiral, racemic terminal allylic alcohol. This catalyst furthermore allows the regioselective (up to 49:1) epoxidation of the terminal double bond in multiply unsaturated terpenes such as myrcene, (S)-citronellene, and (R)-linalool. For the latter two substrates, syn/anti-selectivity of up to 9:1 was observed. Augmented syn/anti selectivity (up to 50:1 syn or 25:1 anti) can be induced in the epoxidation of the chiral substrates (S)-citronellene and (R)-linalool when the "matched" enantiomer of the chiral "Berkessel salalen ligand" is employed.