Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation

The titanium complex of the cis‐1,2‐diaminocyclohexane (cis‐DACH) derived Berkessel‐salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide (“Berkessel‐Katsuki catalyst”). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C−H bonds with hydrogen peroxide. Mechanism‐based ligand optimization identified a novel nitro‐salalen Ti‐catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro‐salalen Ti‐catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1‐decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol‐% only. A cis‐1,2‐diaminocyclohexane (cis‐DACH) Ti salalen complex (R=NO2) was shown to efficiently catalyze enantioselective benzylic hydroxylations with H2O2 as oxidant. Tetralin and indane derivatives are converted to benzylic alcohols with up to 98 % ee. The overoxidation to ketone is negligible. Epoxidation reactions are similarly effective, with 1‐decene being converted to its epoxide in 90 % yield and 94 % ee, at 0.1 mol% catalyst loading.