Dynamic kinetic resolution (DKR) is an attractive method for the complete transformation of a racemic mixture into a single enantiomer. [1] The DKR of secondary alcohols is a prominent example, for which transition-metal-catalyzed racemization is coupled with enzymatic acylation. [2] In particular, B‰ckvall and co-workers have introduced a notable catalyst system that provides a wide range of chiral acetates in good yields and excellent optical purities. [2b±f] However, the catalyst for the racemization of secondary alcohols is activated at high temperature, and needs the corresponding ketones as hydrogen mediators. [3] Thus, the catalyst system requires a thermally stable lipase; p-chlorophenyl acetate has been selected as an acyl donor, [2c] because oxidation of the starting alcohols occurs when the conventional alkenyl acetates are used as acyl donors. [4] derivative has been determined. The decomposition of these complexes suggests rate-limiting loss of nitrous oxide leads to unusual and reactive late-transition-metal terminal oxo intermediates.
Experimental Section
3 a: [Ni(dppf)Cl 2 ], [27] (268 mg) dissolved in CH 2 Cl 2 (15 mL) was treated in one addition with 1 [22] (2 equivalents, 249 mg) suspended in methanol (60 mL). Rapidly a deep orange color developed and the solution volume was immediately reduced to ca 10 mL by evaporation at ambient temperatures. The resulting bright orange crystals were isolated by filtration and washed with portions of cold methanol (4 Â 10 mL) or until the filtrate was colorless. Recrystallization of this product from dichloromethane/ethanol at room temperature returns 243 mg (93 % yield) of 3 a. Elemental analysis calcd (%) C