We examined Me3Al, Et3Al, i-Bu2AlH (DIBAL-H), Me2AlNTf2, allyltrimethylsilane, pinacolborane, 9-borabicyclo[3.3.1]nonane (9-BBN), etc. However, the combined use of these achiral Lewis acid sources to 2B(C6F5)3–(R)-3c showed low reactivities (0–15% yields), and the sole exception was catecholborane. In this regard, the combined use of a stoichiometric amount of catecholborane with chiral phosphoric acid catalyst in the enantioselective reduction of ketones was reported by Antilla. See: Zhang Z, Jain P, Antilla JC.
Angew. Chem. Int. Ed. 2011;
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