New Asymmetric Catalysis by (Salen)cobalt(III) Complexes (Salen=[Bis(salicylidene)ethylenediaminato]={{2,2′-[ethane-1,2-diyl]bis[(nitrilo-N)methylidyne]bis[phenolato-O]}(2−)}) of cis--Structure: Enantioselective Baeyer-Villiger Oxidation of Prochiral Cyclobutanones

Abstract

A series of chiral (salen)cobalt(III) complexes (salen=[bis(salicylidene)ethylenediaminato]={{2,2′-[(ethane-1,2-diyl)bis[(nitrilo-κN)methylidyne]bis[phenolato-κO]}(2−)}) of cis-β structure were prepared and used for the enantioselective Baeyer-Villiger oxidation of prochiral cyclobutanones with hydrogen peroxide as terminal oxidant. Both cationic (salen)cobalt(III) and neutral iodo(salen)cobalt(III) complexes 3–5 and 7–12, respectively, all having a chiral binaphthalenediamine unit, were found to be effective catalysts for the enantioselective Baeyer-Villiger oxidation (Tables 1, 3, and 4). In particular, complex 8 bearing electron-withdrawing F-atoms showed a good level of enantioselectivity (75–79% ee) in the reactions of 3-arylcyclobutanones (Scheme 4). On the other hand, complex 12 bearing tBu groups at C(3) and C(3′) and electron-withdrawing NO2 groups at C(5) and C(5′) (trivial numbering) exhibited a high enantioselectivity of 98% ee in the reaction of a tricyclic cyclobutanone (Table 4).