Chiral Catalysts Dually Functionalized with Amino Acid and Zn2+ Complex Components for Enantioselective Direct Aldol Reactions Inspired by Natural Aldolases: Design, Synthesis, Complexation Properties, Catalytic Activities, and Mechanistic Study

Abstract

Aldolases are enzymes that catalyze stereospecific aldol reactions in a reversible manner. Naturally occurring aldolases include class I aldolases, which catalyze aldol reactions via enamine intermediates, and class II aldolases, in which Zn(2+) enolates of substrates react with acceptor aldehydes. In this work, Zn(2+) complexes of L-prolyl-pendant[15]aneN(5) (ZnL(3)), L-prolyl-pendant[12]aneN(4) (ZnL(4)), and L-valyl-pendant[12]aneN(4) (ZnL(5)) were designed and synthesized for use as chiral catalysts for enantioselective aldol reactions. The complexation constants for L(3) to L(5) with Zn(2+) [logK(s)(ZnL)] were determined to be 14.1 (for ZnL(3)), 7.6 (for ZnL(4)), and 9.6 (for ZnL(5)), indicating that ZnL(3) is more stable than ZnL(4) and ZnL(5). The deprotonation constants of Zn(2+)-bound water [pK(a)(ZnL) values] for ZnL(3), ZnL(4), and ZnL(5) were calculated to be 9.2 (for ZnL(3)), 8.2 (for ZnL(4)), and 8.6 (for ZnL(5)), suggesting that the Zn(2+) ions in ZnL(3) is a less acidic Lewis acid than in ZnL(4) and ZnL(5). These values also indicated that the amino groups on the side chains weakly coordinate to Zn(2+). We carried out aldol reactions between acetone and 2-chlorobenzaldehyde and other aldehydes in the presence of catalytic amounts of the chiral Zn(2+) complexes in acetone/H(2)O at 25 and 37 degrees C. Whereas ZnL(3) yielded the aldol product in 43% yield and 1% ee (R), ZnL(4) and ZnL(5) afforded good chemical yields and high enantioselectivities of up to 89% ee (R). UV titrations of proline and ZnL(4) with acetylacetone (acac) in DMSO/H(2)O (1:2) indicate that ZnL(4) facilitates the formation of the ZnL(4)(acac)(-) complex (K(app)=2.1x10(2) M(-1)), whereas L-proline forms a Schiff base with acac with a very small equilibrium constant. These results suggest that the amino acid components and the Zn(2+) ions in ZnL(4) and ZnL(5) function in a cooperative manner to generate the Zn(2+)-enolate of acetone, thus permitting efficient enantioselective C-C bond formation with aldehydes.