New chiral Schiff base–zinc complexes and their esterolytic catalytic activity
摘要:
A new series of chiral Zn-Schiff base (imine) complexes has been prepared from mono-N-sulfonyl derivatives of (1R,2R)-diaminocyclohexane, N-heterocyclic aldehydes, and zinc salts. The formation and characterization of the (L)ZnX2 complexes was established by NMR. IR and HRESI-MS. Spectroscopic and kinetic evidence indicates that these ligands may be bidentate or tridentate depending on conditions of the medium. The methanolysis of a chiral, racemic picolinate ester catalyzed by the Zn(II)-Schiff base complexes was studied kinetically. The rate constants were found to vary approximately a hundred-fold and in a complex way depending on the imine ligand and the Zn-counter anion, k(obs) =5.0 x 10(-6)-4.8 x 10(-4) M-1 s(-1). A Job plot analysis of ternary complex formation of LZnX2 with two phosphonate transition state analogs suggests that two types of (imine)Zn(picolinate)X ternary complexes may be intermediates and that varying rate-limiting steps may be involved in the LZnX2-catalyzed methanolysis of picolinate esters. (C) 2011 Elsevier B.V. All rights reserved.
An efficient metal‐free oxidative esterification of benzylCHbonds was developed. Using tetrabutylammonium iodide as catalyst and tert‐butyl hydroperoxide as co‐oxidant, benzylic substrates could react smoothly with various carboxylic acids to give the esters with good to excellent yields. The method was also suitable for the O‐protection of N‐Boc amino acids. The reaction mechanism was primarily