Synthesis and coordination ability of tartrate-derived N-alkyl dicarbohydrazides towards Cu2+ ions

Starting from (R,R)-tartaric acid we have synthesized eight N- and N,N-(di-)alkyl dicarbohydrazides 3 and 4, by two general methods, which were able to form two M2L and ML complexes. These target compounds were further used as model nitrogen ligands coordinating Cu2+ ions. Whereas the formation of the first type of complexes is linearly dependent on the increasing electron-donating ability of the appended alkyls, the ML complexes were significantly affected by both electronic and steric effects of the alkyl groups used. These observations were supported by theoretical calculations of both complex types. The target compounds were also employed as chiral nitrogen ligands in the asymmetric version of the Henry reaction. Modest chemical yields and low enantioselectivities were attained for ligand 3a bearing one methyl group on each terminal nitrogen atom. Structure-property relationships were also evaluated to gain useful guidelines for ligand design targeting catalysts for asymmetric reactions.