The (E, Z) Isomerization of C-methoxycarbonyl-N-aryl Chlorohydrazones

Since chlorohydrazones are planar molecules, it is in principle possible to distinguish between their E and Z stereoisomers. Chlorohydrazones known preferentially assume the configuration around C=N double bond, (E, Z) isomerization almost suppressed at room temperature. The lack, or rather difficulty, of such an has been conveniently addressed by in-depth theoretical study seven C-methoxycarbonyl-N-aryl (aryl = phenyl, 4-chlorophenyl, 4-bromophenyl, 4-iodophenyl, 2-chlorophenyl, 2-bromophenyl, 2-iodophenyl). DFT ωB97M-D4/cc-pVTZ calculations these chlorohydrazones, supported XRD determination molecular structure, provided a complete picture processes studied compounds. analysis energetics, geometry, electronic structure (the latter framework Quantum Theory Atoms In Molecules) showed that isomers thermodynamically favored because, within low-energy with extensive π conjugation, electrostatic interactions dipoles C–O, C–Cl, N–H bonds overcome stabilization induced ··· O bond present isomers. We confirmed occurs umklapp mechanism, which –NHAr moiety rotates plane towards linear C=N–N then proceeds other isomer. very slow temperature because interconversion high barriers (≈110 kJ/mol) despite extended electron delocalization transition state.