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

Since chlorohydrazones are planar molecules, it is in principle possible to distinguishbetween their E and Z stereoisomers. Chlorohydrazones are known to preferentially assume the Zconfiguration around the C=N double bond, and their (E, Z) isomerization is almost suppressed atroom temperature. The lack, or rather the difficulty, of such an isomerization has been convenientlyaddressed by the in-depth theoretical study of seven C-methoxycarbonyl-N-aryl chlorohydrazones(aryl = phenyl, 4-chlorophenyl, 4-bromophenyl, 4-iodophenyl, 2-chlorophenyl, 2-bromophenyl,and 2-iodophenyl). DFT !B97M-D4/cc-pVTZ calculations of these C-methoxycarbonyl-N-arylchlorohydrazones, supported by the XRD determination of the molecular structure, provided acomplete picture of the isomerization processes in the studied compounds. The analysis of theenergetics, molecular geometry, and electronic structure (the latter in the framework of the QuantumTheory of Atoms In Molecules) showed that the Z isomers are thermodynamically favored because,within the low-energy planar isomers with extensive ? conjugation, the electrostatic interactionsbetween the dipoles of the C-O, C-Cl, and N-H bonds overcome the stabilization induced by theN-H ··· O bond present in the E isomers. We confirmed that the (E, Z) isomerization occurs by theumklapp mechanism, in which the -NHAr moiety rotates in the molecular plane towards a linearC=N-N configuration and then proceeds to the other isomer. The (E, Z) isomerization is very slow atroom temperature because the umklapp interconversion has high barriers (?110 kJ/mol) despite theextended ? electron delocalization present in the transition state.