Computational Optimization of Alkoxyamine-based Electrochemical Methylation

Abstract

Computational chemistry at the G3(MP2)-RAD//M06-2X/6-31+G(d,p)//SMD level of theory was used to study the oxidation of a test set of methyl adducts of nitroxide radicals and methyl adducts of Blatter's radical, a Kuhn verdazyl and two oxo-verdazyls. The barriers and the reaction energies of the SN2 reactions of the oxidized species with pyridine were also studied with a view to identify species with both low oxidation potentials and low SN2 barriers, so as to broaden the functional group tolerance of in situ electrochemical methylation compared with TEMPO-Me (1-methoxy-2,2,6,6-tetramethylpiperidine). Within the alkoxyamines, the oxidation potentials covered a range of 0.5 V, with trends explicable in terms of electrostatics, ring strain, and charge transfer. The oxidation potentials of oxo-verdazyl adducts, verdazyl adducts, and particularly the methyl adducts of Blatter's radical were considerably low due to the ability of their extensive π-systems to stabilize a positive charge. As expected, the SN2 reaction energies of the oxidized substrate became less favorable as the oxidation potential decreases. Unfortunately, this also meant that the barriers increased due to the excellent Evans-Polanyi correlation (R2 = 0.92). Nonetheless, 7-methoxy-7-azadispiro[5.1.5.836]hexadecane, N,N-di-tert-butyl-O-methylhydroxylamine, and particularly 1-methoxy-2,2,5,5-tetramethylpyrrolidine were identified as suitable candidates for broadening the scope of in situ electrochemical methylation while maintaining comparable kinetics to known reagents.