Dual deoxygenation in an α-ketoimine chelated rhenium(iii) complex: structural and mechanistic interpretations

Abstract

An unprecedented case of dual deoxygenation is demonstrated in rhenium chemistry. It is authenticated that an oxorhenium(V) motif and a chelated diaryl-α-ketooxime ligand undergo concurrent oxygen atom transfer (OAT) to form a triarylphosphine oxide coordinated Re^III–α-ketoimine complex. The two OAT events are mutually dependent. OAT-induced Re^VO → Re^III–OPR₃ conversion must occur prior to the OAT-mediated α-ketooxime → α-ketoimine transformation. The first intramolecular OAT occurs across a free energy barrier of 29.1 kcal mol⁻¹, and subjacent molecular orbital effects related to charge transfer are identified. The N–O bond cleavage of the oxime is induced by oxidative addition at the Re^III centre across a free energy barrier of 25.8 kcal mol⁻¹ to afford a reactive Re^V–hydroxo intermediate. The second intramolecular OAT involves electron transfer between the Re^V-bound hydroxo and PPh₃ moieties. Due to increased nucleophilicity of the hydroxo group, the second OAT is kinetically facile, with a low activation barrier of 8.3 kcal mol⁻¹. Interestingly, while PPh₃ acts as a nucleophile in the first OAT, it behaves as an electrophile in the second. Deoxygenation of diaryl-α-ketooxime is halted upon replacing the oxorhenium(V) motif by a kinetically nonlabile imidorhenium(V) moiety in the Re^V–precursor. In that case, deprotonation of oxime occurs exclusively to generate the Re^V–α-ketooximato complex. The predominance of the C-nitroso form of the oxime in the Re^V–α-ketooximato species is a notable and hitherto unreported feature in rhenium chemistry. The aforementioned reactions of diaryl-α-ketooxime elegantly highlight Re^V-substrate selectivity, which is justified through comprehensive mechanistic analysis.