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

An unprecedented case of dual deoxygenation is demonstrated in rhenium chemistry. It is authenticated that an oxorhenium(V) motif and a chelated diaryl-?-ketooxime ligand undergo concurrent oxygen atom transfer (OAT) to form a triarylphosphine oxide coordinated ReIII�?-ketoimine complex. The two OAT events are mutually dependent. OAT-induced ReV[triple bond, length as m-dash]O ? ReIII�OPR3 conversion must occur prior to the OAT-mediated ?-ketooxime ? ?-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 Image ID:d5dt01782c-t1.gif charge transfer are identified. The N�O bond cleavage of the oxime is induced by oxidative addition at the ReIII centre across a free energy barrier of 25.8 kcal mol?1 to afford a reactive ReV�hydroxo intermediate. The second intramolecular OAT involves electron transfer between the ReV-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-?-ketooxime is halted upon replacing the oxorhenium(V) motif by a kinetically nonlabile imidorhenium(V) moiety in the ReV�precursor. In that case, deprotonation of oxime occurs exclusively to generate the ReV�?-ketooximato complex. The predominance of the C-nitroso form of the oxime in the ReV�?-ketooximato species is a notable and hitherto unreported feature in rhenium chemistry. The aforementioned reactions of diaryl-?-ketooxime elegantly highlight ReV-substrate selectivity, which is justified through comprehensive mechanistic analysis.