Fate of Aryl/Amido Complexes of Rhodium(III) Supported by a POCOP Pincer Ligand: C–N Reductive Elimination, β-Hydrogen Elimination, and Relevance to Catalysis

Abstract

Rhodium complexes supported by the aryl/bis(phosphinite) POCOP pincer ligand undergo reactions that constitute a Rh I /Rh III synthetic cycle for C–N coupling analogous to the classical Pd 0 /Pd II Buchwald–Hartwig chemistry. (POCOP)Rh(Ar)(X) complexes (X = Cl, Br) can be readily obtained by oxidative addition of ArX to the (POCOP)Rh fragment generated in situ from (POCOP)Rh(H)(Cl) ( 1 ) and NaO t Bu. (POCOP)Rh(Ar)(X) complexes react with anilines and diphenylamine in the presence of an equimolar amount of NaO t Bu to give Rh III aryl/amido complexes (POCOP)Rh(Ar)(NHAr′) and (POCOP)Rh(Ar)(NPh 2 ). The intermediate (POCOP)Rh( p -F 3 CC 6 H 4 )(O t Bu) ( 7 ) was isolated and shown to react irreversibly with p -MeC 6 H 4 NH 2 to give (POCOP)Rh( p -F 3 CC 6 H 4 )(NHC 6 H 4 Me- p ) ( 5 ). The latter undergoes reductive elimination of the diarylamine product p -F 3 CC 6 H 4 NHC 6 H 4 Me- p upon heating. The kinetics of this reaction point to a first-order process, and DFT calculations located a transition state for concerted C–N reductive elimination. Complex 1 effected catalytic formation of diarylamines from anilines and aryl chlorides and bromides at 115 °C in the presence of NaO t Bu with modest turnover numbers of <15. In a separate reaction, 5 was degraded by NaO t Bu under catalytic conditions; it is possible that it is one of the reasons for limited catalytic turnover. Reactions of 7 with pyrrolidine and N -methylaniline resulted in the formation of C 6 H 5 CF 3, HO t Bu, and imine complexes of (POCOP)Rh. This ostensibly proceeds via β-hydrogen elimination from the unobserved aryl/amido intermediate, followed by loss of C 6 H 5 CF 3 by C–H reductive elimination. DFT calculations were consistent with this pathway and indicated that it possesses a significantly lower barrier than the concerted C–N reductive elimination.