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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

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posted on 13.10.2014, 00:00 by Samuel D. Timpa, Christopher J. Pell, Jia Zhou, Oleg V. Ozerov
Rhodium complexes supported by the aryl/bis­(phosphinite) POCOP pincer ligand undergo reactions that constitute a RhI/RhIII synthetic cycle for C–N coupling analogous to the classical Pd0/PdII 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 NaOtBu. (POCOP)­Rh­(Ar)­(X) complexes react with anilines and diphenylamine in the presence of an equimolar amount of NaOtBu to give RhIII aryl/amido complexes (POCOP)­Rh­(Ar)­(NHAr′) and (POCOP)­Rh­(Ar)­(NPh2). The intermediate (POCOP)­Rh­(p-F3CC6H4)­(OtBu) (7) was isolated and shown to react irreversibly with p-MeC6H4NH2 to give (POCOP)­Rh­(p-F3CC6H4)­(NHC6H4Me-p) (5). The latter undergoes reductive elimination of the diarylamine product p-F3CC6H4NHC6H4Me-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 NaOtBu with modest turnover numbers of <15. In a separate reaction, 5 was degraded by NaOtBu 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 C6H5CF3, HOtBu, and imine complexes of (POCOP)­Rh. This ostensibly proceeds via β-hydrogen elimination from the unobserved aryl/amido intermediate, followed by loss of C6H5CF3 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.