Fate of Aryl/Amido Complexes of Rhodium(III) Supported by a POCOP Pincer Ligand: C-N Reductive Elimination, beta-Hydrogen Elimination, and Relevance to Catalysis
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2014 American Chemical Society. 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 -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.
