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

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.