A Well-Defined (POCOP)Rh Catalyst for the Coupling of Aryl Halides with Thiols

This article describes a well-defined pincer-Rh catalyst for C–S cross-coupling reactions. (POCOP)­Rh­(H)­(Cl) serves as an active precatalyst for the coupling of aryl chlorides and bromides with aryl and alkyl thiols under reasonable conditions (3% mol cat., 110 °C, 2–24 h, >90% yield). For select substrates, >90% yields were obtained with catalyst loading as low as 0.1%. Key mechanistic intermediates have been isolated and fully characterized, including (POCOP)­Rh­(Ph)­(SPh) (<b>6a</b>) and (POCOP)­Rh­(SPh₂) (<b>6b</b>). The aryl/bis­(phosphinite) (POCOP)Rh system has been shown to favor aryl thiolate reductive elimination at elevated temperatures and in some cases at room temperature, compared with the analogous diarylamido/bis­(phosphine) (PNP)Rh pincer system. Concerted reductive elimination has been studied with <b>6a</b> directly and in the presence of aryl bromide and aryl chloride traps. This investigation demonstrates a clear rate dependence on aryl chloride concentration during catalysis, a dependence that is absent when using aryl bromides. The rate of catalysis is dramatically reduced or brought to zero for <i>ortho</i>-tolyl halides, which can be traced to slower C–S coupling and slower carbon–halogen oxidative addition for <i>ortho</i>-substituted aryls. The influence of the sterics in the thiol component is less straightforward. The S–H oxidative addition product (POCOP)­Rh­(H)­(SPh) (<b>16</b>) has been fully characterized and its reactivity has been examined, resulting in the isolation of the sodium-thiolate adduct (POCOP)­Rh­(NaSPh) (<b>19</b>). The solid-state structure of <b>19</b> shows Na interactions not only with sulfur, but also with a neighboring Rh and the chelating aryl carbon of the pincer framework. The reactivity of <b>16</b> and <b>19</b> indicates that these potential side products should not hinder catalysis

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 Rh^I/Rh^III synthetic cycle for C–N coupling analogous to the classical Pd⁰/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) (<b>1</b>) and NaO^tBu. (POCOP)­Rh­(Ar)­(X) complexes react with anilines and diphenylamine in the presence of an equimolar amount of NaO^tBu to give Rh^III aryl/amido complexes (POCOP)­Rh­(Ar)­(NHAr′) and (POCOP)­Rh­(Ar)­(NPh₂). The intermediate (POCOP)­Rh­(<i>p</i>-F₃CC₆H₄)­(O^tBu) (<b>7</b>) was isolated and shown to react irreversibly with <i>p</i>-MeC₆H₄NH₂ to give (POCOP)­Rh­(<i>p</i>-F₃CC₆H₄)­(NHC₆H₄Me-<i>p</i>) (<b>5</b>). The latter undergoes reductive elimination of the diarylamine product <i>p</i>-F₃CC₆H₄NHC₆H₄Me-<i>p</i> 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 <b>1</b> effected catalytic formation of diarylamines from anilines and aryl chlorides and bromides at 115 °C in the presence of NaO^tBu with modest turnover numbers of <15. In a separate reaction, <b>5</b> was degraded by NaO^tBu under catalytic conditions; it is possible that it is one of the reasons for limited catalytic turnover. Reactions of <b>7</b> with pyrrolidine and <i>N</i>-methylaniline resulted in the formation of C₆H₅CF₃, HO^tBu, 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