title: A well-defined (POCOP)Rh catalyst for the coupling of aryl halides with thiols.
abstract: 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) (6a) and (POCOP)Rh(SPh2) (6b). 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 6a 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 ortho-tolyl halides, which can be traced to slower C-S coupling and slower carbon-halogen oxidative addition for ortho-substituted aryls. The influence of the sterics in the thiol component is less straightforward. The S-H oxidative addition product (POCOP)Rh(H)(SPh) (16) has been fully characterized and its reactivity has been examined, resulting in the isolation of the sodium-thiolate adduct (POCOP)Rh(NaSPh) (19). The solid-state structure of 19 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 16 and 19 indicates that these potential side products should not hinder catalysis.
authors.affiliation: Department of Chemistry, Texas A&M University , College Station, Texas 77842, United States.
authors.forename: Samuel D
authors.initials: SD
authors.lastname: Timpa
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Pell
authors.forename: Oleg V
authors.initials: OV
authors.lastname: Ozerov
mesh_terms: D001554:Benzene / Q000737:chemistry*; D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry*; D006219:Halogens / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D012238:Rhodium / Q000737:chemistry*; D013438:Sulfhydryl Compounds / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D006219:Halogens; D013438:Sulfhydryl Compounds; D012238:Rhodium; D001554:Benzene