Evaluating 1,1'-Bis(phosphino)ferrocene Ancillary Ligand Variants in the Nickel-Catalyzed C-N Cross-Coupling of (Hetero)aryl Chlorides

摘要

Previous reports in the literature have established the utility of 1,1'-bis(diphenylphosphino)ferrocene (DPPF, L-Ph) in the nickel-catalyzed cross-coupling of (hetero)aryl electrophiles with primary or secondary amines. In an effort to evaluate the effect of varying the PR2-donor groups On catalytic performance in such transformations, a series of 10 structurally varied 1,1'-bis(bis(alkyl/aryl)phosphino)ferrocene ancillary ligands (L-X) were systematically examined in selected competitive test cross couplings of (hetero)aryl halides with furfurylamine, morpholine, and indole employing Ni(COD)(2)/L-X catalyst mixtures. In addition to the excellent performance observed for the parent ligand LPh in a number of the test transformations explored, selected dialkylphosphino (e.g., DiPPF, L-iPr) and meta-disubstituted diarylphosphino variants of L-Ph also proved highly effective. In particular, the electron-deficient ligand variant L-CF3 featuring 3,5-bis(trifluoromethyl)phenyl groups on phosphorus was found to exhibit superior catalytic performance relative to L-Ph in most of the test transformations involving the N-arylation of indole. Our efforts to prepare Ni(II) precatalysts of the type (L-X)Ni(o-tolyl)Cl) in analogy with known (L-Ph)Ni(o-tolyl)Cl, by employing several literature methods met with mixed results. Whereas ((LNi)-Ni-iPr(o-tolyl)Cl was prepared straightforwardly and was crystallographically characterized, the use of L-CF3 or ligands featuring tert-butyl (L-tBu, o-tolyl (Lo-tol), or 4-methoxy-3,5dirnethylphenyl (L-OMe) groups on phosphorus under similar conditions resulted in poor conversion to product and/or the formation of poorly soluble materials, highlighting the limitations of this commonly used precatalyst design.