Synthesis and Structure of Intermediates in Copper-Catalyzed Alkylation of Diphenylphosphine
2010-09-06T00:00:00Z (GMT) by
Cu(I) catalysts for alkylation of diphenylphosphine were developed. Treatment of [Cu(NCMe)4][PF6] (1) with chelating ligands gave [CuL(NCMe)][PF6] (2; L = MeC(CH2PPh2)3 (triphos), 3; L = 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene (XantPhos)). These complexes catalyzed the alkylation of PHPh2 with PhCH2Br in the presence of the base NaOSiMe3 to yield PPh2CH2Ph (4). The precursors Cu(dtbp)(X) (dtbp =2,9-di-t-butylphenanthroline, X = Cl (5) or OTf (6)), CuCl, and 1 also catalyzed this reaction, but dtbp dissociated from 5 and 6 during catalysis. Both 2 and 3 also catalyzed alkylation of PHPh2 with PhCH2Cl/NaOSiMe3, but XantPhos dissociation was observed when 3 was used. When CH2Cl2 was used as the solvent for alkylation of PhCH2Cl with precursors 2 or 3, or of PhCH(Me)Br with 2, it was competitively alkylated to yield PPh2CH2Cl (7), which was formed exclusively using 2 in the absence of a benzyl halide. Cu(triphos)-catalyzed alkylation of PhCH(Me)Br gave mostly PPh2CHMePh (8), along with some Ph2P-PPh2 (9), which was also formed in attempted alkylation of dibromoethane with this catalyst. The phosphine complexes [Cu(triphos)(L′)][PF6] (L′ = PH2Ph (10), PH2CH2Fc (Fc = C5H4FeC5H5, 11), PHPh2 (12), PHEt2 (13), PHCy2 (Cy = cyclo-C6H11, 14), PHMe(Is) (Is = 2,4,6-(i-Pr)3C6H2, 15), PPh2CH2Ph (16), PPh2CH2Cl (17)), and [Cu(XantPhos)(L′)][PF6] (L′ = PHPh2 (18), PPh2CH2Ph (19)) were prepared by treatment of 2 and 3 with appropriate ligands. Similarly, treatment of dtbp complexes 5 or 6 with PHPh2 gave [Cu(dtbp)(PHPh2)(X)] (X = OTf (20a) or Cl (20b)), and reaction of PPh2CH2Ph (4) with 1 formed [Cu(PPh2CH2Ph)3][PF6] (21). Complexes 2, 3, 11−14, 16, 17, 19, and 21 were structurally characterized by X-ray crystallography. Deprotonation of diphenylphosphine complex 12 in the presence of benzyl bromide gave diphenylbenzylphosphine complex 16, while deprotonation of 12 in CD2Cl2 gave 17 containing a PPh2CD2Cl ligand. Low-temperature deprotonation of the soluble salt 12-[B(ArF)4] (ArF = 3,5-(CF3)2C6H3) in THF-d8 gave the phosphido complex Cu(triphos)(PPh2) (22). Thermally unstable 22 was characterized by NMR spectroscopy and, in comparison to 12, by density functional theory (DFT) calculations, which showed it contained a polarized Cu−P bond. The ligand substitution step required for catalytic turnover was observed on treatment of 16 or 17 with PHPh2 to yield equilibrium mixtures containing 12 and the tertiary phosphines 4 or 7; equilibrium constants for these reactions were 8(2) and 7(2), favoring complexation of the smaller secondary phosphine in both cases. These observations are consistent with a proposed mechanism for catalytic P−C bond formation involving deprotonation of the cationic diphenylphosphine complex [Cu(triphos)(PHPh2)][PF6] (12) by NaOSiMe3 to yield the phosphido complex Cu(triphos)(PPh2) (22). Nucleophilic attack on the substrate (benzyl halide or CH2Cl2) then yields the tertiary phosphine complex [Cu(triphos)(PPh2CH2X)][PF6] (X = Ph (16) or Cl (17)), and ligand substitution with PHPh2 regenerates 12.