Synthesis and structure of intermediates in copper-catalyzed alkylation of diphenylphosphine.
Cu(I) catalysts for alkylation of diphenylphosphine were developed. Treatment of [Cu(NCMe)(4)][PF(6)] (1) with chelating ligands gave [CuL(NCMe)][PF(6)] (2; L = MeC(CH(2)PPh(2))(3) (triphos), 3; L = 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene (XantPhos)). These complexes catalyzed the alkylation of PHPh(2) with PhCH(2)Br in the presence of the base NaOSiMe(3) to yield PPh(2)CH(2)Ph (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 PHPh(2) with PhCH(2)Cl/NaOSiMe(3), but XantPhos dissociation was observed when 3 was used. When CH(2)Cl(2) was used as the solvent for alkylation of PhCH(2)Cl with precursors 2 or 3, or of PhCH(Me)Br with 2, it was competitively alkylated to yield PPh(2)CH(2)Cl (7), which was formed exclusively using 2 in the absence of a benzyl halide. Cu(triphos)-catalyzed alkylation of PhCH(Me)Br gave mostly PPh(2)CHMePh (8), along with some Ph(2)P-PPh(2) (9), which was also formed in attempted alkylation of dibromoethane with this catalyst. The phosphine complexes [Cu(triphos)(L')][PF(6)] (L' = PH(2)Ph (10), PH(2)CH(2)Fc (Fc = C(5)H(4)FeC(5)H(5), 11), PHPh(2) (12), PHEt(2) (13), PHCy(2) (Cy = cyclo-C(6)H(11), 14), PHMe(Is) (Is = 2,4,6-(i-Pr)(3)C(6)H(2), 15), PPh(2)CH(2)Ph (16), PPh(2)CH(2)Cl (17)), and [Cu(XantPhos)(L')][PF(6)] (L' = PHPh(2) (18), PPh(2)CH(2)Ph (19)) were prepared by treatment of 2 and 3 with appropriate ligands. Similarly, treatment of dtbp complexes 5 or 6 with PHPh(2) gave [Cu(dtbp)(PHPh(2))(X)] (X = OTf (20a) or Cl (20b)), and reaction of PPh(2)CH(2)Ph (4) with 1 formed [Cu(PPh(2)CH(2)Ph)(3)][PF(6)] (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 CD(2)Cl(2) gave 17 containing a PPh(2)CD(2)Cl ligand. Low-temperature deprotonation of the soluble salt 12-[B(Ar(F))(4)] (Ar(F) = 3,5-(CF(3))(2)C(6)H(3)) in THF-d(8) gave the phosphido complex Cu(triphos)(PPh(2)) (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 PHPh(2) 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)(PHPh(2))][PF(6)] (12) by NaOSiMe(3) to yield the phosphido complex Cu(triphos)(PPh(2)) (22). Nucleophilic attack on the substrate (benzyl halide or CH(2)Cl(2)) then yields the tertiary phosphine complex [Cu(triphos)(PPh(2)CH(2)X)][PF(6)] (X = Ph (16) or Cl (17)), and ligand substitution with PHPh(2) regenerates 12.
Cain
Matthew F
MF
Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, USA.
Hughes
Russell P
RP
Glueck
David S
DS
Golen
James A
JA
Moore
Curtis E
CE
Rheingold
Arnold L
AL