Chiral Phosphane Alkenes (PALs): Simple Synthesis, Applications in Catalysis, and Functional Hemilability

A simple synthesis of a chiral phosphane alkene (PAL) involves: 1) palladium-catalyzed Suzuki coupling of 10-bromo-5H-dibenzo[a,d]cyclohepten-5-ol (1) with phenylboronic acid to give quantitatively 10-phenyl-5H-dibenzo[a,d]cyclohepten-5-ol (2); 2) reaction of 2 with Ph(2)PCl under acidic conditions to give a racemic mixture of the phosphane oxide (10-phenyl-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane oxide ((Ph)troppo(Ph), 3), which is separated into enantiomers by using high-pressure liquid chromatography (HPLC) on a chiral column; 3) reduction with trichlorosilane to give the enantiomerically pure phosphanes (R)- and (S)-(10-phenyl-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ((Ph)tropp(Ph), 4). This highly rigid, concave-shaped ligand serves as a bidentate ligand in Rh(I) and Ir(I) complexes. Catalysts prepared from [Rh(2)(mu(2)-Cl)(2)(C(2)H(4))(4)] and (S)-4 have allowed the efficient enantioselective 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyls (Hayashi-Miyaura reaction) (5-0.1 mol % catalyst, up to 95% ee). The iridium complex (S,S)-[Ir((Ph)tropp(Ph))(2)]OTf ((S,S)-6; OTf=SO(3)CF(3)) has been used as a catalyst in the hydrogenation of various nonfunctionalized and functionalized olefins (turnover frequencies (TOFs) of up to 4000 h(-1)) and moderate enantiomeric excesses have been achieved (up to 67% ee). [Ir((Ph)tropp(Ph))(2)]OTf reversibly takes up three equivalents of H(2). The highly reactive octahedral [Ir(H)(2)(OTf)(CH(2)Cl(2))(H(2)-(Ph)tropp(Ph))(2)] could be isolated and contains two hydrogenated monodentate H(2)-(Ph)tropp(Ph) phosphanes, one CH(2)Cl(2) molecule, one triflate anion, and two hydrides. Based on this structure and extensive NMR spectroscopic studies, a mechanism for the hydrogenation reactions is proposed.