Rapid ketone transfer hydrogenation by employing simple, in situ prepared iridium(I) precatalysts supported by "non-N--H" P,N ligands.

The catalytic utility in ketone transfer hydrogenation (TH) of the preformed complexes Ir(cod)(kappa(2)-2-NMe(2)-3-PiPr(2)-indene)X(-) (2 aX(-); X: PF(6), BF(4), and OTf; cod: eta(4)-1,5-cyclooctadiene; OTf: trifluoromethanesulfonate), Ir(cod)(kappa(2)-1-PiPr(2)-2-NMe(2)-indene)OTf(-) (2 bOTf(-)), [Ir(cod)(kappa(2)-2-NMe(2)-3-PiPr(2)-indenide)] (3), and Ir(cod)(kappa(2)-o-tBu(2)P-C(6)H(4)-NMe(2))PF(6) (-) (4PF(6) (-)), as well as of related mixtures prepared from [{IrCl(cod)}(2)] and various P,N-substituted indene or phenylene ligands, was examined. Whereas 2 aX(-), 2 bOTf(-), 3, and related in situ prepared Ir catalysts derived from P,N-indenes proved to be generally effective in mediating the reduction of acetophenone to 1-phenylethanol in basic iPrOH at reflux (0.1 mol % Ir; 81-99 % conversion) in a preliminary catalytic survey, the structurally related Ir catalysts prepared from (o-R(2)P-C(6)H(4))NMe(2) (R: Ph, iPr, or tBu) were observed to outperform the corresponding P,N-indene ligands under similar conditions. In the course of such studies, it was observed that alteration of the substituents at the donor fragments of the supporting P,N ligand had a pronounced influence on the catalytic performance of the derived catalysts, with ligands featuring bulky dialkylphosphino donors proving to be the most effective. Notably, the crystallographically characterized complex 4PF(6) (-), either preformed or prepared in situ from a mixture of [{IrCl(cod)}(2)], NaPF(6), and (o-tBu(2)P-C(6)H(4))NMe(2), proved to be highly effective in mediating the catalytic transfer hydrogenation (TH) of ketones in basic iPrOH, with near quantitative conversions for a range of alkyl and/or aryl ketones and with very high turnover-frequency values (up to 230 000 h(-1) at >50 % conversion); this thereby enabled the use of Ir loadings ranging from 0.1 to 0.004 mol %. Catalyst mixtures prepared from [{IrCl(cod)}(2)], NaPF(6), and the chiral (alphaS,alphaS)-1,1'-bis[alpha-(dimethylamino)benzyl]-(R,R)-2,2'-bis(dicyclohexylphosphino)ferrocene (Cy-Mandyphos) ligand proved capable of mediating the asymmetric TH of aryl alkyl ketones, including that of the hindered substrate 2,2-dimethylpropiophenone with an efficiency (0.5 mol % Ir; 95 % conversion, 95 % ee) not documented previously in TH chemistry.