Rapid Ketone Transfer Hydrogenation by Employing Simple, In Situ Prepared Iridium(I) Precatalysts Supported by “Non‐NH” P,N Ligands

<jats:title>Abstract</jats:title><jats:p>The catalytic utility in ketone transfer hydrogenation (TH) of the preformed complexes [Ir(cod)(κ<jats:sup>2</jats:sup>‐2‐NMe<jats:sub>2</jats:sub>‐3‐P<jats:italic>i</jats:italic>Pr<jats:sub>2</jats:sub>‐indene)]<jats:sup>+</jats:sup>X<jats:sup>−</jats:sup> ([<jats:bold>2 a</jats:bold>]<jats:sup>+</jats:sup>X<jats:sup>−</jats:sup>; X: PF<jats:sub>6</jats:sub>, BF<jats:sub>4</jats:sub>, and OTf; cod: η<jats:sup>4</jats:sup>‐1,5‐cyclooctadiene; OTf: trifluoromethanesulfonate), [Ir(cod)(κ<jats:sup>2</jats:sup>‐1‐P<jats:italic>i</jats:italic>Pr<jats:sub>2</jats:sub>‐2‐NMe<jats:sub>2</jats:sub>‐indene)]<jats:sup>+</jats:sup>OTf<jats:sup>−</jats:sup> ([<jats:bold>2 b</jats:bold>]<jats:sup>+</jats:sup>OTf<jats:sup>−</jats:sup>), [Ir(cod)(κ<jats:sup>2</jats:sup>‐2‐NMe<jats:sub>2</jats:sub>‐3‐P<jats:italic>i</jats:italic>Pr<jats:sub>2</jats:sub>‐indenide)] (<jats:bold>3</jats:bold>), and [Ir(cod)(κ<jats:sup>2</jats:sup>‐<jats:italic>o</jats:italic>‐<jats:italic>t</jats:italic>Bu<jats:sub>2</jats:sub>P‐C<jats:sub>6</jats:sub>H<jats:sub>4</jats:sub>‐NMe<jats:sub>2</jats:sub>)]<jats:sup>+</jats:sup>PF<jats:sub>6</jats:sub><jats:sup>−</jats:sup> ([<jats:bold>4</jats:bold>]<jats:sup>+</jats:sup>PF<jats:sub>6</jats:sub><jats:sup>−</jats:sup>), as well as of related mixtures prepared from [{IrCl(cod)}<jats:sub>2</jats:sub>] and various P,N‐substituted indene or phenylene ligands, was examined. Whereas [<jats:bold>2 a</jats:bold>]<jats:sup>+</jats:sup>X<jats:sup>−</jats:sup>, [<jats:bold>2 b</jats:bold>]<jats:sup>+</jats:sup>OTf<jats:sup>−</jats:sup>, <jats:bold>3</jats:bold>, 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 <jats:italic>i</jats:italic>PrOH at reflux (0.1 mol % Ir; 81–99 % conversion) in a preliminary catalytic survey, the structurally related Ir catalysts prepared from (<jats:italic>o</jats:italic>‐R<jats:sub>2</jats:sub>P‐C<jats:sub>6</jats:sub>H<jats:sub>4</jats:sub>)NMe<jats:sub>2</jats:sub> (R: Ph, <jats:italic>i</jats:italic>Pr, or <jats:italic>t</jats:italic>Bu) 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 [<jats:bold>4</jats:bold>]<jats:sup>+</jats:sup>PF<jats:sub>6</jats:sub><jats:sup>−</jats:sup>, either preformed or prepared in situ from a mixture of [{IrCl(cod)}<jats:sub>2</jats:sub>], NaPF<jats:sub>6</jats:sub>, and (<jats:italic>o</jats:italic>‐<jats:italic>t</jats:italic>Bu<jats:sub>2</jats:sub>P‐C<jats:sub>6</jats:sub>H<jats:sub>4</jats:sub>)NMe<jats:sub>2</jats:sub>, proved to be highly effective in mediating the catalytic transfer hydrogenation (TH) of ketones in basic <jats:italic>i</jats:italic>PrOH, 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<jats:sup>−1</jats:sup> 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)}<jats:sub>2</jats:sub>], NaPF<jats:sub>6</jats:sub>, and the chiral (α<jats:italic>S</jats:italic>,α<jats:italic>S</jats:italic>)‐1,1′‐bis[α‐(dimethylamino)benzyl]‐(<jats:italic>R</jats:italic>,<jats:italic>R</jats:italic>)‐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 % <jats:italic>ee</jats:italic>) not documented previously in TH chemistry.</jats:p>