Chiral Xyliphos Complexes for the Catalytic Imine Hydrogenation Leading to the Metolachlor Herbicide: Isolation of Catalyst–Substrate Adducts

<jats:title>Abstract</jats:title><jats:p>Iridium complexes relevant to the catalytic enantioselective hydrogenation of 2‐methyl‐6‐ethylphenyl‐1′‐methyl‐2′‐methoxyethylimine (MEA‐imine, <jats:bold>1</jats:bold>) in the Syngenta Metolachlor (<jats:bold>3</jats:bold>) process were prepared and characterized. Reaction of the diphosphane (<jats:italic>S</jats:italic>)‐1‐[(<jats:italic>R</jats:italic>)‐2‐(diphenylphosphanyl)ferrocenyl]ethyldi(3,5‐xylyl)phosphane ((<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐Xyliphos, (<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>) with [Ir<jats:sub>2</jats:sub>(μ‐Cl)<jats:sub>2</jats:sub>(cod)<jats:sub>2</jats:sub>] (cod=1,5‐cyclooctadiene) afforded [Ir(Cl)(cod){(<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>}] (<jats:bold>7</jats:bold>), which reacted with AgBF<jats:sub>4</jats:sub> to form [Ir(cod){(<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>}]BF<jats:sub>4</jats:sub> (<jats:bold>8</jats:bold>). Complexes <jats:bold>7</jats:bold> and <jats:bold>8</jats:bold> reacted with iodide to yield [Ir(I<jats:sc>)</jats:sc>(cod){(<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>}] (<jats:bold>9</jats:bold>). When <jats:bold>9</jats:bold> was treated with one and two equivalents of HBF<jats:sub>4</jats:sub>, two isomers of the cationic Ir<jats:sup>III</jats:sup> iodo hydrido complex [Ir(I)(H)(cod){(<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>}]BF<jats:sub>4</jats:sub> were isolated (<jats:bold>10</jats:bold> and <jats:bold>11</jats:bold>, respectively). Complex <jats:bold>9</jats:bold> was oxidized with one equivalent of I<jats:sub>2</jats:sub> to give the iodo‐bridged dinuclear species [Ir<jats:sub>2</jats:sub>I<jats:sub>2</jats:sub>(μ‐I)<jats:sub>3</jats:sub>{(<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>}<jats:sub>2</jats:sub>)]I (<jats:bold>12</jats:bold>). [Ir<jats:sub>2</jats:sub>(μ‐Cl)<jats:sub>2</jats:sub>(coe)<jats:sub>4</jats:sub>] (coe=cyclooctene) reacted with (<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold> to yield the chloro‐bridged dinuclear complex [Ir<jats:sub>2</jats:sub>(μ‐Cl)<jats:sub>2</jats:sub>{(<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>}<jats:sub>2</jats:sub>] (<jats:bold>13</jats:bold>). Complexes <jats:bold>7</jats:bold>–<jats:bold>12</jats:bold> were structurally characterized by single‐crystal X‐ray diffraction and tested as single‐component catalyst precursors for enantioselective hydrogenation of MEA‐imine. Complex <jats:bold>10</jats:bold> and dinuclear complex <jats:bold>12</jats:bold> gave the best catalytic results. Efforts were also directed at isolating substrate– or product–catalyst adducts: Treatment of <jats:bold>8</jats:bold> with 2,6‐dimethylphenyl‐1′‐methyl‐2′‐methoxyethylimine (DMA‐imine, <jats:bold>14</jats:bold>, a model for <jats:bold>1</jats:bold>) under H<jats:sub>2</jats:sub> allowed four isomers of [Ir(H)<jats:sub>2</jats:sub>{(<jats:italic>S</jats:italic>)‐(<jats:italic>R</jats:italic>)‐<jats:bold>4</jats:bold>}(<jats:bold>14</jats:bold>)]BF<jats:sub>4</jats:sub> (<jats:bold>18</jats:bold>–<jats:bold>21</jats:bold>) to be isolated. These analytically pure isomers were fully characterized by 2D NMR techniques. X‐ray structural analysis of an Ir<jats:sup>I</jats:sup>–imine adduct, namely, [Ir(C<jats:sub>2</jats:sub>H<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>(<jats:bold>14</jats:bold>)]BF<jats:sub>4</jats:sub> (<jats:bold>25</jats:bold>), which was prepared by reacting [IrCl(C<jats:sub>2</jats:sub>H<jats:sub>4</jats:sub>)<jats:sub>4</jats:sub>] with [Ag(<jats:bold>14</jats:bold>)<jats:sub>2</jats:sub>]BF<jats:sub>4</jats:sub> (<jats:bold>16</jats:bold>), confirmed the κ<jats:sup>2</jats:sup> coordination mode of imine <jats:bold>14</jats:bold>.</jats:p>