“Chiral-at-Metal” Hemilabile Nickel Complexes with a Latent d¹⁰-ML₂ Configuration: Receiving Substrates with Open Arms

Complexes with highly reactive stereogenic metal centers are of great interest to asymmetric synthesis. Thus, by reacting [Ni­(COD)₂] with 2 equiv of the P-alkene ligand (<i>S</i>)-<b>5</b> ((<i>S</i>)-(+)-<i>N</i>-(3,5-dioxa-4-phosphacyclohepta­[2,1-<i>a</i>;3,4-<i>a</i>′]­dinaphthalen-4-yl)­dibenz­[<i>b</i>,<i>f</i>]­azepine) or (<i>S</i>_<i>P</i><i>,S</i>_<i>C</i>)<i>-</i><b>6</b> ((2<i>S</i>,5<i>S</i>)-(-)-<i>N</i>-(aza-3-oxa-2-phosphabicyclo­[3.3.0]­octan-4-on-2-yl)­dibenz­[<i>b</i>,<i>f</i>]­azepine), the diastereomerically and enantiomerically pure tetrahedral complexes (Δ,<i>S,S</i>)-[Ni­(<b>5</b>-κ<i>P</i>,η²-alkene)₂] (<b>2a</b>) and (Δ,<i>S</i>_P<i>,S</i>_C<i>,S</i>_P<i>′</i><i>,S</i>_C<i>′</i>)-[Ni­(<b>6</b>-κ<i>P</i>,η²-alkene)₂] (<b>2b</b>) were obtained in almost quantitative yields on multigram scales. The Ni atoms showed in both cases stable Δ configurations. Even though these Ni(0) complexes were air stable in the solid state, once dissolved, complex <b>2a</b> readily activated CS₂, alkynes, and enones as the formal d¹⁰-ML₂ fragment [Ni­(<b>5</b>-κ<i>P</i>)₂] (<b>4</b>) to form adducts <b>8</b>–<b>11</b>. This is possible thanks to the decoordination of the hemilabile alkene arms of the P-alkene ligands, and the X-ray crystal structures of the CS₂ and 4-ethynyltoluene adducts confirmed the η² coordination modes of the substrates and the concomitant opening up of the alkene arms of ligand <b>5</b>. The coordination of α,β-unsaturated carbonyl compounds in complexes <b>11a</b>–<b>c</b> was reversible

“Chiral-at-Metal” Hemilabile Nickel Complexes with a Latent d¹⁰-ML₂ Configuration: Receiving Substrates with Open Arms

Complexes with highly reactive stereogenic metal centers are of great interest to asymmetric synthesis. Thus, by reacting [Ni­(COD)₂] with 2 equiv of the P-alkene ligand (<i>S</i>)-<b>5</b> ((<i>S</i>)-(+)-<i>N</i>-(3,5-dioxa-4-phosphacyclohepta­[2,1-<i>a</i>;3,4-<i>a</i>′]­dinaphthalen-4-yl)­dibenz­[<i>b</i>,<i>f</i>]­azepine) or (<i>S</i>_<i>P</i><i>,S</i>_<i>C</i>)<i>-</i><b>6</b> ((2<i>S</i>,5<i>S</i>)-(-)-<i>N</i>-(aza-3-oxa-2-phosphabicyclo­[3.3.0]­octan-4-on-2-yl)­dibenz­[<i>b</i>,<i>f</i>]­azepine), the diastereomerically and enantiomerically pure tetrahedral complexes (Δ,<i>S,S</i>)-[Ni­(<b>5</b>-κ<i>P</i>,η²-alkene)₂] (<b>2a</b>) and (Δ,<i>S</i>_P<i>,S</i>_C<i>,S</i>_P<i>′</i><i>,S</i>_C<i>′</i>)-[Ni­(<b>6</b>-κ<i>P</i>,η²-alkene)₂] (<b>2b</b>) were obtained in almost quantitative yields on multigram scales. The Ni atoms showed in both cases stable Δ configurations. Even though these Ni(0) complexes were air stable in the solid state, once dissolved, complex <b>2a</b> readily activated CS₂, alkynes, and enones as the formal d¹⁰-ML₂ fragment [Ni­(<b>5</b>-κ<i>P</i>)₂] (<b>4</b>) to form adducts <b>8</b>–<b>11</b>. This is possible thanks to the decoordination of the hemilabile alkene arms of the P-alkene ligands, and the X-ray crystal structures of the CS₂ and 4-ethynyltoluene adducts confirmed the η² coordination modes of the substrates and the concomitant opening up of the alkene arms of ligand <b>5</b>. The coordination of α,β-unsaturated carbonyl compounds in complexes <b>11a</b>–<b>c</b> was reversible