Catalytic Enantioselective α-Alkylation of Amides by Unactivated Alkyl Electrophiles.

Carbonyl groups that bear an α stereocenter are commonly found in bioactive compounds, and intense effort has therefore been dedicated to the pursuit of stereoselective methods for constructing this motif. While the chiral auxiliary-enabled coupling of enolates with alkyl electrophiles represented groundbreaking progress in addressing this challenge, the next advance in the evolution of this enolate-alkylation approach would be to use a chiral catalyst to control stereochemistry. Herein we describe the achievement of this objective, demonstrating that a nickel catalyst can accomplish enantioselective intermolecular alkylations of racemic Reformatsky reagents with unactivated electrophiles; the resulting α-alkylated carbonyl compounds can be converted in one additional step into a diverse array of ubiquitous families of chiral molecules. Applying a broad spectrum of mechanistic tools, we have gained insight into key intermediates (including the alkylnickel(II) resting state) and elementary steps of the catalytic cycle.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation.

Outer-sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M-H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square-planar cobalt(II) hydride complex. Photoactivation results in Co-H bond homolysis. The three-coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

Photochemically Driven Reverse Water-Gas Shift at Ambient Conditions mediated by a Nickel Pincer Complex.

The endothermic reverse water-gas shift reaction (rWGS) for direct CO2 hydrogenation to CO is an attractive approach to carbon utilization. However, direct CO2 hydrogenation with molecular catalysts generally gives formic acid instead of CO as a result of the selectivity of CO2 insertion into M-H bonds. Based on the photochemical inversion of this selectivity, several synthetic pathways are presented for CO selective CO2 reduction with a nickel pincer platform including the first example of a photodriven rWGS cycle at ambient conditions.

The elusive abnormal CO2 insertion enabled by metal-ligand cooperative photochemical selectivity inversion.

Direct hydrogenation of CO2 to CO, the reverse water-gas shift reaction, is an attractive route to CO2 utilization. However, the use of molecular catalysts is impeded by the general reactivity of metal hydrides with CO2. Insertion into M-H bonds results in formates (MO(O)CH), whereas the abnormal insertion to the hydroxycarbonyl isomer (MC(O)OH), which is the key intermediate for CO-selective catalysis, has never been directly observed. We here report that the selectivity of CO2 insertion into a Ni-H bond can be inverted from normal to abnormal insertion upon switching from thermal to photochemical conditions. Mechanistic examination for abnormal insertion indicates photochemical N-H reductive elimination as the pivotal step that leads to an umpolung of the hydride ligand. This study conceptually introduces metal-ligand cooperation for selectivity control in photochemical transformations.

Chemical Non-Innocence of an Aliphatic PNP Pincer Ligand.

The synthesis of the divinylamido PNP nickel(II) complex [NiBr{N(CHCHPtBu2 )2 }] is reported. This compound exhibits reversible, ligand-centered oxidation and protonation reactions. The resulting pincer chemical non-innocence can be utilized for benzylic C-H hydrogen atom abstraction. The thermochemistry and kinetics of hydrogen atom transfer were examined.