The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to “traditional” precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal–ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands.

The mechanism of the Morita Baylis–Hillman reaction has been heavily studied in the literature, and a long series of computational studies have defined complete theoretical energy profiles in these reactions. We employ here a combination of mechanistic probes, including the observation of intermediates, the independent generation and partitioning of intermediates, thermodynamic and kinetic measurements on the main reaction and side reactions, isotopic incorporation from solvent, and kinetic isotope effects, to define the mechanism and an experimental mechanistic free-energy profile for a prototypical Morita Baylis–Hillman reaction in methanol. The results are then used to critically evaluate the ability of computations to predict the mechanism. The most notable prediction of the many computational studies, that of a proton-shuttle pathway, is refuted in favor of a simple but computationally intractable acid–base mechanism. Computational predictions vary vastly, and it is not clear that any significant acc...

http://pubs.acs.org/doi/pdf/10.1021/ja5111392

A Case Study of the Mechanism of Alcohol-Mediated Morita Baylis–Hillman Reactions. The Importance of Experimental Observations

On the Basicity of Organic Bases in Different Media

Ionic Liquids as Designer Solvents for Nucleophilic Aromatic Substitutions

Aliphatic amines strongly coordinate, and therefore easily inhibit, the activity of transition-metal catalysts, posing a marked challenge to nitrogen-hydrogen (N-H) insertion reactions. Here, we report highly enantioselective carbene insertion into N-H bonds of aliphatic amines using two catalysts in tandem: an achiral copper complex and chiral amino-thiourea. Coordination by a homoscorpionate ligand protects the copper center that activates the carbene precursor. The chiral amino-thiourea catalyst then promotes enantioselective proton transfer to generate the stereocenter of the insertion product. This reaction couples a wide variety of diazo esters and amines to produce chiral α-alkyl α-amino acid derivatives.

Highly enantioselective carbene insertion into N–H bonds of aliphatic amines

pK a
 Values in DMSO are
reported for ammonium ions derived from amines which have
previously been widely used as nucleophiles in nucleophilic
aromatic substitution reactions; values are also given for four
polynitrodiphenylamines used as indicators

Acidities of Some Substituted Ammonium Ions in Dimethyl Sulfoxide

The reaction of ethyl 2,4,6-trinitrophenyl ether with aniline in dimethyl sulfoxide containing Dabco occurs in two stages. The first gives 5, the σ -adduct intermediate on the substitution pathway,...

Kinetic studies of bold σ -adduct formation and nucleophilic substitution in the reactions of ethyl 2,4,6-trinitrophenyl ether, some phenyl 2,4,6-trinitrophenyl ethers, and phenyl 2,4-dinitronaphthyl ether with aniline in dimethyl sulfoxide

Kinetics of the reaction of phenyl picrates with phenoxide ions in water. Concerted or stepwise

Kinetic and equilibrium studies of ?-adduct formation and nucleophilic substitution in the reactions of morpholine with 1,3,5-trinitrobenzene and some phenyl aryl ethers in dimethyl sulphoxide

Ian A. Robotham

Papers

Acidities of Some Substituted Ammonium Ions in Dimethyl Sulfoxide

Kinetic studies of bold σ -adduct formation and nucleophilic substitution in the reactions of ethyl 2,4,6-trinitrophenyl ether, some phenyl 2,4,6-trinitrophenyl ethers, and phenyl 2,4-dinitronaphthyl ether with aniline in dimethyl sulfoxide

Kinetics of the reaction of phenyl picrates with phenoxide ions in water. Concerted or stepwise

Kinetic and equilibrium studies of ?-adduct formation and nucleophilic substitution in the reactions of morpholine with 1,3,5-trinitrobenzene and some phenyl aryl ethers in dimethyl sulphoxide