The Elusive Palladium-Diazo Adduct Captured: Synthesis, Isolation and Structural Characterization of [(ArNHC-PPh2 )Pd(η2 -N2 C(Ph)CO2 Et)]

The Labile Nature of Air Stable Ni(II)/Ni(0)-phosphine/Olefin Catalysts/Intermediates: EDA-NOCV Analysis

Metal ions-based inorganic-organic hybrid composites are often reported acting as good to excellent catalysts with various substrate scopes under milder reaction conditions. The active catalyst of a catalytic cycle is sometimes proposed to be a short-lived reactive intermediate species. A three coordinate (L-Me)Ni(II) intermediate species [L-Me = O 2 N donor dianionic ligand] can bind with short-lived carbene-ester ligands to produce four coordinate Ni(II) species which can act as carbene transfer intermediates under suitable reaction conditions for C-H functionalization or cyclopropanation reactions. The dissociation of phosphine (PPh 3 ) from the Ni(II) centre of (L-Me)Ni(II)(PPh 3 ) ( 1a ) and binding of short lived carbene esters (:CR 1 -CO 2 R 2 ; R 1 = H, Ph; R 2 = aliphatic group; 2-4 and other carbenes; 5-10 ) to Ni(II) rationalize the phenomenon in solution. Air stable Ni(0)-olefin complexes/intermediates ( 12-18 ) have recently been shown to mediate a variety of organic transformations. This analysis will further help organic/organometallic chemists to rationalize the design and synthesis of future catalysts for organic transformation. EDA-NOCV calculations have been performed to shed light on the stability and bonding of those species. Additionally, our analysis provides a proper reason why the analogous (L-Me)Pd-PPh 3 complex ( 1b ) does not dissociate in solution and hence, a similar catalytic product has not been isolated from identical reaction conditions. The stability and the labile nature of Ni(II/0) complexes has been investigated by state-of-the-art EDA-NOCV analyses.

Well-Defined, Versatile and Recyclable Half-Sandwich Nickelacarborane Catalyst for Selective Carbene-Transfer Reactions

Catalytic carbene-transfer reactions constitute a class of highly useful transformations in organic synthesis. Although catalysts based on a range of transition-metals have been reported, the readily accessible nickel(II)-based complexes have been rarely used. Herein, an air-stable nickel(II)-carborane complex is reported as a well-defined, versatile and recyclable catalyst for selective carbene transfer reactions with low catalyst loading under mild conditions. This catalyst is effective for several types of reactions including diastereoselective cyclopropanation, epoxidation, selective X-H insertions (X = C, N, O, S, Si), particularly for the unprotected substrates. This represents a rare example of carborane ligands in base metal catalysis.

Base Promoted Three-Component Annulation of 4-Diazoisochroman-3-imines with Dimethylsulfonium Ylides: Synthesis of Highly Functionalized Isochromeno[4,3-c]pyridazines

A novel method has been developed to synthesize a unique class of highly functionalized isochromeno[4,3-c]pyridazines. This reaction features an intermolecular functionalization of terminal nitrogen atom of diazo group of 4-diazoisochoman-3-imine with two dimethylsulfonium ylide components, followed by a base promoted 6-exo-trig cyclization step. Readily available starting materials, a broad substrate scope, and operationally simple, mild, and catalyst-free reaction conditions are the prominent features of this method.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Tridentate Nickel(II)-Catalyzed Chemodivergent C-H Functionalization and Cyclopropanation: Regioselective and Diastereoselective Access to Substituted Aromatic Heterocycles

A Schiff-base nickel(II)-phosphene-catalyzed chemodivergent C-H functionalization and cyclopropanation of aromatic heterocycles is reported in moderate to excellent yields and very good regioselectivity and diastereoselectivity. The weak, noncovalent interaction between the phosphene ligand and Ni center facilitates the ligand dissociation, generating the electronically and coordinatively unsaturated active catalyst. The proposed mechanisms for the reported reactions are in good accord with the experimental results and theoretical calculations, providing a suitable model of stereocontrol for the cyclopropanation reaction.

Dimensioning the Term Carbenoid

The current use of the term carbenoid is discussed, particularly in the context of carbene transfer reactions from diazo compounds, in which intermediates of type L_n M=CR¹ R² , or one of its resonance forms, are tagged which such a name. We discuss this issue, on the basis of the data evidencing the metal-carbene nature of those intermediates, as well as the existence of carbenoids of type (N₂ )(M)CR¹ R² en route to the formation to L_n M=CR¹ R^2' from diazo reagents. We propose the exclusive use of the carbenoid term to species of type (X)(M)CR¹ R² with a tetrasubstituted carbon center that upon loss of X afford an effective carbene transfer process.