Class Of Carbenes Research Articles

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents.

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF6 ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC8 results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Bond Dissociation Energies of Carbene-Carbene and Carbene-Main Group Adducts.

A range of carbene structures and their adducts with one another and with a selection of small-molecule electrophiles and nucleophiles were examined at the composite correlated molecular orbital theory G3MP2 level to explore ground-state "carbenic" structures, their stabilities, and reactivities. Differences between carbene general classification as a singlet electrophilic carbene or singlet nucleophilic carbene and their given reactivity are discussed. A key quantity is the carbon-carbon bond dissociation energy for carbene dimers or the carbene-adduct dissociation energy for other species. The carbene dimer bond dissociation energies span a wide range from 10 to 170 kcal/mol. The hydrogenation energies and singlet-triplet splitting were found to correlate best with the carbene's self-dimerization energy, whereas other descriptors do not. The proton and fluoride affinities of the carbenes alone prove inadequate for classifying reactivity among classes of carbenes. The self-dimerization bond dissociation energy, hydrogenation energy, and singlet-triplet splitting of various carbenes, despite sometimes large differences in proton affinity and other indicators of reactivity, provide usable metrics to correlate substantial amounts of thermodynamic and kinetic (reactivity) information regarding these structures.

Recent Advances in Enantioselective Photochemical Reactions of Stabilized Diazo Compounds.

Diazo compounds have proven to be a useful class of carbenes or metal carbenoids sources under thermal, photochemical, or metal-catalyzed conditions, which can subsequently undergo a wide range of synthetically important transformations. Recently, asymmetric photocatalysis has provoked increasing research interests, and great advances have been made in this discipline towards the synthesis of optically enriched compounds. In this context, the past two decades have been the most productive period in the developments of enantioselective photochemical reactions of diazo compounds due to a better understanding of the reactivities of diazo compounds and the emergence of new catalytic modes, as well as easier access to and treatment of stabilized diazo compounds. This review highlights these impressive achievements according to the reaction type, and the general mechanisms and stereochemical inductions are briefly discussed as well.

What Wanzlick Did Not Dare To Dream: Cyclic (Alkyl)(amino)carbenes (cAACs) as New Key Players in Transition‐Metal Chemistry

The first synthesis of cyclic (alkyl)(amino)carbenes (cAACs) was reported in 2005, and since then this class of carbenes has been on a victory tour in main‐group‐element and transition‐metal chemistry. CAACs are easy to synthesize and among the most nucleophilic (σ‐donating) and simultaneously most electrophilic (π‐accepting) carbenes known to date. These properties have led to a vast number of applications of cAACs in main‐group‐element chemistry in, for example, the activation of small‐molecule compounds and enthalpically strong bonds, as well as in the stabilization of highly reactive main‐group‐element compounds. They also proved to possess outstanding ligand properties in transition‐metal chemistry in, for example, the stabilization of unusual low‐valent transition‐metal complexes, leading to enormously highly active but robust catalysts. Here a brief overview of cAAC‐ligated transition‐metal compounds, mainly in low oxidation states, with an emphasis on nickel complexes, is provided, and a personal account of some of the recent developments in the use of cAACs in coordination chemistry and catalysis is given.

Dinitroxide carbenes, a new class of carbenes with autoumpolung character: preparation in solution and stabilization in transition metal complexes.

Dinitroxide carbenes, a new class of carbenes with autoumpolung character: preparation in solution and stabilization in transition metal complexes.

Nucleophilic cyclocarbenes as ligands in metal halides and metal oxides

The nucleophilic cyclocarbene 1,3-dimethylimidazolin-2-ylidene ( 1) reveals universal ligand properties in metal coordination chemistry: in addition to the well-known stabilization of low oxidation-state transition metal fragments, this particular class of carbenes also coordinates with metal halides and metal oxides thus resembling the properties of conventional ether ( O), amine ( N), and phosphane ( P) ligands. Complexes of titanium(IV), zirconium(IV), hafnium(IV), vanadium(II), niobium(IV), tantalum(IV) of general formula MX n L m ( 3a–f) and the rhenium(VII) of formula CH 3ReO 3L 2 ( 3g) are reported (L = 1,3-dimethylimidazolin-2-ylidene).

Direct and Inverse Reactivity‐Selectivity Relationship in the [1.2]‐Addition of Singlet Carbenes to Olefins

The selectivity of carbenes on addition to olefins can—contrary to common belief—increase with increasing reactivity. On the basis of the frontier orbital model two classes of carbenes can be differentiated: those with direct and those with inverse reactivity‐selectivity relationship.