Persistent carbene

A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole. Traditionally carbenes are viewed as so reactive that were only studied indirectly, such as by trapping reactions. This situation has changed dramatically with the emergence of persistent carbenes. Although they are fairly reactive substances, undergoing dimerization, many can be isolated as pure substances. Persistent carbenes tend to exist in the singlet. Their stability is only partly due to steric hindrance by bulky groups. Some singlet carbenes are thermodynamically stable and can be isolated an

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Influence of N-heterocyclic carbenes (NHCs) on the hydrolysis of a diphosphene

Shubhajit Das

We report the influence of N-heterocyclic carbenes (NHCs) on the hydrolysis of a diphosphene TerP?PTer (1; Ter = 2,6-Mes(2)C(6)H(3); Mes = 2,4,6-Me3C6H2), a phosphorus-analogue of an alkene. The diphosphene 1 itself is completely inert towards water. However, NHCs have been found to activate 1 towards ready hydrolysis. While sterically less-encumbered NHCs react with 1 affording NHC-adducts which are in equilibrium with 1 in solution, sterically encumbered NHCs do not bind to 1 at all. Interestingly, in both of these situations hydrolysis of the P-P motif proceeds efficiently. At low temperatures, sterically less-encumbered NHCs are catalytic while the sterically encumbered NHCs play a catalytic role at room temperature. To gain insight on this striking influence of NHCs on the hydrolysis of diphosphene detailed low-temperature P-31-NMR studies along with theoretical calculations have been carried out. In addition, systematic hydrolysis studies of all the NHCs used in this study have also been performed.

ROYAL SOC CHEMISTRY2020

Sequential N–O and N–N Bond Cleavage of N-Heterocyclic Carbene-Activated Nitrous Oxide with a Vanadium Complex

Rosario Scopelliti, Kay Severin, Euro Solari, Alexander Tskhovrebov, Matthew Wodrich

Chemically induced bond cleavage of nitrous oxide typically proceeds by rupture of the N–O bond with concomitant O-atom transfer and liberation of dinitrogen. On a few occasions, N–N bond scission has been observed instead. We report a reaction sequence involving an N-heterocyclic carbene and a vanadium complex that results in cleavage of both the N–O bond and the N–N bond.

2012

Covalent Capture of Nitrous Oxide by N-Heterocyclic Carbenes

Rosario Scopelliti, Kay Severin, Euro Solari, Alexander Tskhovrebov, Matthew Wodrich

A good catch: N-heterocyclic carbenes (NHCs) form stable adducts with nitrous oxide (N2O) under mild reaction conditions. The adducts display unique reactivity as evidenced by an alkylation reaction which leads to a rupture of the NN bond.

Wiley-Blackwell2012

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