Background: N-Heterocyclic carbenes (NHCs) have emerged as ubiquitous species having applications in a broad range of fields, including organocatalysis and organometallic chemistry. Since Arduengo and co-workers first isolated a bottlable NHC, namely imidazol-2-ylidene derivative, these nucleophilic species have attained a prominent place in synthetic organic chemistry. The NHC-induced non-asymmetric catalysis has turned out to be a really fruitful area of research in recent years.
Methods and Results: The isomerization of dimethyl maleate to dimethyl fumarate catalyzed by an N-heterocyclic carbene (NHC), namely 3-benzylbenzothiazolylidene has been investigated experimentally and theoretically. Dimethyl maleate on reacting with 3-benzylbenzothiazolylidene carbene (10 mol%), generated in situ from the reaction of 3-benzylbenzothiazolium bromide with triethylamine in diethyl ether at room temperature under nitrogen atmosphere isomerizes quantitatively to dimethyl fumarate. Theoretical investigation of a model reaction scheme at the DFT level reveals that initial attack of the carbene, which is the rate-determining step, is followed by rotation about the C-C bond in preference to a higher energy path involving proton abstraction. The species so formed splits off the carbene to yield dimethyl fumarate.
Conclusions and Perspective: 3-Benzylbenzothiazolylidene carbene catalyzes isomerization of dimethyl maleate to its trans-isomer. This carbene can be used in other catalytic reactions, such as acyloin condensation and Stetter reaction.