Depending on the type of carbenes used in the reaction with thioketones, formation of different products was observed. In our ongoing studies, attention has been focused on reactions of carbenes (and carbenoids) with lesser studied thiocarbonyl compounds and the non-enolizable thioketones being the favorite models. Among non-heterocyclic, nucleophilic carbenes, dialkoxycarbenes, such as dimethoxycarbene ( 1d) and dibenzyloxycarbene ( 1e), found numerous applications in organic synthesis and served as practically useful models for structural studies ( Figure 1).ĭihalocarbenes, dialkoxycarbenes, and non-enolizable, cycloaliphatic thioketones 2a– d involved in the study. In addition, difluorocarbene ( 1c) is considered an important building block for preparation of fluoroorganic compounds, which are of great importance for medicinal chemistry and related applications. Very recently, new reactions within situ generated carbenes, using non-conventional techniques, such as mechanochemical synthesis or flow-technique, have also been reported.ĭihalocarbenes 1a–b ( Figure 1) occupy a prominent position in the group of electrophilic carbenes, and the Makosza reaction, based on the phase-transfer methodology, offers a straightforward access to synthetically relevant gem-dihalocyclopropanes via their cycloaddition onto ethylenic >C=C< bonds. Great importance of both electrophilic and nucleophilic carbenes, in the current organic and organometallic chemistry, is demonstrated by a large number of original publications and reviews, which have appeared in the last five years. In the last 25 years, along with better known ‘classic’ electrophilic carbenes, the chemistry of nucleophilic analogues, and especially ‘nucleophilic heterocycloc carbenes’ (NHCs), has been under rapid development. Significantly higher reactivity of the thioketone unit in comparison to the ketone group, both present in 3-thioxo-2,2,4,4-tetramthylcyclobutanone molecule, was rationalized in the light of DFT computational study.Ĭarbenes form a group of reactive intermediates in which the central the two-valent carbon atom plays the most important role, determining their structure and chemical behavior. A computational study showed that difluorocarbene tends to react as a nucleophilic species and resembles rather dimethoxycarbene and not typical dihalocarbene species. In contrast, analogous reactions with nucleophilic dimethoxycarbene occur via a single step reaction, which can be considered as the cycloaddition reaction initiated by the attack onto the C=S bond. Remarkably, reactions involving more electrophilic carbenes (dichloro-, and dibromocarbene) proceeds via stepwise mechanism involving thiocarbonyl ylide as a transient intermediate. Obtained results demonstrated that the examined processes exhibit polar nature and in the case of electrophilic dichloro-, and dibromocarbenes are initiated by the attack of carbene species onto the sulfur atom of the C=S group. The molecular mechanisms of addition of dihalocarbenes and dimethoxycarbene to thioketones derived from 2,2,4,4-tetrmethylcyclobutane-1,3-dione were examined on the basis of the DFT wb97xd/6-311g(d,p)(PCM) calculations.
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