Ylides, chalcogen

The chalcogene heterocycles have been used as stable precursors for sulfur-said selenium-cantaining hetero-l,3-dienes in cycloaddition reactions 3//-l,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-1,3-dienes, and 3//-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-substrtuted nitrile ylides [137]... 

The increasing stability toward racemization was supported by theoretical calculations carried out on the dimethyl chalcogen-onium ylides.20 In fact, the... 

The very strong polarization of the Te=X —> Te+ - X (X = C,N,0) bonds accounts for the enhanced electrophilicity of the tellurium center in 7r-telluranes, which markedly exceeds that of other chalcogen centers in respective 7r-chalcogenuranes (ylides, imides, oxides). Such a polarization provides also for the highest nucleophilcity and basicity of the X-centers in ir-telluranes. 

A theoretical study on 5V-methylselenabenzene 192 and its oxygen and sulfur analogues has been carried out using ab initio calculations with the DFT method <2006M1791> (cf. Section 7.11.2, structure 3). The X-H compounds were also evaluated. These structures have 6ji electrons and are homoaromatic with ylide character. Structural parameters were also determined for these structures. The substituent on the chalcogen atom is calculated to be more stable in a pyramidal rather than a planar conformation. 

The [2,3]sigmatropic rearrangement of allylic chalcogen ylides provides a useful method for carbon-carbon bond formation (Eq. 7) ... 

It is worth noting that Doyle and co-workers have reported quite recently a Rh-catalyzed highly enantioselective [2,3]sigmatropic rearrangement of allylic oxonium ylides in place of the allylic chalcogen ylides [51]. 

The reaction between phosphorus ylides and elemental sulfur or selenium has been used to prepare chalcogen-containing heterocyclic species. For example. 

Cyclopropane Formation via Chalcogen-Ylides by Alkylation/Deprotonation... 

Scheme 7.42 Catalytic epoxide (X=0), aziridine (X=iV-Ts or iV-SES) and cyclopropane (X=CH-COY) formation mediated by chalcogen-ylides generated from diazo compounds via metal carbenes (start at upper right corner)...

Aziridine Formation with Chalcogen-Ylides from Carbene Sources... 

Scheme 7.50 Proposed catalytic cycle for epoxide formation with chalcogen-ylides via alkylation / deprotonation /substitution mechanism in a phase transfer system...

Scheme 7.58 Catalytic cycle proposed for the epoxide formation catalyzed by chalcogen-ylides with diazo-derived carbene as ylide source...

The diastereoselectivity of the ylide mediated oxirane formation is connected to the degree of reversibility of the betaine intermediate formed by the reaction of the chalcogen ylide with the carbonyl compound (Scheme 7.62). When the... 

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