Concerted sigmatropic

When dimethyl-2,2-dioxo-5-methyl-177,377-pyrrolo[l,2-r-][l,3]thiazole-6,7-dicarboxylate was heated under flash vacuum pyrolysis at 700°C/10 3mmHg, sulfur dioxide was eliminated and the vinylpyrrole 391 was obtained, which can be explained by allowed suprafacial [1,8]H shifts in the 871 1,7-dipolar system 390. Concerted sigmatropic shifts can only occur when the methyl groups adopt an inward (Z)-conformation (Scheme 57) <2002J(P1)1795, 2004TL3889, 2005JOC6629>. 

FVP treatment of both isolated samples 300 and 301 resulted in a mixture containing 300 + 301 this finding revealed that these two compounds can be mutually transformed into each other under forced conditions. Upon some kinetic measurements (determination of a negative activation entropy), the authors concluded that the ring transformation probably proceeds via a concerted sigmatropic shift to an antiaromatic diazirine intermediate. 

In unsubstituted phenyl ethers, the enolisation is faster than the Cope reaction. This is why the product is predominantly ortho isomer. When both the ortho positions are substituted, the allyl group undergoes a second migration via a concerted sigmatropic mechanism giving a para substituted phenol. This is called para Claisen condensation. 

Allenylsilanes undergo intramolecular additions to appropriately positioned aldehydes, imines, conjugated esters and alkenes to afford various alkynylcyclopentane and cyclohexane derivatives (Eqs. 9.66-9.70) [66]. The reactions are promoted by SnCl4 or by thermolysis. The stereochemistry of these cyclization reactions is consistent with a concerted sigmatropic process as illustrated in Scheme 9.17. 

A number of photochemical 1,3-shifts are known that cannot occur by a concerted (sigmatropic) mechanism, such as the sensitized rearrangement a 3-vinylcydopropene to a cydopentadiene (2.42). There are also a group of reactions for whic h the term bicycle ... 

The latter will consequently undergo the usual concerted sigmatropic process finally resulting in the cycloheptadiene derivative. Due to the higher temperature needs and to the participation of radical intermediates, side reactions are more likely to occur in the trans series. 

This does not mean that rearrangement actually involves the separation and re-attachment of a free radical. Such a stepwise reaction would not be a concerted one, and hence is not the kind of reaction we are dealing with here. Indeed, a stepwise reaction would be a (high-energy) alternative open to a system if a (concerted) sigmatropic rearrangement were symmetry-forbidden. 

The [1,2]-Meisenheimer rearrangement most likely proceeds via a hemolytic dissociation-recombination mechanism, whereas the [2,3]-Meisenheimer rearrangement is a concerted sigmatropic process that goes through a five-membered envelopelike transition state. 

The mechanism of these copper catalyzed isomerizations is not fully understood. A concerted sigmatropic 1,3-shift as Well as a biradical process have been proposed but a decision between these two possibilities could not be made 85). Kinetic measurements at least have shown that the reactions are not monomolecular and that the reaction rates depend on the catalyst concentration. 

Is the following concerted sigmatropic rearrangement thermally allowed ... 

Note that this mechanism lacks the classical intermediates of organic chemistry. It is a concerted sigmatropic reaction that involves shifts in the it framework. 

When both ortho positions are substituted, the allyl group undergoes a second migration via a concerted sigmatropic mechanism, giving the pam-substituted phenol ... 

The aza-Petasis-Ferrier rearrangement is not a true concerted sigmatropic reaction, since it is thought to proceed via C-O bond cleavage of a hemiaminal vinyl ether by the action of an acid catalyst to afford a reactive iminium cation and the enol form of an aldehyde these two intermediates combine subsequently in a Mannich-type process, leading to 3-amino aldehydes (Scheme 40.45) [54]. Recently, Terada and Toda used a BINOL-derived phosphoric acid diester as a catalyst for this interesting reaction [55]. 

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