Stereochemistry cheletropic reactions

The application of the Woodward-Hoffmann rules to cheletropic reactions is not straightforward. In the [2+1] cycloaddition of singlet carbenes to alkenes, the stereochemistry of the alkene is preserved in the product, so the alkene must react suprafacially. The Woodward-Hoffmann rules suggest that the carbene component of this thermal, four-electron reaction must react antarafacially. However, what this means for a species lacking a 77 system is difficult to interpret. 

The most important cheletropic reaction is the addition of singlet carbenes to olefins to make cyclopropanes. Only singlet carbenes will be considered here the pericyclic selection rules cannot be applied to triplet states. The electronic structure of a singlet carbene involves an empty p orbital and a roughly sp hybrid that has two electrons (see, for example, the two lone pair orbitals of the water molecule in Appendix 3). We know from Chapter 10 that singlet carbenes add stereospecifically to olefins, and that the olefin stereochemistry is retained in the cyclopropane product. As such, in the present context, the reaction would be described as suprafacial on the olefin. 

The stereochemistry of cheletropic reactions has been discussed in Section 3.3.4. The linear cheletropic reactions usually offer the best scope for orbital overlap and would normally be expected to be the preferred reaction pathways. Generally the extniaon reactions (i.e. the loss of the species Xyz) are more numerous than cheletropic cyclo-additions because the Xyz species is often a molecule of high thermodynamic stability, and in such cases the reactions are essentially uni-directional. ... 

Some of its special chapters are the Pericyclic Reactions, which includes Cheletropic, Electrocyclic, Sigmatropic and Cycloaddition reactions. The concept of Stereochemistry and Conformation deserve special attention not because they cater to the needs of higher students, but they are immensely useful for candidates trying for UGC and CSIR sponsored competitive examinations, but also those preparing for Union Public Service Commission and State Public Service Commission Exams. The candidates will find the chapters immensely useful and is sure to rouse interest in them in knowing more about mechanistic chemistry. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

Mori et al. have studied the decarboxylation of (39) under a variety of conditions and have found that the conversion to (40) occurs without the involvement of radicals. They suggest that the process is a concerted cheletropic extrusion via the s-cis conformation. A further study has examined the photodecarboxylation of the (5)-ester (39) in unstretched-polyethylene films. The decarboxylation affords (40) with complete retention of the stereochemistry. The yield of product is 98% and the ee is >98%. The photochemical behaviour of the ester in other confining media such as cyclodextrins indicates that cage-escape products are also formed. The irradiation of grandifloric acid (41) at 254 nm in acetonitrile brings about decarboxylation with the formation of epimers. In methanol a different reaction occurs that results in the conversion of the C-methyl group into a carbomethoxy substituent. ... 

Diels-Alder addition of tetrachlorocyclopentadiene dimethylketal to endo-5-norbornene carboxylic acid produces (459). Ketal cleavage (H2SO4), cheletropic loss of CO (reflux in o-dichloro benzene), and aromatization (bromine in hot chlorobenzene) affords the endo-acid (460 X = Cl) which can be dechlorinated by reaction with Ni-Al alloy and alkali to give (460 X = H). The clean retention of stereochemistry in reaction sequence is suggested to be of value in the general synthesis of alicyclic-substituted benzonorbornenes the methyl ester of (460 X = H) can likewise be prepared without epimerization. ... 

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