Cheletropic processes

Since the first report in 1914 of the formation of a butadiene-sulfur dioxide adduct [538], much work has been carried out on the reaction of conjugated dienes with sulfur dioxide and its applications in synthetic strategies. Two pathways can and have been observed a cheletropic reaction (to 2s + -it 4s) yielding 3-sulfolenes and (4 -its + tt 2s) hetero-Diels-Alder addition yielding sultines (see [539] and [540] and references 

The thermal retro-reaction of 3-sulfolenes to dienes and sulfur dioxide occurs under mild conditions (about 120-200°C), and is, as predicted from the Woodward-Hoffmann rules, a disrotatory process so for 2,5-dimethyl-3-sulfolenes [541,542]  

Many uses of the sulfolene reaction for synthetic purposes are associated with this easy stereoselective synthesis of conjugated dienes [109, 203]. The development of this methodology led to new routes for the introduction of electrophiles in the 2 and (or) 5 positions of the sulfolenes 

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Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry analysis in the same way as cycloadditions and sigmatropic processes. In the elimination processes of interest here, the atom X is normally bound to other atoms in such a way that elimination gives rise to a stable molecule. In particular, elimination of S02, N2, or CO from five-membered 3,4-unsaturated rings can be a facile process. 

The molecule of S02 must lie in a plane which bisects the suprafacial cis conformation of the diene. This attack of S02 will be from below, so that a concerted suprafacial, suprafacial addition is allowed. Such reactions are called linear cheletropic processes. 

The concerted suprafacial, suprafacial addition to an alkene as shown above is a disallowed process. So in linear cheletropic process the interaction is suprafacial for both orbitals of the diene and also for HOMO of S02, but it is antarafacial for the LUMO of S02. 

These 6n electron additions and eliminations are apparently concerted and thus come in the category of linear cheletropic process (tt4s + co2s cycloaddition and 7t2v + o2.v + c2s cycloelimination). 

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry restrictions in the same way that cycloadditions and sigmatropic processes are. 

These direct metallation-alkylation reactions of sulfolene and dihydrothiepin dioxide are incentives for future new synthetic applications of the associated cheletropic processes. 

Decomposition of (72) in ethanolic sodium hydrosulphite generates, in situ, an azine which fragments to give nitrogen and cyclo-octatetraene. The yield of cyclo-octatetraene is substantially lower than the yield of cyclo-octadiene from (73). The controversy concerning the origin of this result continues. Anastassiou has re-affirmed the view that there is symmetry-controlled opposition to a linear cheletropic process in (72) but not in (73). 

Just as cycloaddition reactions can be interpreted in terms of orbital-symmetry considerations, elimination reactions which are concerted also require appropriate alignment of orbitals for continuous bonding. The principles of orbital symmetry can then specify what processes can occur in concerted fashion and identify the stereochemical restrictions which are imposed by the concerted process. The number of elimination reactions that have been studied in detail is not large, but there is sufficient information to establish that orbital-symmetry controls are indeed operating. Cheletropic processes are defined as reactions in which two bonds are broken (or formed) to a single atom. 

Main interaction modes between the electron deficient JTjz species and the polyene in cheletropic processes... 

It was shown previously that [2s + 2 ] and [2a + 2a] interactions should be classified together (even inversions), whereas [2s + 2a] or [2 + 2 ] interactions form a separate category (odd inversions). The various possible cheletropic processes may be similarly grouped together, Table 3.1, assuming a constant value for k for the whole set of interactions. 

Cheletropic reactions fill in the gaps in the possible sequence of ring sizes obtainable by cyclo-addition reactions. The cheletropic processes refer to the formation or cleavage of 3, 5, 7. . . membered rings cyclo-additions normally refer to 4, 6, 8. .. membered rings. In fact both processes are closely related. The essential difference is that in the cheletropic reactions both terminal orbital lobes are located at a single atomic centre of one of the participants, and are necessarily orthogonal. In normal cyclo-additions the orbitals concerned are located at the termini of both of the components, and within each component these orbitals are considered to be parallel. 

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