Cheletropic elimination

The synthetic utility of o-quinodimethane generated by cheletropic elimination of S02 has been amply demonstrated by Oppolzer and Nicolaou, who have conducted an intramolecular cycloaddition coupled with the alkylation of 1,3-dihydrobenzo[c]thiophene 2,2-dioxide122. When 1,3-dihydro-l-(4-pentenyl)benzo[c]thiophene 2,2-dioxide (201) prepared from 1,3-dihydrobenzo[c]thiophene 2,2-dioxide and 4-pentenyl bromide is heated in di-n-butyl... 

Cheletropic elimination of the in situ generated phosphinidene complex 8 from the 7-phosphanorbornadiene precursor is believed to be the rate-determining step before 1,2-cycloaddition occurs to the unsaturated hydrocarbon. Without catalysts these are first-order processes that depend only on the concentration of the precursor and not on that of any substrate [47]. The configu-... 

The synthetic utility of o-quinodimethane generated by cheletropic elimination of SO2 has been amply demonstrated by Oppolzer and Nicolaou, who have conducted an intramolecular cycloaddition coupled with the alkylation... 

A good example of a concerted cheletropic elimination is the reaction of 3-pyrroline with IV-nitrohydroxylamine, which gives rise the the diazene 21, which then undergoes elimination of nitrogen. 

The most synthetically useful cheletropic elimination involves 2,5-dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they fragment to give dienes and sulfur dioxide.301 The reaction is stereospecific. For example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of 2,4-hexadiene, respectively, at temperatures of 100°-150°C.302 This stereospecificity corresponds to disrotatory elimination. 

The parent l,4-dihydro-l,4-diazocine (232) and a number of AT,AT-derivatives have been prepared and thoroughly characterized by Prinzbach and Vogel and their colleagues (79AG(E)962, 964, 80CB3161). The benzene diimine precursors have been obtained by two routes. In one case cis benzene dioxide was converted in several steps to the mesyl derivative (225) and thence to the diimine dimesylate. The second proceeds from cis,cis- benzene trioxide to the triimine (227). Two equivalents of acid chloride give the diamides (228) in good yield, and nitrosation with cheletropic elimination of N2O leads to the N,AT -disub-stituted diimines (229). 

Cheletropic elimination of sulfur dioxide from benzo(c)-l,3-dihydrothiophene dioxide (8) was used as a route to c-quinodimethanes [550]. 

Cyclopropene also adds to less reactive, acyclic, dienes218), though it is worth noting that the reaction with cyclohexadiene only proceeds in 10% yield 219). Addition of a range of alkylcyclopropenes to thiophene dioxides leads to cycloheptatrienes, presumably by cheletropic elimination of sulphur dioxide from the intermediate adduct 220) ... 

The reverse of this process is known as cheletropic extrusion (or cheletropic elimination). 

The cheletropic elimination of nitrogen from diazenes 8.1 and 8.2 is a stereospecific reaction (for definition of a stereospecific reaction, see section 1.5). 

The question of enolization in butenolides remains a matter of some theoretical interest and recently, with the advent of lithiation and allied techniques, it has become more pointed. Scandinavian workers have determined the ionization potentials of certain butenolides and found them to lie at or above 9.6 eV a furan-2-ol would be expected to have much lower IP of about 8.4 eV and since no peak was noted in this region it follows that the butenolides are not appreciably enolized in the gas phase.75 At high temperatures (near 500°C) butenolides suffer cheletropic elimination of carbon monoxide giving vinylic ketones in good yield—again ends play no part.76... 

The term "cheletropic reaction" defines reactions in which two a bonds to a single atom are made or broken conceitedly. A widely studied example, theoretically as well as experimentally, is the cheletropic elimination of sulfur dioxide from a cyclic sulfolene to generate stereochemically pure dienes or polyenes. ... 

MDA reaction of a triene generated in situ by thermally induced cheletropic elimination of sulfur dioxide from a substituted 2,5-dihydrothiophene S,5-dioxide has been described18. 

Octahydrophenanthrenes have also been synthesized via IMDA reactions of o-quinodimethane species generated by cheletropic elimination of sulfur dioxide from 1,3-dihy-drobenzo[c]thiophene 2,2-dioxides28,19. 

Scheme 8. First example of cheletropic elimination of SO2 for o-quinodimethane generation (a), and application of the strategy as the key step in Nicolaou s total synthesis of estra-1,3,5(10)-trien-17-one (b).

In general, SO2 extrusion from 3,4-disubstituted sulfolenes is straightforward and there are no stereochemical implications. Alternative methods for SO2 extrusion have been reported. Examples of some sulfolenes which underwent reductive desulfonylation in LiAlH4 are shown in Scheme 6.58 [57]. It was postulated that the cheletropic elimination of SO2 proceeds via thermal cleavage, with LiAlH4... 

Cheletropic elimination of SO2 in a 4n system invoives disrotatory mode... 

Sol 1. (a) On heating, cw-sulfone undergoes cheletropic elimination of SO2 to give stereospecihcally the (2 ,4 )-hexa-2,4-diene. 

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