Elimination, chelotropic

The chelotropic elimination of nitrous oxide from N-nitroso-aziridines is a reaction restricted to this class of compounds... 

Heterocyclic analogs of ortho-quinodimethane have become increasingly popular for Diels-Alder functionalization of C60. Several methods for generating these highly reactive species have been developed however, most attention has been paid to their generation via chelotropic elimination of S02 from het-... 

Electron-deficient (i.e., acyl and sulfonyl) thionitroso compounds have also been prepared and can be used as precursors for 1,2-thiazines <86JCS(Pi)245>. The thiophene ylides (172) react with electron-rich alkenes such as acenaphthylene, and the resulting [4 + 2] adducts undergo a concomitant chelotropic elimination to afford the corresponding acyl or sulfonylthionitroso compounds... 

The third methodology leading to the formation of aromatic compounds is domino [4+2] cycloaddition reaction followed by the chelotropic elimination from the cycloadduct (Scheme 16.18). The intermediate bridged bicyclic cycloadduets are strained, and they undergo elimination leading to aromatization usually under the reaction conditions of the Diels-Alder reaction. 

SCHEME 16.1 Domino [4+2] cycloaddition/chelotropic elimination to aromatics. 

An alkyne may also be used in place of the diene component. With the cyclic sulfone complex 11.174a, this leads to an interesting synthesis of cyclooctatetraenes as the initial cycloadduct 111.76 is photochemically unstable and readily loses SO2 (Scheme 11.59). The adduct may be isolated if uranium glass filters are employed to exclude the high-energy wavelengths. Subsequent photolysis with the more transparent vycor filters causes chelotropic elimination of sulfur dioxide to give the tetraene 11.177. 

Oxidation to the sulfone followed by the Ramberg-Backlund reaction (halogenation, intramolecular displacement, chelotropic elimination of SO ) then dehvered 14, which was selectively reduced, leading to 15. 

Scheme 4.2 Diels-Alder cycloaddition of tetrachlorothiophene dioxide (4) and /V-ethyI maleimide (5) yields a high energy intermediate (6) that decomposes spontaneously by elimination of sulfur dioxide. The initially formed product is subsequently oxidized in situ. Transition states for cycloaddition and chelotropic S02 elimination closely resemble the hexachloronorbornene derivative 3 used as a hapten to elicit antibody 1 E9.

Extrusion (or elimination) of sulfur from thiiranes and thiirenes is a facile process. Virtually all thiiranes and thiirenes, as well as their V-oxides and V,V-dioxides, undergo thermal extrusion of the sulfur moiety with increasing facility according to the trend S << SO < S02. Sulfur monoxide produced in this way can be utilized in cycloaddition processes, for example, from 2 in refluxing toluene with 2,3-dimethyl-l,3-butadiene in a chelotropic addition <1997JOC8366>. 

The last procedure is particularly noteworthy beirause of the mild reaction conditions, which are compatible with various functional groups. The chelotropic, thermal elimination of sulfur dioxide, (547) (542), one of the oldest routes to o-quinodimethanes. remains attractive owing to the... 

Three main strategies have been employed for the prodnction of heterocyclic orffto-quinodimethanes a 1,4-elimination, the chelotropic loss of snlfur dioxide from a 2,5-dihydrothiophene S. iS -dioxide and the electrocyclic ring opening of a cyclobnteno-heterocycle each of these is illustrated diagramatically below. 

Additions to Electron-deficient Species. Diazomethane will also add to highly electrophilic species such as sulfenes or im-minium salts to give the corresponding three-memheredring heterocycles. When the reaction is performed on sulfenes, the products are episulfones which are intermediates in the Ramherg-Backlund rearrangement, and are therefore precursors for the s)mthesis of alkenes via chelotropic extrusion of SO2. The sulfenes are typically prepared in situ by treatment of a sulfonyl chloride with a mild base, such as Triethylamine (eq 47). Similarly, the addition of diazomethane to imminium salts has been used to methyle-nate carbonyls. In this case, the intermediate aziridinium salt is treated with a strong base, such as Butyllithium, in order to induce elimination (eq 48). 

For the latter reaetion, a mechanism involving a thermally allowed (2+4) cycloaddition followed by electron transfer (oxidation of Mn to Mn ) and also thermally allowed chelotropic (2+2+2) elimination has been suggested ... 

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