Chelotropic extrusion

Hoffmann and Schiittler (75CB844) attempted to generate methanoylcarbene by high-temperature chelotropic extrusion from a bicycloheptadiene derivative, but the only identified product was a very low yield of benzene. 

Hoffmann and Schiittler showed by C labelling (75CB844) that (81) generated by chelotropic extrusion from a bicycloheptadiene derivative did not isomerize to oxirene. This is as expected from MO calculations (Section 5.05.6.2). 

Scheme 3. The highly-ordered transition state associated with the Diels-Alder reaction has made it a natural target for work in the field of catalytic antibodies [15]. Indeed, Hilvert [15a] has reported recently the first successfully antibody-catalyzed Diels-Alder reaction. Monoclonal antibodies elicited to the boxed hapten 14 provided both acceleration of and multiple turnover of the cycloaddition between tetrachlorothiopene dioxide and IV-ethyl maleimide. The initial adduct decomposes by the chelotropic-extrusion of sulfur dioxide affording the dihydrophthalimide derivative 13...

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). 

Thus, ergocalciferol is reacted with sulfur dioxide at -10 °C, and then the hydroxy-group is sUylated. The chelotropic extrusion of sulfur dioxide produces the (5 ,7 )-isomer. Allylic hydroxylation by Barton s method with selenium dioxide and JV-methylmorpholine JV-oxide gives a mixture of (IS)- and (lH)-isomers in the ratio of 7 1. SUylation of the hydroxy-group at the 1-position and crystallisation lead finally to the (lS,5 )-hydroxy-derivative in pure form. The yield over all the reaction stages is around 28-35 %. [102,103]... 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

Finally, it should be of interest that the rearrangements of several cyclic benzylic sulfinates have also been described in the literature by Durst46 and Hogeveen47, and seem to proceed by a special two-step mechanism retro-Diels-Alder extrusion of S02, followed by its chelotropic addition to the unstable quinodimethane intermediate (e.g. equation 10). 

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>. 

An alternative synthetic strategy was used involving the use of a masked diene as a sulfolene intermediate that, when heated, undergoes tandem chelotropic SO2 extrusion followed by a Diels-Alder cyclization (Scheme 20). The final result is the construction of a C-5 epimer of the... 

Quantitative M.O. models were used in a discussion of the photoelectron spectrum of thiiran 1,1-dioxide. As compared with dimethyl sulphone, all antibonding S—C cr-orbitals increase in energy and bonding S—C a-orbitals decrease in energy because of the relative compression of the S—C bonds in the cyclic sulphone. The chelotropic reactions of thiiran 1,1-dioxide have been compared with the formation of alkene-oxygen Tr-complexes. Activation parameters for the extrusion of sulphur dioxide from thiiran 1,1-dioxide have been obtained. ... 

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