Episulfone intermediate

The Michael induced Ramberg-Backlund reactions gives a mixture of ( )- and (Zy dienes via extrusion of S02 from the episulfone intermediate 57 (equation 55)47. 

Treatment of a-halosulfones 357 bearing a -hydrogens with base gives olefins by extruding SO2 from the episulfone intermediates 359. The reaction was found by Ramberg and Backlund in 1940 and named after them, and it has been very useful for the preparation of tailor-made olefins. The reaction has been investigated in detail " and utilized widely for olefin syntheses. An excellent review on this reaction by Paquett covers the literature up to 1975, so only recent studies are dealt with in this section. The generally accepted mechanistic path is shown below. 

Finally, the most significant mechanistic feature of the Ramberg-Backlund rearrangement is the stereoselective formation of ds-olefin products, as a result of the preferential cis-positioning of the pair of R groups in the episulfone-forming transition state, variously attributed to London forces , to diastereoselectivity in carbanion formation and to steric attraction . However, with the use of stronger bases such as potassium t-butoxide °, the trans-olefin predominates (equation 52), apparently due to prior epimerization of the kinetically favoured cts-episulfone, and subsequent loss of the sulfur dioxide. Similarly, when the episulfone intermediates possess unusually acidic... 

S02. There is much evidence for this mechanism,311 including the isolation of the episulfone intermediate,312 and the preparation of episulfones in other ways and the demonstration that they give olefins under the reaction conditions faster than the corresponding a-halo sulfones.313 Episulfones synthesized in other ways (e.g., 6-62) are reasonably stable compounds but eliminate S02 to give olefins when heated or treated with base. 

The Ramberg-BScklund reaction (B-77MI50600) of a-halosulfones with bases goes via an episulfone intermediate (e.g. Scheme 11). The reaction with a.a -dihalosulfoxides or o ,a -dihalosulfones gives good yields of thiirene 1-oxides and thiirene 1,1-dioxides (Scheme 134) (79JA390, 7008(50)65). A variation is the reaction of an a,o ,a , a -tetrabromosulfone with phosphines which also yields thiirene 1,1-dioxides (74JOC2320>. 

Sutherland, A. G., Taylor, R. J. K. The first Isolation of an episulfone Intermediate from a Ramberg-Backlund reaction. Tetrahedron Lett. 1989, 30, 3267-3270. 

To date, there is no report on the isolation of an episulfone intermediate in the Ramberg-Backlund reaction. Taylor and Sutherland have recently succeeded in the isolation of an episulfone50. Thus, when iodo-sulfone 89 was treated with potassium t-butoxide, episulfone 90 was obtained as a white crystalline solid in 69% yield as illustrated in equation 48. Treatment of 90 with excess potassium t-butoxide at — 20 °C to room... 

The vinylogous RBR is a variation whereby either a vinyl(a-bromoallq l)sulfone or a bronioallyl(allq l)sulfone is treated with base, delivering, via a vinyl-substituted episulfone intermediate, a 1,3-diene tScheme... 

The Michael-induced RBR exploits the electrophilic nature of vinyl-substituted sulfones. Upon nucleophilic attack, the alkene double bond migrates to form an episulfone intermediate by displacement of an a-halide fScheme R.12T The product alkene, resulting from subsequent SO2 extrusion, is transposed by one position from the original alkene. The a-halide may... 

The (Z)-alkene selectivity seen in the RBR product 56 is unusual for stilbenes, in that isomerization of the relatively acidic episulfone intermediate t5 ically ensures that the (E)-isomers exclusively prevail (see Section 8.41. The reversal of alkene stereoselectivity in this case has been proposed to result from intramolecular promotion of the sulfur dioxide extrusion from the episulfone by the adjacent alkoxide. ... 

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