Sulfur trioxide 4+2 cycloaddition reactions

Phenyliodonium sulfate 289, which is obtained from the reaction of iodo-sobenzene and sulfur trioxide at -50°C, behaves as a 1,4-dipole in the reactions with several alkenes (86ZOR450) to give intermediate [4 + 2] cycloadduct 290. Decomposition of this unstable adduct affords cyclic sulfates 291 (Scheme 73). Likewise, another 1,4-dipole 292 undergoes cycloaddition reaction to give cyclic sulfone 293 (88ZOR888). The 1,4-dipole 292 thermally decomposes to give 294 (86DOK1374) (Scheme 74). 

Sulfur trioxide is a strong Lewis acid comparable in strength with such acids as GaCl3, BC13j and, according to some data, even SbCl5 [4]. Non-catalysed reaction of S03 with terminal polyfluoroalkenes results in formation of /J-sultones, which is currently viewed as a concerted electrophilic 2 + 2 cycloaddition reaction [4], This process has been intensively studied for over 30 years and data on this reaction has been reviewed [173]). 

The only methodology pertinent to the formation of 1,2- and 1,3-oxathietanes and their oxidized forms involving S-C and O-C bond formations is that for the preparation of /3-sultones. It is now well established that [7t2s+Jt2s] cycloaddition of sulfur trioxide with an alkene generates /3-sultones in moderate to excellent yields (Equation 1). Indeed, the previous version of this chapter detailed numerous examples of the formation of /3-sultones (especially fluorinated compounds) by the reaction of SO3 with an appropriate alkene at elevated temperatures <1996CHEC-II(1B)1083>. 

Sultones such as 2 are formed in high yield from the cycloaddition reaction of sulfur trioxide and polyfluorinated alkenes. e.g. 

A reaction closely related to the [2 + 2] cycloaddition of thiocarbonyl S.S-dioxides is the addition of sulfur trioxide-dioxane complex to alkenes. This occurs stereoselectively and from (Z)- and (E)-2-butene the corresponding cis- and /ra .s -3,4-dimethyl-1,2-oxathietane 1,1-dioxides 11 are obtained121. 

Methyl N-sulfonylurethane (2) undergoes [2 + 2] cycloaddition to certain hetero-atomic tr-systems to give intermediates which spontaneously lose sulfur trioxide, in an overall quasi -Wiltig reaction. Thus diphenylcyclopropenone (6) reacts with (2) in THF at 30 to give (7) in quantitative yield. 

Chlorosulfonyl isocyanate6 (CSI) (57) is a very reactive compound which was first prepared from cyanogen chloride (56) and sulfur trioxide (Graf, 1956) (Scheme 31). CSI (57) undergoes [2+2] cycloaddition reactions with various alkenes initial adducts like (58) and (59) may be hydrolysed to the b-lactams (60) and (61). The cycloaddition reaction also occurs with enamines (62) to give products like (63) and (64) (Scheme 32). 

The reaction of olefins with SO3 is exothermic, and often only secondary reaction products are isolated. Attempts to control the cycloaddition reaction involve the use of dioxane- or pyridine-S03 complexes. However, to generate the monomer from the cyclic trimer, freshly distilled SO3 at low temperatures is more effective. Also, oleum seem to have an advantage over the pure sulfur trioxide in [2+2] cycloaddition reactions. ... 

Across other double bonds In the reaction of fluorosulfonyl-Af-sulfinylamine with sulfur trioxide an initial reaction across the N=S bond occurs to form the [2+2] cyeloadduet 67, which disproportionates to give sulfur dioxide and FS02N=S02. The latter dimerizes via a [2+2] cycloaddition across the S=N bond to give a symmetrical dimer 68. 

Beware though that 74 on treatment with sulfur trioxide at room temperature explodes A criss-cross [3+2] cycloaddition to give l,2,3-oxathiazolo[4,5-d][l,2,3]oxathiazole-2,2,5,5-tetroxide 77 in 83 % yield is observed in the reaction of sulfur trioxide with dicyanogen 76... 

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