Sulfonylation, alkyl radicals

This sequence of formation of radical cation which is followed by a C—S bond scission into alkyl radical and alkyl sulfonyl cation was previously suggested by the same authors for the radiolysis of polyfolefin sulfonefs in the solid state72 and was confirmed by scavenger studies73. Scavengers are ineffective in crystalline solids such as dialkyl sulfones and hence could not be used in this study. 

The C—S bond in the sulfonyl radical RS02 is weak and therefore the reaction of the alkyl radical with the sulfonyl radical is reversible. The decay of the sulfonyl radical is an endothermic reaction. This peculiarity explains the existence of the optimal temperature for sulfoxidation. The increase in temperature lowers the steady-state concentration of sulfonyl radicals and, therefore, increases the chain termination by the reaction of the alkyl radical with dioxygen. 

The sulfonyl radical is unstable and dissociates via C—S bond back to the alkyl radical and sulfur dioxide. The rate constant of this reaction for the cyclohexylsulfonyl radical was calculated from the kinetic data on the chain decomposition of cyclohexylsulfonyl chloride [2]. This decay of cyclohexylsulfonyl chloride initiated by DCHP occurs according to the following chain mechanism [29,31] ... 

The desired products were obtained in satisfactory to excellent yields by using only 1.2 equivalents of the allylsulfones with primary, secondary and tertiary alkyl radicals. Many different types of allylic sulfones bearing an ester group, a sulfonyl group, and a bromine atom react equally well (Scheme 46). The whole transformation represents formally a reductive allylation or hy-droallylation of alkenes. 

These two components of the ESR spectrum may be attributed to an alkyl radical and a sulfonyl radical. The presence of two radicals can be confirmed by power saturation experiments. The spectrum at high microwave power shows increased intensities for the lines due to the alkyl radical and... 

An intramolecular 2-alkylation was also observed in a sulfonyl free radical induced addition-cyclization <95SL763>. A key intermediate in a new synthesis of pallescensin A (a biologically active labdane diterpene) was prepared by a cationic cyclization reaction with a furan <95SYN1141>. The sonochemical Barbier reaction was extended to carboxylate salts. 2-Furanylketones 10 can be obtained by sonication of a mixture of furan, lithium carboxylate, an alkylchloride, and lithium in THF <95JOC8>. 

RSOO are generated by the hydrogen abstraction from thiols by OH radicals, alcohol radicals, etc., producing thiyl radicals (RS ) which on reaction with oxygen produce RSOO [Eqs. (34) and (35)]. Alternatively, reaction of alkylsulfonyl chloride with e q produces alkyl sulfonyl peroxyl radicals [Eq. (36)]. 

It is noteworthy that this Cl reagent can be applied to a wide range of organic radicals involving rather stable radicals such as a-keto radical, a-alkoxyalkyl radical, and the benzyl radical for which the aforementioned radical formylation system with CO cannot be applied. The high reactivity of phenyl sulfonyl oxime ether is supported by kinetic studies [29]. The approximate rate constants for the addition of a primary alkyl radical to this phenyloxime ether was determined to be k=9.6xl0 M s at 25 °C, which is 1.8 times faster than the addition to aery-... 

In similar fashion to the decarboxylative sulfonylation described above, the trapping of the intermediate alkyl radical by an electron-withdrawing isocyanide is a kinetically favorable process, and hydrolysis of the resultant adduct can then lead to the structure of the original carboxylic acid, thereby providing a method for the preparation of labeled carboxylic acids as indicated [25]. A disadvantage of this... 

Generation of non-stabilized alkyl radicals. Malacria has developed several new cascade reactions taking advantage of selective hydrogen atom abstractions by alkenyl radicals [145]. Recently, he achieved the preparation of a linear triquinane from an acyclic precursor. The last steps of the reaction sequence are a hydrogen abstraction by a vinyl radical at the /i-position of the sulfone followed by the elimination of a sulfonyl radical, affording the final product in 50% yield (Scheme 44)... 

The sulfonylation of alkyl radicals plays an important role in the copolymerization of olefins with sulfur dioxide [53a]. In a recent study, Shevlin [53b] has shown that the cyclooligomerization of diallyl malonate could be controlled by the addition of a chain transfer agent such as PhSH (Eq. 22). 

C. Ollivier and P. Renaud, A novel approach for the formation of carbo-nitrogen bonds Azidation of alkyl radicals with sulfonyl azides, J. Am. Chem. Soc., 123 (2001) 4717-4727. 

Essentially, the process as developed by the Esso Company was the passing of sulfur dioxide and oxygen through various straight chain hydrocarbons in the presence of a source of y radiation. Alkyl radicals produced by irradiation of the hydrocarbon add sulfur dioxide, forming an alkyl sulfonyl radical ... 

Direct addition of alkyl radicals to sulfur dioxide usually results in sulfonyl RSO2 rather than oxysulfinyl ROSO radicals in spite of the fact that the latter products are more stable thermochemically [43,44]. The explanation for this is a substantial activation energy (19.9 kcal mol" ) required for oxygen addition [43] while addition at the sulfur atom requires almost no energy of activation [43-45]. For this reason oxygen-addition to sulfur dioxide does not normally compete with the majority of other free radical processes (see below). Experimentally both processes have been observed during the reaction of gaseous sulfur dioxide with several types of carbon-based radicals chemically anchored to the silica surface [44] ... 

As exemplified in the following cases, this addition reaction has often been used as the first step in the preparation of vinyl sulfones. The adduct must therefore undergo a subsequent elimination reaction. A base is needed to perform dehydrohalogenation. The selenides resulting from the addition of selenosul-fonates can be eliminated very smoothly through oxidation. Thus, Yus and co-workers [104] have based a very simple method for the preparation of j9-sulfonyl-a,j9-unsaturated compounds on the addition of tosyl iodide to Michael acceptors (equation (50)) whereas Barton et al. [105] have shown that the addition of phenylseienotosylate to electron-rich olefins, catalyzed by Ru(II) complexes, is efficient (equation (51)). An alternative approach is the Ce(IV)-mediated addition of arylsulfinates to electron-rich olefins which proceeded directly to the olefinic product resulting from oxidative elimination of the intermediate alkyl radical (equation (52)) [106]. 

In an early experiment, Abramovitch and Breslow observed the formation of traces of alkyl azides during Curtius-type rearrangement of sulfonyl azides. This formation was rationalized by a direct reaction between alkyl radicals and sulfonyl azides. Roberts examined the reaction of aryl and alkyl sulfonyl azides with allylstarmanes in order to... 

Ueno and coworkers10 have found that the facile displacement of sulfonyl group from a-alkylated allyl p-tolyl sulfones 18 by tri-n-butyltin radical in the presence of 2,2 -azobis[2-methylpropanenitrile] (AIBN) occurs smoothly in refluxing benzene (equation 11). In contrast, vinyl sulfones undergo the radical substitution reaction to give vinylstannanes in the presence of AIBN at a higher temperature11. 

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