Electrophilic additions sulfur dioxide

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

Oxathiolane 3,3-dioxide (332) metallates in its 2-position to yield an anion which reacts with various electrophiles (alkyl halides and carbonyl compounds) to give substituted oxathiolanes (333) in good to excellent yield (79TL3375). Pyrolysis of these alkylated products affords the corresponding aldehydes or 2-hydroxyaldehydes in addition to sulfur dioxide and isobutylene (Scheme 71). The oxathiolane (332) thus becomes another member of the already burgeoning class of carbonyl anion equivalents. 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

An anomalous cycloaddition is the insertion of a carbene into an alkene. Some cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can illustrate with the drawing 6.127 for the suprafacial addition of sulfur dioxide to a diene, and with the drawing 6.128 for the 8-electron antarafacial addition of sulfur dioxide to a triene. The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial on the alkene with singlet electrophilic carbenes [see (Section 4.6.2) page 149]. This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.129. 

For several years, research conducted at California State University Fresno has focused on the utilization of carbon monoxide as an alternative to sulfur dioxide and other preservatives and sterilants as well (Muller et al., 1996). Carbon monoxide shares several of its chemical attributes with sulfur dioxide. Carbon monoxide has an electron deficiency on the carbon atom similar to the electron deficiency on sulfur in SOg. Additionally, the carbon has a pair of electrons available which can attack any Lewis acid and/or electrophile. Thus, CO can act very much like SOg and, in addition, has the properties of a strong electrophile. Further, once a covalent bond has been formed between carbon monoxide and one of its target molecules, it is usually a carbon-carbon bond which by its nature is nonpolar and thus difficult to hydrolyze. By comparison, carbon-sulfur bonds are polar and thus easily hydrolyzable. Compared with SO2, no carbon monoxide can be freed by hydrolysis (in either strong acid or base) from its... 

Numerous studies have been devoted to the addition of RSO2X to olefins. The propagation steps for these chain processes are shown in Scheme 13. The main factors controlling the reactivity of sulfonyl radicals are polar effects and the reversibility of the addition. Sulfonyl radicals exhibit an electrophilic character with respect to addition to olefins [98]. Evidence for the reversibility came early from the observation of the isomerization of cis- and rran -2-butenes during their copolymerization with sulfur dioxide and their Cu(I)-mediated reaction with benzenesulfonyl chloride [99]. Therefore, the chain transfer of alky] adduct radica]s has to compete effectively with -elimination of sulfonyl radicals (see Scheme 13). Selenosulfonates, sulfonyl halides, and sulfonyl cyanides are the most popular substrates that meet this property. 

In summary, sulfonamides are most commonly prepared by the reaction of amines with sulfonyl halides. Aryl sulfonyl chlorides may be accessed from C-H bonds by chlorosulfonylation, from C-S bonds by oxidation, from C-N bonds by diazotization, or from C-X bonds by metalation. Approaches to all l sulfonamides are more limited as they are typically prepared by either oxidative chlorination of thiols or addition of organometallic nucleophiles to sulfur electrophiles. Traditional sulfonamide preparation has frequently necessitated harsh reagents and conditions, but the development of Pd-catalysed approaches and discovery of new sulfur dioxide sources allow for operationally simple sulfonamide synthesis under mild conditions. Future directions in sulfonamide synthesis will likely involve the direct C-H installation of sulfonamides without the use of hazardous reagents. 

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