Sulfur dioxide electrophiles

The above-mentioned trends indicate an electrophilic nature of the insertion of SO2 into metal-carbon bonds. Furthermore, the reactivity dependence on the steric properties of R is consistent with a backside approach of the sulfur dioxide electrophile (e.g., 78). 

Two-component methods represent the most widely applied principles in sulfone syntheses, including C—S bond formation between carbon and RSOz species of nucleophilic, radical or electrophilic character as well as oxidations of thioethers or sulfoxides, and cheletropic reactions of sulfur dioxide. Three-component methods use sulfur dioxide as a binding link in order to connect two carbons by a radical or polar route, or use sulfur trioxide as an electrophilic condensation agent to combine two hydrocarbon moieties by a sulfonyl bridge with elimination of water. 

Beside these free radical reactions of sulfur dioxide, its electrophilic reactions generating sulfinates with organometallic compounds453,454 or sulfinic acids with arenes under Friedel-Crafts conditions455 are well known. To complete these three-component syntheses, the sulfinates prepared first are transformed to sulfones by reactions with appropriate electrophiles, discussed earlier in this chapter, i.e. equation 82. 

As formal a, /i-unsaturated sulfones and sulfoxides, respectively, both thiirene dioxides (19) and thiirene oxides (18) should be capable, in principle, of undergoing cycloaddition reactions with either electron-rich olefins or serving as electrophilic dipolarophiles in 2 + 3 cycloadditions. The ultimate products in such cycloadditions are expected to be a consequence of rearrangements of the initially formed cycloadducts, and/or loss of sulfur dioxide (or sulfur monoxide) following the cycloaddition step, depending on the particular reaction conditions. The relative ease of the cycloaddition should provide some indication concerning the extent of the aromaticity in these systems2. 

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). 

The mechanism and synthetic scope of reactions of monohaptopropargyl-iron complexes has been described earlier (see Schemes 30-33 in Section IV,A,2). By using sulfur dioxide and sulfur trioxide as electrophilic reagents, it is possible to synthesize metal-containing heterocycles in the reduced 1,2-oxathiole category (sultines and sultones) (Scheme 119).182-184 An... 

Indeed, where reactions at a ring carbon take place under relatively mild conditions, special circumstances are at work. For example, 2,6-tert-butylpyridine combines with sulfur trioxide in liquid sulfur dioxide at -10 C to give the corresponding 3-sulfonic acid (Scheme 2.3). An explanation is that the bulky tert-butyl groups prevent access of the large electrophile to N-1. Steric hindrance is much less at C-3 and sulfonation is diverted to this site using the free pyridine as the substrate. 

Reactions of trifluorovinyllithium with a variety of electrophiles such as proton, halogen, trialkylsilyl chloride, trialkyltin chloride, methyl iodide, carbon dioxide, sulfur dioxide afforded the corresponding trifluorovinylated derivatives [111, 113,114] (Scheme41). 

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 higher homologue of 3-sulfolene, 2,7-dihydrothiepin-l,1-dioxide could be similarly metallated at low temperatures. Reaction with reactive electrophiles followed by extrusion of sulfur dioxide led [549] to the corresponding hexatriene derivatives, as in the stereoselective synthesis of (Z,E)-l,3,5-decatriene (considered as an undecatriene in the publication) shown here. 

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... 

A carboxyl group usually hinders electrophilic attack. It is, therefore, at first sight surprising to note that picolinic acid hydrochloride reacts with thionyl chloride to give 4-chloropicolinic acid in yields of up to 55%.182-184 Under more severe conditions a 35% yield of 4,6-dichloropicolinic acid is obtained.182 Sulfur dioxide in the reaction mixture favors 4-chloropicolinic acid formation.183 This suggests that the chlorination may actually involve a nucleophilic attack by chloride ion upon a complexed pyridinium salt1 ... 

Protonation of the unstable intermediate (by the HC1 just produced) gives an electrophile powerful enough to react even with the weak nucleophile Cl" (low pi Hi poor nucleophilicity). The tetrahedral intermediate that results can collapse to the acyl chloride, sulfur dioxide, and hydrogen chloride. This step is irreversible because SC)2 and HCl are gases that are lost from the reaction mixture. 

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