Radical phenylsulfonyl

Remarkable solvent effects on the selective bond cleavage are observed in the reductive elimination of cis-stilbene episulfone by complex metal hydrides. When diethyl ether or [bis(2-methoxyethyl)]ether is used as the solvent, dibenzyl sulfone is formed along with cis-stilbene. However, no dibenzyl sulfone is produced when cis-stilbene episulfone is treated with lithium aluminum hydride in tetrahydrofuran at room temperature (equation 42). Elimination of phenylsulfonyl group by tri-n-butyltin hydride proceeds by a radical chain mechanism (equations 43 and 44). 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

There is clear evidence that the addition of a sulfonyl radical to an olefin is a reversible reaction. This has been demonstrated for the case of but-2-ene, where cis-trans isomerization of the olefin accompanies addition63,64. Furthermore, 18a and 18b radicals generated by the reaction of Bu3Sn with erythro- and tfireo-2-bromo-3-(phenylsulfonyl)butane, respectively, eliminate benzenesulfonyl radicals to some extent, even at — 67 °C, to form 2-butenes in a non-stereospecific manner, providing also indication that the rotation about the central C—C bond is faster than the /1-cleavage process65. 

Radical allylation of B-alkylcatecholboranes using easily available allyl-sulfones has been described [109-111]. By using phenylsulfones, the fragmentation produces a stable phenylsulfonyl radical that reacts with B-alkyl-... 

Thermal intramolecular [2 + 2]-cydoadditions of phenylsulfonyl-substituted allenes 33 gave 34 stereoselectively. An initial carbon-carbon bond formation occurred at the central allenic carbon and the proximal olefmic carbon. The resulting non-allylic radical 35 is unstable and cyclizes rapidly which may account for the high stereoselectivity [30]. 

It should be noted that addition of the tributyltin radical to 1-fluoro-1-(phenylsulfonyl)ethene provides phenyl vinyl sulfone as the only isolated product. However, 2-trimethylsilyl-1-fluoro-1-(phenylsulfonyl)ethene reacts with tributyltin hydride in the presence of AIBN to provide (E)-2-trimethylsilyl-1-fluoro-1-tributylvinylstannane. The vinylstannane is an equivalent for the synthon "H2C=CF " providing a convenient route to 2-fluoro-1-alkenes.8 The trimethylsilyl group can be removed with potassium fluoride in dimethyl sulfoxide-water or oxalic acid-methanol at the end of the reaction sequence. 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

Recently, Kim and colleagues have described a new efficient method for the preparation of a-keto esters 48 via a free-radical acylation approach using (phenylsulfonyl) methoxycarbonyl oxime ether 46 as carbonyl equivalent radical acceptor (Scheme 28). The oxime 46 was conveniently prepared from readily available methylphenylsulfonyl acetate 44 by a two-step sequence (via oxime 45) as shown in Scheme 28. Nitrosation of 44 with isoamyl nitrite in the presence of sodium methoxide gave oxime 47 in 78% yield. 

This approach involves additions of alkyl radicals to C=N bonds and snbseqnent fi-expulsion of the phenylsulfonyl group (as a radical) which react with bis(trialkyl)tin to propagate a chain. The phenylsulfonyl group lowers the energy of the LUMO of the radical acceptor, increasing the rate of addition of alkyl radicals . 

Recent advances in radical reactions have greatly benefited from the efficiency of organotin reagents as mediators. Radical reaction of alkyl iodides with trifluoromethyl phenylsulfonyl oxime ether 69 and hexamethylditin at 300 nm in benzene afforded the corresponding trifluoromethyl oxime ethers 70 in high yields (Scheme 37) . 

Among the many variants that add two heteroatoms, those that add phenylsulfonyl groups are particularly useful because they proceed in high yield under mild conditions and provide functionality for subsequent synthetic transformations. The examples contained in Scheme 88 illustrate some recent applications.230-232 In contrast to most radical additions to alkynes, the additions of ArS X are often highly stereoselective. 

The combination of carbon monoxide with sulfonyl oxime ethers allow for a set of multicomponent coupling reactions involving consecutive Cl/Cl-type coupling, a rare class of radical multicomponent reactions. In Scheme 6.27, examples of three-, four-, and five-component coupling reactions are shown [46], In these reactions, allyltin is not incorporated into the product, but serves as an acceptor of the phenylsulfonyl radical and a source of the tributyltin radical, which delivers the radical chain. 

Although one can equally well envision a mechanism whereby a tin radical adds to the carbonyl oxygen to furnish a ketyl radical, this intermediate would have to eliminate a phenylsulfonyl radical and form... 

We conceived that, if the alcohol was to be deoxygenated to a radical, an olefinic linkage would be formed by -elimination of a phenylsulfonyl radical. The xanthate function is a convenient radical source and gave the olefin in about 80% yield in toluene under reflux using AIBN or benzoyl peroxide as initiators and Ph2SiH2.22... 

A large amount of woik has been accomplished using mixed sulfiir/selenium reagents, such as seleno-sulfides and selenosulfones. One example of selenosulfide addition via radicals is selenotbiolactoniza-tion. When selenosulfide (15) is treated with AIBN, a mixture of y-seleno-substituted thiolactones is produced (equation 9). Although these lactonizations result in mixtures of stereoisomers, they usually can be separated. In addition to these reports, selenosulfones have been used to form allenic sulfones, alkynic sulfones and (phenylsulfonyl)(Uenes (S< me4). ... 

Kim and Kim [14] have reported the addition of alkyl radicals derived from alkyl iodides to phenylsulfonyl oxime ether 107 attached to Wang resin (Scheme 22). Treatment of 107 with a range of alkyl iodides and hexam-ethylditin with irradiation at 300 nm gave the expected adducts such as 109, in moderate to good yield after cleavage from the solid support. 

The authors also investigated the feasibility of a radical cyclization-capture sequence [14] using immobilized phenylsulfonyl oxime ether 107 and iodide 110 (Scheme 23). Upon treatment with hexamethylditin, 110 undergoes a sequence of two 5-exo cyclizations followed by capture of the resultant radical by resin 107. Cleavage from the support gave 111 in moderate yield. 

Scheme 22 Alkyl radical additions to phenylsulfonyl oxime ethers by Kim and Kim [ 14]...

The hydroboration of acetylenes (3) with diisopinocampheylborane (IpC)2BH in THF led after refunctionalisation and transesterification to the olefins (4a, b, c) isolated in good yields. Monooxidation with mCpBA led to the sulfoxide (4d) whereas the sulfone (4e) was obtained with two equivalents of mCpBA. The same sulfone (4e) could also be obtained in an excellent overall yield by radical addition of phenylsulfonyl iodide to the pinacol ester of vinylboronic acid followed by a dehydroiodination in the presence of Et2N (87 % overall yield). The carboxylic ester (4a) could be transformed into the corresponding carboxylic acid (4f) (79 % yield) 11 which led to the acid chloride (4g) by treatment with freshly distilled thionyl chloride at 0°C (91 % yield), p-keto vinylboronates are easily accessible by oxidation of the corresponding protected allylic alcohol according to the following scheme ... 

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