Thiophenoxy radical

The instability of the 2,4,6-triphenyl-thiophenoxy radical is probably not due to stereochemical reasons but to the fact that the preferred delocalization of the sulfur unpaired electron into the aryl-system which requires a partial C—S double bond is unfavored. The S—S-dimerization giving 9 is, therefore, the prefered reaction. 

Dehydrogenation of thiophenols with TPPO gives disulfides. Thiophenoxy radicals, even in the case of 2,4,6-triphenylthiophenol, have not been observed by ESR spectroscopy 145 >. 

No successful attempts to observe the spectrum of the thiophenoxy radical or its unhindered substituted analogues have been reported in solution studies. Smentowski (1963) found that the p-chlorobenzene-thiolate anion reacts with nitrosobenzene to give the spectrum of the latter s radical-anion (flow system) and a high yield of the disulphide thep-chlorothiophenoxy radical is apparently formed as an intermediate but dimerizes too rapidly for spectroscopic detection. The spectra of both aromatic and aliphatic thiol radicals have, however, been observed when the species, generated by ultraviolet irradiation of the corresponding disulphides, are trapped in the solid state (Smissman and Sorensen, 1965 Windle et al., 1964). 

In 1972, Lewis and Winstein reported that the reaction of a,a-dimethylallyl phenyl sulfide (1) with thiophenol in the presence of tert-butyl hydroperoxide gave the isomeric compound 7,7-dimethylallyl phenyl sulfide (3) (Scheme 1) [1]. It was proposed that this reaction occurred by addition of thiophenoxy radical to the terminal end of the alkene to produce radical intermediate 2. This radical then underwent y9-scission with loss of the tertiary thiophenoxy group to form the rearranged alkene 3. 

Displacements by thiophenoxide ion have an interesting possibility - the nucleophile can attack one electron at a time, transferring an electron to produce a thiophenoxy radical while reductively cleaving the electrophile to form an alkyl radical. Then the two radicals, in a solvent cage, can couple (Fig. 1.23). In an exploration of this process, called the SET mechanism, we used thiophenoxide with the sodium salt of p-carboxybenzyl iodide, and with the corresponding mesylate. We saw that there was a large acceleration by added ethanol in the iodide case, but not with the mesylate. We proposed that in the iodide displacement this reflected the conversion of thiophenoxide ion, with its delocalized charge, into the much more hydrophobic thiophenoxy radical at the transition state. Other evidence as well supported the SET mechanism. The carbon-iodine bond is more easily reductively cleaved than is the carbon-mesylate bond. 

In spite of the low regioselectivity observed in Eq. (3), we were able to develop a number of pyrrolizidine alkaloid syntheses through rational modifications of the cyclization substrate. Our first examples of pyrrolizidine alkaloid syntheses are outlined in Scheme 1 [7]. Imide 7 was prepared from succinimide and 3-butyn-l-ol in 6 steps. Reduction of 7 followed by hydroxy-thiophenoxy exchange gave cyclization substrate 8. Treatment of 8 with TBTH gave a 71% isolated yield of pyrrolizi-dinone 9, which was converted to isoretronecanol in 4 steps. The radical cyclization step also gave small amounts of the C] isomer of 9, reduction product 10 (5%), and... 

Two separate syntheses of ( - - )-heliotridine (850) employ the strategy of an intramolecular addition of an a-acylamino radical to an alkyne (Scheme 125) [188,189]. Selective reduction of 857 to 858 followed by acetylation (859) and acetoxy-thiophenoxy exchange affords the radical precursor 860. 

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