Thiocarbonyl compounds radical reactions

Application of radical reactions to organic synthesis has recently received much atrendon, and various important reacdons have been discovered in this field. Alkyl halides, sulfides, seleniJes, and thiocarbonyl compounds have been used as precursors to alkyl radicals. Some examples are illustrated in Scheme 7.18. ... 

The main problem is how to generate free radicals at low temperatures. It was discovered this can be done by using the trialkylborane-oxygen redox couple. Prior to the studies on thiocarbonyl compounds, Furukawa and Tsuruta (68) had used a mixture of trialkylboranes and oxygen for vinyl polymerizations, and studies by Fordham and Sturm (69) and Zutty and Welch (70) had confirmed them as free-radical polymerizations. For the fluorothiocarbonyl work (39), it was shown that at - 78° C the reaction of a trialkylborane and oxygen proceeds cleanly to an alkylperoxydialkylborane, V. 

Thiocarbonyl compounds for which the C=S group is linked to two heteroatoms, as in thionocarbonates, thionocarbamates, thioureas and trithiocarbonates, have been in general less directly involved as intermediates in synthesis, and we have considered here only xanthates which intervene in the Chugaev reaction (see Section 3.1) and Barton radical reactions (see Section 3.2). They are traditionally prepared by reaction of an alkoxide anion, generated from the alcohol and a strong... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

D. H. R. Barton, D. O. Jang, and J. Cs. Jaszberenyi, The invention of radical reactions. Part XXXI. Diphenylsilane A reagent for deoxygenation of alcohols via their thiocarbonyl derivatives, deamination via isonitriles, and dehalogenation of bromo- and iodo-compounds by radical chain chemistry, Tetrahedron, 49 (1993) 7193-7214. 

The mechanism of this reaction proceeds via an a-hydride abstraction with formation of an intermediate chloroxonium ion pair, under conditions where the free-radical chlorination is suppressed by the lack of light855. The above cyclic ethers are smoothly isomer-ized in presence of aluminum chloride to 2,2,4-trichlorobutanal856 and 2,2,5-trichlorohep-tanal857, respectively. Methylthio(thiocarbonyl) compounds have also been chlorinated by... 

Radical stabilization energies for a wide variety of carbon-centered radicals have been calculated at G3(MP2)-RAD or better level. While the interpretation of these values as the result of substituent effects on radical stability is not without problems, the use of these values in rationalizing radical reactions is straight forward. This is not only true for reactions involving hydrogen atom transfer steps but also for other reactions involving typical elementary reactions such as the addition to alkene double bonds and thiocarbonyl compounds. 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

Radical dehydroxylation is most effective for secondary alcohols, including those derived from carbohydrates, in which traditional methods such as tosylation (or mesylation) and L1A1H4 reduction often fail. The reaction tolerates many different functional groups, as illustrated in the reduction of the thiocarbonyl compound 8 (4.8). ... 

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