Thiones, group transfer reactions

Whereas additions of carbon radicals to alkene moieties are the best characterized homolytic additions, carbon radicals are known to add to a wide range of unsaturated systems. These include polyenes, alkynes, arenes, heteroarenes, carbon monoxide,isonitriles, °° ° nitriles, ° imines and derivatives, ° ° aldehydes,nitrones, and thiones. ° Many of these reactions, such as addition of an alkyl radical to a carbonyl group, ° are thermodynamically unfavorable and readily reversible, and they form the basis of composite group-transfer reactions discussed below. 

Pyrimidin-2(l//)-ones (85 X = O) and -thiones (85 X = S) on reduction with sodium borohydride afford mixtures of lH-3,4- (86) and l//-3,6-dihydropyrimidinones (87), the product ratios depending on the acidity of the medium, and some further reduction to the fully reduced pyrimidinones may also occur. Similar results are obtained with LAH as the reductant, although l-arylpyrimidine-2-thiones mainly form the corresponding 3,6-dihydropyridine-2-thiones (88), especially if a hydroxy or methoxy group occupies an ortho position in the aryl substituent. This suggests that the group may complex with the reagent and facilitate an intramolecular hydride transfer reaction. L-Selectride reduces N-benzyl-... 

Thiocarbonyl compounds are converted into the corresponding carbonyl derivatives in good yield (70-99%) by their reaction with concentrated aqueous sodium hydroxide and dichloromethane in the presence of tetra-n-butyiammonium hydrogen sulphate [4], The reaction is general for thioeslers, thioamides, thioureas and thiones (Table 3.28), and no reaction occurs in the absence of the phase-transfer catalysts. The reaction is also aided by the initial 5-methylation of the thiocarbonyl group [5]. 

It should be pointed out that the treatment of phosphorus or sulfur donors bearing ferrocenyl groups with diiodine results in the formation of molecular charge-transfer adducts without simultaneous oxidation of the ferrocenyl groups (Durfey et al. 2000, Gridunova et al. 1982). At the same time, the reaction of 4-ferrocenyl-l,3-dithiolene-2-thione with diiodine leads to a charge-transfer complex in which the iron has been oxidized from Fe" to Fe ". This was confirmed by magnetic measurements and Moessbauer spectroscopy (Allen et al. 2003). 

Electron-rich and electron-poor olefins influence the outcome of the thietane reaction. Olefins containing electron-withdrawing groups react only with the n,n state of the thiones to produce the four-membered S-ring (Eq, 110 jjjg mechanism is described as an electron transfer from... 

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