Dithiols from carbonyl compounds

This reaction is based on a stoichiometric reaction of multifunctional olefins (enes) with thiols. The addition reaction can be initiated thermally, pho-tochemically, and by electron beam and radical or ionic mechanism. Thiyl radicals can be generated by the reaction of an excited carbonyl compound (usually in its triplet state) with a thiol or via radicals, such as benzoyl radicals from a type I photoinitiator, reacting with the thiol. The thiyl radicals add to olefins, and this is the basis of the polymerization process. The addition of a dithiol to a diolefin yields linear polymer, higher-functionality thiols and alkenes form cross-linked systems. 

The stoichiometric equivalents of halofluorides have been recently applied to transform alkylene dithioacetals into gcm-difluorides.70-71 Dithioacetals such as 1,3-dithiolanes and 1,3-dithianes arc readily obtained from the corresponding carbonyl compounds by the reaction with ethane-1,2-dithiol or propane-1,3-dithiol in the presence of the complexes boron trifluoride-bis(acetic acid) or boron trifluoride-diethyl ether. Using a two-step procedure, a range of aldehydes and ketones can be converted into gem-difluorides under mild conditions. 

Two thiolate radical anions can couple to give the dithiolate precursor, 5-49, of the second product. This is analogous to the well-documented coupling of ketyls (radical anions derived from one-electron reduction of carbonyl compounds), which results in the formation of pinacols (1,2-diols). 

Dithians 20 illustrate this approach and have been remarkably popular considering the problems in their use. They can be made from aldehydes with propane-1,3-dithiol and Lewis acid catalysis. They are deprotonated with BuLi and react with alkyl halides, epoxides, and carbonyl compounds (E+) to give 22 and hence 19 after hydrolysis. The hydrolysis is by no means easy there are many methods and this alone should warn us that none is very good.6... 

With hydrogen sulfide in the absence of hydrogen, however, aldehydes and ketones give geminal dithiols, the pressures required varying from 35 to 8500 atm according to the nature of the carbonyl compound 392 these dithiols can also be obtained without use of pressure and at room temperature in dimethylformamide as solvent containing a basic catalyst such as morpholine or butylamine.393... 

Another example of polymer-supported A1 based Lewis acid is cross-linked polystyrene-supported aluminum triflate (79). Cross-linked polystyrene-supported AICI3 (72) was easily converted into (79) by treatment with triflic acid. This catalyst was applied to dithioacetalization of carbonyl compounds and transdithioacetal-ization of acetals (Scheme 19.19) [44]. From benzaldehyde the corresponding dithioacetal (84) was obtained in the presence of the polymeric catalyst (79) in 98% yield in 30 minutes. The same product was also obtained from the dimethyl acetal (85) in 94% yield. Chemoselectivity of the polymeric catalyst was also demonstrated in Scheme 19.19. Aldehydes reacted faster in the presence of ketone (34) to give the dithioacetals with (79). Aliphatic ketone (89) exclusively reacted with dithiol (81) in the presence of aromatic ketone (34). These chemoselectivities were higher than those obtained from the reactions using nonsupported Al(OTf)3. 

The reaction of dithiocarbamates and diselenocarbamates with a-halogenated carbonyl compounds has proved to be a useful first step in the synthesis of l,3-dithiole-2-thiones and their analogues. Thus 2-dimethylamino-l,3-dithiolium salts (38) > and diselenolium salts (39) " are readily obtainable by this route, and these salts are converted into thiones (40 X = Y = S or Se, Z = S) or selones (40 X = Y = S or Se, Z = Se) with hydrogen sulphide or hydrogen selenide. The dihydroxybenzo-l,3-dithiole-2-thione (41) has been prepared from p-benzoquinone and dithiocarbamic acids. ... 

As mentioned above (Section 4.31.2.1), 3-alkylidene-l,2-dithioles (3d) may possess considerable ionic character consistent with contributions from structures (62), and condense with suitable carbonyl or other compounds. Condensation reactions of 3-aIkyI-l,2-dithiol-ylium salts (105) (66ZC321, 64CI(L)461, 80AHC(27)15l> are readily explained in terms of initial proton transfer from the salt to the substrate, as in the formation of salts of type... 

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