Dithiols, from carbonyls

Dithioketals, especially the cyclic dithiolanes and dithianes, are also useful carbonyl-protecting groups. These can be formed from the corresponding dithiols by Lewis acid-catalyzed reactions. The catalysts that are used include BF3, Mg(03SCF3)2, Zn(03SCF3)2, and LaCl3.100 S-Trimethylsilyl ethers of thiols and dithiols also react with ketones to form dithioketals.101... 

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. 

Thioacetals, arising from the reaction of an aldehyde or ketone with either two moles of a thiol or one mole of a 1,2- or 1,3-dithiol, are useful both as a means of protection of a carbonyl group (Section 5.8.8, p. 625) and as important intermediates in organic synthesis. 

In particular, 1,3-dithiane prepared from dimethoxymethane (methylal) and pro pane-1,3-dithiol in the presence of boron trifluoride-etherate,237 and 2-alkyl-1,3-dithianes prepared similarly from aldehydes,2383 are important acyl anion equivalents. These and other uses are discussed in Sections 5.7.5, p. 596, and 6.6.1, p. 909. A wide-ranging review of the reversal of polarity of the carbonyl group through the formation of these sulphur-containing reagents has emphasised their value in organic synthesis.2388... 

The prominent feature is the absence of the usual carbonyl frequencies in the 1620-1720 cm-1 range, together with one or more strong bands in the 1500-1610 cm-1 range. The location of the carbonyl frequency is difficult to obtain because the modes are somewhat mixed, as it is for pyran-4-ones.124 The carbonyl character of absorption bands has been deduced from solvent effects, as shown in Table II for some (5-aryl-l,2-dithiol-3-ylidene)acetophenones (94). Two bands are more-or-less affected by solvent effects in the 1525-1560 cm-1 range. 

The formation of an isothiazole ring from the thiophene system (5) is of interest. Probably ammonia breaks the sulfur-carbonyl bond and then adds to the second carbonyl group, giving an imine (8), which is then oxidized by the hydrogen peroxide. A number of similar rearrangements and interconversions of isothiazole, thiophene, and dithiole systems are known and will be discussed later (Section II,C,3). 

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... 

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... 

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