Vinyl sulfides reduction

To gain understanding of the interdependence between the olefin reduction and the sulfoxide reduction, the saturated sulfoxide 52 was prepared and treated with BH3-THF. No reaction was observed under the similar conditions (Scheme 5.18). The unactivated vinyl sulfide 16 was also not reactive toward BH3-THF. These results indicated that sulfoxide and olefin were reduced simultaneously, not independently. Again this phenomenon was unexpected and pointed to the unique nature of this reaction. 

Perhaps the most useful type of alkene substrates for these reactions are enol ethers, enol esters and vinyl sulfides. Silyl enol ethers have excellent electron-donor properties, with an ionization potential of about 8 eV and an oxidation potential in various solvents of approximately 1.0-1.5 V vs SCE161. These compounds are easily synthesized by reaction of an enolate with a chlorosilane. (A very recent report synthesized a variety of silyl enol ethers with extremely high stereochemical yield, using the electrogenerated amidate of 2-pyrolidinone as the base.)162 An interesting point is that the use of oxidative or reductive cyclization reactions allows carbonyl functionalities to be ambivalent, either oxidizable or reducible (Scheme 65)163. 

Desulfuration.2 Vinyl sulfides arc reduced to the corresponding alkene by isopropylmagnesium bromide in the presence ol [(C6H5)3P]2NiCl2. No overreduction in observed, as frequently noted in reductions with Raney nickel. 

Ethylenic cation radicals are also capable of rotating around the double bond. At the same time, the main specificity of ethylenic cation radical reduction consists of the high selectivity of the reaction. One-electron oxidation was developed as a strategy for selective and efficient reduction of relatively ionizable functionalities, including conjugated dienes, styrenes, and vinyl sulfides (Mirafzal et al. 1993). Reduction is highly sensitive to substrate ionizability and permits selective reduction of the more ionizable function in a difunctional compound. 

Cycloalkenyllithiums.3 Alkenyllithiums are usually prepared by reductive lithiation of trisylhydrazones of ketones with butyllithium, but this method fails with the hydrazones of cyclic ketones. However, the cycloalkenyl sulfides, prepared by reaction of cyclic ketones with thiophenol, can be reductively lithiated with LDBB at -78°. This lithiation fails in the case of cyclopentenyl sulfides, but is useful in the case of the vinyl sulfides obtained from 6-, 7-, and 8-membered cycloalkanones. 

Since the sulfide substituents can be removed reductively, the cyclisations of 273 and 276 are synthetically equivalent to cyclisations onto disubstituted double bonds, giving compounds such as 275.142>143 Like the corresponding cyclisations onto monosubstituted alkenes they are highly stereoselective, with the sense of stereoselectivity being solvent-dependent the products 274 and 277 are trans as shown for a cyclisation conducted in THF in pentane complete cis selectivity is obtained. Similar cyclisations onto vinyl sulfides have been used to explore the stereochemical course of the anionic cyclisation reaction, and are discussed below.146... 

Dihydrobenzo[b]thiophene may be obtained by reduction of ben-zo[b]thiophene (Section III,A,1) by treating 2-o-(benzylthio)phenyIethanol with SOCl2-pyridine (91%),208 and by cyclization of o-(methylthio)phenyl-carbene at 250°C (45%) the last procedure is mainly of academic interest.304 Substituted phenyl vinyl sulfides undergo photocyclization to the expected... 

The present preparation Illustrates a general and convenient method for a two-step deoxygenation of carbonyl compounds to olefins. Related procedures comprise the basic decomposition of p-toluenesulfonylhydrazones,2 the hydride reduction of enol ethers,3 enol acetates,9 enamines,3 3 the reduction of enol phosphates (and/or enol phosphorodlamidates) by lithium metal in ethylamine (or liquid ammonia),33 the reduction of enol phosphates by titanium metal under aprotic conditions,32 the reduction of thioketals by Raney nickel,33 and the reduction of vinyl sulfides by Raney nickel in the presence of isopropylmagnesium bromide.3 ... 

When hemiacetal formation is followed by reduction, sulfides can be obtained from aldehydes in a one-pot manner under mild reaction conditions (eq 15). Vinyl sulfides can be directly prepared from silyl enol ethers upon treatment with PhSTMS in the presence of equimolar amounts of BF3 -OEt2 (eqs 16 and 17). Ketones under the same reaction conditions give only trace amount of vinyl sulfides. 

Finally, reductive cleavage of an a-chloroether provides access to vinyl sulfide 85 which corresponds to the C(9)-C(14) fragment of the macrocyclic lactams trienomycin and mycostrienin (Scheme 17). The same group have used related reductive cleavage methodology... 

C-S bond formation through reaction between aryl halide (Cl, Br, I) and aromatic or aliphatic thiols has been catalysed by a Ni-NHC complex under basic conditions. The reaction is of wide scope and high efficiency. Vinyl sulfides have been synthesized by hydrothiolation of alkynes catalysed by Rh-NHC complexes. Mechanistic investigations have established that the catalysis proceeds via an oxidative addition of the S-H bond to Rh(I) followed by alkyne insertion and reductive elimination. Interestingly, the regioselectivity could be controlled by the nature of the complex (mono or dinuclear precursors), and the use of a strong donor such as the NHC prevents the deactivation of the catalyst. 

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