Lithium carbon-sulfur bonds

Sulfonic acids are prone to reduction with iodine [7553-56-2] in the presence of triphenylphosphine [603-35-0] to produce the corresponding iodides. This type of reduction is also facile with alkyl sulfonates (16). Aromatic sulfonic acids may also be reduced electrochemicaHy to give the parent arene. However, sulfonic acids, when reduced with iodine and phosphoms [7723-14-0] produce thiols (qv). Amination of sulfonates has also been reported, in which the carbon—sulfur bond is cleaved (17). Ortho-Hthiation of sulfonic acid lithium salts has proven to be a useful technique for organic syntheses, but has Httie commercial importance. Optically active sulfonates have been used in asymmetric syntheses to selectively O-alkylate alcohols and phenols, typically on a laboratory scale. Aromatic sulfonates are cleaved, ie, desulfonated, by uv radiation to give the parent aromatic compound and a coupling product of the aromatic compound, as shown, where Ar represents an aryl group (18). 

Carbon-sulfur bonds can be formed by the reaction of elemental sulfur with a lithio derivative, as illustrated by the preparation of thiophene-2-thiol (201) (700S(50)104). If dialkyl or diaryl disulfides are used as reagents to introduce sulfur, then alkyl or aryl sulfides are formed sulfinic acids are available by reaction of lithium derivatives with sulfur dioxide. 

Formation of a-Sulfinyl carbanions has been widely investigated17. Several bases were found to be suitable for the generation of these species, e.g., methyllithium and lithium diisopropyl-amide. Butyllithium and rm-butyllithium, however, must be used with caution since they can cause cleavage of the carbon sulfur bond, resulting in an exchange of the ligand at sulfur by a nucleophilic displacement28-29. This method has been used for the preparation of optically active alkyl methyl sulfoxides 28. 

Cohen and coworkers reported on dilithio species of ketone ,/ -dianions, in which cyclopropyllithium constitutes the / -carbanion portion10. In the examples given in Scheme 9, the first lithium enolate was produced by proton-abstraction with lithium 2,2,5,5-tetramethylpiperidide (LTMP), and the second involved the reductive cleavage of the carbon-sulfur bond by lithium 4,4 -di-tczt-butylbiphenylide (LDBB). 

The homolytic cleavage of the carbon-sulfur bond generates an intermediate radical 17 that adopts an axial orientation. The second electron transfer gives a kinetic a-oriented and configurationally stable lithium reagent 18, which is protonated with retention of configuration. Again, the transient radical defines the stereochemical outcome of the overall process. 

Benzyl phenyl sulfides can be sequentially metallated and alkylated. The cleavage of the carbon-sulfur bond in the resulting product leads to a benzyllithium and lithium phenylthiolate. Alkylation of the former organometallic then produces an alkylbenzene (Scheme 15), but competing alkylation on the thiolate is however observed. 

The allylation of a-metalloallyl sulfones followed by reduction of the carbon-sulfur bond, usually with lithium in ethylamine, ° ° ° or with Na/Hg ° ° (the first cited method being the most se-... 

The different reactivities of the two substituents on C-l allow selective replacement and conversion of them. Alkyl monothio-acetals [proposed as intermediates in mercury(II)-catalyzed demer-captalation reactions—see Section IV,l,b] have been prepared from a-bromothioethers by the combined action of an alcohol and silver(I) carbonate the introduction of S-nucleophiles is discussed in Section 11,6. Reduction of 81 by lithium aluminum hydride effects hydrogen-olysis of the carbon-halogen bond, whereas the action of Raney nickel on the derived S-ethyl O-methyl monothioacetal specifically cleaves the carbon-sulfur bond to afford the pentaacetate of 1-0-methyl-D-galactitol.327... 

The reduction of the carbon-sulfur bond in (1) with lithium naphthalenide also produces an alkenation reagent (eq 2). This protocol is limited, however, to use with nonenolizahle ketones and aldehydes. Lithiohis(trimethylsilyl)methane, the active intermediate in this reaction, can also he produced through the deprotonation of bis(trimethylsilyl)methane. 

Cleavage of a carbon to sulfur bond by a lithium reagent is a well known entry to organolithium reagents. This process has been used to synthesize a-nitrogen carbanions, and hence SMA derivatives.67 165 This study has been extended to tellurium and selenium starting materials as well.166... 

In contrast with unreactive, unfunctionalised terminal alkenes, allylic and homoallylic ethers (22, 24) and alcohols (20) from which the product organolithiums (21, 23, 25) can be chelated in a (preferably) five-membered, oxygen-containing ring, carbolithiate rapidly and cleanly.23 Coordination overrides any preference for the lithium to be bonded to the primary carbon, but cannot overcome the unfavourability of forming a tertiary organolithium - 26 gives 27, but 28 cannot be carbolithiated. Coordination to sulfur in similar thioethers 29 works too. 

An alternative synthetic procedure to lithium arenethiolates is the insertion reaction of elemental sulfur (S8) into the lithium-carbon bond of a suitable or-ganolithium reagent in THF at low temperature (Eq. 12). 

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