Lithium thiolates

Quite a number of asymmetric thiol conjugate addition reactions are known [84], but previous examples of enantioselective thiol conjugate additions were based on the activation of thiol nucleophiles by use of chiral base catalysts such as amino alcohols [85], the lithium thiolate complex of amino bisether [86], and a lanthanide tris(binaphthoxide) [87]. No examples have been reported for the enantioselective thiol conjugate additions through the activation of acceptors by the aid of chiral Lewis acid catalysts. We therefore focussed on the potential of J ,J -DBFOX/ Ph aqua complex catalysts as highly tolerant chiral Lewis acid catalyst in thiol conjugate addition reactions. 

Gasking and Whitham described the one-pot preparation of 1-silylated 3,3-di-methyl-substituted allenyl sulfides 307 (Scheme 8.82) [170]. Treatment of alkyne 305 with lithium thiolate generates allenyllithium species 306, which is subsequently silylated by trimethylsilyl chloride. Formation of lithiated intermediate 306 is based on a procedure developed by Clinet and Julia [171]. 

TABLE 13. Synthesis of vinyl sulfides (185) by the reaction of magnesium alkyU-dene carbenoids with lithium thiolates... 

Tomioka et al. reported the asymmetric Michael addition of lithium thiolates catalyzed by chiral aminoether 31 (Scheme 8D. 18) [39]. Thus, in the presence of catalytic amounts of 31 (10 mol %) and lithium 2-(trimethylsilyl)thiophenolate 32-Li (8 mol %), thiol 32 (3 equiv.) reacted with a,p-unsaturated esters at -78°C in toluene-hexane solvent to give the Michael adduct with up to 97% ee. In the ahsence of 31, the reaction of thiophenol proceeded in only 0.5% yield at room temperature. A monomeric complex consisting of 31 and lithium is proposed as the key reactive species in this asymmetric reaction. The trimethylsilyl group at the ortho-po-sition of the thiol moiety in 32 contributes to the formation of the stereochemically defined monomeric chelated structure, wherein the lithium cation is coordinated with the three heteroatoms of the tridentate ligand 31. The reactions of acyclic /nmv-a,P-unsaturated esters (R1 = Me, Et, Pr, Bu, Bu, PhCH9 R2 = H) proceeds with high enantioselectivity in... 

It may be possible to prepare lithium thiolate derivatives of living anionic polymers using higher ratios of lithium to S8, since yec-butylthiolate was obtained in ca. 60% yield from. scc-butyllithium in benzene when the ratio was 8 350). 

Ashby, E. C. Park, W. S. Goel, A. B. Su, W.-Y. Single-electron transfer in reactions of alkyl halides with lithium thiolates./. Org. Chem. 1985, 50, 5184-5193. 

Lithium acetylides and lithium thiolates have been shown to add to magnesium alkylidene carbenoids yielding conjugated enynes and vinyl sulfides.94... 

While the major use for palladium catalysis is to make carbon-carbon bonds, which are difficult to make using conventional reactions, the success of this approach has recently led to its application to forming carbon-heteroatom bonds as well. The Overall result is a nucleophilic substitution at a vinylic or aromatic centre, which would not normally be possible. A range of aromatic amines can be prepared direcdy from the corresponding bromides, iodides, or triflates and the required amine in the presence of palladium(O) and a strong alkoxide base. Similarly, lithium thiolates couple with vinylic triflates to give vinyl sulfides provided lithium chloride is present. 

The process was recently extended to the asymmetric cyclization of unsaturated esters 158 with the use of a lithium thiolate of 10-mercaptoisoborneol, 159, as an initiating chiral thiolate, thus providing a new methodology for the asymmetric building of chiral carbocycles (Scheme 50)m. 

Lithium thiolates add well to activated olefins, generating /i-sulfuri/cd lithium eno-lates175. Those can be used in tandem inter-176 or intra-177 molecular addition-aldolisation processes. In the presence of a catalytic amount of bi- or tridentate chiral ligands178, this addition becomes enantioselective and the newly created asymmetric center(s) can be almost totally controlled, provided the thiolate and the enone structures are well chosen (Scheme 40)179 l82. 

SCHEME 77. Tandem lithium thiolate enantioselective Michael addition kinetic protonation181... 

A remarkable example of tandem conjugate addition-aldol reaction has been recently reported by Tomioka and coworkers. The transient lithium enolate, generated by 1,4-addition of benzyl lithium thiolate onto the corresponding a,-unsaturated ester182,595, is followed by an intramolecular aldol tandem cyclization, resulting in a five-membered... 

This coupling is equally applicable to alkyl-, aryl-, and vinyllithium reagents. The products are obtained in generally good yield and in 98-100% isomeric purity. The reaction can also be used to prepare vinyl sulfides from vinyl halides and a lithium thiolate (equation II). 

The reaction of thiol esters with lithium ynolates (equation 67) takes place by a route different than the one shown in equation 65 for alcohol esters. Thiol esters (162) undergo a two-carbon homologation to S-keto thiol esters 165 in good yield. Intermediates 163 undergo a two-step rearrangement to a S-keto thiol ester enolate (165), via elimination of lithium thiolate to yield a ketene (164), followed by the nucleophilic attack of the thiolate on 164. Finally, the homologated S-keto thioester (165 ) is obtained on acidification of the reaction mixture . ... 

Conversion to Thioesters. The transformation of A7-acyl imides into thioesters with lithium thiolate reagents proceeds with exceptional selectivity for the exo carbonyl moiety even in exceptionally hindered cases. A recent application of this reaction in a complex setting has been reported (eq 66). This transformation is significant in that the normally reliable peroxide hydrolysis procedure proved to be nonselective. The recently reported high yield reduction of thioesters to aldehydes enhances the utility of these thioester intermediates. 

Driven in part by interest in potential materials applications,the number of i-block complexes containing ligands with sulfur donors has increased tremendously since the 1980s. The structures of hundreds of such compounds are now known, even though the first crystal structure of a lithium thiolate did not appear until 1985. ... 

Reports that thermolysis of metal chalcogenolate complexes offers a low-temperature route to the synthesis of novel solid state materials (32) is also stimulating research in this area. As is the case with copper(I) thiolates, lithium thiolates are often aggregated species (Section III.C) and from the few magnesium thiolate structures known (Section III.D) one sees a preference for monomeric and dimeric formulations when bulky substituents are present. 

Structurally Characterized Lithium Thiolate, Selenolate. and Tellurolate Species... 

The bonding mode of the sulfur atoms in lithium thiolate complexes is often not easily identifiable from the coordination geometry of these atoms (156). Ab initio calculations on the bonding in complexes [MSH] (M = alkali and alkaline earth metal) indicate that this bonding is highly ionic (157). 

Interestingly, four examples exist of mixed-metal thiolate complexes in which one or more lithium thiolate moieties are present within an aggregate that also contains a lanthanide or actinide metal thiolate unit (Table XIV). These species... 

Structurally Characterized Mixed-Metal Lithium Thiolates... 

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