Substitution alkenyl sulfides

Nucleophilic substitution of a,/3-epoxysilanes followed by the Peterson elimination is valuable for the stereoselective synthesis of alkenes.3 The reactions with lithium phenylsulfide and diphenylphosphide form alkenyl sulfides and alkenylphosphines, respectively, in a stereospecific manner. 7-Metallo-a,/ -epoxysilanes are isomerized to a-siloxyallylmetals by anionic ring opening and subsequent Brook rearrangement (Equation... 

Free-radical addition to the terminal methylene group of a vinylcyclopropane 1 affords an a-substituted cyclopropylmethyl radical which rearranges to produce a 1 -substituted pent-2-ene. The most frequently used radical precursors are thiols, additions of which lead to alkenyl sulfides 2. ... 

Similar methylenation of amides, imides, and thiol esters produces the corresponding heteroatom-substituted olefins (Table 4.4). Since the olefination of imides is greatly affected by steric hindrance, an unsymmetrical imide is monomethyl-enated at the sterically less hindered carbonyl group when treated with 3 (entry 3). The low basicity of 3 allows the transformation of a thiol ester to the corresponding alkenyl sulfide without racemization (entry 4). 

Scheme 4.44. Preparation of enol ethers and alkenyl sulfides by the use of a-heteroatom-substituted carbene complexes...

Alkyllithium reagents can also be generated by reduction of sulfides.32 Alkenyl-lithium and substituted alkyllithium reagents can be prepared from sulfides,33 and sulfides can be converted to lithium reagents by the catalytic electron transfer process described for halides.34... 

AIkylthio)allylritanium reagentS, RSCH=CHCH2TiL (l).9 The reagents are prepared by deprotonation of allylic alkyl (aryl) sulfides with sec- or r-butyllithium followed by addition of Ti(0-/-Pr)4 at - 78°. They can react with carbonyl compounds at the a- or "/-position. a-Adducts predominate in reactions with a- and /1-mono- and disubstituted sulfides, whereas /-adducts predominate in reaction with /-substituted sulfides. The a-adducts show high eryr/iro-selectivity. The products are useful precursors to alkenyl oxiranes and to 2-(arylthio)-l,3-butadienes. 

A chromium carbene generated in situ firom C1CI2 and a dihaloalkyl daivative, has been utilized to form a variety of ( )-disubstituted alkene compounds (480). The general reaction is summarized in equation (110). Alkenyl halides, sulfides and silanes, as well as dialkyl-substituted alkenes, have been synthesized by this method. 

Ma recently demonstrated that 2-haloanilides can undergo coupling with metal sulfides subsequent intramolecular condensation affords substituted benzothia-zoles (3). Thioamides and thioureas also serve as precursors to benzothiazoles (vide infra). In addition to S -arylation reactions, alkenyl halides are also effective... 

An efficient synthesis of variously substituted 2,3-dihydrothiophenes was developed that involved a copper-catalyzed S-alkylation and S-alkenyl-ation of sodium sulfide with 1,4-diiodoenes to yield 2,3-dihydrothiophenes in high yields (13TL1475). The 2,3-dihydro derivatives were converted to the appropriate thiophene by oxidation. 

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