Alkynes thiocarbonylation

More recent studies have shown that the regioselectivity of alkyne thiocarbonylation depends on both the metal-ligand combination and the solvent with Pd(0 Ac)2/dppb in THF affording the linear a, P-unsaturated thioester 4 as the dominant product, whereas Pd(OAc)2/dppp in CH2CI2 was highly regioselective for the branched isomer S (Equation 10.3) [11]. 

If an alkyne (or activated alkene) is present in the solution the sulfenic acid, from (57), or the alkanethiosulfoxylic acid (R SSOH), from (58), can be trapped, as shown in (59) and (60), respectively, and the course of the decomposition is relatively straightforward. [The thiocarbonyl compound formed in (57) normally forms polymer.]... 

The versatility of these [4+2] heterocyclization reactions is a consequence of the wide range of ene and diene components which can be used. In addition to alkenes and alkynes functioning as ene components, a variety of heterodienophiles is available such as electron-deficient imines (e.g. equation 89), nitriles e.g. equation 90), electrophilic carbonyl compounds (e.g. equation 91), thiocarbonyl compounds (e.g. equation 92), singlet oxygen (e.g. equation 93), nitroso compounds (e.g. equation 94), sulfenylsulfonamides (e.g. equation 95) and azo compounds (e.g. equation 96). Many of these reactions proceed with excellent regioselectivity and stereoselectivity, probably because in many instances they involve... 

Besides alkoxy-, amino-, and amidocarbonylation reactions, a carbonylation process using a thiol as nucleophile, that is, thiocarbonylation, has been extensively studied.It has been shown that the thiocarbonylation takes place with 1-alkynes, " " " propargyl alcohols,allenes, " 1,3-dienes, propargylic mesylates, " and bicyclopropylidene . 

Evidence has been presented591 that favours the involvement of thiirane intermediates in the formation of ( , )-divinyl sulfides (455) from the reaction of thiocarbonyl ylides (454) with activated alkynes, while the reaction of l-alkynyl-2,3-epithio alcohols (456) with a catalytic amount of Hg(II) has been shown to afford592 substituted thiophenes (457) (see Scheme 117). 

Azole approach. 5-JT[l,3,4]Thiadiazolo[3,2-a]pyridin-5-ones (723) can be prepared by 1,3-dipolar cycloaddition reactions between electron-deficient alkenic or alkynic dipolarophiles and the thiocarbonyl ylide dipole present in anhydro-5-hydroxy-2-methyl-6-phenylthiazolo[2,3-6][l,3,4]thiadiazolium hydroxide (720). Sulfur is extruded from the original acetylene adduct (722) whereas H2S is eliminated from the alkene adduct (721) to form the same product (723) (79JOC3808). 

Numerous structures containing the thiocarbonyl ylide dipole are conceivable. Incorporation of the thiocarbonyl ylide dipole into a bicyclic heterocyclic system is possible by the conversion of the cyclic thione (203) into the ring-fused mesoionic system (204). The thiocarbonyl ylide dipole (205) undergoes cycloaddition with both alkenic and alkynic electron-poor dipolarophiles in refluxing benzene or xylene so that, after extrusion of hydrogen sulfide or sulfur, respectively, from the initial 1 1 cycloadducts (206) and (207), a ring-fused pyridinone is formed. The method has been used for the annelation of pyridinones to the imidazole, 1,2,4-triazole, thiazole and 1,3,4-thiadiazole systems... 

Addition of alkynic 1,3-dipolarophiles to the thiocarbonyl ylide systems (48) gives the adducts (49) which readily lose sulfur giving the benzoheterocycles (50) and this is a useful synthetic route to these systems. The heterocycles (50) have also been made by elimination of hydrogen sulfide or methylamine from analogous alkenic adducts. 

The benzo derivative (128) reacts as a thiocarbonyl ylide. Addition of N-(p-tolyl)maleimide gives a mixture of the exo (71%) and endo (16%) adducts (129 Ar=p-tolyl), which in hot acetic acid eliminate hydrogen sulfide giving the pyrido[l,2-a]benzimidazole (130 Ar=p-tolyl). Analogous 1 1 cycloadducts (131) are formed with dimethyl maleate, dimethyl fumarate, methyl crotonate and methyl acrylate. In contrast to the transformation (129) —> (130), treatment of the adducts (131 R = H, Me) with hot acetic acid gives the tetracyclic compounds (133) via the benzimidazole derivatives (132 R = H, Me). Reaction with alkynic 1,3-dipolarophiles gives pyrido[l,2-a]benzimidazole (134) by desulfurization of the primary adducts (80CL1369). 

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