Acetates alkynes: acidic hydrogen

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

A closely related synthesis utilizes alkynic epoxides, which add hydrogen sulfide in the presence of a base to form thiophenes in good yield. For example, when the epoxide (184) was stirred in an aqueous solution of barium hydroxide with slow addition of hydrogen sulfide gas, and the product extracted after neutralization with acetic acid, the substituted thiophene (186) was obtained in 60-70% yield (53ZOB1569). In these experiments R1 = Me or Et, R2 = H or Me and R3 = Ph. The same synthetic method was used to produce a thiophene having an optically active substituent, with relatively little racemization. In this case R1 = EtC HMe while R2 and R3 = H, but the yield was equivalent (73JOC2361). This reaction also undoubtedly proceeds via an intermediate (185), a structure related to a 1 -mercapto-1,3-butadiene. 

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). 

Acylsilanes may serve as synthetic equivalents of cai boxylic acids when treated with alkaline hydrogen peroxide. Especially attractive is the one-pot conversion of the ethynyl group of 1-alkynes into monosubstituted acetic acids via sequential silylation, hydroboration, and oxidation, as depicted below. 

---