Dialkylacetylenes reduction

It was observed in 1941 that with sodium in liquid ammonia, called dissolving-metal reduction, different dialkylacetylenes were converted to the corresponding trans alkenes in good yields and with high selectivity 195... 

In fact, reduction by lithium of dialkylacetylenes without the possibility of such stabilization effect yields exclusively the trans alkenes.482... 

Dialkylacetylenes usually cannot be reduced electrochemically up to —3.0 V which is the limit in most solvents suitable for these experiments. However, the three seven-membered cyeloalkynes, (31) and its corresponding S-oxide and S,S-dioxide, exhibit irreversible reduction waves at —2.93, —2.78 and —2.83 V vs. saturated calomel electrode, although the corresponding cyclooctynes are not reduced up to —3.0 V182). 

Reduction of acetylenes. Alkyl phenylacetylenes are reduced by chromium(II) perchlorate-triethylamine to cw-olefins in high yield. Terminal acetylenes are also reduced readily, but dialkylacetylenes are not reduced. 

An acetylene may be reduced to an olefin by sodium in liquid ammonia, ° by electrolytic reduction at a spongy nickel cathode, or by partial hydrogenation over metal catalysts. Catalysts for the hydrogenation include nickel, ° iron, colloidal palladium, and palladium on barium sulfate or calcium carbonate. Pure trans olefins are obtained from dialkylacetylenes by reduction with sodium in liquid ammonia. The yields ate better than 90%. Catalytic hydrogenation leads to mixtures of cis and trans olefins in which the cis isomers predominate. ° Mono- and di-arylacetylenes have also been reduced. ... 

The reduction of a carbon-carbon multiple bond by the use of a dissolving metal was first accomplished by Campbell and Eby in 1941. The reduction of disubstituted alkynes to c/ s-alkenes by catalytic hydrogenation, for example by the use of Raney nickel, provided an excellent method for the preparation of isomerically pure c -alkenes. At the time, however, there were no practical synthetic methods for the preparation of pure trani-alkenes. All of the previously existing procedures for the formation of an alkene resulted in the formation of mixtures of the cis- and trans-alkenes, which were extremely difficult to separate with the techniques existing at that time (basically fractional distillation) into the pure components. Campbell and Eby discovered that dialkylacetylenes could be reduced to pure frani-alkenes with sodium in liquid ammonia in good yields and in remarkable states of isomeric purity. Since that time several metal/solvent systems have been found useful for the reduction of C=C and C C bonds in alkenes and alkynes, including lithium/alkylamine, ° calcium/alkylamine, so-dium/HMPA in the absence or presence of a proton donor,activated zinc in the presence of a proton donor (an alcohol), and ytterbium in liquid ammonia. Although most of these reductions involve the reduction of an alkyne to an alkene, several very synthetically useful reactions involve the reduction of a,3-unsaturated ketones to saturated ketones. ... 

The mechanism of dissolving metal reductions depends on the nature of the solvent and the nature of the substrate. The proposed mechanism for the reduction of dialkylacetylenes by sodium in HMPA in the presence of a proton donor is illustrated in equation (18). The addition of an electron to the triple bond of (45) is proposed to produce the rran -sodiovinyl radical (46), or the corresponding radical anion (47), which undergoes protonation by the added alcohol to produce the radical (48). Further reduction of (48) by sodium produces the rrans-sodiovinyl compound (49), which on protonation produces the trans-a -kene (50). In the absence of a proton donor, the reduction of (45) with sodium in HMPA results in the formation of a mixture of cis- and trans-2- and 3-hexenes. Control studies showed that the isomerization products 2- and 3-hexene are not formed by rearrangement of the cis- or frans-3-hexenes. It was concluded that the starting alkyne (45) acts as a reversible proton donor reacting with an intermediate anion or radical anion to produce the delocalized anion (51) which is then protonated to produce the al-lene (52). Reduction of the allene (52), or further rearrangement to the alkyne (53) followed by reduction, then leads to the formation of the mixture of the cis- and trans-2- and 3-hexenes (equation 19). ... 

In the initial studies by Campbell and Eby it was noted that 3- and 4-octyne, 3-hexyne and 5-decyne could be efficiently reduced to the corresponding rram-alkenes in good yields and with remarkably high stereoselectivity. Shortly thereafter, Henne and Greenlee reported the quantitative reduction of 1-alkynes to 1-alkenes using sodium in ammonia in the presence of ammonium ion. In the absence of ammonium ion, however, extensive metallation of the 1-alkyne occurs. In the presence of ammonium ion dialkylacetylenes are inefficiently reduced (extensive hydrogen evolution occurs, in which sodium is consumed). 

Alkynes undergo reduction to 1-alkenes, which in turn are further reduced to the corresponding alkanes. However, when the reduction of phenylacetylene is carried out in the presence of a stoichiometric amount of FeCb, or a catalytic amount of NiCk, at -40 C for a short period of time, styrene is formed in excellent yield (very minor amounts of overreduction also occur). When the reductions are carried out at room temperature for 24 h, however, excellent yields of ethylbenzene are obtained (equations 41 and 42). The diarylacetylene, diphenylacetylene (106), is reduced to only dr-stilbene by L1A1H4 in the presence of Fe chloride (equation 43). Dialkylacetylenes, such as (107), are cleanly reduced to dr-al-kenes by LiAlH4 in the presence of Ni chloride (equation 44). The use of the other transition metal chlorides in the reduction of alkynes results in the formation of small amounts of the rranr-alkenes in addition to the predominant dr-alkene. " ... 

The Cr(II) amine complex, formed by rapid addition of an aqueous solution of Cr(Il) perchlorate to a cold solution of ethylenediamine or Et3N in DMF, reduces terminal and alkylphenylacetylene, but not dialkylacetylenes to the corresponding olefin with a high selectivity for the cis-isomer. Propargyl alcohols undergo facile reduction, but with stereoselectivity, the [Cr -ethylenediamine] complex favoring tranj-addition, whereas [Cr -NEt3] favors di-addition °. Tranj-additions is the exclusive pathway in the absence of amine, e.g. ... 

Two groups of workers have reported a new synthesis of the acetylenic linkage from carboxylic acid esters. The key step is the reductive elimination of the enol phosphate of a /3-keto-sulphone with sodium amalgam or sodium in liquid ammonia. The novelty of this procedure is that it is eminently suitable to the synthesis of some important classes of acetylenes, such as dialkylacetylenes and... 

---