Dialkylacetylenes

The rates of bromination of dialkylacetylenes are roughly 100 times greater than for the corresponding monosubstituted alkynes. For hydration, however, the rates of reaction are less than 10 times greater for disubstituted derivatives. Account for this observation by comparison of the mechanisms for bromination and hydration. 

Treatment of dialkylacetylenes with 1 equivalent of TTN in aqueous acidic glyme resulted in formation of acyloins in high yield [Eq. (30)] the suggested mechanism of this transformation, shown in Scheme 24, is... 

A Ag/Pd-cathode hydrogenates 2-butyne-1,4-diol and acetylene dicarboxylic acid exclusively to the cis-olefin [323]. Similar results were obtained at a Cu net covered with spongy silver [324]. With dimethyl butynedioate the cis/trans ratio of the product dimethyl butenedioate on a Pd black cathode decreased with increasing pH both in electrolytic and catalytic hydrogenation [325]. On the other side at a Hg cathode a trans addition to alkynes occurs [326]. In methy-lamine/liCl, dialkylacetylenes are reduced to trans-olefins. Nonconjugated aromatic internal acetylenes are selectively reduced to aromatic trans-olefins [327]. 

Dialkylacetylenes. A convenient method for the prepn of dialkylacetylenes, RC CR, is described in which the process involves interaction of sodium acetylide, sodamide alkyl halides in iq ammonia. Intermediate products are not isolated, thereby saving substantial time as compared with older methods. The following pure compds were prepd ... 

T1(N03)3 is a versatile oxidant since it converts diarylacetylenes to 1,2-diketones and dialkylacetylenes to a-hydroxy ketones 716... 

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... 

The hydrogenations of dialkylacetylenes are of particular interest from a stereochemical viewpoint since, as noted in Sect. 4.3, the adsorbed state of the acetylene is expected to adopt a c/s-configuration and, consequently, upon hydrogenation to yield the cis-olefin. Wide use of this fact has been made in preparative organic chemistry as noted by Burwell [192] and by Campbell and Campbell [193]. Although early studies of the catalytic hydrogenation of disubstituted acetylenes [194—196] revealed the formation of trans- as well as c/s-olefins, it was generally assumed that the trans-isomer was formed by isomerisation of the cis-ole-fin. However, more recent studies have shown that this view may have... 

As mentioned in Section 3.3.1, allenes 169 are not formed from alkenylpalladium 167. However, aryl-substituted allenes 187 are obtained predominantly by the coupling of aryl bromides with dialkylacetylenes 186 [80]. 

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). 

Angle strained cycloalkynes undergo electrophilic additions typical for open-chain dialkylacetylenes, at a faster rate. 

Bridged cations have been indicated as intermediates of the addition of bromine to dialkylacetylenes on the basis of the exclusive trans mode of the reactions. Bridged cations are in each case likely to be formed in the addition of sulphenyl derivatives to alkyl- as well as to arylacetylenes. The retention of trans configuration and the anchimeric assistance effects observed in SNl-type reactions of j8-arylthiovinyl sulphonates indicate that the bridged geometry is always favoured over a linear one. 

With dialkylacetylenes, the products of hydrolysis and oxidation are cis-alkenes and ketones, respectively. 

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. ... 

Mono- and di-alkylacetylenes are prepared from sodium alkydes and primary alkyl halides which lack branching on the second carbon atom. The branched primary halides as well as secondary and tertiary halides undergo dehydrohalogenation to olefins by the basic alkyde. The alkydes are best prepared from the acetylenes and sodium amide in liquid ammonia.The yields of 1-alkynes are frequently 70-90% when alkyl bromides are employed as alkylating agents. Dialkylacetylenes... 

A recent synthesis of methylenomycin-B (74), based on these observations, features an allylation/car-bonylation (69) - (70) followed by a C-1—C-2 bond/nickel acyl insertion (70) -> (71) and a final meth-oxycarbonylation (71) -> (72). Thus 2-butyn-l-ol (1,2-dialkylacetylenes are inert) afforded in one synthetic operation a 1 4 mixture (78%) of regioisomeric cyclopentenones (72) and (73) which was converted to the antibiotic (74) (Scheme 16). ... 

Dialkylacetylenes give acyloins in aqueous media and z-methoxyketoncs in methanol ... 

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. " ... 

Further reports have appeared on the reaction of trivalent phosphorus compounds with acetylene dicarboxylates. In the first, alkyl diphenylphosphinites (e.g.57) are shown to react with dialkylacetylene dicarboxylates (e.g.58) in the presence of carbon dioxide to form 1,2-oxaphosphol-3-enes (e.g.59) which in the presence of excess phosphinite decompose via (60) to give di-ylids (e.g.61). On the other hand, the phosphoranes (62) from phosphonites and phosphites react with a further phosphorus component to give the ylids (63) which are readily converted by treatment with alcohol into phosphonates (65) apparently via ketene intermediates (64) as evidenced by and isotopic tracer studies. ... 

Pinnau I, He Z, and Morisato A. S3mthesis and gas permeation properties of poly(dialkylacetylenes) containing isopropyl-terminated side-chains. J. Membr. Sci. 2004 241 363-369. 

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