Hexynes isomerization

Pukhnarevich et al. (55) showed that the structure of silanes, R3SiH (in which R3 was 15 combinations of R = Me, Et, n-Pr, i-Pr, i-Bu, Me3SiO—, C1CH2, EtO, i-PrO, and Ph) had very little effect on the ratio of isomeric adducts that form by their addition to 1-hexyne with chlo-roplatinic acid according to Eq. (60). 

The thermal isomerization of higher terminal alkynes also delivered some allene, from 1-hexyne and 1-heptyne, for example, some 1,2-diene was formed [30]. With an ,/l-unsaturated unit in the alkyne 9, a photochemical isomerization to 10 was successful but delivered only a low yield and 11 as a significant side-product [31]. These reactions tolerate different functional groups alcohols, ethers or, as in 12, tertiary amines and nitriles have been used (Scheme 1.5) [32, 33],... 

Murakami and colleagues132 studied the Diels-Alder reactions of vinylallenes with alkynes catalyzed by a rhodium complex. When a vinylallene lacking substituents at the vinylic terminus was reacted with a terminal alkyne, 1,3,5-trisubstituted benzenes were obtained, the reaction between vinylallene 197 and 1-hexyne (198) being a representative example (equation 55). The reaction was proposed to proceed via a rhodacycle which afforded the primary Diels-Alder adduct via reductive elimination. Aromatization via isomerization of the exocyclic double bond led to the isolation of 199. 

Knox et al. (31) have also reported that cyclic 1,3-dienes react in a Diels-Alder fashion with the acetylene adduct, 4 (eq. 26). The Mo-Mo distance in 10 is 2.504(1)A, commensurate with a MosMo triple bond as required by the 18-electron rule. Acyclic dienes do not give 10, but with hexa-2,4-diene, 4 yields ethyne and the hexyne complex from the isomerized diene (eq. 27). 

Moore and Peters have observed an interesting rearrangement reaction during the electrolysis of 1-phenyl-l-hexyne in DMF at a Hg cathode. When the acetylene was reduced at coucentrations of less than 2mM, 1-phenylhexane was formed by the usual 4e process however, at higher concentrations (2-1 OmM) the acetylene underwent an isomerization to l-phenyl-l,2-hexadiene (2). This allene, being readily reduced, was converted initially into a mixture of alkenes and finally into 1-phenylhexane (Scheme 5). 

Our second example concerns the cathodic reductions of 6-X-l-phenyl-1-hexynes in DMF - . 1-Phenyl-1-hexyne (> 0002m) first isomerizes to l-phenyl-1,2-hexadiene which is reduced in steps, or (<0-002m) is reduced in one 4e-step to 1-phenylhexane . At the higher concentrations, a radical anion could be detected by cyclic voltametry (100 V/s ), although the usual measurements indicate irreversibility . Here the concentration dependence is attributed to competition between further reduction (low m) and isomerization (high m) of the starting material. 

For 6-chloro-l-phenyl-l-hexyne (X = Cl, > 0 002m), isomerization to the allene is promoted at the cathode and subsequent stepwise reduction (equation 79) to three... 

In the presence of [Cp2Mo2(CO)4], 3 > [Ni2(COD)2( JL-ri2-RC R)],223 = and [Ni4(RNC)4( X3-V-RC=CR)3], alkynes are converted to ds-alkenes. The alkenes once formed no longer bind to the complex and hydrogenation does not proceed further to give alkanes or even isomerized alkenes. Cocondensation of lanthanoid metal atoms with internal alkynes generates lanthanoid complexes of alkynes, which are potential catalysts for hydrogenation. [ Sm(l-hexyne) n] or [ Er(3-hexyne) n] catalyzes hydrogenation of hex-3-yne to ds-hex-3-ene (97% cis) at room temperature and atmospheric pressure of H2. ... 

Hydroboration of 1-hexyne with bis(tran5-2-methylcyclohexyl)borane followed by treatment with iodine and aqueous NaOH results in the transfer of one alkyl group from boron to the adjacent carbon. The hydroxide-mediated deboronoiodination gives isomerically pure cA-alkenes. ... 

The complex [(OC)4Fe(//-PPh2)Pd( -Cl)]2 is a selective catalyst for the isomerization of 1-octene to 2-octene and the hydrogenation of 1-hexyne in the presence of 1-hexene. At 448 K, under 100 atm H2, 93% of a sample of 1-hexyne in benzene was reduced to hexene and only 3% to hexane. This is unexpected because palladium is usually an excellent catalyst for the hydrogenation of olefins. It also catalyzes the carbonylation of 1-octene under mild conditions (348 K, 50 atm). The total yield of esters was ten times greater than with [PdCl2(PPh3)2] as a catalyst. This bimetallic complex was also an effective catalyst for the carbonylation of 1,5-cyclooctadiene. ... 

Hydroboration of terminal alkynes, e.g. 1 -hexyne, 1 -octyne or cyclohexylacetylene, with a dialkylborane, such as bis(l, 2-dimethylpropyl)borane, followed by copper(I)cyanide and copper(II) acetate in HMPA containing a trace of water, gives isomerically pure ( )-l-cyanoalk-l-enes (equation 29)133. Successive treatment of 1-bromo-l-alkynes with dialkylboranes and sodium methoxide results in the borinate esters 208, which are converted into ( )-alkenes of greater than 99% isomeric purity by protonolysis. The action of alkaline hydrogen peroxide on the borinates produces ketones (equation 30)134. 

The action of the complex 570 on various alkynes gives diverse products ethyl propiolate affords a mixture of all three possible triethyl benzenetricarboxylates, phenylacetylene yields mainly the trans-QnyviQ 571 3-hexyne behaves similarly, but 1-octyne isomerizes to 2-octyne . The palladacyclopentadiene 572 catalyses the co-trimerization of hex-3-yne... 

Phenyl-1-propyne (55) underwent facile formal intermolecular hydroamination, affording the allylic amine 56 in high yield at 0 "C in the presence of AcOH or benzoic acid. In this reaction, at first, Pd-catalyzed isomerization of 55 to pheny-lallene (57) occurs by addition-elimination of H-Pd-OAc to internal alkyne 55, and then the allene 57 is converted to jr-allylpalladium intermediate 58 by hydropal-ladation. The final step is a well-known amination to produce the allylic amine 56. As an intramolecular version, 2-(2-phenylpropenyl)pyrrole (60) was obtained from l-phenyl-7-amino-l-hexyne 59 [16,16a]. Similarly Pd/benzoic acid-catalyzed hydroalkoxylation of 55 with (—)-menthol (61) afforded the allylic ether 62 [17]. 

Takeuchi has shown that the stereoselectivity of alkyne hydrosilylation can be controlled by both the ligand and the solvent (Equation 16.31). addition of the Si-H bond across the alkyne has been observed for the hydrosilylation of hexyne by triethyl-silane in ethanol solvent in the presence of a catalyst generated from the combination of [Rh(COD),] " and PPhj. In contrast, trans addition occurs for the same reaction in the same medium with a neutral rhodium catalyst lacking added phosphine. Trans addition also occurs in the presence of Wilkinson s catalyst in toluene. (Equation 16.32). As discussed in Section 16.3.5.2.4 below, trans addition likely results from the isomerization of a vinyl intermediate. -... 

KNH2/AI2O3 is effective for the isomerization of alkynes. Thus, 1-hexyne is isomerized exclusively to 2-hexyne in a 92% yield in dioxane at 333 K in 20 h. The isomerization of alkyn-l-yl alcohol to a, /J-unsaturated ketones proceeds over solid-base catalysts (72). Thus, in the following reaction, the a, /3-unsaturated... 

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