Acetylenic triple bonds

Hydrogenations involving consecutive reactions are common in the organic process industry and even in the hydrogenation of fats. In the fine chemicals industry we have examples of acetylenic (triple) bonds to be selectively converted to olefinic (double) bonds. Lange et al. (1998) have shown, for the comversion of the model substance 2-hexyne into cis-2-hexene, how catalytically active microporous thin-film membranes can accomplish 100% selectivity. This unusual selectivity is attributed to avoidance of backmixing. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Going to extremes, the reactivity of internal acetylenic triple bonds compared with terminal olefinic double bonds was also checked. Diallyl ethers of commercial 2-butyne-l,4-diol and 3-hexyne-2,5-diol are available in high yield by phase transfer etherification. They are reacted under essentially the same conditions as those described in section 3.1, with the double bond now being in 100 percent excess at the beginning (Eq. 4). 

This disadvantage can be ruled out by spacers between the allylic and the acrylic group [15], but the selectivity in favor of the allylic group is not improved. An acetylenic triple bond instead helps to clarify the situation. 2-Propynoxyethyl acrylate, available in 90 % yield from ethoxylated propargylic alcohol by esterification, is hydrosilylated very smoothly only at the triple bond, leaving the acrylic side virtually untouched (Eq. 5). 

Independently Volpin17 synthesized diphenyl cyclopropenone from diphenyl-acetylene and dibromo carbene (CHBr3/K-tert.-butoxide). This reaction principle of (2 + 1) cycloaddition of dihalocarbenes or appropriate carbene sources ( caibenoids ) to acetylenic triple bonds followed by hydrolysis was developed to a general synthesis... 

Metal-catalyzed C-H bond formation through isomerization, especially asymmetric variant of that, is highly useful in organic synthesis. The most successful example is no doubt the enantioselective isomerization of allylamines catalyzed by Rh(i)/TolBINAP complex, which was applied to the industrial synthesis of (—)-menthol. A highly enantioselective isomerization of allylic alcohols was also developed using Rh(l)/phosphaferrocene complex. Despite these successful examples, an enantioselective isomerization of unfunctionalized alkenes and metal-catalyzed isomerization of acetylenic triple bonds has not been extensively studied. Future developments of new catalysts and ligands for these reactions will enhance the synthetic utility of the metal-catalyzed isomerization reaction. 

Substitution of an acetylene triple bond for the terminal double bond provides an easy entry to /-progesterone (32) [9] (Scheme 13.3.7) ... 

Hydroxy-terminated polyester (HTPS) is made from diethylene glycol and adipic acid, and hydroxy-terminated polyether (HTPE) is made from propylene glycol. Hydroxy-terminated polyacetylene (HTPA) is synthesized from butynediol and paraformaldehyde and is characterized by acetylenic triple bonds. The terminal OH groups of these polymers are cured with isophorone diisocyanate. Table 4.3 shows the chemical properties of typical polymers and prepolymers used in composite propellants and explosives.E4 All of these polymers are inert, but, with the exception of HTPB, contain relatively high oxygen contents in their molecular structures. 

Similar reactions applied to transition metal-acetylene complexes appear capable of separating the 2 carbon atoms originally linked by the acetylenic triple bond 18). Thermal isomerization of metal-acetylene complexes may achieve the same result, showing how metal clusters can catalyze scrambling reactions of acetylenes, e.g.. 

These covalent bond lengths are reasonably constant among molecules, as the paraffin C—C bond usually has a length of 154 pm, the olefin C=C double bond has a length of 134 pm, and the acetylenic triple bond has a length of 120 pm. The C—H bond is 109 pm in a paraffin and 105 pm in an acetylene. 

Other algal carotenoids contain acetylenic triple bonds. For example, alloxanthin has the following structure at both ends of the symmetric molecule. The symmetric carotenoids canthaxanthin and astaxan-thin have oxo groups at both ends ... 

The current trend for vinyl chloride monomers is toward ethylene as the hydrocarbon raw material, replacing electrochemical acetylene, and it promises to continue. Electrochemical acetylene will be phased out almost completely, except in special cases. The high activation energy level required for forming the acetylene triple bond precludes design of a low cost process for its formation. In addition, the difficulties encountered in handling such a highly reactive material will deter its use. 

Both main group and transition metal elements interact with the acetylenic triple bond in a variety of reactions, including hydrogenation, hydrometallation, hydration and cycloadditions. Notably, in most reactions the cyclopropane ring remains intact. 

Structural analysis of the homopolymers by spectroscopic methods confirmed that the diynes had undergone [2 + 2 + 2] polycyclotrimerizations by forming new benzene rings from their acetylenic triple bonds. The ratio of the 1,2,4- to 1,3,5-isomers of the trisubstituted benzene rings was estimated to be 2.2 1. Careful evaluation of the 111 NMR spectra unveiled that the number of terminal triple bonds in the final hb-PAs was much smaller than that in an ideal hyperbranched structure produced by the diyne polycyclotrimer-ization. This result suggests that intra-sphere ring formation might have been involved in the cyclotrimerization polymerization. 

The hb-PAEs of hb-P13 and hb-P15 contain NLO-active azo-functionalities, which are soluble, film-forming, and morphologically stable (Tg > 180 °C). Their poled films exhibited high SHG coefficients ( 33 up to 177pm/V), thanks to the chromophore-separation and site-isolation effects of the hyperbranched structures of the polymers in the three-dimensional space (Table 5) [28]. The optical nonlinearities of the poled films of the polymers are thermally stable with no drop in d33 observable when heated to 152 °C (Fig. 8), due to the facile cross-linking of the multiple acetylenic triple bonds in the hb-PAEs at moderate temperatures (e.g., 88 °C). 

Haufiler, M. and Tang, B. Z. Functional Hyperbranched Macromolecules Constructed from Acetylenic Triple-Bond Building Blocks. Vol. 209, pp. 1-58... 

Amyl alcohol was selected as a solvent with a convenient boiling point for the dehydrohalogenation. Acetylenic triple bonds are attacked by water in the presence of strong alkali or strong acid. 

Partial shielding by a triple bond. When the acetylenic triple bond is aligned with the magnetic field, the cylinder of electrons circulates to create an induced magnetic field. The acetylenic proton lies along the axis of this field, which opposes the external field. 

In general, the acetylenic triple bond is highly reactive toward hydrogenation, hydroboration, and hydration in the presence of acid catalyst. Protection of a triple bond in disubstituted acetylenic compounds is possible by complex formation with octacarbonyl dicobalt [Co2(CO)g Eq. (64) 163]. The cobalt complex that forms at ordinary temperatures is stable to reduction reactions (diborane, diimides, Grignards) and to high-temperature catalytic reactions with carbon dioxide. Regeneration of the triple bond is accomplished with ferric nitrate [164], ammonium ceric nitrate [165] or trimethylamine oxide [166]. 

By hydrogenation of the acetylenic triple bond, butynediol can be converted into butenediol and into the particularly important butanediol. Partial hydrogenation is effected with noble metal catalysts under mild conditions, whereas total hydrogenation is usually carried out over heterogeneous Co and Ni catalysts. 

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