Hexynes hydrogen bonding

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. 

In an extension to this work, treatment of the template with 5-hexynal under the standard dehydrating conditions furnished the cycloadduct 301 in good yield (68). However, structural analysis of both the product and the azabicyclo[3.3.0]octane-3-carboxylic acid, derived by hydrogenation of the double bond, followed by... 

A stereospecific synthesis for cw-3-hexen-l-ol starts with the ethylation of sodium acetylide to 1 -butyne, which is reacted with ethylene oxide to give 3-hexyn-l-ol. Selective hydrogenation of the triple bond in the presence of palladium catalysts yields cw-3-hexen-l-ol. Biotechnological processes have been developed for its synthesis as a natural flavor compound, e.g., [12]. 

Hexyne has the triple bond in the middle of a carbon chain and is termed an internal alkyne. If, instead, an alkyne with the triple bond at the end of the carbon chain, a 1-alkyne or a terminal alkyne, were used in this reaction, then the reaction might be useful for the synthesis of aldehydes. The boron is expected to add to the terminal carbon of a 1-alkyne. Reaction with basic hydrogen peroxide would produce the enol resulting from anti-Markovnikov addition of water to the alkyne. Tautomerization of this enol would produce an aldehyde. Unfortunately, the vinylborane produced from a 1-alkyne reacts with a second equivalent of boron as shown in the following reaction. The product, with two borons bonded to the end carbon, does not produce an aldehyde when treated with basic hydrogen peroxide. 

The strongest C—H bond is that of a 1-alkyne, where the hydrogen is bonded to an A/ -hybridized carbon. Therefore, the C—H stretch of a 1-alkyne occurs at the highest wavenumber of all C—H bonds, near 3300 cm-1. The spectrum of 1-hexyne is shown in Figure 13.7. 

The interaction of a lanthanide metal with a substrate such as 3-hexyne could occur in several ways 60) by it complex formation, by oxidative addition into a C—H bond, or by reduction involving radical species. Subsequent lanthanide metal vapor studies were designed to test some of these possibilities. Co-condensation of lanthanide metal vapor with reagents containing acidic hydrogen atoms, e.g., terminal alkynes, demonstrated that oxidative addition of C—H was a viable reaction [Eqs. (32) and (33)] 51). These reactions also provided access to a new class of... 

Hydrogenation of Compounds with C C Bonds 1-Hexyne could be hydrogenated using a PVP-stabihzed Rh nanopartides dispersion previously described in Section 11.3.1.1.1 [15]. 

W.J.Evans (33,34) prepared some divalent organolanthanides by co-condensation at low temperature of lanthanide metal vapours with unsaturated hydrocarbons (cyclopentadienes, alkynes) containing acidic hydrogen. Some organolanthanides showed catalytic activity. Thus, Sm(C Me ) (THF) catalyzes hydrogenation of 3-hexyne into cis-hexene (cis trans > 99 1 under mild conditions 25°C, 1 atm of hydrogen. The reaction is believed to involve the addition of a hydride Ln-H to the triple bond followed by hydrogenolysis with H (35). The same complex polymerizes ethylene (35). /... 

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