Hydrogenation of propyne

Anpo M, Shima T, Kodama S Kubukawa Y (1987) Phtotcatalytic hydrogenation of propyne with water on small-particle titania size quantization effects and reaction intermediates. J Phys Chem 91 4305... 

Acetylene is the most widely used alkyne. Alkynes undergo the same reactions as alkenes. Because a triple bond coimects the two carbon atoms, addition of an atom initially forms a double bond. The addition of a second atom converts the double bond into a single bond. For example, hydrogenation of propyne to propane is represented as ... 

A recent study of the hydrogenation of propyne on Pd/Zr02 showed residual absorptions from surface species after evacuation. These were attributed to di-cr and propylidyne adsorbed species, but no spectra were illustrated (113). SER spectra of the alkynes but-2-yne, pent-l-yne, and pent-2-yne on a gold electrode have been measured (83). At -0.4 V the nC=C modes are lowered by about 110 cm 1 from the values for the free alkynes. For the alk-2-ynes the shift increases by a further 10 cm"1 upon a change of the potential to +0.4 V. The spectrum of but-2-yne has two principal component bands, at 2134 and 2109 cm-1 in the pC=C region, and also shows a downward shift for the nCH3 modes. 

Conversion of a terminal alkyne to its alkynylsilane prevents loss of the relatively acidic terminal hydrogen (pKa of ethyne c. 25) during later synthetic steps. For example, the terminal hydrogen of propyne was masked whilst its propargylic anion was used in a synthesis of Cecropia juvenile hormone, a chemical which plays ail important role in insect development (Figure Si5.2). 

Characterisation of the carbonaceous deposit formed during the selective hydrogenation of propyne over paliadium/zirconia... 

A The alkynyl hydrogen of propyne is acidic and hence the propyne will form an organometallic derivative. The Grignard derivative will react with carbon dioxide to give the carboxylic acid. Cis alkenes are formed by the catalytic hydrogenation of alkynes using a Lindlar catalyst. Hence a propynyl alcohol is a precursor to compound (b). This could be obtained by the addition of a propyne anion to propanone ... 

J.T. Vehrli, O.J. Thomas, D.L. Trlmn, M.S. Walnvrlghc and N.W. Cant, Selectivfl Hydrogenation of Propyne over Supported Copper Catalysts Influence of Support, submitted to Appl> Catal. 

The hydrogenation of molecules of the type HC =CR, where R is an alkyl group, is a largely neglected field. The only mechanistic studies reported to date are the hydrogenation of propyne in the gas phase (78) and of 1-butyne (a) in alcoholic solution (57) and (b) in the gas phase (90). Only the last mentioned study has employed an isotopic tracer. 

Selective Hydrogenation of Propyne over Supported Copper Catalysts Influence of Support, submitted to Appl Catal. 

We have just seen that a conjugated diene is more stable than an isolated diene. Now we need to see why a cumulated diene is less stable than an isolated diene. Cumulated dienes are unlike other dienes in that the central carbon is sp hybridized since it has two tt bonds. In contrast, all the double-bonded carbons of isolated dienes and conjugated dienes are sp hybridized. The sp hybridization gives the cumulated dienes unique properties. For example, the —AH° for hydrogenation of allene is similar to the —AH° for the hydrogenation of propyne, a compound with two sp hybridized carbons. 

Figure 7.4 The results of a PASADENA experiment on the in situ hydrogenation of propyne with pHj over lr(l) (Vaska s) complex immobilized on phosphine-modified silica gel. The H MAS NMR spectrum with corrected baseline is shown in the figure.

Schiff base complex immobilized on a metal-organic framework (MOF) material, designated as lRMOF-3-SI-Au, was shown to be stable under reactive conditions [102,103]. It was used for the gas-phase hydrogenation of propyne and propylene, and PHIP effects were successfully observed for the reaction products (propylene and propane, respectively), with stereoselective syn addition of H2 observed in the hydrogenation of propyne to propylene (Figure 7.7). The reactions were carried out at 130 °C, with no evidence of the metal complex reduction under these conditions. The reaction yields were relatively low, and the signal enhancement factors provided by PHIP were moderate (about 16), which could be the result of an enhanced nuclear spin relaxation of the reaction products in contact with the porous matrix of the MOF support. 

Hydrogenation of other gaseous substrates on supported Pt catalysts was addressed using pH2 as well. Hydrogenation of propyne over Pt/y-Al203 catalyst was shown to produce PHIP in the products of both syn and anti addition of H2 to the substrate [90]. Thus, supported Pt metal catalysts achieve a non-stereoselective pairwise addition of H2 to propyne. The likely reason for this is the ability of supported metal catalysts to readily isomerize propylene. Non-stereoselective pairwise Hj addition to produce 1-butene was also observed in the hydrogenation of 1-butyne [15]. For this substrate, other products are possible. They include 2-butene (cis- and tram-) and butane, and all were shown to exhibit PHIP effects. A detailed scheme describing the formation of all these hyperpolarized products has been proposed [15]. 

Figure 7.10 H NMR spectra detected during the heterogeneous hydrogenation of propyne over Pd/ll7C (a,b) and Pd/C (c) catalysts with normal Hj (b) and pHj (a,c). Reference [101] - adapted by permission of The Royal Society of Chemistry.

The selective hydrogenation of propyne to propene has been studied over porous ultraflltration polymeric membranes of polyacrylonitrile (PAN), polyetherimide (PEI) or polyamideimide (PAI) or over poly(acrylic acid) membranes, treated with palladium acetate. Similarly, porous PVDF membranes, also loaded with palladium particles, were used in hydrogenation flow-through experiments of methylenecyclohexane to methylcy-clohexanone. The hydrogenation of viscous liquids such as vegetable oils could also be performed in flow-through experiments over porous polymeric membranes loaded with palladium or platinum particles. " ... 

Ziegler S, Theis J and Fritsch D (2001), Palladium modified porous polymeric membranes and their performance in selective hydrogenation of propyne , J Membr Sci, 187,71-84. 

Groschel L, Haidar R, Beyer A, Colfen H, Frank B and Schomacker R (2005), Hydrogenation of propyne in palladium-containing polyacryUc acid membranes and its characterization , Ind Eng Chem Res, 44,9064-9070. 

Traced by in situ NMR, the PHIP was observed in the gas-phase hydrogenation of propyne catalyzed by nano-Pd/IL [BMIm][PFg]/activated carbon fibers (Pd /SILP/ACF) [75]. This phenomenon was observed when normal was replaced with para-hydrogen-enriched Hj gas (ortho-H rpara-H 1), Figure 2.31. 

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