Deuterium, reaction with acetylene

The first reaction is only possible if Z)(CH) in acetylene <112-2 kcal. If HgH is formed directly in the primary step the upper limit to Z)(GH) is raised to 120-7 kcal. The reaction of deuterium atoms with acetylene may provide additional evidence. Geib and Steacie 1 8 found the activation energy to be <5 kcal. There are two possible mechanisms ... 

It is generally agreed that the kinetics and the distributions of deuter-ated products from the reactions of alkynes or alkadienes with deuterium are satisfactorily interpreted in terms of the consecutive addition of two hydrogen atoms, of unspecified origin, to the adsorbed hydrocarbon to yield the monoolefin. The identity of the distributions of deuteroethyl-enes from the reaction of acetylene with equilibrated and non-equil-ibrated hydrogen—deuterium mixtures also provides strong evidence for such a mechanism [91]. 

The interaction of an alkyne or alkadiene with deuterium leads to the formation of deuteroalkenes whose isotopic composition yields valuable information regarding possible reaction mechanisms. In an attempt to interpret in detail the deuteroalkene distributions, two approaches have been used. The first, due to Bond [163], is a simplified version of the general theory proposed by Kemball for the hydrogenation of ethylene (see Sect. 3.4) and has been used to interpret the results of the reaction of acetylene with deuterium [163—165]. The method comprises a steady state analysis of the reaction scheme... 

Among the early systemmatic studies of the metal-catalysed hydrogenation of acetylene were those of Sheridan et al. [158,168—170] who investigated the kinetics and product distributions over pumice-supported metals. Subsequently, the reaction has been extensively studied by Bond et al. [9,165,171—175] over pumice- and alumina-supported metals and metal powders. The reaction of acetylene with deuterium over nickel [91, 163] and alumina-supported noble Group VIII metals [164,165] has also been investigated. 

The reaction of acetylene with deuterium has been studied over alumina-supported noble Group VIII metals [164,165], whilst over nickel-pumice catalysts the reaction of perdeuteroacetylene with hydrogen has been investigated [163]. In both of these studies, the deuteroethylene distributions have been interpreted in terms of the steady state analysis discussed in Sect. 4.4. Typical deuteroethylene distributions together with the values of p, q and s are shown in Table 16. 

An unsymmetrical acetylenic intermediate is formed from 3-bromo-2-cyclooctenone (177) which gave on reaction with base in methanol-d a substituted methoxyether (179) with over 90% vinylic deuterium (Eaton and Stubbs, 1967). The intermediate was shown to be 2-cyclo-... 

The reaction sequences shown in equations 4 and 5 involve the reduction of the appropriate acetylenic tetrahydropyranyl (THP) ethers with (Ph3P)3RhCl and deuterium gas. 

Intermolecular reactions of propargylic alcohols with a-methylstyrene gave the corresponding 1-hexene-5-ynes in moderate yields with complete regioselectivity (Scheme 7.30). The incorporation of a deuterium atom at the C-6 position (acetylenic terminal carbon) of the product and a substantial isotope effect (kH/fco = 4) were observed when a-methylstyrene-methyl-dj was used in place of a-methylstyrene. It is considered that the Cp-Cy double bond of an allenylidene complex reacts with a-methylstyrene, where the allenylidene complex works as an enophile, to afford the corresponding vinylidene complex via an allenylidene-ene reaction, as shown in Scheme 7.30. 

This was explained by the involvement of a vinylidene complex that is also in agreement with the migration of the acetylenic hydrogen to C-2 observed by deuterium labeling. The stereoselective reaction requires the use of EtjN and a slight excess of the alkyne. 

In comparison with the extensive studies of acetylene itself, the hydrogenation of monoalkylacetylenes has received much less attention. The hydrogenation of methylacetylene over pumice-supported metals [170, 181,184,185] and over metal powders [181—185], has been studied. No studies of the reaction of propyne with deuterium have been reported. 

The production of acetylene as the major gaseous product in the reaction of butadiyne (equation 9) occurs in part via C2H radicals when short wavelength radiation is used , but with 254 nm radiation it is suggested that a molecular process occurs in a non-linear excited state. The evidence for this additional process is that no C2HD is formed when perdeuteriopropyne is present as a source of deuterium to trap the radicals. 

To prove this course, they carried out some investigations with d(1)-heptyne(l). The cyclization reaction is slower with the deuterated acetylene and, in the presence of Ni(CO)2(PPh3)2, there is deuterium exchange between D(l)heptyne(l) and pentyne(l). This apparently indicates that the catalysis initiates with the breaking of the =C—H bond. Recent studies with platinum support this hypothesis even if, as in this case, the t compound is more stable than the hydride. In fact, Pt(PPh3)2(HC=CPh) can also be obtained by the following ways 38, 83) ... 

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