Reactions of Other Unsaturated Compounds

Jochem Henkelmann, Jan-Dirk Arndt, Roland Kessinger 

Alkynes are highly reactive building blocks in synthesis which, despite the fact that their positive enthalpy of formation (acetylene Hp = +229.4 kJ/mol) [1] makes them metastable at room temperature, react only at elevated temperature, under increased pressure, and in the presence of suitable catalysts. Under these conditions they are able to take part in a large number of reactions, which are subdivided below into two main groups reactions with retention of or with transformation of the triple bond. For clarity there is further division, in accordance with conventional practice, into the basic reactions of vinylation, ethynylation, carbonylation, and cyclization, although these do not reflect the variety of reaction paths and mechanisms. The cyclization reactions are excluded from the following review since they are dealt with in detail in Section 3.3.8. 

Only some of the syntheses with acetylene which are practiced industrially are carried out with homogeneous metal catalysis initially it was predominantly the late transition metals Fe, Co, Ni, Cu, Zn, Cd, Hg, and Pd which were employed. In recent times the use of the noble metals Rh, Ru, Pd, and Pt has opened up a multiplicity of new reaction possibilities for alkynes. These methods have quickly found their way into the toolkit of the synthetic organic chemist. 

Reactions of acetylene with retention of the triple bond lead to valuable building blocks for synthesis, which can be made to undergo further functionalization. In industry, the most important examples of ethynylation are C-C linkages by addition of acetylene to carbonyl compounds. 

In volume terms, the most important ethynylation product is butynediol. It is prepared with copper acetylide catalysis [5] from aqueous formaldehyde and acetylene (eq. (1)). Heterogeneous copper catalysts, on support materials, are most commonly used for this reaction. 

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