Acetylenic triple-bond building blocks

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

Tang BZ (2008) Construction of functional polymers liom acetylenic triple-bond building blocks. Macromol Chem Phys 209 1303-1307... 

Vinyl acetate is one of many compounds where classical organic chemistry has been replaced by a catalytic process. It is also an example of older acetylene chemistry becoming outdated by newer processes involving other basic organic building blocks. Up to 1975 the preferred manufacture of this important monomer was based on the addition of acetic acid to the triple bond of acetylene using zinc amalgam as the catalyst, a universal reaction of alkynes. 

Substituted cyclopropyl rings conjugated with a triple bond system have recently received attention as C5 building blocks. The procedure described here is a modification of the decarboxylation-elimination reaction for the preparation of a.3 acetylenic acids from enol sulfonates of acyl malonates. Addition of aqueous alkali to the enol sulfonate of diethyl cyclopropyl carbonyl malonate gives cycl opropyl propiol ic acid, but the yield is 1 ow. 

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. 

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. 

Suzuki-Miyaura cross-coupling polymerization of 1,4-bis((Z)-2-bromovinyl)benzenes with aryl-bis-boronic acids. The interest has been in an alternative approach, where rather than building a PPV with a pre-ordained stereochemistry, a postpolymerization yyn-selective reduction on a poly(phenylene ethynylene) (PPE) is used [125]. This scheme has the advantage that high molecular weight PPEs can be synthesized using either Pd-catalysis or alkyne metathesis. This route could also potentially allow for the access to an additional array of PPVs that are uniquely accessible from PPEs. The transformation of the triple bonds in PPEs and other acetylene building blocks to alkenes has considerable potential. 

Figure 5.3.10 summarizes the most important industrial applications of acetylene. Note that acetylene produced from CaC2 was the most important base chemical up to the 1960s and many important intermediates were produced at that time by addition reactions to the triple bond of acetylene. Since then acetylene has been increasingly replaced as key building block by ethylene. Ethylene production is sig-niflcantly cheaper than acetylene production and technical chemists have learnt over the years to produce the same intermediate compounds from ethylene rather than from acetylene. 

Haussler M, Qin A, Tang BZ (2007) Acetylenes with multiple triple bonds a group of versatile A -type building blocks for the construction of functional hyperbranched polymers. Polymer 48 6181-6204... 

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