Propyne trimerization

In contrast to the work of Peterson (38) with 3-hexyne in trifluoroacetic acid (vide supra), no trimeric adducts were observed in the hydrogen halide additions to propyne (48). 

The so-called trimerization of propynal in the presence of piperidine acetate provides a synthesis of 4-ethynyI-4//-pyran-3,5-dicarbaldehyde (149) (50LA(568)34> it should be noted that the structure proposed for the product in the original work has been corrected (64CB1959). In the absence of moisture, the reaction fails and it seems likely that the synthesis involves hydration of the alkyne to the divinyl ether. Finally, condensation with the third molecule of the aldehyde results in cyclization to the product (Scheme 20). 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

The products of the radiolysis of butyne-2, propyne, pentyne-2, hexyne-3, and butyne-1, have been determined by Rondeau et Dimers, trimers and tetra-mers, in decreasing importance, are the most significant but many minor products including aromatic compounds, are observed. A free-radical mechanism is suggested. 

The olefin metathesis reaction is also suitable for alkynes. In the presence of a metathesis catalyst alkynes can undergo one of the three following reactions (1) cyclo-trimerization (e. g., trimerization of propyne into 1,3,5- and 1,2,4-trimethylbenzene) (2) polymerization, e. g., of phenylalkynes (eq. (9)) and (3) metathesis (rupture and reformation of carbon-carbon triple bonds), e. g., eq. (10). 

Moreover, it has been shown (Imamura et al. 1995a) that such rare-earth imide or imide-like species exhibit oligomerization activity of alkynes. Selective cyclic dimerization and trimerization of propyne and ethyne to cyclohexadiene and benzene occur at 453 K during the oligomerization, respectively, in which the active catalysts are characterized as rare-earth imides induced by the thermal treatment of R/C. 

---