4-Methyl-phenylacetylene cation

A cationic rhodium complex-catalyzed codimerization of 1,3-dienes with alkynes gives the corresponding cyclohexadienes in good yields with high regioselectively, as exemplified in the reaction of 2-methyl-l,3-butadiene with phenylacetylene (Eq. 12) [31]. 

The cationic binuclear complex [(Mes3PAu)2Cl]BF4 has been found to catalyse the reaction of 2-methyl- or 2-pentyl-furan (78) with phenylacetylene, pent-l-yne, or hept-l-yne (79) to afford the products of a twofold hydroarylation of the alkyne (80).120... 

Silylene 1 is an unusually versatile catalyst for alkene and alkyne polymerization. The list of compounds polymerized by 1 includes ethene, propene, 1-hexene, styrene, dimethylbutadiene, vinylidene chloride, vinyl ethyl ether, methyl methacrylate, and phenylacetylene. The polymerization does not seem to take place by any of the usual mechanisms, anionic, cationic or free-radical. Instead it somewhat resembles coordination polymerization, as observed for Ziegler-Natta type catalysts. Silylene 2 also catalyzes the polymerization of 1-hexene, but the polymerization is 10 to 100 times slower than with 1. 

Further on, the cationic 18-election Ru-vinylidene complexes (e.g. 39, 40, and 41), introduced by Grubbs and co-workers (Scheme 25) were screened for their catalytic properties attesting that they performed well only with a small range of olefin substrates. More effective in metathesis, several neutral and cationic 16- and 18-electron Ru-tridentate complexes, were reported by van Koten et al. as obtained from reaction of [RuCl2(NN N)(PPh3)] (where NN N is the 2,6-bis[(dimethylamino)methyl]pyridine ligand) with two equivalents of AgPFJ, in the presence of an excess of phenylacetylene (isolated yield 95%). 

Conjugated dienes can be selectively hydrated to ketones in the presence of cationic ruthenium complexes with bipyridyl ligands. The role of ruthenium is to catalyze the isomerization of allylic alcohols formed by the addition of water to diene. This method allows one to convert butadiene to methyl ethyl ketone in high yield [187]. Hydration of triple bonds is one of the oldest catalytic processes of organic chemistry. Though this reaction has no industrial value, it can serve as a tool of fine organic synthesis. The hydration can be catalyzed by rhodium salts under phase-transfer conditions [188]. The more exotic process of the hydrolysis of phenylacetylene to toluene and carbon monoxide catalyzed by ruthenium complex should also be mentioned [189] ... 

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