Alkynes, activation dimerisation

Cyclotrimerisation, Dimerisation and Nucleophilic Substitution. These three reactions are promoted by a class of catalysts where the bimetallic mechanism is generally very well understood. In this section the diversity of alkyne activation processes is highlighted through the characterisation of intermediate structures or their analogues. 

Numerous systems have been examined for photochromic activity and many of these involve the isomerisation of aromatic ring systems a monograph has been published which surveys many of these rearrangements. A short review, written in Chinese, on the photo-dimerisation of aromatic compounds and the photoaddition of enones and alkynes to aromatic rings has been reported. ... 

One of the most well-studied bimetallic catalysts used for the C-C coupling of alkynes are the thiolato-bridged diruthenium complexes 7 (Scheme 5) [22]. In the presence of NH4BF4 these complexes catalyse the head-to-head dimerisation of a number of terminal alkynes to selectively yield Z-enynes [23]. In contrast, related monometallic Ru catalysts typically yield a mixture of E- and Z-isomers, with the E-isomer more commonly favoured [24-26]. Previous work has shown that diruthenium complexes such as 7 are exceptionally robust due to the strong bridging ability of the thiolate ligands, which results in retention of the dinuclear core during reaction [27]. The proposed mechanism for the dimerisation reaction involves a concerted activation process where both Ru centres activate one alkyne each via the catalytic cycle shown in Scheme 5. Initial coordination of the first alkyne yields the vinylidene intermediate 8. The second alkyne then coordinates to... 

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