Alkyne dimerization

This type of alkyne dimerization is also catalyzed by certain nickel complexes, as well as other catalysts and has been carried out internally to convert diynes to large-ring cycloalkynes with an exocyclic double bond. ... 

In another type of alkyne, dimerization is the reductive coupling in which two molecules of alkyne, the same or different, give a 1,3-diene ... 

When desired vinylidene-mediated pathways are not sufficiently favorable. Group 9 metal catalysts can access a set of typical side-reaction pathways. Alkyne dimerization to give conjugated enynes or higher oligomers is often observed. Polysubstituted benzenes resulting from [2 + 2 + 2] alkyne cyclotrimerization are also common coproducts. Fortunately, the selectivity of rhodium and iridium catalysts can often be modulated by the variation of spectator ligands. 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

Miyaura and coworkers have disclosed an iridium-catalyzed alkyne dimerization reaction whose stereochemical outcome is determined by ligand structure (Equation 9.7) [21]. 

Scheme 9.10 lr(l)-catalyzed alkyne dimerization leading to butatriene derivatives. 

Table 9.11 Diiridium complex 58-catalyzed alkyne dimerization.

Chatani and coworkers published an efficient method for the Rh(I)-catalyzed anti-Markovnikov hydroamination of terminal alkynes using either primary or secondary amines [58]. This reactivity had been observed earlier in the course of their studies on hydrative alkyne dimerization (Equation 9.8). 

Scheme 10.19 Proposed catalytic cycles for terminal alkynes dimerization.

The radical anion of /3-trimethylsilylstyrene also undergoes dimerization but coupling takes place at the carbons a to silicon 33). The kinetics of the alkyne dimerization, followed by ESR, showed the reaction to be second order in radical anion 43). With Li+, Na+, K+, or Rb+ as the counterions, the rate increases in the order Si < C < Ge 45). Consistent with the increased stability of the trimethylsilyl-substituted radical anion, the radical anion of 1,4-bis(trimethylsilyl)butadiyne, produced by reduction with Li, Na, K, Rb, or Cs in THF is stable at room temperature even on exposure to air, whereas the carbon analog, 1,4-di-r-butyl-1,3-butadiyne radical anion, dimerizes by second-order kinetics at -40° (42). The enhanced stability of the trimethylsilylalkynyl radical anions has been attributed to p-drr interactions (42). 

Another reaction of 1-alkynes that extends the carbon chain is a coupling reaction in which the alkyne dimerizes under the influence of a cuprous salt, usually cuprous ammonium chloride ... 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

Cp Ru [14] and TpRu [20] complexes have also been studied in depth. As represented in Scheme 2c, the catalytic alkyne dimerization proceeds via coordinatively unsaturated ruthenium alkynyl species. Either a direct alkyne insertion and/or previous vinylidene formation are feasible pathways that determine the selectivity. The head-to-tail dimer cannot be formed by the vinylidene mechanism, whereas the E or Z stereochemistry is controlled by the nature of the alkynyl-vinylidene coupling. 

It is noteworthy that alkyne dimerization may also lead to butatriene derivatives RCH=C=C=CHR. This product was first found for t-butylacetylene with... 

Although alkyne dimerization reactions have been observed with a variety of transition metal catalysts, selectivity is highly dependent not only on the metallic center but also on the ancillary ligands and the substrates. 

Scheme 13. Alkyne dimerization and coupling with L-glutamic acid catalyzed by[Cp RuCI(COD)].

Scheme 9 Acetylene activation via jr-complexation with copper(I) and proposed mechanism for the formation of diacetylene bond via cuprated alkyne dimers...

Terminal alkynes can undergo several types of interaction with ruthenium centres. In addition to the formation of ruthenium vinylidene species, a second type of activation provides alkynyl ruthenium complexes via oxidative addition. When these two types of coordination take place at the same metal centre, the migration of the alkynyl ligand onto the Ca atom of the vinylidene can occur to form enynyl intermediates, which upon protonation by the terminal alkyne lead to the formation of enynes corresponding to alkyne dimerization... 

Insertion and condensation reactions of alkynes have been well known for many years (1,191). Alkyne dimerization to form either cyclobutadiene (192) or metallocyclopentadiene products (193), trimerization to arenes (194), condensation of two alkynes and carbon monoxide to cyclo-pentadienones (195), and condensation of alkyne and isonitriles to cyclo-pentadienimines (196) are common reactions. This section surveys alkyne insertion products originating from monomeric Mo(II) and W(II) reagents. 

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