Dimerization of Two Alkyne Units: Model Studies, Intermediate Trapping Experiments, and Kinetic Studies

By means of high level quantum chemical calculations (B2PLYPD and CCSD­(T)), the dimerization of alkynes substituted with different groups such as F, Cl, OH, SH, NH₂, and CN to the corresponding diradicals and dicarbenes was investigated. We found that in case of monosubstituted alkynes the formation of a bond at the nonsubstituted carbon centers is favored in general. Furthermore, substituents attached to the reacting centers reduce the activation energies and the reaction energies with increasing electronegativity of the substituent (<i>F</i> > OH > NH₂, Cl > SH, H, CN). This effect was explained by a stabilizing hyperconjugative interaction between the σ* orbitals of the carbon-substituent bond and the occupied antibonding linear combination of the radical centers. The formation of dicarbenes is only found if strong π donors like NH₂ and OH as substituents are attached to the carbene centers. The extension of the model calculations to substituted phenylacetylenes (Ph–CC–Y) predicts a similar reactivity of the phenylacetylenes: <i>F</i> > OCH₃ > Cl > H. Trapping experiments of the proposed cyclobutadiene intermediates using maleic anhydride as dienophile as well as kinetic studies confirm the calculations. In the case of phenylmethoxyacetylene (Ph–CC–OCH₃) the good yield of the corresponding cycloaddition product makes this cyclization reaction attractive for a synthetic route to cyclohexadiene derivatives from alkynes

Enediyne Dimerization vs Bergman Cyclization

High-level quantum chemical calculations reveal that the dimerization of enediynes to 1,3-butadiene-1,4-diyl diradicals is energetically more favored than the corresponding Bergman cyclization of enediynes. Moreover, the activation barrier of both reactions can be drastically reduced by the introduction of electron-withdrawing substituents like fluoro groups at the reacting carbon centers of the triple bonds

Model Studies on the Dimerization of 1,3-Diacetylenes

By means of high-level quantum chemical calculations (B2PLYPD and CCSD­(T)), the dimerization of 1,3-diacetylenes was studied and compared to the dimerization of acetylene. We found that substituted 1,3-diacetylenes are more reactive than the corresponding substituted acetylenes having an isolated triple bond. The most reactive centers for a dimerization are always the terminal carbon atoms. The introduction of a test reaction allows the calculation of the relative reactivity of individual carbon centers in phenylacetylene, phenylbutadiyne, and phenylhexatriyne. A comparison shows that the reactivity of the terminal carbon atoms increases with increasing numbers of alkyne units, whereas the reactivity of the internal carbon atoms remains very low independent of the number of alkyne units

Au(I)-Catalyzed Dimerization of Two Alkyne UnitsInterplay between Butadienyl and Cyclopropenylmethyl Cation: Model Studies and Trapping Experiments

In recent years, Au­(I)-catalyzed reactions proved to be a valuable tool for the synthesis of substituted cycles by cycloaromatization and cycloisomerization starting from alkynes. Despite the myriad of Au­(I)-catalyzed reactions of alkynes, the mono Au­(I)-catalyzed pendant to the radical dimerization of nonconjugated alkyne units has not been investigated by quantum chemical calculations. Herein, by means of quantum chemical calculations, we describe the mono Au­(I)-catalyzed dimerization of two alkyne units as well as the transannular ring closure reaction of a nonconjugated diyne. We found that depending on the system and the method used either the corresponding cyclopropenylmethyl cation or the butadienyl cation represents the stable intermediate. This circumstance could be explained by different stabilizing effects. Moreover, the calculation reveals a dramatic (>10¹²-fold) acceleration of the Au­(I)-catalyzed reaction compared to that of the noncatalyzed radical variant. Trapping experiments with a substituted 1,6-cyclodecadiyne using benzene as a solvent at room temperature as well as studies with deuterated solvents confirm the calculations. In this context, we also demonstrate that trapping of the cationic intermediate with benzene does not proceed via a Friedel–Crafts-type reaction

Dimerization of Substituted ArylacetylenesQuantum Chemical Calculations and Kinetic Studies

The dimerization of substituted arylacetylenes is a very interesting tool to generate 1,3-butadiene 1,4-diradicals. Recently, it was shown that electron-withdrawing groups attached to the triple bond reduce the activation barrier and increase the stability of the diradical intermediates. Here, we investigate the influence of the π donor character of substituents, which are bound to the aryl system, on the dimerization reaction of arylacetylenes. Both quantum chemical calculations and kinetic studies reveal that the higher the π donor character of substituents, the lower the activation barrier. The highest observed difference between the model systems amounts to 4.0 kcal/mol, which represents an acceleration by a factor of 700. However, according to the calculations the π donor character of the substituents increases the diradical character of the intermediates