Acidity of Acetylenes and Cu+-Acetylenes Complexes from Electronic Structure Calculations

Beattie, Jeffrey W., and Jonathan E. Stevens

Often reactions are catalyzed by the presence of metal ions. The role of copper in catalyzing the Sonagashira reactions1,2,3, a multistep coupling reaction which results in the linking of a terminal alkyne to an organic halide, is not completely understood. Researchers speculate that it proceeds with the deprotonation of a terminal alkyne, which is theoretically facilitated by the copper (I) cation.

            This work uses ab initio molecular orbital and density functional calculations to understand the impact of complexation of Cu+ with acetylene and phenylacetylene. Modeling is done in the gas phase and in organic solvents. Results suggest that the resulting complex, which forms through coordination of π electrons to the Cu+ ion, is stable. The energy required to deprotonate the complex is reduced compared to that of the alkyne alone, hence increasing the acidity.

 

1. Chinchilla, R.; Nájera, C. The Sonogashira Reaction:  A Booming Methodology in Synthetic Organic Chemistry. Chem. Rev. 2007, 107, 874-922.

2. Stephens, R. D.; Castro, C. E. The Substitution of Aryl Iodides with Cuprous Acetylides. A Synthesis of Tolanes and Heterocyclics. J. Org. Chem. 1963, 28, 3313-3315.

3. M. J. Mio, private communication