Gas-Phase Acidity of C2H2 and CU+-C2H2 Complexes From Electronic Structure Calculations

Stevens, Jonathan, and Jeffery Beattie

Reactions can often be catalyzed by the presence of metal or metal ions. This work is a preliminary study on the role of copper atoms in Sonagashira1 reactions, a type of multistep reaction coupling reaction which results in the linking of a terminal alkyne to an organic halide. The complete mechanism for this reaction is unknown, but researchers speculate2,3 that it proceeds through deprotonation of the alkyne, a process which is facilitated by complexation of the alkyne to a copper (I) cation.

This work implements ab initio molecular orbital and density functional calculations to study the impact of complexation with Cu+ on the acidity of the simplest alkyne, C2H2. Our early gas-phase results show that Cu+-C2H2 forms a stable complex through coordination of p electrons in C2H2 to the metal ion. The energy required for removal of a proton from C2H2 is substantially reduced by this complexation, resulting in an increase in the estimated acidity equilibrium constant. Future work in this area will treat the effect of solvent and explore the possibility of use of other metals than Cu as catalysts in Sonagashira coupling reactions.

REFERENCES

1. Chinchilla, R.; Nájera, C. Chem. Rev. 2007, 107, 874-922

2. Stephens, R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 3313-3315

3. M Mio, private communication