Dissolving metal reduction of acetylenes: a computational study

Area of Science:

• Physical Chemistry
• Computational Chemistry

Background:

• The reduction of alkynes to alkenes is a fundamental organic transformation.
• Liquid ammonia is a common solvent for such reductions, influencing reaction pathways.

Purpose of the Study:

• To computationally investigate the mechanism of the two-electron, two-proton reduction of alkynes to trans-alkenes.
• To model the influence of liquid ammonia as a solvent on the reduction process.

Main Methods:

• High-level ab initio single-point computations were performed.
• The polarizable continuum model (PCM) was used to simulate liquid ammonia.
• Two approaches were used: PCM alone and PCM with ten explicit ammonia molecules.

Main Results:

• Relative energies of key species (alkyne, radical anion, vinyl anion, dianion, alkene) were calculated.
• The two computational methods yielded nearly identical results.
• The radical anion is likely bent in ammonia.
• Trans stereochemistry is probably determined at the vinyl anion stage.

Conclusions:

• The vinyl anion is the likely determinant of trans stereochemistry in alkyne reduction.
• Explicit solvent molecules have a negligible effect on relative energies compared to the PCM model.
• Solvent effects of ammonia were analyzed in relation to gas-phase properties.