Cobalt- and Silver-Promoted Methylenecyclopropane Rearrangements

Area of Science:

• Organometallic Chemistry
• Reaction Mechanism Studies
• Computational Chemistry

Background:

• The methylenecyclopropane rearrangement is a fundamental organic transformation.
• Understanding factors that influence its reaction rate is crucial for synthetic applications.
• Previous studies have explored various catalytic systems for this rearrangement.

Purpose of the Study:

• To investigate the rate-enhancing effects of alkyne-cobalt complexes on methylenecyclopropane rearrangement.
• To explore the role of silver cations in promoting the same reaction.
• To elucidate the mechanistic pathways involved using computational studies.

Main Methods:

• Experimental kinetic studies of methylenecyclopropane rearrangement in the presence of alkyne-Co2(CO)6 complexes.
• Kinetic studies with silver cations, including a series of aryl-substituted methylenecyclopropanes.
• Density Functional Theory (DFT) calculations to model transition states and intermediates.

Main Results:

• Alkyne-Co2(CO)6 complexes significantly enhance the rearrangement rate, attributed to transition state stabilization.
• Computational analysis indicates spin delocalization onto cobalt atoms stabilizes the biradical intermediate.
• Silver cations also accelerate the reaction, with rate enhancements correlating to substituent electronic effects (σ+).

Conclusions:

• Both alkyne-cobalt complexes and silver cations effectively catalyze methylenecyclopropane rearrangement.
• Stabilization of reaction intermediates, particularly biradicals, via spin delocalization is a key mechanistic feature.
• The precise mechanism of silver catalysis (biradical vs. carbocation intermediate) requires further investigation.