Novel diborane-analogue transition structures for borane reactions with alkyl halides

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Organic Reaction Mechanisms

Background:

• Borane is a reducing agent with applications in organic synthesis.
• Understanding the reduction mechanisms of alkyl halides and formaldehyde by borane is crucial for synthetic planning.

Purpose of the Study:

• To investigate the reaction mechanisms of alkyl halides and formaldehyde reduction by borane using computational methods.
• To elucidate the role of transition structures and solvation effects in these reductions.

Main Methods:

• Ab initio calculations
• Density Functional Theory (DFT) methods
• Transition state modeling
• Solvation effect analysis using dimethyl ether and dimethyl sulfide

Main Results:

• Optimized transition structures for alkyl halide reduction by borane resemble diborane, featuring bridging hydrogen atoms.
• Similar bridging transition structures were identified for formaldehyde reduction, though not the lowest energy pathway.
• Solvation by dimethyl ether/sulfide disrupted bridging in chloromethane reduction and led to ligand dissociation in formaldehyde reduction.

Conclusions:

• High calculated activation free energies for alkyl halide reduction correlate with their low observed reactivity with borane.
• Solvation significantly influences the reduction mechanisms and transition state geometries.
• Computational insights provide a mechanistic basis for the differing reactivities observed in borane reductions.