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A computational model for the dimerization of allene.

Sarah L Skraba¹, Richard P Johnson

¹Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, USA.

The Journal of Organic Chemistry

|December 4, 2012

Summary

Computational studies confirm allene dimerization to 1,2-dimethylenecyclobutane occurs via diradical intermediates, not a concerted mechanism. This stepwise process explains stereochemical outcomes in racemic versus enantiopure allene dimerization.

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Realistic Membrane Modeling Using Complex Lipid Mixtures in Simulation Studies

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Published on: September 1, 2023

Area of Science:

Computational chemistry
Organic reaction mechanisms

Background:

Allene dimerization to 1,2-dimethylenecyclobutane is a known reaction.
The mechanism, whether concerted or stepwise via diradicals, has been debated.

Purpose of the Study:

To computationally investigate the mechanism of allene dimerization.
To determine if the reaction proceeds through diradical intermediates or a concerted pathway.
To elucidate the stereochemical preferences observed in allene dimerization.

Main Methods:

High-level ab initio computations were performed using CCSD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p) level of theory.
Transition states and intermediates for allene dimerization were located and characterized.
Energy barriers for key steps of the proposed mechanism were calculated.

Main Results:

Two diastereomeric transition states for C2-C2 allene dimerization were identified, with barriers of 34.5 and 40.3 kcal/mol.
Both transition states lead to a singlet bisallyl (tetramethyleneethane) diradical intermediate.
The bisallyl diradical exhibits low barriers for planarization and methylene rotation, facilitating subsequent ring closure.
The diradical closure occurs via conrotatory motion with a barrier of 15.7 kcal/mol.

Conclusions:

Computational results strongly support a stepwise dimerization mechanism involving diradical intermediates.
The findings align with experimental observations regarding the preferential dimerization of racemic 1,3-disubstituted allenes.
The mechanism allows for the retention of stereochemical information, explaining observed product distributions.