Conformer-specific polar cycloaddition of dibromobutadiene with trapped propene ions

TL;DR

This study investigates the detailed mechanism of a polar cycloaddition reaction between dibromobutadiene and propene ions, revealing a competition between concerted and stepwise pathways through experimental and computational methods.

Contribution

It introduces a novel experimental approach combining molecular beams and trapped ions to explore conformational effects in gas-phase cycloadditions, highlighting a mechanistic borderline case.

Findings

Both conformers of the diene are reactive with similar rates.

Reaction proceeds via a competition of concerted and stepwise pathways.

The mechanistic nature is a borderline case between concerted and stepwise.

Abstract

Diels-Alder cycloadditions are efficient routes for the synthesis of cyclic organic compounds. There has been a long-standing discussion whether these reactions proceed via stepwise or concerted mechanisms. Here, we adopt a new experimental approach to explore the mechanistic details of the model polar cycloaddition of 2,3-dibromo-1,3-butadiene with propene ions by probing its conformational specificities in the entrance channel under single-collision conditions in the gas phase. Combining a conformationally controlled molecular beam with trapped ions, we find that both conformers of the diene, gauche and s-trans, are reactive with capture-limited reaction rates. Aided by quantum-chemical and quantum-capture calculations, this finding is rationalised by a simultaneous competition of concerted and stepwise reaction pathways, revealing an interesting mechanistic borderline case.