The pressure-dependence of the dimerization of 1,3-butadiene: Experimental and theoretical volumes of activation and reaction

Abstract

The volume of activation found for the [4+2]-Diels-Alder cyclodimerization of 1,3-butadiene leading to 4-vinylcyclohexene turned out to be substantially lower than that found for the competing [2+2]-cyclodimerization leading to trans-divinylcyclobutane (ΔΔ₀ ^#= −12cm³mol⁻¹). The [4+2]-cyclodimerization of Z, Z-1,4-dideuterio-l,3-butadiene shows only 3 % loss of stereochemistry at 1 bar and <1 % at 6.8–8.0 kbar. These findings show good evidence for a stereospecific pericylic Diels-Alder mechanism competing with a small amount of nonstereospecific stepwise reaction which is almost completely suppressed at high pressure. The volumes of activation and reaction for the various dimerization pathways of 1,3-butadiene are calculated by a Monte-Carlo computer simulation. The good agreeement between experimental and simulated data confirms the hypothesis that configurational effects (e.g. different packing of cyclic and acyclic states) are important for the explanation of activation and reaction volumes.