Despite an extensive literature, controversy about the Diels-Alder reaction mechanism still exists. For the thermal dimerization of 1,3-butadiene, for example, a concerted one-step [I] or two-stage [2] mechanism, a mixture of orbital symmetry allowed and forbidden cycloadditions 131, as well as a two-step diradical pathway [4-61 have been suggested.
The observed Arrhenius parameters of some gas-phase Diels-Alder additions with cyclohexa-1,3-diene and their reverse reactions 171 all seem to support a similar diradical pathway.1 To test this mechanistic interpretation of kinetic results, we undertook a stereochemical study of the gas-phase addition of trans-2-butene (TB) to cyclohexa-173-diene (CHD) and its reverse reaction. The stereochemistry of the addition of cis-2butene (CB) to CHD was investigated too.
After vacuum distillation and preparative chromatography, the final purity of CHD (Organica) was better than 99.4%. T B (Fluka AG) and CB (L'air Liquide) were distilled in the vacuum line and contained less than 0.02% CB and 0.7% T B respectively. Trans-5,6-dimethylbicyclo [2,2,2] oct-2-ene (TDMBO) and cis-5,6-dimethylbicyclo[2,2,2]oct-2-ene (CDMB0)2 were synthesized under pressure in the liquid phase [8] and identified by infrared [8], nuclear magnetic resonance [8], and mass spectrometry. TDMBO, which was chromatographically pure (impurity less than 0.01%), was used to perform some retro Diels-Alder reactions. Moreover, TDMBO and CDMBO were both used to identify the gas-phase Diels-Alder products. The vacuum line and static reaction system were as described previously [g]. After reaction, the heavier fraction (containing TDMBO and CDMBO) was analyzed by gas chromatography at 80°C on an 8-m column (1/8 inch) 300h tricresylphosphate on chromosorb P (45-60 mesh). The lighter fraction (containing TB and CB) was analyzed at 25°C This diradical pathway assumes that the formation of the first bond as well as the ring By chromatographic analysis, no distinction could be made between endo, endo-and exo, closure of the diradical intermediate can be rate determining. exo-adducts. Therefore, both stereoisomers will be referred to as CDMBO.