Evidence for a diradical intermediate in the Diels–Alder reaction of 2‐butene with cyclohexa‐1,3‐diene

Abstract

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-2-butene (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.

Kinetics of the diels–alder addition of ethene to cyclohexa‐1,3‐diene and its reverse reaction in the gas phase

“…The biradical mechanism (6) would then be excluded. This is, however, not consistent with the lost of stereospecificity observed for the additions of cis-and trans-2-butene to CHD [4], so that for the reaction of E with CHD a diradical ring closure with no or a very small activation energy seems appropriate.…”

“…This mechanistic hypothesis is supported strongly by a recent stereochemical study of the additions of cis-and trans-2-butene to CHD and the reverse Diels-Alder reaction of trans-5,6-dimethyl-bicyclo[2.2.2]oct-2-ene [4].…”

“…The gas phase Diels-Alder additions of propene and acrolein to cyclohexa-1,3-diene (CHD) and their reverse reactions probably proceed through a biradical intermediate [l-31. This mechanistic hypothesis is supported strongly by a recent stereochemical study of the additions of cis-and trans-2-butene to CHD and the reverse Diels-Alder reaction of trans-5,6-dimethyl-bicyclo[2.2.2]oct-2-ene [4].…”

The Low Energy of Concert in Many Symmetry‐Allowed Cycloadditions Supports a Stepwise‐Diradical Mechanism

“…. Trans‐2‐butene cycloadds at 600° to cyclohexadiene with 6.85% rotation ; E con < 4.3. Hexachlorocyclopentadiene undergoes DA reactions with six dienophiles with 27–95% rotation (v.i.…”

ChemInform Abstract: Kinetics and Mechanism of the Thermal Reactions of endo‐ and exo‐5‐Cyanobicyclo(2.2.2)oct‐2‐ene and Methyl‐Substituted Derivatives in the Gas Phase

“…They also suggest that the position of the rate-determining transition state depends on the amount of stabilizing energy in the concerned biradicals. Indeed, it would lie between (i) the cycloadduct and the biradical when the biradical is only stabilized by the ally1 resonance in the ring (BO [21 and alkyl-substituted derivatives [1,5,14,16] and ethene elimination in this work) and (ii) the biradical and diene and dienophile when it is further stabilized by the resonance energy of a substituent (cyano-and vinyl-substituted BOs [3,6,13, and CHD elimination in this work] and dimers of CHD [151).…”