EVIDENCE FOR PERICYCLIC AND STEPWISE PROCESSES IN THE CYCLODIMERIZATION OF CHLOROPRENE AND 1,3-BUTADIENE FROM PRESSURE-DEPENDENCE AND STEREOCHEMISTRY - EXPERIMENTAL AND THEORETICAL VOLUMES OF ACTIVATION AND REACTION

From the pressure dependence of cyclodimerization of chloroprene, it h as been concluded that volumes of activation may be useful for the dis tinction between competing concerted and stepwise cycloadditions. Acco rdingly, one of the [4 + 2] cyclodimers, 1,4-dichloro-4-vinylcyclohexe ne (3), should be formed in a stepwise Diels-Alder reaction involving a diradical intermediate analogously to the [2 + 2] cyclodimerizations leading to cis- and trans-1,2-dichloro-1 ,2-divinylcyclobutane (5 and 6). A stereochemical analysis using (E)-1-deuteriochloroprene (1-D-E shows, indeed, a nonstereospecific course for the formation of the for mal Diels-Alder adduct 4 (cis:trans ratio of 59.6:40.4), as expected f or a stepwise process, and confirms the conclusion drawn from pressure dependence. The mechanism of the Diels-Alder dimerization of 1,3-buta diene (13) is elucidated by an investigation of the effect of pressure and the stereochemistry. The activation volume found for the Diels-Al der dimerization leading to 4-vinylcyclohexene (16) turned out to be s ubstantially lower than that found for the competing [2 + 2] cyclodime rization leading to trans-1 ,2-divinylcyclobutane (17) (Delta Delta V = -13.3 cm(3)/mol). The dimerization of (Z,Z)-1,4-dideuterio-1,3-buta diene (13-1,4-D-Z) shows only 3% loss of stereochemistry in the format ion of 16-D at 1 bar and <1% at 6.8-8 kbar (cis:trans ratios of 97:3 a nd >99:<1, respectively). These findings provide good evidence for a s tereospecific pericyclic Diels-Alder mechanism competing with a small amount of nonstereospecific stepwise reaction which is almost complete ly suppressed by high pressure. Volumes of activation and reaction are calculated for the Diels-Alder reaction of ethene with 1,3-butadiene (13) and the various dimerization pathways of 1,3-butadiene (13) by a Monte Carlo computer simulation using the model of hard spheres to des cribe the various ground and transition structures. The good agreement between calculated and experimental data shows that volumes of activa tion and reaction cannot be explained by properties of single molecule s. They require consideration of configurational effects (e.g., the di fferent packing of cyclic and acyclic states) for their interpretation . Thus, activation volumes can provide important information on transi tion-state geometries.