Contributions of Tunneling in 8π-6π Electrocyclic Cascade Reactions of Bicyclo[4.2.0]octa-2,4-diene Moieties

Six-electron electrocyclic reactions usually require relatively high temperatures; however recent research has shown that such reactions can occur at significantly lower temperatures in biosynthetic and biomimetic pathways. Pathways resulting in bicyclo[4.2.0]octa-2,4-diene moieties arise from thermally allowed 8π-6π electrocyclization cascade reactions of 1,3,5,7-octatetraenes, as in the biosynthesis of endiandric acids, elysiapyrones, and numerous other natural products. We report multidimensional tunneling calculations to explore the possible contribution of heavy-atom tunneling (e.g. by carbon) to biosynthetic pathways and biomimetic syntheses, and thus to provide a more complete picture of biochemical kinetics. M06-2X/cc-pVDZ calculations on the 8π-6π cascade cyclizations of methylated octatetraene model systems suggest that at 240 to 340K, tunneling contributions were low at 20% to 10% during the 8π electrocyclization, and moderate at around 32% to 17% during the 6π electrocyclization. These values suggested tunneling contributes significantly to––but does not dominate––the thermally allowed 8π-6π cascade