Effect of a Methoxy Substituent on the Vinylcyclobutane Carbon Migration

Abstract

Over the temperature range 250-300 degrees C, 8-exo-methoxybicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-exo- and 5-endo-methoxybicyclo[2.2.2]oct-2-enes, 2a and 2b, respectively, with a clear preference for the si product: si/sr = 3.2. Both 1a and its 8-endo epimer 1b experience appreciable epimerization and fragmentation. A long-lived intermediate with weakly interacting diradical centers, one of which is stabilized by a methoxy substituent, can account for all such observations.

Gas Phase Thermal Reactions of exo-8-Cyclopropyl-bicyclo[4.2.0]oct-2-ene (1-exo)

“…The si/sr ratios for the more conformationally labile bicyclo[4.2.0]oct-2-enes are lower than those reported for bicyclo [3.2.0]hept-2-enes [3]. The relative kinetic importance of the observed exit channels for bicyclo[4.2.0]oct-2-enes labeled with a deuterium [4], methyl [5], or methoxy [3] at a migrating carbon is k ep > k f ≥ k 13 .…”

“…The si/sr ratios for the more conformationally labile bicyclo[4.2.0]oct-2-enes are lower than those reported for bicyclo [3.2.0]hept-2-enes [3]. The relative kinetic importance of the observed exit channels for bicyclo[4.2.0]oct-2-enes labeled with a deuterium [4], methyl [5], or methoxy [3] at a migrating carbon is k ep > k f ≥ k 13 . The abbreviation k ep represents the rate of epimerization or one-centered stereomutation at C8; k f , the rate of direct fragmentation; k 13 , the total rate of [1,3]-sigmatropic migration including both si and sr products.…”

“…Woodward and Hoffmann, in their Conservation of Orbital Symmetry treatise, formally classified this type of skeletal rearrangement as a [1,3]-sigmatropic carbon migration [1]. For a given vinylcyclobutane leading from a migrating carbon OPEN ACCESS to a migration terminus, four discrete products could be formed by si, sr, ai, and ar routes.…”

“…The relative kinetic importance of the observed exit channels for bicyclo[4.2.0]oct-2-enes labeled with a deuterium [4], methyl [5], or methoxy [3] at a migrating carbon is k ep > k f ≥ k 13 . The abbreviation k ep represents the rate of epimerization or one-centered stereomutation at C8; k f , the rate of direct fragmentation; k 13 , the total rate of [1,3]-sigmatropic migration including both si and sr products. This order is markedly different from the observation that [1,3]-carbon shifts afford the dominant products in the corresponding bicyclo[3.2.0]hept-2-enes [3,6,7].…”

“…The abbreviation k ep represents the rate of epimerization or one-centered stereomutation at C8; k f , the rate of direct fragmentation; k 13 , the total rate of [1,3]-sigmatropic migration including both si and sr products. This order is markedly different from the observation that [1,3]-carbon shifts afford the dominant products in the corresponding bicyclo[3.2.0]hept-2-enes [3,6,7]. An early review of [1,3]-carbon rearrangements offered a significant prediction of the role of exit channels such as fragmentation and epimerization in the mechanistic assessment process: "The nature of exit channels such as fragmentations and stereomutations, which are undoubtedly mediated by diradical transition structures, are important in the formulation of a consistent mechanistic framework" [2].…”