RESEARCH ARTICLE Stereoselectivity in a series of 7‐alkylbicyclo[3.2.0]hept‐2‐ enes: Experimental and computational perspectives Phyllis Leber | Katherine Kidder | Don Viray | Eric Dietrich‐Peterson | Yuan Fang | Alexander Davis Department of Chemistry, Franklin & Marshall College, Lancaster, PA, USA Correspondence Phyllis Leber, Department of Chemistry, Franklin & Marshall College, Lancaster, PA 17604‐3003, USA. Email: pleber@fandm.edu Funding information Franklin & Marshall College Hackman Fund; American Chemical Society Petro- leum Research Fund, Grant/Award Num- ber: 55266‐UR4 Abstract Rate constants for overall decomposition (k d ) for a series of exo‐7‐ alkylbicyclo[3.2.0]hept‐2‐enes are relatively invariant. For the alkyl substitu- ents ethyl, propyl, butyl, isopropyl, and t‐butyl, the ratio of the rate constant for [1,3] sigmatropic rearrangement to the rate constant for fragmentation, k 13 /k f , is significantly lower than k 13 /k f = 150 observed for exo‐7‐ methylbicyclo[3.2.0]hept‐2‐ene. Regardless of the size and mass of the alkyl group, the stereoselectivity of the [1,3] carbon migration appears to be quite stable at 80% to 89% suprafacial inversion (si), an observation consistent with conservation of angular momentum but not conservation of orbital symmetry. This global result comports with the phenomenon of “dynamic matching” espoused by Carpenter and collaborators for [1,3] sigmatropic rearrangements in general. KEYWORDS [1,3] carbon migration, [1,3] sigmatropic rearrangement, bicyclo[3.2.0]hept‐2‐enes, computational analysis, first‐order kinetics, vinylcyclobutane thermal chemistry 1 | INTRODUCTION Bicyclo[3.2.0]hept‐2‐ene (1) undergoes thermal isomeri- zation to norbornene (2) in the gas phase. This rearrange- ment has served as a historical exemplar of a [1,3] sigmatropic carbon migration. Competitive isomerization and fragmentation processes at temperatures in excess of 300°C convert 1 to its isomer 2 or directly to fragments cyclopentadiene and ethylene (Scheme 1). [1,2] Determina- tion of an accurate rate constant k 13 for the [1,3] rear- rangement 1‐to‐2 and the associated Arrhenius parameters is complicated by the observation that 2 expe- riences facile Diels‐Alder cycloreversion (retro Diels‐ Alder reaction) to cyclopentadiene and ethylene. Despite the large experimental error in activation energy for the [1,3] process, Cock and Frey conclude that the activation parameters for the sum (k 13 + k f ) are close to the expected values “if both processes have a biradical mech- anism” and therefore that “a two‐step pathway is competitive.” [1] 1.1 | Criteria of concert: energetics and stereoselectivity Given the dramatic impact surrounding the publication of The Conservation of Orbital Symmetry the previous year, the tone of the Cocks and Frey paper is all the more remarkable. Woodward and Hoffmann recognized that a “two step, non‐concerted path” [3] for the [1,3] sigmatropic rearrangement of vinylcyclopropane to cyclopentene was thermodynamically consistent with an activation energy ca. 50 kcal/mol, a value virtually identi- cally to that reported by Cocks and Frey. Indeed, of the Received: 30 May 2018 Revised: 19 June 2018 Accepted: 19 June 2018 DOI: 10.1002/poc.3888 J Phys Org Chem. 2018;e3888. https://doi.org/10.1002/poc.3888 © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/poc 1 of 12