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d,l-and meso-2,2'-bis[ 1 -methylene-1,2,3,4-tetrahydronaphthalenes] 8 and 9 were synthesized, and the activation energy parameters for their unimolecular [3,3] sigmatropic rearrangement to l,2-bis(3,4-dihydro-l-naphthalenyl)ethane (13) were determined. The d,l diastereomer is constrained to undergo Cope rearrangement in the chair conformation while the meso diastereomer is constrained to the boat. At 150 °C kdf fcmeM = 7 X 106. A comparison of activation energy parameters allows quantification of the effect of 2,5-diphenyl substituents on the chair-boat energy difference. Comparison with suitable bis(methylenecycloalkane) models reveals that the enthalpy of activation for the chair and boat transition states is lowered the same amount (7.5-8.5 kcal/mol). This result relegates secondary orbital interactions to a relatively minor role as a cause of the chair-boat energy difference. These results do not support the notion that the chair and boat topologies proceed by different mechanisms. The anomalous entropies of activation of the chair and boat transition states can be understood in terms of a resonance hybrid model comprised of two extreme forms (loose and tight), with substituents altering their relative contributions. A simple force field model is used to account for the major differences in energy between the chair and boat transition states.