Origin of the preference for the chair conformation in the Cope rearrangement. Effect of phenyl substituents on the chair and boat transition states

Abstract

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.

Do Secondary Orbital Interactions Really Exist?

“…However, an experimental study on the effect of phenyl substituents has shown that the energy difference between boat and chair transition states cannot be justified in terms of these interactions. 17 In fact, a simple force field model that takes into account only nonbonding steric interactions (MM2) is able to justify the differing stabilities of the two transition states. In summary, it can be concluded that if sufficient experimental or theoretical evidence were known at that time, serious doubts regarding the effectiveness of SOI would surely have been raised.…”

Substituent Effects and Nearly Degenerate Transition States: Rational Design of Substrates for the Tandem Wolff−Cope Reaction

“…To explain why the ketene group stabilizes 11 and 13 to nearly equal energies, we first note that Cope transition states can be viewed as a resonance hybrid of aromatic and diradical forms ) in model transition states 14 − 16 , we see that a diradical form contributes significantly to 16 -chair but not 16 -boat, a difference consistent with divergent radical-chair vs. aromatic-boat pathways in the ketene Cope reaction.…”

Attempts to prepare an all-carbon indigoid system

“…As far as the mode of formation of 23 is concerned, the isomerization formally is a [3.3]sigmatropic rearrangement (Cope rearrangement). Since the rearrangement of structurally similar compounds [ 24 ], including the parent system hexa-1,5-diene, requires much higher temperatures than those given in Scheme 5 , we assume that the metalorganic reagent (or products derived therefrom) play a role in the isomerization.…”