T-BUTYL-SUBSTITUTED CYCLOPENTADIENONES

Monomeric 3-t-butyl- and 2,4-di-t-butylcyclopentadienones (3 and 18) have been isolated. Their ultraviolet spectra are similar to that of fulvene and probably correspond closely to that of cyclopentadienone. The unusually high field chemical shifts of τ 5.07 and 3,50 for the α and β protons of 18 are thought to reflect largely an increased π-electron density at the respective olefinic carbon atoms relative to those in cyclopentenone. The isolation of monomeric 2-t-butylcyclopentadienone (26) was not successful because of the pronounced reactivity of 26 toward dimerization. Diels-Alder dimerizations of 18, 3, and 26 occur stereoselectivey with the formation of endo dimers and with relative rates of 1, 5 × 106, and >108. The stereochemical courses and relative rates of these dimerizations appear to be controlled mainly by nonbonded steric effects. A rate constant of 108 M−1 sec−1 for the dimerization of cyclopentadienone at 30°, which approaches that of about 1010 M−1 sec−1 for diffusion control, was estimated from a combination of second-order perturbation and electrostatic interaction theories and experimental rate constants for dimerizations of several five-membered ring dienes and dienophiles. The small energy gap between the highest occupied and lowest unoccupied π-MO´s of cyclopentadienone is thought to be mainly responsible for the large magnitude of this estimated rate constant. Rates of dimerizations of several derivatives of cyclopentadiene were estimated and a qualitative explanation was given for the unusually high reactivity of the ethylene ketal of cyclopentadiene toward. © 1969, American Chemical Society. All rights reserved.