Frontier-controlled cycloaddition reactions of cyclopentadienones having electron-donating or -attracting substituents: configuration of adducts and kinetic studies

Abstract

The stereochemistry of the main adducts resulting from the cycloadditions of 2,5-dimethoxycarbonyl- and 2,5-diethyl-3,4-diphenylcyclopentadienones [CPC (1a) and EPC (1b)] with para-substituted styrenes (2a–h), norbornadiene (7), norbornene (8), 1,4-dihydro-1,4-epoxynaphthalene (9), maleic anhydride (10), and N-phenylmaleimide (11), was established as having the endo configuration. In order to define the substituent effects on the rate of the Diels–Alder reaction, a kinetic study of the reactivities of (1a, b) with a series of styrenes (2a–h) was carried out. The log k/k_H values for the cycloadditions of (1a) with (2a–h) were correlated with Okamoto–Brown's σ_p⁺ constants and the ρ value was –0.941. This result suggests that the cycloaddition is classified as a Diels–Alder reaction with inverse electron demand. By contrast, in the correlation for the cycloaddition of (1b) with (2a–h), two lines with markedly different slopes were observed, which can be ascribed to a Diels–Alder reaction with neutral electron demand. The initial cycloadditions of (1a) and (1b) with tropone (29) gave the [4 + 6]π adducts (32a,b). Subsequently the adduct (32b) underwent oxy-Cope rearrangement to (35). The cycloadditions of (1a) and (1b) with 2-chlorotropone also gave oxy-Cope-type rearrangement products (34a,b).