Substituent Effects on the Photogeneration and Selectivity of Triaryl Vinyl Cations

Abstract

The photochemical reactions of a series of triaryl vinyl halides 1X in acetic acid and in acetonitrile have been studied using product analysis as a function of the time of irradiation. The quantum efficiencies of formation of the products derived from the photogenerated vinyl cations 1(+) depend on the alpha-aryl substituent, the beta-aryl substituent, the leaving group X (= bromide or chloride), and the temperature at which the irradiations are carried out. Hammett correlation or noncorrelation of the alpha-aryl substituent effects with (excited-state) substituent constants indicates that the ions 1(+) are formed directly from the excited states of 1X by heterolytic cleavage of the carbon-halogen bond. Homolytic cleavage, yielding radicals 1(*), is a parallel process: the partitioning into ion and radical occurs in the excited state. This conclusion is corroborated by the leaving group effect and the temperature effect. By performing the photochemical reactions of 1X in the presence of HOAc and/or NaOAc as well as the labeled common halide ion (82)Br(-) or (36)Cl(-), the relative reactivities of the cations 1(+) toward these nucleophiles were determined. The selectivities follow the Reactivity-Selectivity Principle. The temperature effect data show that the reactions of the cations with the anionic nucleophiles are (de)solvation controlled and their reactions with the neutral nucleophile activation controlled.

BF3·Et2O-Induced Decomposition of Ethyl 2-Diazo-3-hydroxy-3,3-diarylpropanoates in Acetonitrile: A Novel Approach to 2,3-Diaryl β-Enamino Ester Derivatives

“…To substantiate this electronic effect, the relative migratory aptitude of the aryl groups, indicated as the logarithm of the 20 / 19 ratio, was analyzed with their respective parametrized resonance effects ( R eff ), inductive effects ( F ), and Hammett σ values . As shown in Figure , the data fit well to R eff ( r 2 = 0.91), indicating that an aryl with electron-donating group (for mesomeric effect) is endowed with a higher tendency to migrate at the more electrophilic carbon, in agreement with the α-vinyl cation formation . On the contrary, no correlation was observed either with the inductive effects ( F ) or with the Hammett σ values (data not shown), thus ruling out that the resonance is the only electronic parameter which influences the aryl migration process.…”

Phenanthrene Synthesis by Palladium-Catalyzed Benzannulation with o-Bromobenzyl Alcohols through Multiple Carbon–Carbon Bond Formations

“…Alkynes 5a and 5i ; and aryl halides 6a – 6g were obtained from commercial suppliers. ( Z )-β-Halostyrenes 1a , 1b , 1c , 1g , 1h , and 1i ; o -bromobenzyl alcohols 2a – 2c and 2d ; benzochromene 4 ; alkynes 5b , 5c , 5d , 5e , 5f , 5g , and 5h ; and chrysene 10 were known compounds.…”

Synthesis of Multisubstituted Triphenylenes and Phenanthrenes by Cascade Reaction of o-Iodobiphenyls or (Z)-β-Halostyrenes with o-Bromobenzyl Alcohols through Two Sequential C–C Bond Formations Catalyzed by a Palladium Complex

“…( Z )-β-Halostyrenes 9b , 9h , , 9i , and 9j were prepared by literature procedures. Spectroscopic data of 9b , 9c , 9d , 9h , 9i , and 9j can be found in the literature.…”