Rate constants for the reactions of benzaldehyde-derived iminium ions with C-nucleophiles (enamines, silylated ketene acetals, and enol ethers) have been determined photometrically in CH3CN solution and used to determine the electrophilicity parameters E of the cations defined by the correlation log k(20°C) = s(N)(E + N) (Mayr, H.; et al. J. Am. Chem. Soc. 2001, 123, 9500-9512). With electrophilicity parameters from E = -10.69 (Ar = p-MeOC6H4) to E = -8.34 (Ar = p-CF3), the iminium ions Ar-CH═NMe2(+) have almost the same reactivities as analogously substituted arylidenemalononitriles Ar-CH═C(CN)2 and are 10 orders of magnitude more reactive than the corresponding aldehydes. The rate constants for the reactions of iminium ions with amines and water in acetonitrile are 10(3)-10(5) times faster than predicted by the quoted correlation, which is explained by the transition states which already experience the anomeric stabilization of the resulting N,N- and O,N-acetals.
The generation of carbon radicals by halogen-atom and group transfer reactions is generally achieved using tin and silicon reagents that maximize the interplay of enthalpic (thermodynamic) and polar (kinetic) effects. In this work, we demonstrate a distinct reactivity mode enabled by quantum mechanical tunneling that uses the cyclohexadiene derivative γ-terpinene as the abstractor under mild photochemical conditions. This protocol activates alkyl and aryl halides as well as several alcohol and thiol derivatives. Experimental and computational studies unveiled a noncanonical pathway whereby a cyclohexadienyl radical undergoes concerted aromatization and halogen-atom or group abstraction through the reactivity of an effective H atom. This activation mechanism is seemingly thermodynamically and kinetically unfavorable but is rendered feasible through quantum tunneling.
Diaryliodonium salts are well-established compounds in free radical chemistry and are already used as photoinitiators (free radical or cationic polymerization), but the presence of counter anions is a strong drawback. Indeed, a counter anion is always required (e.g., SbF6−) leading to potential toxicity issues or release of HF. In the present paper, counter anion-free and fluoride-free aryliodonium salts are proposed, that is, aryliodonium ylides (AY) are studied here as new and efficient additives for radical chemistry and an example is provided for the camphorquinone (CQ)/amine based photoinitiating systems (PISs) for the polymerization of thick (1.4 mm) and thin (20–13 µm) methacrylates under air and blue light irradiation. The newly proposed PISs, for example, CQ/amine/AY, presented excellent polymerization performances and good bleaching properties were obtained after polymerization. Real-time Fourier transform infrared spectroscopy (RT-FTIR) was used to monitor the photopolymerization profiles. The chemical mechanisms involved were investigated using electron spin resonance (ESR).
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