Free Energy Relationships for Reactions of Substituted Benzhydrylium Ions: From Enthalpy over Entropy to Diffusion Control

Abstract

Second-order rate constants k(2) for the reactions of various donor- and acceptor-substituted benzhydrylium ions Ar(2)CH(+) with π-nucleophiles in CH(2)Cl(2) were determined by laser flash irradiation of benzhydryl triarylphosphonium salts Ar(2)CH-PAr(3)(+)X(-) in the presence of a large excess of the nucleophiles. This method allowed us to investigate fast reactions up to the diffusional limit including reactions of highly reactive benzhydrylium ions with m-fluoro and p-(trifluoromethyl) substituents. The rate constants determined in this work and relevant literature data were jointly subjected to a correlation analysis to derive the electrophilicity parameters E for acceptor-substituted benzhydrylium ions, as defined by the linear free energy relationship log k(2)(20 °C) = s(N)(N + E). The new correlation analysis also leads to the N and s(N) parameters of 18 π-nucleophiles, which have only vaguely been characterized previously. The correlations of log k(2) versus E are linear well beyond the range where the activation enthalpies ΔH(++) of the reactions are extrapolated to reach the value of ΔH(++) = 0, showing that the change from enthalpy control to entropy control does not cause a bend in the linear free energy relationship, a novel manifestation of the compensation effect. A flattening of the correlation lines only occurs for k(2)> 10(8) M(-1) s(-1) when the diffusion limit is approached.

In Situ-Generated Glycinyl Chloroaminals for a One-Pot Synthesis of Non-proteinogenic α-Amino Esters

“…Several phenolic derivatives were selected to compare their innate reactivity (electronic and steric factors), while heteroaromatic substrates also showed a broad scope of reactivity as demonstrated by the range of temperature utilized (entries 1–5). For example, anisole ( N = −1.2, CH 2 Cl 2 ), 27 furan ( N = 1.4, CH 2 Cl 2 ), 28 anthracene ( N = 2.0, CH 2 Cl 2 ), 29 and bromoresorcinol and a chalcone substrate showed a very modest reactivity in CHCl 3 as anticipated from the corresponding nucleophilic factor ( N ); therefore, these substrates were subjected to more efficient reaction conditions in CH 3 CN (method B). We then observed a complete switch of reactivity for the most electronically rich arenes, for which the mildest reaction conditions in CHCl 3 (method A) afford the best results in terms of yield and regioselectivity (entries 6–12).…”

IDPi‐Katalyse

“…[20] Seit dem ersten Bericht 1978 von Herold und Hoffmann über eine enantioselektive Va riante, [21] bei der ein Campher-abgeleitetes chirales Allylboronat als Reagenz verwendet wurde,f olgte eine Unzahl enantioselektiver Allylierungsmethoden auf Basis sowohl chiraler Auxiliare [22] als auch chiraler Katalysatoren. Dieses Reagenz, das erstmals 1976 von Hosomi und Sakurai füreine Lewis-Säure-vermittelte Allylierung von Aldehyden und Ketonen verwendet wurde, [23] ist ein sehr schwaches Nukleophil [24] und bedarf daher starker Lewis-Säuren füre ine ausreichende Aktivierung der elektrophilen Carbonylverbindungen. Dieses Reagenz, das erstmals 1976 von Hosomi und Sakurai füreine Lewis-Säure-vermittelte Allylierung von Aldehyden und Ketonen verwendet wurde, [23] ist ein sehr schwaches Nukleophil [24] und bedarf daher starker Lewis-Säuren füre ine ausreichende Aktivierung der elektrophilen Carbonylverbindungen.…”

Reactivity scales for quantifying polar organic reactivity: the benzhydrylium methodology

“…Most E, N, and s N parameters derived for these reference compounds in 2001 by least-squares minimization remained unchanged until today, and only E-values for few highly reactive benzhydrylium ions as well as the nucleophile-specific parameters derived from reactions with these electrophiles were adjusted in 2012. 13,14 In order to characterize more reactive nucleophiles, e.g., carbanions, less reactive compounds were needed as reference electrophiles. It was straightforward to use quinone methides for .…”