Defining the Synthetic Scope of ortho-Quinone Methides by Quantifying Their Electrophilicity.

Abstract

A series of aryl-substituted ortho-quinone methides (oQMs) was synthesised and structurally characterised. Kinetic studies of the nucleophilic additions of carbanions (reference nucleophiles) to oQMs were used to determine second-order rate constants k2 for the carbon-carbon bond forming reactions (20°C, DMSO) at the oQMs' exocyclic π-bond. Analysing the kinetic data by the linear free energy relationship lg k2 = sN(N+E) revealed the Mayr electrophilicities E of the oQMs. The electrophilicities E of oQMs correlate linearly with Hammett substituent constants and experimentally determined reduction potentials Epred as well as with quantum-chemically calculated methyl anion affinities (MAAs), which provides valuable tools for prediciting the reactivity of further types of oQMs. Embedding the oQMs in Mayr's reactivity scales enables to predict novel nucleophilic reaction partners for oQMs and can productively be used to prepare simple Michael adducts as well as 4+2 or 4+1 cyclisation products as demonstrated in this work by several novel reactions with neutral or negatively charged C-, N-, and S-nucleophiles.