Experimental and spectroscopic evidence of the hidden triplet transition state of quinone methide: A new reactivity paradigm

Abstract

Quinone methides (QMs), a versatile class of reactive intermediates known for centuries, structurally have predominantly been shown in two states - neutral and zwitterionic, acting as an excellent electrophilic synthon that readily reacts with a wide range of nucleophiles. Herein, we unequivocally reveal the latent triplet biradical canonical form of the para-quinone methide through experiments and are complemented by spectroscopic techniques. Notably, this canonical form exhibits typical radical-like reactivity, including addition reaction with alkene as well as alkynes, formation of adduct with TEMPO, hydrogen atom transfer reactions, etc. Leveraging this diradical structural feature, we have constructed a variety of scaffolds with both synthetic and pharmacological significance, such as s-cis dienes and diaryl- or triaryl-methane. Furthermore, the synthesized products can be readily transformed into tetra-substituted m-alkenes.