Diversity of reactivity modes upon interplay between Au(iii)-bound isocyanides and cyclic nitrones: a theoretical consideration

Abstract

Metal-bound isocyanides demonstrate a very rich chemistry towards 1,3-dipoles of the allyl anion type such as nitrones. Depending on the metal centre, dipole nature and substituent in isocyanides, cycloadducts, imine + isocyanates or metallacycles may be formed. In this work, the reasons for such diversity, intimate details of the reaction mechanism and main factors determining the chemoselectivity in the reaction between Au(III)-bound isocyanides ([AuCl₃(CNR)]) and cyclic nitrones ⁻O–⁺C(H)XX (XXX = CH₂CH₂CMe₂, CH₂CH₂CH₂, OCH₂CMe₂, CH₂OCMe₂ and CH₂CH₂O) are analysed in detail by theoretical (DFT) methods. The formation of cycloadducts is controlled by the LUMO_π*(CN) of isocyanides and it occurs in a stepwise manner via the initial nucleophilic addition of nitrone at the C atom of CNR (except for nitronate ⁻O–⁺C(H)CH₂CH₂). The imine + isocyanate products are formed upon oxygen transfer from nitrones to isocyanides, and N–O bond cleavage is the main factor determining this process. A metallacycle is formed upon decomposition of the cycloadduct, and this process includes deprotonation of the oxadiazoline CH group/N–O bond cleavage and Cl⁻ elimination/cyclization. The main factor controlling the metallacycle formation is the acidity of the endocyclic CH group in the cycloadduct. Effects of the substituent R in CNR and the nitrone nature on the reactivity are analysed.