Selective 1,2-insertion of carbodiimides and substrate-divergent silyl group migration at 1-metallacyclobuta-2,3-dienes

Abstract

The study of new, unusual structures and reactivity patterns is of great interest for various field of organic and organometallic chemistry. Metallacycles of electropositive transition metals represent an excellent platform for this purpose. In this work, we present the synthesis, characterisation, and reactivity toward carbodiimides of a set of ring-strained group 4 ansa-metallocene-based 1-metallacyclobuta-2,3-dienes Cp′₂M(Me₃SiC₃SiMe₃) (M = Ti, Zr; Cp′₂ = indenyl or tetrahydroindenyl based ansa-Cp ligand). In line with experimental studies, computational analysis of the electronic structure of new and previously reported 1-metallacyclobuta-2,3-dienes for the first time shows that the biradical character of these species not only depends on the metal, but also strongly on the nature of the cyclopentadienyl ligands. In reactions with carbodiimides RNCNR, depending on the substituents of the carbodiimide, either 2-aza-1-metallacyclobut-3-ene (R = iPr, Cy, and p-Tol) or 2-aza-1-metallacyclohexa-2,4,5-triene (R = SiMe₃) complexes were produced. Mechanistic studies by DFT calculations reveal that 1,2-insertion of the carbodiimide followed by migration of a SiMe₃ group of the metallacycle occurs for R = iPr, Cy, p-Tol, whereas for R = SiMe₃ insertion is followed by SiMe₃ migration along the carbodiimide. The reaction of a 2-aza-1-metallacyclobut-3-ene complex with benzophenone produces a zirconocene amidopyridine complex through a series of unprecedented bond activation and formation events, once again demonstrating the potential of early transition metal complexes for activation and transformation of small molecules.