Reactivity of Pnictaalumenes Towards 1,3-Dipole Molecules

Alkynes undergo 1,3-dipolar cyclization reactions with organic azides giving 1,2,3-triazoles. Pnictaalumenes RPn═AlR' are the isoelectronic congeners of alkynes, hence a similar reactivity toward 1,3-dipole molecules is expected. Herein, we report the reactions of ^DippTerPn═AlCp* (^DippTer  =  2,6-(2,6-iPr₂C₆H₃)-C₆H₃, Cp*  =  [Me₅C₅], Pn  =  P, As) toward aryl azides. Instead of the anticipated phosphaaluminatriazole products from a [3+2] cyclization, the formation of four-membered AlN₂Pn ring systems of the general form ^DippTerPn(μ-NR)₂AlCp* was observed. Depending on the steric profile of the azide, different outcomes were observed: a) N_α insertion into the Pn═Al bond accompanied by twofold N₂ elimination, b) N_γ insertion under preservation of the N₃ motif, or c) mixed N_a/N_γ products. The bonding situation is complex and best described as Al(III) complexes with NPnN-amino-amide-type ligands. A twofold insertion also occurs with the isovalent electronic diazoalkane Me₃Si(H)C═N₂, yielding imine-substituted AlN₂Pn derivatives. In the case of its P-congener, two thermal rearrangements were observed, which first result in a AlN₃P ring species with a terminal Al─CN unit, followed by a second slower rearrangement to an aluminate cyanido complex with an unusual NNN pincer-type ligand framework. The current study contrasts well-established [3+2] cyclization reactions between alkynes and 1,3-dipoles and provides access to unprecedented ligand motifs.