Cyclisation of biscarbenoids - a novel mode of cyclobutadiene stabilisation.

The reaction of the vicinal biscarbenoid Pip-C[triple bond, length as m-dash]C-Pip with dimethyltin dichloride yields a unique tetraamino-substituted cyclobutadienyl system featuring a dative C-Sn interaction. DFT investigation of the reaction mechanism revealed that coordination of the stannyl fragment to the nucleophilic carbon leads to a metal-stabilised zwitterion, allowing for [2+2] cycloaddition under thermal conditions. The compound features a homoaromatic π-system comprising the three sp2 -hybridised carbon atoms of the four-membered ring as a consequence of charge separation.