Atom-economical ring-contraction of pyridazine and phthalazine induced by an oxygen-bridged Ga/P FLP

The oxygen-bridged frustrated Lewis pair (FLP) Bis₂Ga-O-P^tBu₂ (Bis = CH(SiMe₃)₂; GaOP) is capable of activating and further transforming multiple nitrogen-containing substrates. Remarkably, the reaction with pyridazine leads to an instantaneous, atom-economical ring-contraction at ambient temperature, producing the GaOP·3-hydro-2-imino-pyrrole adduct. Further rearrangement to the GaOP·2-amino-pyrrole adduct occurs due to an imine-enamine tautomerism. Heating this mixture to 70 °C yields the GaOP·5-hydro-2-imino-pyrrole adduct via a 1,3-hydrogen shift. Using phthalazine instead results in the formation of a stable 1,2-addition product across the N-C bond, whereas a subsequent ring-contraction is thermally inducible, yielding the GaOP·9-hydro-2-imino-isoindole adduct. For cis-azobenzene, a 1,2-addition to both nitrogen atoms is observed, while tests for 1,2,4,5-tetrazine derivatives and compounds with N-N single bonds, including hydrazines, show different types of decomposition. Strikingly, ring-opening of the three-membered, N-containing ring in p-tosylaziridine is demonstrated, where by a six-membered ring is formed, as it is the case for the capture of N-sulfinylaniline.