Characteristic Features of Neutral Cyclopropenylium‐Like Compounds: Synthesis, Structure, and Chemical Behavior of a Cyclopropenylioborate Betaine

Bis(propynyl)zirconocene was treated with B(C 6 F .5 ) 3 and then with two equivalents of tert‐butyl isocyanide to yield the organometallic three‐membered ring product 8 . Subsequent hydrolysis gives the substituted (2‐iminioethylidene)cyclopropeneborate betaine system 9 that exhibits a pronounced cyclopropenylioborate betaine character. This follows from its characteristic structural features (X‐ray crystal‐structure analysis of the isomer cis ‐ 9 ) and its chemical features. Complex 9 undergoes a very facile cis/trans isomerization by formal rotation around the exocyclic partial CC‐double bond (ΔG + ≈ 17 kcal mol −1 ). Compound 9 reacts cleanly in methanol solution with tert‐butyl isocyanide by insertion and ring enlargement to form the four‐membered ring system 12 . Compound 12 was also characterized by X‐ray diffraction.