CHARACTERISTIC FEATURES OF NEUTRAL CYCLOPROPENYLIUM-LIKE COMPOUNDS - SYNTHESIS, STRUCTURE, AND CHEMICAL BEHAVIOR OF A CYCLOPROPENYLIOBORATEBETAINE

Bis(propynyl)zirconocene was treated with B(C6F5)(3) and then with two equivalents of tert-butyl isocyanide to yield the organometallic thre e-membered ring product 8. Subsequent hydrolysis gives the substituted (2-iminioethylidene)cyclopropeneborate betaine system 9 that exhibits a pronounced cyclopropenylioborate betaine character. This follows fr om its characteristic structural features (X-ray crystal-structure ana lysis of the isomer cis-9) and its chemical features. Complex 9 underg oes a very facile cis/trans isomerization by formal rotation around th e exocyclic partial CC-double bond (Delta G(rearr)double dagger, appro ximate to 17 kcal mol(-1)). Compound 9 reacts cleanly in methanol solu tion with tert-butyl isocyanide by insertion and ring enlargement to f orm the four-membered ring system 12. Compound 12 was also characteriz ed by X-ray diffraction.