W. Ahlers et al., CHARACTERISTIC FEATURES OF NEUTRAL CYCLOPROPENYLIUM-LIKE COMPOUNDS - SYNTHESIS, STRUCTURE, AND CHEMICAL BEHAVIOR OF A CYCLOPROPENYLIOBORATEBETAINE, Chemische Berichte, 130(8), 1997, pp. 1079-1084
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.