NITRILE ADDITION TO (ETA-4-BUTADIENE)ZIRCONOCENE - THE FORMATION OF ORGANOMETALLIC AND METAL-FREE HEXATRIENEDIAMINE DERIVATIVES

(eta-4-Butadiene)zirconocene adds one equivalent of benzonitrile to form the five-membered azazirconacyclopentene derivative 7, which contains an NH group in the ring and a vinyl substituent at the alpha-carbon center. The reaction of 7 with an additional equivalent of benzonitrile furnishes the chiral nine-membered metallacycle [Cp2ZrN(H)C(Ph)=CHCH=CHCH2C(Ph)=N] (12) which may be isomerized to the thermodynamically favored tautomer [Cp2ZrN=C(Ph)CH2CH=CHCH2C(Ph)=N] (13). The Gibbs activation energy of the enantiomerization of the trans-cycloalkene-like 13 is DELTA-G(not-equal)(323 K) = 15.5 +/- 0.3 kcal mol-1. The analogous conformational equilibration of 12 has a lower activation barrier of DELTA-G(not-equal) (236 K) = 12.1 +/- 0.3 kcal mol-1; complex 13 is characterized by X-ray diffraction. Hydrolysis of 13 yields the conjugated 1,6-diamino-1,6-diphenylhexatriene 16 which is characterized by X-ray diffraction.