Reactions of Functional Groups at the Periphery of Group 4 Metallocene Frameworks: Selective Formation of a Cp-Bonded Azaboretidine Derivative by a Hydroboration Route

6-Amino-6-methylfulvene has been coupled with benzoic acid by treatment with 7-azahydroxybenzotriazole and DCC. Subsequent two-fold deprotonation, at the NH and 6-methyl groups, generated the vinylidene-bridged Cp/benzoylamido ligand 5. Treatment of 5 with ZrCl4(THF)2 gave the spiro-metallocene complex 3a (characterized by X-ray crystal structure analysis), while reaction of 5 with (Et2N)2ZrCl2(THF)2 yielded the bridged half-sandwich complex 3b. Treatment of 3a with two molar equivalents of the strongly electrophilic borane HB(C6F5)2 resulted in hydroboration of the exocyclic CC double bond with concomitant intramolecular N–B adduct formation to yield the azaboretidine-type systems 7 (three diastereoisomers, one of which has been characterized by X-ray diffraction analysis). Similarly, hydroboration of 3b with HB(C6F5)2 yielded the azaboretidine derivative 9. Activation of the complexes 7 and 9 with methylalumoxane led to active homogeneous Ziegler-type propene polymerization catalysts.