Bis(enamino-Cp) Group 4 metal complex chemistry: developing a Mannich-type carbon–carbon coupling reaction at the bent metallocene famework

Treatment of 6-methyl-6-dimethylaminofulvene (2a) with methyl lithium in ether results in a deprotonation reaction to yield the enamino-substituted lithium cyclopentadienide reagent 3a. Its reaction with zirconium tetrachloride (0.5 molar equivalents) results in the formation of the [1-dimethylaminobutadien-1,3-diyl-bis(cyclopentadienyl)]zirconium dichloride product 4a. The unsaturated ansa-metallocene is probably formed via a Lewis acid-catalyzed Mannich-type carbon–carbon coupling reaction of an in situ generated bis[(dimethylaminoethenyl)cyclopentadienyl]ZrCl2 intermediate. A number of related examples of this condensation reaction at the intact Group 4 bent metallocene framework is described, using different amino-substituents and Ti, Zr and Hf systems. Three examples of the unsaturated ansa-metallocene systems were characterized by X-ray diffraction. In a few cases it was possible to isolate the open (enamino-Cp)2ZrCl2 complexes. In the case of, e.g., (piperidinoethenylcyclopentadienyl)2ZrCl2 (13c) treatment with a catalytic quantity of the Lewis acid ZrCl4 or the Brønsted acid View the MathML source[PhNMe2H+][BPh4-] resulted in a clean conversion to the respective Mannich condensation product, here the ansa-metallocene complex 4c. The strongly electrophilic borane HB(C6F5)2 did not catalyze the CC-coupling reaction under the applied reaction conditions but added cleanly to the enamino-Cdouble bond; length as m-dashC double bond of, e.g., 13c to yield a cyclic nitrogen donor-stabilized hydroboration product (19).