Developing some functional group chemistry at the Group 4 bent metallocene frameworks

Four series of reactions were explored that are aimed at the development of a functional group chemistry at the sensitive bent metallocenes of the Group 4 metals. First, we have tested and employed the olefin metathesis reaction. Two [C5H4–(CH2)n–CHCH2]2ZrCl2 complexes (with n=4 and n=1) were treated with the Cl2(PCy3)2RuCHPh catalyst in dilute solution. Intramolecular metathesis took place readily with liberation of ethene to yield the corresponding ansa-metallocene complexes [C5H4–(CH2)n–CHCH–(CH2)n–C5H4]ZrCl2 that were isolated in ca. 28% (n=4, trans-CHCH–) or ca. 50% (n=1, cis-CHCH–) yield. Intermolecular olefin metathesis could also be effected using either a first or a second generation Grubbs catalyst to metathetically couple a variety of (L)Cl2M(C5H4–CH2CHCH2)-type titanium or zirconium complexes to yield the respective dimetallic products. Alkenyl substituents were attached at the ligand stage at the indenyl 2-positions by means of a nickel-catalyzed cross-coupling reaction. Transmetallation eventually gave the bis(2-alkenylindenyl)ZrCl2 complexes. Photolysis of three examples of this type of complexes (with 1-alkenyl substituents being methyl, cyclohexyl, or phenyl) resulted in a very efficient intramolecular [2 + 2] cycloaddition to yield the respective substituted cyclobutylene-bridged ansa-metallocenes. These complexes were MAO activated to give very active homogeneous metallocene catalysts for, e.g., ethene/1-octene copolymerization. The corresponding (s-trans-butadiene)-ansa-zirconocene complexes feature a pronounced C–H/phenylene π-interaction that probably helps to stabilize the s-trans-diene complex relative to its s-cis-isomer. Deprotonation of 7-dialkylaminofulvenes provides an easy access to enamino-cyclopentadienides. Transmetallation under carefully controlled conditions gave the respective bis(enamino-C5H4)ZrCl2 complexes. Their treatment with a catalytic amount of a Lewis acid (TiCl4) or a Brønsted-acid ([HNMe2Ph+][BPh4−]) led to a rapid intramolecular Mannich-type carbon–carbon coupling reaction that gave novel, very rigid C3-bridged, –NR2 functionalized ansa-metallocene systems. The analogous, readily performed Mannich-coupling reaction starting from 1,1′-diacetylferrocene and a variety of secondary amines provided an interesting and useful novel entry to [3]ferrocenophane systems. Catalytic hydrogenation of the unsaturated bridge followed by additional functionalization opened a novel pathway to ferrocenophane-based chelate ligands (some optically active) for stereoselective catalytic transformations. Eventually, examples of direct attack of the electrophilic boranes B(C6F5)3 or HB(C6F5)2, respectively, to the Cp rings of bis(cyclopentadienyl)zirconacyclopentadienes are described, leading to novel types of Cp-borylated Group 4 metallocene complexes.