1,3-Doubly Bridged Group 4 Metallocenes by Intramolecular Reductive Coupling of Pendant Olefins

The new precursors M(NMe2)2Cl2(dme) (M = Zr (3), Hf (4)) react with the dilithium derivative of the silyl-bridged bis(1-indenyl) ligand bearing pendant 4-pentenyl groups Li2[3,3‘-5R2−SBI] (2) in toluene at −35 °C to provide the corresponding bis(dimethylamido) metallocenes with rac/meso ratios of 6−7:1. Subsequent treatment of the substantially rac-enriched zirconocene mixture with excess TMS−Cl in dichloromethane in the presence of a catalytic amount of anhydrous HCl provides rac-[3,3‘-5R2−SBI]ZrCl2 (7) in 40% isolated yield free from its meso isomer. Similar reactions of Li2[SBI]·OEt2 and of Li2[EBI]·OEt2 with 3 and 4 also lead to selective formation of the rac-metallocenes with rac/meso ratios as high as 20:1 for [SBI]Zr(NMe2)2 (10). Sodium amalgam reduction of rac-[3,3‘-5R2−SBI]ZrCl2 (7) in THF results in the intramolecular tail-to-tail coupling of the pendant olefins to provide the crystallographically characterized α,α‘-disubstituted zirconacyclopentane 13 in 95% isolated yield. In contrast, reduction of rac-[3,3‘-5R2−SBI]HfCl2 (8) (prepared from 2 and HfCl4 in toluene) leads to both the symmetric α,α‘-disubstituted and the unsymmetric α,β‘-substituted hafnacyclopentanes (14 and 15, respectively) in a 1:1 ratio. Zirconacycle 13 reacts with [HNMe2Ph]+[OTf]- to give the monotriflate 17, in which one of the 4-pentenyl arms is released and the other is formally protonated at its terminal carbon atom to give this thermally stable secondary alkyl complex. Formation of 17 is best understood as proceeding through a mono-olefin intermediate 16, which can be trapped with 2-butyne to form the corresponding zirconacyclopentene 18. Reaction of 13 with the organometallic Lewis acid B(C6F5)3 results in the formal abstraction of a hydride from the 3-position of one arm to form [HB(C6F5)3]- and a THF-bound π-allyl cation 19, in which one pendant alkene is also free.