Bis(trimethylsilyl)acetylene complexes of titanocenes and zirconocenes: Their recent chemistry and reactions with Lewis acids

The chemistry of the metallocene his (trimethylsilyl) acetylene complexes [Cp2M(L)(eta(2)-Me3SiC2SiMe3)] [M = Ti, without L (1); M = Zr, L = THF (2a); M = Zr, L = pyridine (2b)], the pentamethylcyclopentadienyl complexes [Cp*M-2(eta(2)-Me3SiC2SiMe3)] [M = Ti (3), Zr (4)], and the ethylenebis(tetrahydroindenyl) (ebthi) complexes rac-[(ebthi)M(eta(2)-Me3SiC2SiMe3)] [M = Ti (5), Zr (6)] has been described in several reviews. A fine-tuning of the reactions is possible by varying the Cp ligands (Cp, Cp*, ebthi), additional the co-ligands (THF, pyridine), and/or metals (Ti, Zr), and it has been pointed out that these complexes offer a number of compelling advantages over other, similar, widely used metallocene reagents. In this microreview recent (2000-2004) examples of the chemistry of complexes 1-6 are summarized, and some special interactions with Lewis acids are described. In these reactions the metallocene cores "Cp'Ti-2" and "Cp'Zr-2" (Cp' = substituted Cp ligands) are mostly formed after dissociation of the alkyne. As examples of this, a new selective zirconocene coupling route to large, functionalized macrocycles has been established from 2b and polyynes, complex 1 catalyses the first anti-Markovnikov hydroamination of terminal alkynes, complexes 1 and 3 selectively form polynuclear titanium complexes by C-C coupling of N-containing heterocycles, and complexes 1, 2b, 3, and 6 promote selective C-H and C-F bond-cleavage reactions of fluorinated N-containing heterocycles. Beside this recent chemistry, this microreview also focuses on the reactions of the above-mentioned complexes with the Lewis acids diisobutylaluminum hydride and tris(pentafluorophenyl)borane. For example, complexes 1, 2b, and 6 react with iBu(2)AlH to give heterodimetallic complexes containing tetracoordinate planar carbon atoms. The titanium complex 1 reacts with B(C6F5)(3) with elimination of the alkyne and gaseous hydrogen to give an electrophilic substitution of the Cp or Cp* ligand in which zwitterionic Ti-III complexes are formed. The zirconium complexes 4 and 6 do not liberate hydrogen in the reaction with B(C6F5)(3), although, after C-H activation of one methyl group of the Cp* or Cp part of the ebthi ligand and a B-C bond formation, zwitterionic complexes containing alkylidene groups with agostic interactions with the metal center are obtained. Complex 5 reacts with B(C6F5)(3) with Si-C bond cleavage of the alkyne and formation of an alkynylboranate [Me(3)SiCequivalent toCB(C6F5)(3)](-), which is complexed by the "rac-(ebthi)Ti" fragment to give a zwitterionic complex. Some of the formed heterodinuclear complexes and the zwitterionic complexes are active catalysts in the ring-opening polymerization of lactones, cyclic carbonates, and epoxides, as well as the polymerization of ethylene. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).