Titanocenes characteristics

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

Bis(arene)zirconium complexes, characteristics, 4, 697 Bis(arylamido)pyridines, with Hf(IV), 4, 792 Bis(aryl) bridges, in complexes with chromium(VI), 5, 346... 

The highly unstable, active titanocene 8, reported by van Tame-len et al., also forms a blue complex with dinitrogen, [(CjH Ti] (25) (Section III,C). Interestingly, the visible absorption spectrum of this titanocene 8 is identical with that of -(rj1 T75-CjsH4)(i -CsHs)3Ti2 (10) (bands at 486 and 640 nm) (3). Unfortunately however for comparative purposes, the reported spectrum of 8 does not cover the near infrared region at —1120 nm where 10 shows a characteristic intervalence absorption band. 

J. Albertsen, Synthesis and characteristics of new chalcogen-rich heterocycles with organoelement compounds of the 4th main group and purification of bisorganic polysulfanes using titanocene chalcogenide chelate complexes. Doctoral Dissertation, Technical University of Berlin, 1993. 

To study the effects of water and other solvents on titanocene(III)-mediated processes we used the transannular cychzation of epoxygerma-crolides as a model reaction [47]. Thus, we found that in anhydrous, non-halogenated solvents such as THF the reaction led selectively to decalins with an exocyclic double bond (Scheme 5). In an aqueous medium (THF/H2O), however, the characteristic lime green color of Cp2TiCl turned deep blue and the main product was a reduced decalin (Scheme 5). Under these conditions, water (either H2O or D2O) proved to be more effective than the toxic and expensive hydrogen-atom donor 1,4-cyclohexadiene for the reduction of tertiary radicals [47]. This is an unusual phenomenon in free-radical chemistry [48-50], subsequently exploited by us for the selective reduction of aromatic ketones as we shall see later [51,52]. 

Hydrolysis of titanocene produces the complex [Cp(C5H4)TiOH]2 X-ray crystallography has established that this has the fulvalene structure (1) characteristic of the titanocene molecule (Vol. 4, p. 2) and that the titanium atoms are bridged by the two hydroxy-groups/ ... 

All effective catalysts for the asymmetric reduction of prochiral C=N groups are based on complexes of rhodium, iridium, ruthenium, and titanium. Whereas in early investigations (before 1984) emphasis was on Rh and Ru catalysts, most recent efforts were devoted to Ir and Ti catalysts. In contrast to the noble metal catalysts which are classical coordination complexes, Buchwald s a sa-titanocene catalyst for the enantioselective hydrogenation of ketimines represents a new type of hydrogenation catalyst [6]. In this chapter important results and characteristics of effective enantioselective catalysts and are summarized. 

Infrared spectroscopic studies of the reactants and products were carried out employing FTIR and KBr pellets. Bands characteristic of the presence of both the amylose and titanocene moieties are present. For instance, the products exhibit infrared bands characteristic of amylose (all bands given in cm-1) at about 1650, 1480, 1440, 1360, 1275, 1240, 1160, 1100, 810 and 760 and bands characteristic of the Cp Ti moiety at 1405, 1030 and 855. A band assigned to the Ti-O-C grouping is present at 1130. The ratios of the peak intensities of the two components also corresponds roughly to a 1 1 stoichiometry. 

By virtue of their absorption characteristics, many of the compounds listed in Table 10.4 can be employed in conjunction with visible light sources. As the research in organometallic chemistry gained momentum, the potential advantages of organometallic complexes as photoinitiators were also explored, and two such compounds, a ferrocenium salt and a titanocene, were commercialized (see Chart 10.3). 

Yan, X. W., Wang, J. D., Shan, Y. B., and Yang, Y. R. 2006. Characteristics of titanocene catalyst supported on palygorskite for ethylene polymerization. Chinese Chemical Letters 17 653-656. 

---