Titanocene system

Various by-side interactions can also improve the efficiency of II Dye and III Dye PISs (see below) in for example the coumarin derivative/titanocene system [ALL 04],... 

Titanocene Dihalides and Pseudohalides. When both acido ligands X in the titanocene system (CsH5)2TiX2 were represented either by the halide ligands F, Cl, Br, or by pseudohalide ligands, e.g. NCS or pronounced antitumor activity was found... 

A further extension of the modification of the titanocene system is the total replacement of one cyclopentadienyl ring in (C5H5)2TiX2 by an additional acido ligand Y which may be identical with the other two addo ligands X or different from them With XXXVIII-XL, weak antiproliferative activity was retained. 

The neutral 3 dx metallocenes are thus known for x = 3 — 8, but the d9 copper complex has thus far resisted preparation, and the d2 titanocene has been found (54) to be both diamagnetic and dimeric, and is therefore excluded from consideration here. A number of cationic species, corresponding formally to Ti(Cp)2+, and V(Cp)2+, systems are however well known, but it seems very probable that these do not possess pseudo-axial symmetry (see (41) for further discussion), and very recently it has been demonstrated (55) that stable V(Cp)2+ complexes cannot be isolated without the coordination of an additional ligand to the metal. The parent systems are therefore limited to V(Cp)2, Cr(Cp)2, Mn(Cp)2, Fe(Cp)2, Co(Cp)2, and Ni(Cp)2 and the cationic species to Cr(Cp)2+, Fe(Cp)2+, Co(Cp)2+, and Ni (Cp)2+> and the d-d spectra of these systems are now considered individually. 

The asymmetric hydrogenation of acyclic imines with the ansa-titanocene catalyst 102 gives the chiral amines in up to 92% ee.684,685 This same system applied to cyclic imines produces the chiral amines with >97% ee values.684,685 The mechanism of these reductions has been studied 686... 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

It has been reported that the hydrogenation of imine ArC(Me)=NCH2Ph proceeds with enantioselectivity of up to 96% when Rh(I)-sulfonated BDPP is used in a two-phase system. However, the asymmetric reaction of ON bonds with ruthenium(II) catalyst is rather rare.99 Willoughby and Buchwald100 demonstrated a titanocene catalyst that shows good to excellent enantioselectivity in the hydrogenation of imine. 

Ketoximes and aldoximes Ketoximes and aldoximes are uniquely substituted C=N systems that react differently with titanocene and zirconocene. Aliphatic and alicyclic O-silylated ketoximes R2C=N OSiMe3 react with complex 1 with elimination of the alkyne and N—O bond cleavage to give imido-silanolates 85 [45],... 

Table 12.1. Titanocene-catalyzed formation of [3.3.0] systems. For the sake of clarity, only the major isomer is shown.

Using the catalytic system described above, the enantioselective opening of meso epoxides could also be pursued. Although many excellent examples of ring-opening of meso epoxides by Sn2 reactions have recently been reported, the reaction planned here is conceptually different [40]. In the SN2 reaction, the path of the incoming nucleophile has to be controlled. In the titanocene-catalyzed reaction, the intermediate radical has to be formed selectively. If an intermediate similar to that invoked in the Bartmann ring-open-... 

Another synthetic application of olefin metathesis using a thioacetal-titanocene(II) system is the ring-closing olefin metathesis (RCM) of carbene complexes possessing an olefin moiety, e. g. 33 (Scheme 14.17). The success of the RCM apparently depends on the substituents at the carbon—carbon double bond (i. e. the substituent(s) on the resulting carbene complex 34). 

Another titanium-based reagent for the methylenation of carbonyl compounds is that prepared from dibromomethane/zinc/titanium tetrachloride and related systems (Scheme 14.25) [48]. These systems transform a wide variety of carboxylic acid derivatives to terminal olefins in the same way as titanocene-methylidene does. 

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