Cyclopentadienyl-titanium reaction

Sulfur imides with a single NR functionality, S5NR (6.12), SeNR (6.13) (R = Oct), " SgNH (6.14), ° and S9NH (6.15) ° are obtained by a methodology similar to that which has been used for the preparation of unstable sulfur allotropes, e.g., S9 and Sio. Eor example, the metathesis reaction between the bis(cyclopentadienyl)titanium complexes 6.8-6.10 and the appropriate dichlorosulfane yields 6.14 and 6.15 (Eq. 6.4). °... 

A more effective control of both simple diastereoselectivity and induced stereoselectivity is provided by the titanium enolate generated in situ by transmetalation of deprotonated 2,6-dimethylphenyl propanoate with chloro(cyclopentadienyl)bis(l,2 5,6-di-0-isopropylidene-a-D-glucofuranos-3-0-yl)titanium. Reaction of this titanium enolate with aldehydes yields predominantly the. yyw-adducts (syn/anti 89 11 to 97 3). The chemical yields of the adducts are 24 87% while the n-u-products have 93 to 98% ee62. 

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

Triazacyclohexane also gives rise to very active catalysts with the use of chromium [13] as do ligands of the type RS(CH2)2NH(CH2)2SR [14], The latter coordinate in a meridional fashion, while the former can only coordinate in a facial fashion. Recently examples using cyclopentadienyl titanium complexes [15] and tantalum have been reported [16], The diversity of the chromium systems and the new metal systems show that very likely more catalysts will be discovered that are useful for this reaction, including 1-octene producing catalysts (1-octene is in high demand as a comonomer for ethene polymerisation for certain grades of polyethylene). 

Interestingly, the metallocene-activated C-F activation is not limited to tertiary C F bonds (in contrast to the Na/benzophenone system). Perfluorocyclohexane has been defluorinated in the presence of bis(r/5-cyclopentadienyl)zirconium(IV) chloride or bis(t/5-cyclopentadienyl)-titanium(IV) chloride. This reaction is accompanied by hydrogen transfer from the solvent onto the substrate yielding 1,2,4,5-tetrafluorobenzene as product.210... 

Methylenetriphenylphosphorane, 254 Titanium(IV) chloride-Lithium aluminum hydride, 310 (Z)-Alkenes By elimination reactions Dichlorobis(cyclopentadienyl)-titanium, 102 Hydrogen peroxide, 145 By hydrogenation of carbon-carbon triple bonds... 

Organocopper reagents, 207 Organovanadium reagents, 219 By desilylation of vinylsilanes Tetrabutylammonium fluoride, 286 By desulfurization of vinyl sulfides and related sulfur compounds Sodium dithionite, 281 Elimination reactions By dehalogenation Dichlorobis(cyclopentadienyl)-titanium, 102... 

Bis(cyclopentadienyl)titanium(II) dicarbonyl complexes, preparation and reactivity, 4, 250 Bis(cyclopentadienyl)titanium(II) dinitrogen complexes, preparation and reactivity, 4, 250 Bis(cyclopentadienyl)titanium halides ligand metathesis reactions, 4, 537 olefin polymerization, 4, 538 organic reactions, 4, 540 properties, 4, 530 reductions, 4, 532 synthesis, 4, 510... 

While it is not the purpose here to review all Cp-Ti compounds, there is obviously a very close relationship between CpTi moieties and their rearrangement product (CsH TiH via ring-to-titanium a-hydrogen shift, as has been alluded to above. The nature of the Ti-H species involved in reactions between low-valent titanium compounds and dinitrogen is not always clear. There seems little doubt that some form of bis(cyclopentadienyl) titanium is an active intermediate in dinitrogen coordination reactions (31, 37-39), but there is as yet no conclusive evidence that a Ti-H species is involved in the initial reaction (11), despite some claims to the contrary (40). The existence of different forms of Cp2Ti is referred to again below. 

A summary of the reactions involving the cyclopentadienyl titanium hydrides is given in Scheme 1 (page 276). 

The polymerization of a-olefins, promoted by homogeneous Ziegler-Natta catalysts based on bis(cyclopentadienyl)titanium(IV) or analogous zirconium compounds and aluminum alkyls, occurs simultaneously with a series of other reactions that greatly complicate the kinetic interpretation of the polymerization process (see Scheme 3). 

Most titanium(IV) alkyls tend to be reduced by aluminum alkyls in a complicated sequence of reactions accompanied by evolution of alkane and alkene. The catalytic activity of the bis(cyclopentadienyl)titanium-aluminum complexes is associated with the titanium alkyl. Hence, it is very interesting to investigate the mechanism of any reductive reaction. In order to study side reactions in the absence of polymerization, highly alkylated systems completely free of halogen are preferred. Moreover, reduction takes place much faster, the higher the alkyl-group content of the added aluminum alkyl. 

Addition of traces of chloride in the form of bis(cyclopentadienyl)-titanium dichloride lowered the yield of polyethylene and initiated the known reduction reaction (129). Finally, it was found that polyethylene formation was caused by traces of water ( 10-8 mol%). Consequently, the yield increased to 500,000 g polyethylene per gram of titanium when two equivalents of trimethyl- or triethylaluminum previously treated with one equivalent of water was added to dimethylbis(cyclopentadienyl)ti-tanium (Table VII). 

Since this time, more detailed studies indicate that metal-N2 complexes indeed are formed in these reactions. Examples have been isolated by careful adjustment of conditions. That these reactions proceed to near-stoichiometric production of NH3 and that the bis-r/5-cyclopentadienyl titanium core (and its pentamethyl-substituted analogue) limits the coordination possibilities at the metal held promise of relatively easy determination of both the structure of the com-... 

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

Complexes of (7)-C5H5)2Ti(III)R, where R may be an aryl, i)3-aUyl, or T 2-cyclopentadienyl group, are known. The aryl (40) and 7j3-allyl (41) complexes are prepared by reaction of bis(i)-cyclopentadienyl)titanium monochloride, or dichloride, with the appropriate Grignard reagent. 

A bis(i7-cyclopentadienyl)titanium(III) alkyl dinitrogen complex was prepared by reaction of [(i7-C5H5)2TiCl]2 with isopropylmagnesium chlo-... 

From cyclopentadienyl titanium complexes 49 and di-p-tolylcarbodiimide, a product 50, derived from a reductive coupling reaction, is obtained. ... 

Bis(dialkylainino)alane8-Dicblorobis(cyclopentadienyl)titanium, HAJ(NR2)2-Cp2TiQ2. ITie alanes are prepared in quantitative yield by the reaction shown in equation (I). 

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