Titanium atom chlorides

In a communication by Tanimoto and coworkers, ketene O-alkyl O -trunethylsilyl acetals 49 provide either a-nitroso esters 50 or their oximes 51 on reaction with nitric oxide or isoamyl nitrite in the presence of titanium(IV) chloride (Scheme 29). These reactions seem to provide a relatively direct way to introduce a nitrogen substituent at the a-carbon atom of carboxylic esters. 

Aromatic substitution (see Substitution reactions at aromatic carbon atoms) Aromatization of six-membered rings Titanium(IV) chloride-Diethylaluminum chloride, 309... 

Note that only ca 1% of the titanium atoms introduced with TiCl3 into the system give rise to the formation of surface active sites (since most of the titanium atoms remain inside the solid TiCl3 particles) [40], However, such active sites exhibit rather low stereospecificity in propylene polymerisation. The activity and stereospecificity of catalysts based on the / -TiCl3 modification also depend on the type of alkyl aluminium compound used as the activator. The application of triethylaluminium leads to a catalyst of much higher activity but of much lower stereospecificity, and on account of this diethylaluminium chloride is used for the polymerisation of propylene and other a-olefins, while triethylaluminium (and also triisobutylaluminium) is used for ethylene polymerisation [28],... 

Fig. 17.42. Reduction of epoxides with electron-transferring agents and 1,4-cyclohexadiene as the H atom donor. The more sterically hindered C-0 bond cleaves in a regioselective fashion, namely the Csec-0- instead of the Cprim-0 bond in the upper example—stoichiometric amounts of bis(cyclopentadienyl)titanium(III) chloride (CpJiCl) act as electron carrier—or the Ctert-0- instead of the C rin -0 bond in the bottom example, respectively (electron carrier catalytic amounts of Cp2TiCl/stoichiometric amounts of manganese powder).

The reactivity of halogenated alkenes in the electrophilic hydrofluorination reaction decreases with an increasing number of halogen atoms. Thus polyhaloalkenes are rather resistant to the electrophilic addition of hydrogen fluoride.Therefore, the hydrofluorination needs catalysis, with boron trifluoride(Table 6), tantalum(V) fluoride (Table 5), antimony(V) chloride, niobium(V) fluoride,titanium(IV)chloride or molybdenum(V) chloride. ... 

Friedrich et al. [45] discovered that a catalytic amount of titanium(IV) chloride as a Lewis acid greatly facilitates cyclopropanation reactions of alkenes by the system CFl2Br2-Zn-CuCl. The Lewis acid catalyst might bind to the oxygen atom of the allylic alcohol present as the (iodomethyl)zinc alkoxide, and thus increase the electro-philicity of the methylene group [46]. 

Thus, the experimental data show that the composition of catalytic systems does not influence the stereoregularity of the corresponding polymer fractions but only their relative content. Hence, the stereospecificity of the active centers of these catalysts is the same including the one-component catalyst TiCl2. This confirms the concepts on the monometallic character of AQ in the Ziegler-Natta catalysts. The possible existence of chiral titanium atoms on the titanium chloride surface was studied by Cossee and Arlman ... 

Indirect replacement of the a-hydrogen atom of carboxylic esters by the nitroso group is remarkable. This procedure uses ketene 0-alkyl O -silyl acetals (1), generated from carboxylic esters, which are treated with nitric oxide or isopentyl nitrite in the presence of titanium(IV) chloride. In the absence of an a-hydrogen a-nitroso esters (2) are obtained. a-Nitroso esters with an a-hydrogen undergo isomerization to oximes of a-keto esters (3 equation 1). Similarly, silyl enol ethers of aldehydes or ketones can be used instead of the carbonyl compound itself for nitrosation. Thus, treatment of enol ether (4) with nitro-syl chloride gives the a-nitroso aldehyde (5 equation 2), which is quite stable at 0 C, but dimerizes at room temperature. 

The reactivity of water with both carbanion and carbocation intermediates is well known and recognised, but until recently it was generally believed that water is inert towards free radicals. Some years ago, Cuerva et al. by chance observed that tertiary radicals were reduced effectively in the presence of bis(cyclopentadienyl)titanium(III) chloride and water. Now the authors have solid evidence to show that water really acts as a complete hydrogen atom source rather than a simple proton donor for radical reductions mediated by Ti(III) and, presumably, other metals that react by single electron transfer (Scheme 8.7).6... 

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