Titanium cations: generation

In addition to MAO, boron compounds based on tris(pentafluorophenyl)boron and its derivatives, typically dimethylanilinium tetrakis(pentafluorophenyl) borate, have been used as cocatalysts for sPS polymerizations (40,41). Although MAO has been used in large molar excesses relative to the titanium complex, the boron compounds may be used in roughly equimolar amounts to the titanium catalyst. The boron cocatalyst reacts with a titanium alkyl species, either by protonation in the case of dimethylanilinium tetrakis(pentafluorophenyl)borate or by alkyl group abstraction in the case of tris(pentafluorophenyl)boron, to generate a titanium cationic species with a borate counterion (74-76). The esr spectral evidence has been reported for these systems, supporting a titanium(III) cationic active species (76). 

One-electron reduction or oxidation of organic compounds provides a useful method for the generation of anion radicals or cation radicals, respectively. These methods are used as key processes in radical reactions. Redox properties of transition metals can be utilized for the efficient one-electron reduction or oxidation (Scheme 1). In particular, the redox function of early transition metals including titanium, vanadium, and manganese has been of synthetic potential from this point of view [1-8]. The synthetic limitation exists in the use of a stoichiometric or excess amount of metallic reductants or oxidants to complete the reaction. Generally, the construction of a catalytic redox cycle for one-electron reduction is difficult to achieve. A catalytic system should be constructed to avoid the use of such amounts of expensive and/or toxic metallic reagents. 

Several catalysts and initiator systems have been tested for the polymerization of GlcAnBzl3, including the following Lewis acids boron trifluoride and its etherate, phosphorus pentafluoride, titanium tetrachloride, and antimony pentachloride and pentafluoride. Several cationic initiators have also been used, including (triphenylmethyl) antimony hexachloride, 2,3,4,6-tetra-O-acetyl-D-glucopyranosyl hexa-fluorophosphate, acetyl hexafluorophosphate, pentamethylbenzyl hexa-fluorophosphate (most of which were generated in situ), and triethyl-... 

Serpone, N. (2006). Is the band gap of pristine Ti02 narrowed by anion- and cation-doping of titanium dioxide in second-generation photocatalysts Journal of Physical Chemistry B, 110(48), 24287-24293. 

Alkenylsilanes and -stannanes, and arylsilanes and -stannanes are useful reagents for transfer of an sp -carbon unit to electrophiles under titanium catalysis. Epoxides are opened by TiCE to generate cationic carbon, which is successfully trapped with bis(trimethylsilyl)propene as an aUcenylsilane (Eq. 122) [305]. Other Lewis acids, for example ZnCla, SnCU, and BF3 OEt2, proved less satisfactory. Cyclic epoxides such as cyclopentene and cyclohexene oxides gave poorer yields. An intramolecular version of this reaction proceeded differently (Eq. 123) [305]. Eqs (124) and (125) illustrate diastereoselective alkenylation and arylation of (A,0)-acetals that take advantage of the intramolecular delivery of alkenyl and aryl groups [306], Cyclic ethers... 

Most organometallic EDA complexes of arenes with titanium tetrachloride [116] in solution also follow the general reaction scheme in Eq. 15 in that no net chemical reaction is observed upon charge-transfer irradiation due to rapid back electron transfer (A et 10 ° s ). For example, the transient absorption spectrum of bro-moanthracene (BrAnt) cation radical generated by 532-nm laser excitation of the [BrAnt, TiCU] complex in cyclohexane (see Figure 7) decays completely to the spectral baseline within about 1 ns (see inset) due to back electron transfer [116], (Eq. 18) ... 

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