Titanium complexes, electron-transfer reactions

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) ... 

The use of bis-Cp titanium complexes as catalytic systems for reductive transformations via one-electron transfer in organic reaction has been reviewed.1... 

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

As shown in Scheme 18.1, the reaction mechanism for the photocatalytic direct decomposition of NO over the isolated tetrahedral titanium oxide species can be proposed, that is, two NO molecules are able to adsorb onto these oxide species as weak ligands to form reaction precursors. Under UV irradiation, the charge-transfer excited complexes of the oxides [Ti +-0 ] are formed. Within their lifetimes, the electron transfers from the Ti site, on which the photo-formed electrons are trapped, into the anti-ir -bonding orbital of the NO molecule, and the electron transfers simultaneously from the n-bonding orbital of another NO molecule into the 0 site, where... 

A plausible reaction pathway for double alkylation of alkenes is outlined in Scheme 9. It is assumed that 16 reacts with RMgX to generate the titanium(III) complex 19. One-electron transfer from 19 to alkyl bromide leads to the cleav-... 

Several titanium(IV) complexes are efficient and reliable Lewis acid catalysts and they have been applied to numerous reactions, especially in combination with the so-called TADDOL (a, a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) (22) ligands [53-55]. In the first study on normal electron-demand 1,3-dipolar cycloaddition reactions between nitrones and alkenes, which appeared in 1994, the catalytic reaction of a series of chiral TiCl2-TADDOLates on the reaction of nitrones 1 with al-kenoyloxazolidinones 19 was developed (Scheme 6.18) [56]. These substrates have turned out be the model system of choice for most studies on metal-catalyzed normal electron-demand 1,3-dipolar cycloaddition reactions of nitrones as it will appear from this chapter. When 10 mol% of the catalyst 23a was applied in the reaction depicted in Scheme 6.18 the reaction proceeded to give a yield of up to 94% ee after 20 h. The reaction led primarily to exo-21 and in the best case an endo/ exo ratio of 10 90 was obtained. The chiral information of the catalyst was transferred with a fair efficiency to the substrates as up to 60% ee of one of the isomers of exo3 was obtained [56]. 

The use of titanium and zirconium complexes as electron shuttles to transfer electrons from excess aluminum present in the reaction solution to the fluorinated substrate has been described,210 e.g. formation of perfluoronaphthalene from perfluorodecahydronaphthalene. 

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