Titanium alcoholates

Biodiesel is a mixture of methyl esters of fatty acids and is produced from vegetable oils by transesterification with methanol (Fig. 10.1). For every three moles of methyl esters one mole of glycerol is produced as a by-product, which is roughly 10 wt.% of the total product. Transesterification is usually catalyzed with base catalysts but there are also processes with acid catalysts. The base catalysts are the hydroxides and alkoxides of alkaline and alkaline earth metals. The acid catalysts are hydrochloride, sulfuric or sulfonic acid. Some metal-based catalysts can also be exploited, such as titanium alcoholates or oxides of tin, magnesium and zinc. All these catalyst acts as homogeneous catalysts and need to be removed from the product [16, 17]. The advantages of biodiesel as fuel are transportability, heat content (80% of diesel fuel), ready availability and renewability. The... 

The compound also may be prepared by other methods. These include ignition of barium and titanium alcoholates in an organic solvent treatment of tetraethyl titanate or other alkyl ester of titanium with an aqueous solution of barium hydroxide and ignition of barium titanyloxalate. 

Cyclopropylamines and cyclopropanols can be prepared from alkyknagnesium ha-lides. The reaction is catalyzed by titanium alcoholates and its mechanism includes the formation of a dialkoxytitanacyclopropane 270, which reacts with a carbonyl compound or nitrile (Scheme 22). The use of chiral titanium alcoholates allows the reaction to be performed with up to 78% ee (equation 171) . 

For instance, in the polymerization of 1.3-pentadiene in the presence of a catalyst prepared from optically active titanium alcoholates and diethylaluminum monochloride, the diolefin might give a complex with a titanium atom before the polymerization (92) the steric structure of the complexed pentadiene molecule might be determined by the optically active alkoxy groups still bound to the titanium atom. However, the case of. the polymerization of diolefins is much more complex than the case of the polymerization of vinyl monomers owing to the possible... 

In this chapter, the oxidations with hydroperoxides mediated or catalyzed by chiral titanium alcoholates will be discussed first. Then, results obtained with various chiral metal-Schiff bases... 

Table VI summarizes important homogeneous Ziegler catalysts. The best known are the systems based on bis(cyclopentadienyl)titanium(IV), titanium alcoholates, vanadium chloride, or chromium acetylacetonate with trialkylaluminum or alkylaluminum halides.

Formation of cyclopropylamines and cyclopropanols from alkylmagnesium halides and esters, amides, or nitriles, catalyzed by titanium alcoholates, was discovered by Kulinkovich in 1989. The reaction mechanism includes the formation of dialkoxytitanacyclopropanes, which reacts with a carbonyl compound or a nitrile (Scheme 109). 

If an alkene is present in the reaction mixture the intermediate, titanacyclopropane, undergoes facile ligand exchange, giving new titanium species, which react further in the catalytic cycle. Thus, various alkenes can be involved in this reaction, inter- or intramolecularly, allowing the preparation of numerous cyclopropane derivatives. In the presence of a chiral titanium alcoholate the reaction can be performed with good enantioselectivity (Scheme 110).319... 

Of greater interest, gem-dimethylation of ketones is possible with reactions conducted in the temperature range — 30 to 22° by a two-step sequence involving addition followed by methylation of the intermediate titanium alcoholate. The method is widely applicable, although subject to steric effects, and yields are generally in the range 60 85%. In addition, tertiary alcohols and tertiary alkyl chlorides are methylated by reactions conducted at room temperature in yields of about 75%.2 gem-Dihaloalkanes can also be converted into gcm-dimcthylalkanes.1... 

Sharpless and Katsuki described a very general method for the asymmetric epoxidation ofallylic alcohols (Scheme 9) [53]. The titanium alcoholate was initially used in stoichiometric amounts, though it was subsequently found it was possible to run the reaction catalytically in the presence of molecular sieves [54]. This method soon became a routine reaction in synthesis, because of its gener-... 

Titanium alcoholates of diols 2.50 generated by exchange with (/-PrO TiC are also potent catalysts, and the most efficient catalyst is derived from 2.50 (R = Me, R = Ar=Ph). These catalysts promote the diastereo- and enantioselective cycloaddition of cyclopentadiene or acyclic dienes with some a,P-unsaturated imides 7.68 (R = H, Rg=Me, MeOOC) [45, 778] or of acyclic dienes with substituted benzoquinones at -78°C [1572, 1573] (Figure 9.20). Reactions occur at 0°C in the presence of molecular sieves, and only catalytic amounts of these alcoholates are used. Intramolecular cycloadditions also give useful sdectivities under these conditions [1574]. However, the process for preparation of the catalyst varies according to the reaction [778, 1573], and this is veiy important to the observation of high selectivities. 

Namely a thin film of polycrystalline T1O2 was deposited onto a substrate by thermal decomposition of titanium alcoholate solution. During the heat treatment the film was cracking and numerous pores and crevices were formed in it producing high surface roughness. Efficiency of the PEC cell with the above mentioned sensitizer and the Br" - Br3 redox couple amounted at 4%. 

The development of other efficient methods for the enantioselective oxidation of sulfides to sulfoxides has been pursued for many years. Various chiral additives and metal complexes have been investigated. However, most of these systems have disadvantages such as formation of sulfone, long reaction times, unsatisfactory yields and poor enantiomeric excesses. Diethyl tartrate remains the additive (combined with titanium alcoholates) which gives the best results in terms of enantioselectivity. Since 1989, it has been found that other chiral ligands can be efficient in asymmetric induction. 

A new class of siliceous microporous materials with varying metal contents and high porosities has recently been presented by Maier et al. [105,106]. These materials which are X-ray amorphous and where no crystalline structure can be seen in the transmission electron microscope are synthesized via a sol-gel-route under hydrothermal conditions without the use of template molecules. The simple reaction of silicon alcoholates and titanium alcoholates which are mixed in ethanol and the condensation of which is induced by the addition of hydrochloric acid, produces a gel that becomes solid after a few days. The following careful calcination process under inert atmosphere and milling of the resulting solid gives the porous material. Detailed analysis of the pore structure para-... 

The natural products gutta-percha and balata consist of tra w-l,4-polyisoprene. With the aid of vanadium trichloride and triethylalumium, tra w-l,4-polyisoprene can be produced with 98% trans-, A enchainments [133,258]. The optimal Al/V ratio is the range of 5 to 7. The activity can be increased by the addition of small amounts of ether, heterogenerization on supports (kaolin, Ti02), or blending with titanium(III) chloride or titanium alcoholates [259-261]. Further catalysts featuring lower activity, however, are allylnickel iodide, trisallylchormium on silica, or complexes of neodymium [262 265]. 

Chemically more interesting are transesterifications e.g., of methylchrysanthemates with a substituted furfurol acetate in the presence of sodium methoxide [769], of permethric acid ethylester with metaphenoxy benzyl alcohol and sodium methoxide at elevated temperatures [770, 771] or the transesterification of the acetate of the latter alcohol, (but also with other acetates of pyrethroid alcohols [772]) in the presence of titanium alcoholates to give permethrin 372 [774,775]. In the last reaction an originally cis/trans 58/42 ratio changed to cis/trans 44/56. 

Cyclopropylamines and cyclopropanols of type 209 can be prepared using alkylmagnesium halides (Kulinkovich reaction). This reaction requires the presence of titanium alcoholates. Its mechanism includes the formation of a dialkoxytitaniumcyclopropane 210, which adds to a carbonyl compound or nitrile (Scheme 2-82). The use of chiral titanium alcoholates affords enantioenriched products such as 211 with 78% ee using a TADDOL derivative (Scheme 2-83). ... 

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