Titanium complexes addition

Aqueous solutions containing titanium(IV) give an orange-yellow colour on addition of hydrogen peroxide the colour is due to the formation of peroxo-titanium complexes, but the exact nature of these is not known. 

These mixed phosphate ester titanium complexes or their amine salts are useful as fuel additives to help maintain cleanliness of carburetors and inhibit surface corrosion. Chloride-free mixed alcohol phosphate esters can be obtained if a tetraalkyl titanate is used (101). 

Chiral Titanium Complexes. Chiral titanium complexes are useful for the enantioselective addition of nucleophiles to carbonyl groups ... 

Mixtures of a titanium complex of saturated diols, such as TYZOR OGT, and a titanium acylate, such as bis- -butyl-bis-caproic acid titanate, do not have a yellowing or discoloring effect on white inks used to print polyolefin surfaces (506). The complexes formed by the reaction of one or two moles of diethyl citrate with TYZOR TPT have an insignificant color on their own and do not generate color with phenol-based antioxidants (507). The complexes formed by the addition of a mixture of mono- and dialkyl phosphate esters to TYZOR TBT are also low color-generating, adhesion-promoting additives for use in printing polyolefin films (508). 

A synthetically useful diastereoselectivity (90% dc) was observed with the addition of methyl-magnesium bromide to a-epoxy aldehyde 25 in the presence of titanium(IV) chloride60. After treatment of the crude product with sodium hydride, the yy -epoxy alcohol 26 was obtained in 40% yield. The yyn-product corresponds to a chelation-controlled attack of 25 by the nucleophile. Isolation of compound 28, however, reveals that the addition reaction proceeds via a regioselective ring-opening of the epoxide, which affords the titanium-complexed chloro-hydrin 27. Chelation-controlled attack of 27 by the nucleophile leads to the -syn-diastereomer 28, which is converted to the epoxy alcohol 26 by treatment with sodium hydride. 

A combination of diethylzinc with sulfonamides 18 or 19 offers another possibility for the enantioselective acetate aldol reaction39,41. The addition of silyl enol ethers to glyoxylates can be directed in a highly enantioselective manner when mediated by the binaphthol derived titanium complex 2040. 

The use of chiral titanium complexes as catalysts is also possible. As shown in Scheme 5-26, the Sharpless titanium catalyst gave ees up to 53% for the addition of diethyl phosphite to benzaldehyde. 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

Dianionic bis(amide) ligands bearing additional donor atoms have been described by several researchers. High activities for ethylene polymerization are observed for pyridyldiamido zirconium complexes such as (42) (1,500gmmol-1 bar-1 h-1),145 although the corresponding titanium complex is much less active.146... 

Additionally, it has been shown that novel benzylidene titanium complexes of type 74 react with polymer-bound carboxylic esters to form the corresponding enol ethers (Scheme 28).79... 

Addition of the (l-silylalkyne)titanium complex to carbonyl compounds and imines occurs at the (3-position to the silyl group, as shown in Fig. 9.2. However, the reaction with sBuOH takes place exclusively at the carbon—titanium bond a to the silyl group to give the (P-silylalkenyl)titanium species, as in Eq. 9.5 (values in square brackets denote the regioselectivity) [24], where the vinyl—titanium bond is visualized by the outcome of the iodi-nolysis. The overall reaction can therefore be regarded as the hydrotitanation of silylace-... 

By choosing an appropriate titanium complex, a /ra r-isomer of 1,2-cyclohexanols can be prepared selectively. Because intramolecular pinacol coupling of hexanedials with Sml2 usually produces t-isomers of cyclohexane-1,2-diols, the titanium-mediated reaction complements the samarium-mediated cyclization (Equation (17)). In addition, when a /-butyl group fixes the conformation of the substrate, one of the diastereomers is produced selectively (Equation (18)). ... 

The enantiopure acyclic and cyclic allyl sulfoximines 13 and 14, respectively, required for the synthesis of the corresponding titanium complexes 1 and 2, are available from sulfoximine 12 [13] and the corresponding aldehydes and cycloal-kanones by the addition-elimination-isomerization route, which can be carried... 

Reaction of the bis(allyl) titanium complexes 16 and 18 with aldehydes occurs in a step-wise fashion with intermediate formation of the corresponding mono (allyl) titanium complex containing the alcoholate derived from 4 and 5 as a ligand at the Ti atom. Then the mono(allyl)titanium complexes combine with a second molecule of the aldehyde. Both the bis (allyl) titanium complexes and the mixed mono(allyl)titanium complexes react with the aldehydes at low temperatures with high regio- and diastereoselectivities. Interestingly, control experiments revealed that for the reaction of the bis (allyl) titanium complexes with the aldehyde to occur the presence of Ti(OiPr)4 is required, and for that of the intermediate mono(allyl)titanium complexes the addition of ClTi(OiPr)3 is mandatory (vide infra). 

Mechanistic studies of asymmetric amplification using a chiral amino alcohol catalyst have continued to dale"1 151 155. In the case of the chiral titanium complex, the observed asymmetric amplification was influenced by the method of preparation of the catalyst153. Asymmetric amplification is also observed in the catalytic addition of diphenylzinc to ketones100. 

In 1977, an article from the authors laboratories [9] reported an TiCV mediated coupling reaction of 1-alkoxy-l-siloxy-cyclopropane with aldehydes (Scheme 1), in which the intermediate formation of a titanium homoenolate (path b) was postulated instead of a then-more-likely Friedel-Crafts-like mechanism (path a). This finding some years later led to the isolation of the first stable metal homoenolate [10] that exhibits considerable nucleophilic reactivity toward (external) electrophiles. Although the metal-carbon bond in this titanium complex is essentially covalent, such titanium species underwent ready nucleophilic addition onto carbonyl compounds to give 4-hydroxy esters in good yield. Since then a number of characterizable metal homoenolates have been prepared from siloxycyclopropanes [11], The repertoire of metal homoenolate reactions now covers most of the standard reaction types ranging from simple... 

Some of the metallic constituents were somewhat soluble in the propane-oil portion, but all tended to be concentrated in the asphaltic portion. Although vanadium (0.02% by weight in the original crude petroleum) was present in all fractions, the greater part was found in the cyclohexane and benzene fractions. From the similarity of the absorption spectra of the vanadium concentrates from petroleum and those of synthetic vanadium porphyrin complexes, Skinner arrived at the conclusion that vanadium compounds from the petroleum of the Santa Maria Valley Field in California exist as porphyrin complexes. Additional metallic constituents were detected by Skinner as these became concentrated in the various solvent fractions, including aluminum, titanium, calcium, and molybdenum. 

In contrast to V-heteroaldehyde and -ketone complexes, r -heteroalde-hyde complexes are less prone to cyclo addition reactions. The reactivity of these complexes is characterized by the insertion of C N, C=0, and C=N bonds into the M-C(heteroaldehyde) bond as has been demonstrated with the examples of zirconium and titanium complexes. 

Lewis acid-promoted asymmetric addition of dialkylzincs to aldehydes is also an acceptable procedure for the preparation of chiral secondary alcohol. Various chiral titanium complexes are highly enantioselective catalysts [4]. C2-Symmet-ric disulfonamide, chiral diol (TADDOL) derived from tartaric acid, and chiral thiophosphoramidate are efficient chiral ligands. C2-Symmetric chiral diol 10, readily prepared from 1-indene by Brown s asymmetric hydroboration, is also a good chiral source (Scheme 2) [17], Even a simple a-hydroxycarboxylic acid 11 can achieve a good enantioselectivity [18]. 

Bernadi and Scolastico, and later Evans in a more effective manner, indicated that the enantioselective addition reaction using silyl enol ethers can be catalyzed by Lewis acidic copper(II) cation complexes derived from bisoxazolines [38-40]. In the presence of the copper complex (S,S)-14 (10 mol %), silyl enol ethers derived from thioesters add to alkylidenemalonates or 2-alkenoyloxazo-lidone in high ees (Scheme 12). Bernadi, Scolastico, and Seebach employed a titanium complex derived from TADDOL for the addition of silyl enol ethers to nitroalkenes or 2-cyclopentenone [41-43], although these are stoichiometric reactions. 

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