Titanium derivatives

Sdanediols, eg, (CgH )2Si(OH)2 and H0Si(CgH )20Si(CgH )20H, yield four-and-six-membered rings with titanium alkoxides. Pinacols and 1,2-diols form chelates rather than polymers. The more branched the diol molecule, the more likely are its titanium derivatives to be soluble and even monomeric. 

The metallosiloxanes are liable to hydrolysis but the rate of hydrolysis is very dependent on the metal used in the polymer. Examination of three low molecular weight metallosiloxanes indicates the relative rates of hydrolysis to be 2220 27.2 1 for the tin, aluminium and titanium derivatives respectively. Russian workers state that polyorganometallosiloxanes are being used industrially but little detailed information is available. 

Primary 1-lithio-2-alkenyl diisopropylcarbamates are not configurationally stable in solution. However, under properly selected conditions, the ( )-sparteine complex of the 5-enantiomer crystallizes, leading to a second-order asymmetric transformation6 77-78 132. The suspension is converted to the tri(isopropoxy)titanium derivative with inversion of the configuration, which is shown to have enantiomeric purities up to 94% (Section D.l.3.3.3.8.2.3.). 

For further conversions via the titanium derivative see Section D.l. 3.3.3.8.2.3. The complex of the trimethylsilyl derivative was subjected to an X-ray structure analysis6. 

The sense of chirality transfer from chiral titanium derivatives takes the expected... 

Good chemical yields were obtained with the lithium derivative whereas very high diastereose-lectivities were obtained with the titanium derivative. These adducts were readily hydrolyzed with 0.1 N hydrochloric acid to enantiomerically enriched nitro amino acids, methyl 2-amino-3-methyl-4-nitrobutanoates. 

Since the catalytical behaviour of titanium derivatives depends on their degree of condensation, these catalysts are used under conditions where the water concentration is very low, for instance in transesterifications or in the last steps of esterifications. However, the amounts of water required to hydrolyse these compounds are so low (less than 0.5 ppm for Ti(OBu)4) that hydrolysis is probably the determining phenomenon in most studies. It seems that before Fradet and Marshal230 the contribution of this side effect has not been taken into consideration. 

Such interactions are probably very important in the catalytic action of titanium derivatives since they induce the formation of positive charges on the carbon atom of the acid carbonyl group, thus favoring the attack by the alcohol. 

Lagally, P. "Preparation and Stabilization of Water Soluble Reactive Titanium Derivatives," US Patent 2,950,174(1960). 

Titanium derivatives were shown to serve as cross-coupling partners for aryl triflates chlorides or bromides, with better tolerance to functional groups than Grignard reagents although specific expensive N,P-chelating ligands are required (Equation (7)) 171... 

At this point it should be noted that this mechanism is unexpected. Simple platinum alkyls decompose by 3-elimination whenever possible and there are no well-established examples of a-elimination [10]. All previous studies have indicated that metallacyclobutanes decompose by 3-elimination, even for tantalum and titanium derivatives for which a-elimination is a frequent mechanism for decomposition of the simple alkyls [11, 12]. There is even a labelling study which appears to prove the 3-elimination mechanism for decomposition of platinacyclobutanes (equation 3) [13]. 

Cydopentadienyl Titanium Derivatives for Carbonyl Olefination/Olefin Metathesis... 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Kagan et al. were the first to report the corresponding enantioselective catalytic hydrogenation using chiral metallocene derivatives [94, 95]. By using menthyl- and neomenthyl-substituted cyclopentadienyl titanium derivatives in the presence of activators (Scheme 6.5) [96], these authors observed low ee-values (7-14.9%) for the catalytic hydrogenation of 2-phenyl-l-butene into 2-phenylbutane. In contrast, no enantiomeric excess was obtained with the corresponding zirconocene derivatives. 

In a different study, intermediate 331 was transformed into its titanium derivative (by treatment with triisopropoxytitanium chloride) in order to discriminate between carbonyl compounds. It was found that this reagent reacted preferentially or exclusively with aldehydes in the presence of ketones (which were recovered unaltered) or with aliphatic aldehydes in the presence of aromatic ones. ... 

The most important reaction, the homoaldol reaction of the titanium derivatives, proceeds as an efficient syn-S process and will be discussed separately (see Section IV.C.5). Reactions with further electrophiles will be presented very briefly. The silylation of primary substrates 302 by different chlorotriorganosilanes proceeds with good a-selectivity and with inversion of the configuration ... 

As far as the epoxidation of allylic alcohols with chiral titanium derivatives is concerned, there is agreement in the literature that the oxygen transfer step can be described as in equation 16. An /j -bound alkylperoxo moiety is present in the coordination sphere of the metal, and a preliminary coordination of the substrate through its hydroxylic function to the titanium center is required. 

Typical examples referring to titanium derivatives are alkoxides with TBHP and titanosilicate (in particular TS-1) in the presence of H202. Based on this latter system, ENICHEM" commercialized a procedure for hydroxylation of phenol to cathecol and hydroquinone. Other activated arenes are also hydroxylated by TS-1 and hydrogen peroxide". Interestingly, for TS-1 catalysis a mechanism similar to that proposed... 

Ruthenium, cobalt and halogen are the key elements of this catalysis (2), although ruthenium in combination with halogen-containing zirconium and titanium derivatives is also effective (3). In the case of the Ru-Oo couple, the highest yields of acetic acid may generally be achieved with ruthenium oxide, carbonyls and complex derivatives in combination with various cobalt halides dispersed in low-melting quaternary phosphonium halide salts (2). 

First, when 1,1-dihaloalkanes are reduced with low-valent titanium derived from TiCU and zinc, it is necessary to add a catalytic amount of PbCU (or Pb) for reproducibility (Equations (5) and (6)). Two kinds of zinc powder are available for laboratory use electrolytic zinc derived by hydrometallurgy and distilled zinc derived by pyrometallurgy. 

Mukaiyama, Shiina and co-workers used a reagent generated by L1AIH4 reduction of a low-valent titanium derived... 

Vicinal diols of 10-membered ring enynes are also generated by a low-valent titanium derived from TiCls and zinc-copper couple in DME (Equation (21)). ... 

For Michael addition to nitroolefins, the lithio or titanium derivatives have been found to be useful [87AG(E)480], Hydrolysis gives a-amino--y-nitrocarboxylate esters (Scheme 69). 

Dialkylbis(cyclopentadienyl)silanes can be used in a similar manner, as in the following preparation of a titanium derivative (R = Me) (96) ... 

The alcoholysis of other derivatives than alkoxides themselves (method 4) has not found any broad application in the case of titanium derivatives however, the use of Ti(NMe2)4 for this purpose has been reported [1579], Phenoxides... 

The corresponding hafnium compound behaves similarly. Attempts to make the titanium derivative using this method failed. Primarily titanium is reduced, as indicated by the color change of the solution from green to brown. Variation of the reaction conditions did not provide better results, in all cases only bi(undecamethylcyclohexasilanyl) could be identified among the reaction products. 

Alkyltrisidialkylamino)titanium derivatives, R Ti(NR2)3. ) These reagents are readily available by reaction of halotris(dialkylamino)titanium with organolithium or -magnesium compounds. The reagents, prepared in situ in ether, react with nonenolizable aldehydes to give tertiary amines (1), possibly via an iminium salt (equation I). With enolizable aldehydes and ketones, the reaction results in enamines after nonaqueous workup. 

The hydroamination of alkynes is an efficient way to obtain aldimines with the advantage of avoiding formation of by-products. As shown in Scheme 8.65, the method has been developed into a multicomponent synthesis of a-branched amines. Aldimines 154 are formed using a titanium derivative as catalyst and reacted in situ with an organolithium reagent [141]. 

Acetic acid, glacial, dehydration for use in preparation of titanium derivative of acetylacetone, 2 119... 

The best catalysts for this reaction are inorganic acids (H2SO, HC1), organic acids (p-toluenesulfonic, methanesulfonic) and metal compounds such as tin and titanium derivatives - e.g. tetraisopropyl titanate. To achieve good yields of products, not only is a catalyst generally necessary but also the means to drive the equilibrium to the right as written in the reactions above. 

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