Titanium, bis

A number of additional metal-catalyzed epoxidations have been reported in the past year. Platinum is a rarely used catalyst in oxidation reactions. The use of chiral Pt-catalyst 2 in the epoxidation of terminal alkenes provides the epoxide products in moderate yield and enantiomeric excess <06JA14006>. The chiral hydroxamide 3 is used with a Mo catalyst to provide the epoxide product in excellent yields and moderate enantioselectivity <06AG(I)5849>. A bis-titanium catalyst, 4, has also been used to epoxidize the usual set of alkenes with H202 as the oxidant <06AG(I)3478>. 

Dipolarophiles D5. Electron-deficient alkenes based on acrolein and its analogs are widely used as dipolarophiles. To carry out asymmetrical 1,3-dipolar cycloadditions between various nitrones and acrolein, the bis-titanium catalyst (543) (Fig. 2.37) was used as the chiral Lewis acid (Table 2.22) (754a). 

The bis(titanium)hydrazido complex formed by HCl reaction with (h -Cp)2TiN2MgCl in MeOH reacts further with HCl to produce ... 

Kii, S., Maruoka, K. Catalytic enantioselective allylation of ketones with novel chiral bis-titanium(IV) catalyst. Chirality 2003,15, 68-70. 

The reduction of double bonds (C=X, X=C, O, N etc.) with H2 or various hydrides such as R3 SiH, R3 SnH or BH3 complexes can be facilitated or mediated by titanium Lewis acids including TiCU, Ti(O Pr)4 or its derivatives [286]. The selectivity of the reduction can be tuned by the change of Lewis acid species. For example, the reduction of a ketone and its dimethyl acetal by Bu3SnH showed the latter to be more reactive than the former in the presence of TiCU, but the reactivity was reversed by use of bis-titanium isopropoxide [287] (Scheme 14.122). 

Maruoka s chiral bis-titanium Lewis acid (12) [24] can be regarded as an excellent example of chiral LLA. In this catalytic system, it is proposed that the Lewis acidity of one titanium center might be enhanced by the intramolecular coordination of the oxygen atom of isoproxy group to the other titanium (Scheme 1.18). 

Scheme 1.18 Plausible formation of LLA species from chiral bis-titanium Lewis acid (12).

Noteworthy is the fact that the use of Ti(IV)-(S)-BINOL complexes resulted in low yields with moderate enantioselectivities, implicating the importance of enhanced Lewis acidity in chiral bis-titanium catalyst (12). 

Scheme 1.19 Enantioselective 1,3-dipolar cycloaddition reactions of nitrones catalyzed by chiral bis-titanium catalyst (12).

Conventional synthetic schemes to produce 1,6-disubstituted products, e.g. reaction of a - with d -synthons, are largely unsuccessful. An exception is the following reaction, which provides a useful alternative when Michael type additions fail, e. g., at angular or other tertiary carbon atoms. In such cases the addition of allylsilanes catalyzed by titanium tetrachloride, the Sakurai reaction, is most appropriate (A. Hosomi, 1977). Isomerization of the double bond with bis(benzonitrile-N)dichloropalladium gives the y-double bond in excellent yield. Subsequent ozonolysis provides a pathway to 1,4-dicarbonyl compounds. Thus 1,6-, 1,5- and 1,4-difunctional compounds are accessible by this reaction. 

The earliest Ziegler-Natta catalysts were combinations of titanium tetrachloride (T1CI4) and diethylalummum chloride [(CH3CH2)2A1C1] but these have given way to more effective zirconium based metallocenes the simplest of which is bis(cyclopentadi enyl)zirconmm dichlonde (Section 14 14)... 

Examples of photothermoplasts include polyacrylates, polyacrylamides, polystyrenes, polycarbonates, and their copolymers (169). An especially well-re searched photothermoplast is poly(methyl methacrylate) (PMMA), which is blended with methyl methacrylate (MMA) or styrene as a monomer, and titanium-bis(cyclopentadienyl) as a photoinitiator (170). 

Phenols react readily with tetraalkoxides to give highly colored (yeUow to orange) titanium tetraphenoxides (6). TYZOR KTM [83897-99-8] is the bis-cresyl titanate derived from TYZOR TBT. The tetracresyl titanate [28503-70-0] is also available commercially. 

Titanium chelates are formed from tetraalkyl titanates or haUdes and bi- or polydentate ligands. One of the functional groups is usually alcohoHc or enoHc hydroxyl, which interchanges with an alkoxy group, RO, on titanium to Hberate ROH. If the second function is hydroxyl or carboxyl, it may react similarly. Diols and polyols, a-hydroxycarboxyflc acids and oxaUc acid are all examples of this type. P-Keto esters, P-diketones, and alkanolamines are also excellent chelating ligands for titanium. 

TYZOR LA [65104-06-5] (5), an aqueous solution of the ammonium salt of the titanium bis-lactate complex, is prepared from two equivalents of lactic acid to one of TYZOR TPT. The by-product isopropyl alcohol is removed by distillation and the resultant solution is neutralized with ammonium hydroxide. 

The orange-red titanium acetylacetone chelates are soluble in common solvents. These compounds are coordinately saturated (coordination number equals 6) and thus much more resistant to hydrolysis than the parent alkoxides (coordination number 4). The alkoxy groups are the moieties removed by hydrolysis. The initial product of hydrolysis is beheved to be the bis-hydroxy bis-acetylacetone titanate, (HO)2Ti(acac)2, which oligomerizes to a... 

Reaction of TYZOR DC and 1,3-propanediol gives titanium 1,3-propylenedioxide bis(ethyl acetoacetate) [36497-11-7J, which can be used as a noncorrosive curing catalyst for room-temperature-vulcanizing siUcone mbber compositions (99). Similar stmctures could be made, starting with titanium bis-acetylacetonates, such as that shown in stmcture (9). 

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). 

Bis(ethyl)titanium(IV) chloride [2247-00-9] M 177.0. Crystd from boiling toluene. 

Sulfur imides with a single NR functionality, S5NR (6.12), SeNR (6.13) (R = Oct), " SgNH (6.14), ° and S9NH (6.15) ° are obtained by a methodology similar to that which has been used for the preparation of unstable sulfur allotropes, e.g., S9 and Sio. Eor example, the metathesis reaction between the bis(cyclopentadienyl)titanium complexes 6.8-6.10 and the appropriate dichlorosulfane yields 6.14 and 6.15 (Eq. 6.4). °... 

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