Titanium complexes, reaction with

Reaction of the bis (allyl) titanium complexes -16 with saturated and unsaturated aldehydes at -78 °C in the presence of 1.1 equiv of ClTi(OiPr)3 afforded the corresponding Z-anti-configured homoallyl alcohols 4 with >98% regioselectivity and >98% diastereoselectivity in good yields (Scheme 1.3.6) [14]. 

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). 

Table 1.3.1 Reaction of the acyclic mono(allyl)titanium complexes -19 with aldehydes.

The reaction of the acyclic bis(allyl)titanium complexes 16 with the imino esters 23a-c in the presence of Ti(OiPr)4 and ClTijOiPrjs at low temperatures proceeded with >98% regioselectivity and >98% diastereoselectivity and gave the corresponding T-syn-configured unsaturated a-amino acid derivatives E-24 in good yields (Scheme 1.3.11) [21, 22]. 

Table 1.3.5 Reaction of the cyclic mono(allyl)titanium complexes 20 with the imino ester 23c.

L = O/Pr, = aiiyisuifoximine Scheme 1.3.13 Reactivity scheme for the reactions of the bis(allyl)titanium complexes 16 with aldehydes. 

A titanium complex (1) with a salen ligand is an efficient catalyst for the enan-tioselective epoxidation of alkenes with hydrogen peroxide as the terminal oxidant. The participation of a titanium-peroxo species, activated by hydrogen bonding, in the reaction, has been postulated.73... 

A mechanistically obscure transformation occurs upon treatment of the tetramethylfulvene titanium complex 161 with methallyl Grignard, producing bridged titanacyclobutane complex 162. This reaction is proposed to proceed by intramolecular alkylation at the central carbon of an 7]4-fulvene, 7]3-methallyl intermediate, but with due consideration... 

The intramolecular coordination chemistry of substituted bis-Cp titanium complexes, dealing with the syntheses, reactions, structures, and some applications in homogeneous and catalytic reactions, has been summarized.1134... 

Previously, Pasini [27] and Colonna [28] had described the use chiral titani-um-Schiff base complexes in asymmetric sulfide oxidations, but only low selec-tivities were observed. Fujita then employed a related chiral salen-titanium complex and was more successful. Starting from titanium tetrachloride, reaction with the optically active C2-symmetrical salen 15 led to a (salen)titani-um(IV) dichloride complex which underwent partial hydrolysis to generate the t]-0x0-bridged bis[(salen)titanium(IV)] catalyst 16 whose structure was confirmed by X-ray analysis. Oxidation of phenyl methyl sulfide with trityl hydroperoxide in the presence of 4 mol % of 16 gave the corresponding sulfoxide with 53% ee [29]. 

Molecular sieves (MS) can have a detrimental effect on the enantioselectivity when dienophiles with a free hydroxyl group e.g., juglone (lib) are used. The ee-value drops from 85% for the addition of 1,4-naphlhoquinone (11a) to 1-acetoxy-l,3-butadiene (3c) to 9% for addition of juglone (lib). The latter reaction proceeds with the MS-free binaphthol-titanium complex 21 with 96% ee37. 

Treatment of the titanium complex (149) with two equivalents of bisulfenyl chloride afforded the bicyclic compound (150) (mp 204°C dec.) in 30-40% yield (Scheme 13) <9lCB214l>. Reaction of (149) with SCI2 and S2C12 gave (151) and (152) in 40% total yield in a ratio of 1 1 (Scheme 13)... 

Catalytic asymmetric halogenation reactions are still rarely studied. Togni et al. developed the efficiency of [Ti(TADDOLato)] complexes 18 in combination with the fluorinating agent Selectfluor in the catalytic fluorination of P-ketoesters. In 2004, this group executed the asymmetric chlorination of P-ketoesters using titanium complexes 18 with (dichloroiodo)toluene to generate enantiomerically enriched a-chlorinated products 19 (Scheme 5) [35]. 

The key step of the reaction mechanism is reaction of the titanium complex 118 with the a,co-enone to give the titanaoxacyclopentane 140, which reacted with diphenylsilane by the cleavage of the Ti-0 bond to afford the alkyltitanium compound 141. Reductive elimination furnished the siloxane 138 and the catalyst entered the cycle again (Scheme 62). 

Several structures of the transition state have been proposed (I. D. Williams, 1984 K. A. Jorgensen, 1987 E.J. Corey, 1990 C S. Takano, 1991). They are compatible with most data, such as the observed stereoselectivity, NMR measuiements (M.O. Finn, 1983), and X-ray structures of titanium complexes with tartaric acid derivatives (I.D. Williams, 1984). The models, e. g., Jorgensen s and Corey s, are, however, not compatible with each other. One may predict that there is no single dominant Sharpless transition state (as has been found in the similar case of the Wittig reaction see p. 29f.). 

Similar to IFP s Dimersol process, the Alphabutol process uses a Ziegler-Natta type soluble catalyst based on a titanium complex, with triethyl aluminum as a co-catalyst. This soluble catalyst system avoids the isomerization of 1-butene to 2-butene and thus eliminates the need for removing the isomers from the 1-butene. The process is composed of four sections reaction, co-catalyst injection, catalyst removal, and distillation. Reaction takes place at 50—55°C and 2.4—2.8 MPa (350—400 psig) for 5—6 h. The catalyst is continuously fed to the reactor ethylene conversion is about 80—85% per pass with a selectivity to 1-butene of 93%. The catalyst is removed by vaporizing Hquid withdrawn from the reactor in two steps classical exchanger and thin-film evaporator. The purity of the butene produced with this technology is 99.90%. IFP has Hcensed this technology in areas where there is no local supply of 1-butene from other sources, such as Saudi Arabia and the Far East. 

Titanium Complexes of Unsaturated Alcohols. TetraaHyl titanate can be prepared by reaction of TYZOR TPT with aHyl alcohol, followed by removal of the by-product isopropyl alcohol. EbuUioscopic molecular weight determinations support its being the dimeric product, octaaHoxydititanium. A vinyloxy titanate derivative can be formed by reaction of TYZOR TPT with vinyl alcohol formed by enolization of acetaldehyde (11) ... 

Reaction with Lactones. Hydroxycarboxyhc acid ester complexes of titanium are formed by reaction of a tetraalkyl titanate with a lactone, such as P-propiolactone, y-butyrolactone, or valerolactone (35). For example. 

Sohd, water-soluble a-hydroxycarboxyhc acid and oxaUc acid titanium complexes can be formed by reaction of the acid and a tetraaLkyl titanate in an inert solvent, such as acetone or heptane. The precipitated complex is filtered, rinsed with solvent, and dried to give an amorphous white soHd, which is water- and alcohol—water-soluble (81,82). 

---