Schiff base titanium complex

Xn approach to 5-hydroxy-3-ketoesters by the condensation of diketene with aldehydes n lives a titanium alkoxide derived from the complex Schiff base 14. ... 

In 2005, Katsuki reported that the dimeric titanium tetradentate Schiff base complex 7, prepared from the salen 6 and titanium tetraisopropoxide... 

Belokon et al.83 have investigated the formation of the homo-and bimetallic titanium complexes with di-Schiff base ligands, by means of FT NMR spectroscopy. The ligands have been shown to adapt the ds-p configuration in titanium (IV) complexes. Analysis of the 1H NMR spectra has allowed determination of the population of the homobimetallic complexes derived from two different Ti(IV) complexes [34],... 

Iu search for efficieut aud greeuer processes over the past few years various heterogeneous catalysts such as titanium incorporated mesoporous molecular sieves [45,46], Schiff-base complexes supported on zeolite [47] and Zn(II)-Al(III) layered double hydroxide (LDH) [48], oxomolybdenum(VI) complexes supported on MCM-41 and MCM-48 [49], polyoxometallate supported materials [50], Co and Mn-AlPO s [51] etc. have been developed and studied for the catalytic epoxidatiou of a-pinene. Many of these processes suffer from drawbacks and limited applicability due to the poor conversion and also the selectivities. Sacrificial aldehydes are often used as an oxygen acceptor in such processes to achieve reasonable yield and selectivities but, this procedure leads to an increase in the E-factors and decrease in the atom economy [51]. 

Employing silyated quadridentate Schiff bases, e.g. 0,0 -bis(trimethylsilyl)-JV,JV -ethylenebis(salicylideneimine),105 a range of titanium(IV) complexes Ti(j/5-C5H5)(salen)L (25 L = Cl, OMe, SMe, NMe2, SnPh3, acac) have been prepared. 

M. Hayashi, Y. Miyamoto, T. Inoue, and N. Oguni, Enantioselective trimethylsilylcyanation of some aldehydes catalyzed by chiral Schiff base-titanium alkoxide complexes, J. Org. Chem. 58 1515 (1993). 

Schiff base -complexes of titanium (TITANIUM COMPOUNDS - ORGANIC] (Vol 24)... 

Carreira et al. used a chiral BlNOL-derived Schiff base-titanium complex as the catalyst for the aldol reactions of acetate-derived ketene silyl acetals (Scheme 8C.29) [64a]. The catalyst was prepared in toluene in the presence of salicylic acid, which was reported to be crucial to attain a high enantioselectivity. A similar Schiff base-titanium complex is also applicable to the carbonyl-ene type reaction with 2-methoxypropene [64b], Although the reaction, when con-... 

Chiral Schiff bases derived from salicylaldehyde, when complexed to titanium(IV), catalyse enantioselective addition of diketene (101) to aldehydes (RCHO R = Ph, alkyl, alkenyl) to yield 5-hydroxy-/ -keto esters (102), with up to 84% ee 19... 

The source of enantioselection in the trimethylsilylcyanation of benzaldehyde using chiral Schiff base-titanium(IV) complexes has been investigated by computation.270... 

The macrocyclic chemistry of tetradentate Schiff base complexes has been known for long time. However, the successful use of such a complex as an enantioselective catalyst in epoxidation reactions is a relatively recent finding. In these reactions complex 9.9 or an analogue is used. One of the possible routes for the synthesis of intermediate 9.2 of Table 9.1 involves the use of a similar catalyst. While complex 9.9 works well with unfunctionalized alkenes, for the epoxidation of allylic alcohols, dialkyl tartarates, 9.10, are the preferred ligands. As we shall see, the mechanisms of epoxidation in these two cases are different. Also for the tartarate-based system titanium is the metal of choice (see Section 9.3.3). 

In the same year, Fujita s group63 reported the asymmetric oxidation of aryl methyl sulfide by hydroperoxides (TBHP, CHP) and an optically active catalyst formed by a Schiff base-oxovanadium(IV) complex 32, giving (S)-sulfoxides in low ee (up to 40%) (Fig. 4). Later, they developed64 a more promising approach using 33, a binuclear Schiff base-titanium(IV) complex (4 mol% equiv) to catalyze the asymmetric oxidation of methyl phenyl sulfide by trityl hydroperoxide in methanol at 0 °C. The (ft)-methyl phenyl sulfoxide was obtained with 60% ee. 

Before leaving the area of oxene chemistry, there is one further system worthy of mention the manganese Schiff-base complexes. The Schiff-base complexes were prepared in response to the Katsuki-Sharpless system for stereospecific epoxidation (Figure 2.19).57 The Katsuki-Sharpless system consists of titanium(IV) isopropoxide and ( + )- or (—)-diethyl tartrate with... 

Narasaka has reported that TADDOL-Ti dichloride catalyzes the asymmetric addition of trimethylsilylcyanide to aromatic and aliphatic aldehydes (Sch. 63) [148]. The reactions proceed only in the presence of MS 4A. In reactions with aliphatic aldehydes a chiral cyanotitanium species obtained by mixing of the TADDOL-Ti dichloride and trimethylsilylcyanide before addition of the aldehydes acts as a better chiral cyanating agent and affords higher enantiomeric excesses. Chiral titanium complexes obtained from an alcohol ligand and salicylaldehyde-type Schiff bases and a salen ligand have been reported to catalyze the asymmetric addition of hydrogen cyanide or... 

The same conversion is successfully catalyzed by using in-situ generated complexes of Ti(OPr )4 and tridentate Schiff bases (Stmcture 54), which are derived from substituted salicylaldehydes with chiral aminoalcohols [85]. Another similar chiral reagent is derived from reaction of titanium tetraisopropoxide and the Schiff base of 3,5-di-tert-butylsalicylaldehyde and (5)-valinol. The mechanism and stereoselectivity of these chiral Lewis acids are discussed by Corey and co-workers. Other chiral Ti Schiff base complexes have been employed in asymmetric TMSCN addition to benzaldehyde [85]. 

The Ti-Cp bond is cleaved by reaction of Cp2TiCl2 with the lithium salt of Schiff base to give racemic complexes of titanium.1188... 

Schiff Bases. An extension of work on VO and UOl" complexes of optically active Schiff bases has resulted in the isolation of two series of titanium complexes of stoicheio-metry. TiOSB and TiOSB,HA [SB = quadridentate Schiff base dianion from sali-cylaldehyde and ( —)-propylenediamine, ( —)-butanediamine, meso-butanediamine, (-)-cyclohexylamine, or (-l-) Stilbenediamine A = CIO4, C2O4, or Cl]. The low solubilities of the complexes and an i.r. band at ca. 800 cm attributable to v(Ti—... 

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]. 

Oguni also reported that the titanium complex of a chiral amino alcohol whose amino group was modified with a salycylal-type Schiff base as shown in 11 (Fig. 1) catalyzed the cyanosilylation of aromatic and aliphatic aldehydes [44,45,... 

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