Titanium complexes catalyst performance

Binaphthol-derived titanium complexes [64], prepared from chiral ligands 65 (Figure 3.13), also performed very well in the cycloadditions of conjugated aldehydes with cyclic and acyclic dienes. Judging from the absolute configurations of endo and exo adducts, this catalyst should cover the re-face of carbonyl on its u tz-coordination to s-trans a,/l-unsaturated aldehydes, and hence dienes should approach selectively from the si-face. 

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. 

This section presents a summary of the currently preferred conditions for performing titanium-catalyzed asymmetric epoxidations and is derived primarily from the detailed account of Gao et al We wish to draw the reader s attention to several aspects of the terminology used here and throughout this chapter. The terms titanium tartrate complex and titanium tartrate catalyst are used interchangeably. The term stoichiometric reaction refers to the use of the titanium tartrate complex in a stoichiometric ratio (1(X) mol %) relative to the substrate (allylic alcohol). The term catalytic reaction (or quantity) refers to the use of the titanium tartrate complex in a catalytic ratio (usually 5-10 mol %) relative to the substrate. 

When catalytic asymmetric allylation was attempted with a catalytic amount of chiral titanium complexes, BINOL-TiCl2 or BIN0L-Ti(0-/-Pr)2 the reaction was found to be slow. The reaction was performed satisfactorily when BIN0L-Zr(0-/-Pr)2 was employed as catalyst in the presence of molecular sieves (Eq. 18) [19a]. 

Lewis add complexes formed by the reactions of various aminoalcohols with Et2AlG [778, 824] or by the reaction of Et2Zn with a chiral sulfamide [806] have displayed a low efficiency in the asymmetric condensations of ketene and thioketene silyiacetals derived from acetic acid with aldehydes. Disappointing se-lectivities have also been observed with some binaphtol-titanium complexes [778]. However, Mikami and Matsukawa [1296] recently performed the enantioselective condensation of various aldehydes with acetic acid derivatives in the presence of a chiral binaphtol-titanium complex. Good selectivities were observed when the reaction was performed at 0°C in toluene (Figure 6.95). Quaternary ammonium fluorides derived from cinchona alkaloids have been proposed as catalysts to perform additions of enoxysilanes derived from ketones to PhCHO, but the observed selectivities are modest [1303],... 

The Sharpless epoxidation of allyl alcohols 3.16 by /erf-butyl hydroperoxide under catalysis with chiral titanium complexes is a very popular method that has frequently been used in industry [811, 812, 853], This epoxidation was initially developed with stoichiometric amounts of tartrate catalysts. Today, it is usually performed in the presence of catalytic amounts of Ti(0/ -Pr)4 and diethyl or diisopropyl tartrate (2R,3R)- or (25,35)-2.69 (R = Et or r-Pr). The reactions are conducted at or near room temperature in the presence of molecular sieves. Several... 

Sharpless epoxidation of allyl alcohols (Sharpless, 1985, 1988 Pfenninger, 1986 Rossiter, 1985 Woodard et al., 1991 Finn and Sharpless, 1991 Corey, I990a,b), an example of which is included in Table 9.6, is perhaps the most recent and one of the most remarkable applications of asymmetric catalysis. The reaction is normally performed at low temperatures (-30 to 0°C) in methylene chloride with a titanium complex consisting of a chiral component [diethyl tartrate (DET) or diisopropyl tartrate (DIPT)] and a titanium salt (titanium tetraisopropoxide) as the catalyst. The beauty of the synthesis is that both enantiomers of the tartrate are available so that either form of the product can be prepared in more than 90% ee. 

In addition to their work on DA reaction, Seebach et al. also studied polymeric titanium catalyst 82,83 (see Figure 7.7) in the [3 + 2] cycloaddition of 3-crotonoyloxazolidinone and diphenyl nitrone (Table 7.5). The use of soluble catalysts 81a,b,d (entries 1-3) and supported-titanium catalysts 83a,b,d (entries 4-6) afforded similar performances in terms of yield, dia-stereoselectivity and enantioselectivity in favour of adduct 89a. Interestingly, the enantioselectivity was better in heterogeneous conditions with polymeric TADDOL-titanium complex 83d (entry 6, 56% enantiomeric excess) than using soluble catalysts (entries 1-3). 

The Sharpless epoxidation of allylic alcohols by hydroperoxides uses as mediator [45] or as catalyst [46] a chiral titanium complex obtained from the combination Ti(OPr )4/diethyl tartrate (DET) in 1 1 ratio. Kinetic resolution of P-hydroxysulfides was also observed, but without diastereoselectivity for the product P-hydroxysulfoxides [47]. We found that the Sharpless reagent deactivated by 1 equivalent of water allows the enantioselective oxidation of aryl methyl sulfides into sulfoxides to be performed with ee s up to 90% [4S-50]. The best reagent combination proved to be Ti(0Pr )4/DET/H20 = 1 2 1. Independently, Modena et al. obtained similar enantioselectivities with the combination Ti(OPr )4/DET in 1 4 ratio [51]. These two combinations are sometimes referred to as the Kagan reagent and the Modena reagent, respectively. They will be considered successively. 

MAO activated mono-cyclopentadienyl titanium complexes containing one alkoxide a-ligand appear to have a superior catalyst performance as compared to the analogous trichlorides, Cp TiCl3 CpTiCl2(OR)/MAO systems have polymerization activities that are double or... 

Besides the cocatalyst, there are some other additives that are capable of influencing the performance of catalyst systems during styrene polymerization. The addition of a proper amount of triisobutylalu-minum (TIBA) to various titanium complex/MAO systems increases the catalyst activity, whereas the addition of TMA or triethylaluminum (TEA) inhibits polymerization. A relatively high molar ratio of TIBA/MAO will cause a reduction in activity and a sharp decrease in the molecular weights of sPS. When a haloalkylaluminum such as AlEt2Cl is used with the CpTiCb/MAO system, only atactic polystyrene is produced by a noncoordination polymerization process, that is, a cationic... 

Mikami reported that BINOL derived titanium complex efficiently catalyzed the aldol reaction of silyl enol ether with excellent control of both absolute and relative stereochemistry [106] (Scheme 14.37). The reaction was proposed to proceed via a prototropic ene reaction pathway that is different from that of Mukaiyama aldol condensation. A cyclic antiperiplanar transition-state model was proposed to explain the pref erential formation of the syn diastereomer from either (E)- or (Z)-silyl enol ethers [106]. Further modifications of the catalyst system include the use of perfluorophenols and other activating additives [107], or performing the reaction in supercritical fluids [108]. Furthermore, the nucleophile could be extended to enoxysilacyclobutane derivatives [109]. 

The DA reaction of methacrolein with 1,3-dienol derivatives was catalyzed by BINOL derived titanium complexes, giving the endo adduct with high enantiose-lectivity [155]. The reaction was performed in the absence of MS, since MS can act as an achiral catalyst in this reaction and compromise the product enantiomeric excess. Similarly, the DA reaction of juglone with butadienyl acetate catalyzed by BINOL-Ti complex proceeds in only 9% ee in the presence of MS, whereas the reaction proceeds in 76-96% ee in the absence of MS to provide the endo adducts useful for the synthesis of anthracyclines and tetracycline [155,156). The drawback of the above method is the instability of the addition products, owing to their high tendency to aromatize. Corey improved this DA reaction with quinone monoacetal as a supplant of juglone or benzoquinone [157] (Scheme 14.64). 

Since the previous titanium-based catalyst was not able to perform a-olefin polymerization, it was obviously interesting to investigate other soluble active systems to further study the problem of stereoregulation. That was done by Zambelli [Makromol Chemie, 112, 160 (1968)] with the complex obtained from VCI4 and AIR2CI (Fig. 21) in the presence of either an excess of A1 derivative or a Lewis base ligand, e.g. anisole. 

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