Titanium complexes with //-ligands

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

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],... 

Chiral titanium complexes with a, a, a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) ligands are versatile auxiliaries in the Lewis acid catalyzed alcoholysis of racemic 4-(arylmethyl)-2-phenyl-5(477)-oxazolones 234, providing the corresponding enantiomerically enriched N-protected amino acid esters 235 (Scheme 7.73). The enantioselectivity of the reaction is dependent on the solvent, temperature, and chiral ligand. Selected examples of the alcoholysis of saturated 5(477)-oxazolones are shown in Table 7.21 (Fig. 7.23). 

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). 

Uemura et al. [49] found that (R)-1,1 -binaphthol could replace (7 ,7 )-diethyl tartrate in the water-modified catalyst, giving good results (up to 73% ee) in the oxidation of methyl p-tolyl sulfoxide with f-BuOOH (at -20°C in toluene). The chemical yield was close to 90% with the use of a catalytic amount (10 mol %) of the titanium complex (Ti(0-i-Pr)4/(/ )-binaphthol/H20 = 1 2 20). They studied the effect of added water and found that high enantioselectivity was obtained when using 0.5-3.0 equivalents of water with respect to the sulfide. In the absence of water, enantioselectivity was very low. The beneficial effect of water is clearly established here, but the amount of water needed is much higher than that in the case of the catalyst with diethyl tartrate. They assumed that a mononuclear titanium complex with two binaphthol ligands was involved, in which water affects the structure of the titanium complex and its rate of formation. 

Homoleptic tetrazincates, characteristics, 2, 346 Homoleptic titanium(III) complexes, with -ligands, 4, 282 Homoleptic trizincates, characteristics, 2, 345 Homoleptic vanadium(III) alkyl complexes synthesis and structure, 5, 12 VMes3 reactivity, 5, 12... 

Polyethylene-Wock-poly(clhylcnc-co-norbornene) (PE-fo-P(E-co-NBl ) block copolymer was successfully synthesized by a titanium complex with two non-symmetric bidentate /J-cnaminokclonalo ligands [136,137]. Bis(pyrrolide-imine)titanium complex also has the ability to produce the PE-fo-P(E-co-NBE) block copolymer. PE-fo-PS was synthesized via sequential monomer addition during homogeneous polymerization with bis(phenoxy-imine)metal catalysts [138]. 

Alkenes without functional groups are difficult to hydrogenate enantioselec-tively with noble-metal catalysts. Titanium complexes with a C2 symmetric chiral ligand framework, as in the ansa-titanocene (22-XVI), reduce aryl-substituted C=C bonds in very high isolated and optical yields 19... 

A small number of titanium complexes with heterocychc ligands have been prepared. Reaction of TiCl4(THF)2... 

A titanium complex with a pyrazolylborate ligand was studied by Campbell and Malanga [19]. Similarities between the cyclopentadienyl ligand and the hydridotris(pyrazolyl)borate ligand have been noted for transition metal complexes. Catalyst efficiencies are much lower than those of the analogous penta-methylcyclopentadienyl complexes. 

K. Barry Sharpless (bom 1941) received his PhD in 1968 at Stanford University. Since 1990 he is W. M. Keck Professor of Chemistry at the Scripps Research Institute in La Jolla, USA. Among several other important discoveries. Sharpless developed catalysts for asymmetric oxidations. In 1980 he achieved the catalytic asymmetirc oxidation of allylic alcohols to chiral epoxides by utilizing titanium complexes with chiral ligands (e. g. Section 3.3.2). One of the many applications of chiral epoxides is the use of the epoxide (R)-glycidol for pharmaceutical production of beta-blockers. Sharpless received the Nobel prize for chemistry in 2001 together with Knowles and Noyori. 

Titanium complexes with the tropidinyl ligand as Cp-equivalent and containing a phosphinimido or ketimido ligand have been described (Scheme 243).650... 

Scheme 350 shows the structures of a number of mono-Cp titanium complexes with more elaborately substituted aryloxo ligands. The compounds are formed by the reaction of Cp TiCl3 with 1 equiv. of the substituted phenol in the presence of an excess of pyridine or by treatment of the lithium phenoxide with Cp TiCl3 some of them have been... 

Allyltitanium compounds and Ti enolates derived from mono-Gp chloro titanium complexes with two chiral alkoxo ligands add to aldehydes with high enantioface discrimination.973... 

Pyrrolyl ligand and analogous N-heterocycles are isoelectronic with the Cp ring. Titanium complexes with this type of ligands have been described (Section 4.05.7). 

Brintzinger and coworkers produced a titanium complex with a binaphthyl linked ansa metallocene ligand [56[. Upon activation with nBuLi, this complex catalyzed the reductions of B and 2 phenyl 1 pyrroline in 76 and 98% ee, respectively. Titanium catalysts with chiral amido cyclopentadienyl ligands have also been reported, but were considerably less stereoselective than the metallocene based ones [57]. 

Figure 2 is the representation which contains more data since r 2 - BH4 ligands are known for all the first-row transition metals. Titanium, copper and chromium are the metals for which more examples of q2 coordination can be found. For copper, all the examples correspond to complexes with only one BH4- ligand. While for titanium, complexes with more than one tetrahydrob-orate have also been reported. 

Titanium complexes with asymmetrically substituted cyclopentadienyl ligands can be used as chiral hydrogenation catalysts. The complexes [LfTiCl2] (L = 15,16) were prepared. A [(15)2(TiCl2]/BuLi catalyst under 1 atm H2 reduces 2-phenylbut-l-ene in up to 33% ee. The hydrogenation was used as a test reaction to measure the asymmetric induction achieved by the new ligands. ... 

Self-supported titanium complexes with linked bis-BINOL ligands were used as an alternative approach for the immobilisation of catalysts, as shown in enantioselective sulfide oxidation (see Section 7.2.2). The same ligands were used with success in asymmetric carbonyl ene reactions. The chiral metal-bridged polymer 76, derived from ent-lOa, titanium tetraisopropoxide and water (Scheme 7.45), catalysed the ene reaction between 68b and 71, to give R)-72 in 88% yield and 88% enantiomeric excess. The catalyst can be reused at least five times without affecting its efficiency. 

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