Bis-titanium catalyst

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). 

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).

Although there are many reports on the enantioselective catalytic double bond isomerization of functionalized achiral alkenes, that of alkenes bearing an isolated double bond have had limited success. The use of a chiral bis(indenyl)titanium catalyst 5 containing a chiral bridging group realized the highly enantioselective isomerizations of unfunctionalized achiral alkenes with up to 80% ee (Equation (27)).90... 

The initial work on the asymmetric [4-1-2] cycloaddition reactions of A -sulfinyl compounds and dienes was performed with chiral titanium catalysts, but low ee s were observed <2002TA2407, 2001TA2937, 2000TL3743>. A great improvement in the enantioselectivity for the reaction of AT-sulfinyl dienophiles 249 or 250 and acyclic diene 251 or 1,3-cyclohexadiene 252 was observed in the processes involving catalysis with Cu(ll) and Zn(ii) complexes of Evans bis(oxazolidinone) (BOX) ligands 253 and 254 <2004JOC7198> (Scheme 34). While the preparation of enantio-merically enriched hetero-Diels-Alder adduct 255 requires a stoichometric amount of chiral Lewis acid complex, a catalytic asymmetric synthesis of 44 is achieved upon the addition of TMSOTf. 

It was 1996 when Buchwald and Hicks reported the first example of an asymmetric PKR involving a catalytic amount of a chiral titanocene complex. The titanium catalyst (6 ,6 )-(EBTHI)Ti(GO)2 (EBTHI = ethylene-1,2-bis( 7 -4,5,6,7-tetrahydro-l-indenyl)) obtained in situ by treatment of (6 ,6 )-(EBTHI)TiMe2 under CO pressure was efficient for the formation of enantiomerically enriched carbocyclization adducts. ... 

This method is comparable to similar, catalytic Sim-mons-Smith-type methods employing the titanium TADDOL catalyst 20 (95 5 er) or the Ci-symmetric bis-sulfonamide catalyst 32 (93 7 er) for the cyclopropanation of the allylic alcohol 22 (eq 6). However, due to the preliminary nature of these earlier investigations, substrate scope and generality have not been extensively documented. All of the aforementioned methods are limited by their dependence on the allylic alcohol functionality. Only one method for Simmons-Smith-type cyclopropanation of other substrate classes has been developed. Use of a stoichiometric, chiral dioxaborolane [CAS 161344-85-0] additive allows for selective cyclopropanation of allylic ethers, homo-ally lie alcohols and allylic carbamates. ... 

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

The authors devised the new bidentate titanium catalyst (anthraquinone-l,8-di-oxy)bis(triisopropoxytitanium) (55) and utilized it for the simultaneous coordination of carbonyl substrates [59]. Comparison of the reactivity and selectivity with the corresponding monodentate titanium catalyst 56 in several synthetic examples genuinely demonstrates the high double-activation ability of 55 toward carbonyls under catalytic conditions as illustrated in Scheme 1-20. 

Atactic polypropene has been synthesized with homogeneous catalytic systems based on mono-Cp trialkoxo titanium complexes activated by MAO.951 Syndiotactic polystyrene has been synthesized with different mono-Gp trialkoxo titanium derivatives activated by MAO and AlMe3, and the catalytic efficiency has been compared with bis-Cp titanium catalysts.952 The titanium ligands affect both catalytic activity and stereoregularity of the polypropylene obtained. For the CpTi(OPrn)3/MAO system, factors influencing the propylene polymerization, such as temperature, Al/Ti molar ratio, and monomer pressure, have been studied. 

Gel-type poly(styrene-co-divinylbenzene) beads have been used as a carrier to encapsulate bis-Cp titanium catalysts through a simple swelling-shrinking procedure. These catalytic species are homogeneously distributed in the PS bead particle and exhibit high and stable ethylene polymerization and ethylene/1-hexene co-polymerization activity.1236... 

One of the most remarkable aspect on the bis-Cp titanium derivative chemistry has been the production of new and unprecedented variety of polyolefins. The use of this type of complexes as Ziegler-Natta pre-catalyts for the olefin polymerization has opened new possibilities to produce polyolefins with different properties, and significant effort has been devoted to the design of new bis-Cp catalyst structures. This section summarizes simple aspects related to the polymerization of ct-olefins catalyzed by bis-Cp titanium complexes containing a cr-Ti-C bond. A more comprehensive review of the catalytic applications of titanium complexes in the a-olefin polymerization processes is covered in Chapter 4.09. 

The synthesis, structural determination, and reactivity of ( -symmetric ethylene-bridged titanium compounds has been investigated (Scheme 645). The complexes have been characterized by X-ray crystallography and applied as catalysts for the enantioselective isomerization of alkenes, though they prove to be less enantioselective than the known chiral ansa-bis-Ind titanium catalyst.1667... 

Differently, in the case of secondary chain growth with bis(phenoxy-imine)titanium catalysts, Coates and coworkers reported that chain release occurs exclusively by /3-H hydride transfer from the terminal methyl. This generates an allylic end group as shown in Scheme 15,160 and it has been utilized to produce functionalized syndiotactic propylene oligomers.258... 

Chiral titanium catalyst (2) for enantioselective isomerization of alkenes.2 The chiral titanium catalyst, an a . a-bis(indenyl)titanium dichloride (2), is prepared by the reaction of the dimesylate of 1 with indenyllithium to form a product that is con-... 

As previously demonstrated for the ROP of rac-LA, titanium complexes supported by benzotriazole-phenolate 23-28/ a mixture of rac- and meso-amino bis(alkanolate) 40/41 and anatrane 66-69 ligands proved to be efficient catalysts for the ROP of rac-BBL to give poly(3-hydro g butyrate) (PHB). The most notable features of these catalysts are the high polymerisation activity and productivity in solvent-free reactions at 80 °C (rac-BBL Ti = 200, 95-99% conv. after 9-48 min), combined in some cases with high stereoselectivity (i.e. in syndiotactic PHB, up to Pr = 0.73) and narrow PDIs (1.19-1.36). Note that up to now there is no report of isospecific polymerisation of rac-BBL using titanium catalysts. 

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