Meso zirconium complex

The meso-tetraphenylporphyrin complex [Zr(TPP)Cl2] has been prepared by reaction of H2TPP with ZrCl4 in boiling benzonitrile. The electronic spectrum of [Zr(TPP)Cl2] and the facile kinetics of TPP dissociation in acidic media suggest that the zirconium atom lies considerably out of the plane of the TPP ligand with both chlorine atoms being located on the same side of the TPP plane.708... 

The spectra of the tetra-T)3-allyl complexes of Mo, W, and Zr (Table XI) show significant differences. Whereas that of the zirconium complex has only two signals (one for the meso and one for the terminal carbon atoms), the spectra of the molybdenum and tungsten complexes consist of three... 

Along with the development of chiral Lewis acid catalysts, a chiral trialkanolamine (42) has been used to prepare the catalyst (43) (Eq. 19). By use of this zirconium complex as a catalyst, enantioselective addition of the azide to meso epoxides was achieved [20a]. Thus, the oxirane ring was opened by /-PrMe2SiN3 to give the adduct (44) with high enantioselectivity (Eq. 20). In another example, a diamide ligand (45), which behaves as a tetradentate ligand, was used to achieve a similar reaction (Eqs 21 and 22) [20b]. 

The use of zirconium complexes derived from tartramides 3.19 in asymmetric epoxidation of homoallylic alcohols does not result in any improvement over the related to titanium analogs [808]. A zirconium complex prepared from Zr(Otert-Bu)4 and (S,S,S)-triisopropylam3ne 3.22 in the presence of water catalyzes the asymmetric ring opening of meso-epoxides by /-PiMe2SiN3 (ee 85%), while related titanium complexes are less efficient [805,831]. 

In 1992, Nugent [12] reported a significant advance in the Lewis acid-promoted opening of meso epoxides (Scheme 2). A novel chiral zirconium complex catalyses epoxide opening with hindered silyl azides iii excellent ee. The catalyst is derived from zirconium tert-h xi-oxide and the tetradentate C3-symmetric ligand 1, available by combination of (5)-pro-pene oxide with (S)-l-aminopropan-2-ol. Al-... 

Less acidic than Ti and Zi chloroderivatives, MeTi(OPr )3 perfoims chelation-controlled addition to chiral alkoxy ketones as well as or better than organomagnesium compounds, but fails to chelate to aldehydes or hindered ketones. Should the formation of a cyclic chelation intermediate be forbidden, the reaction is subject to nonchelation control, according to Ae Felkin-Anh (or Comforth) model. Under these circumstances, the ratio of the diastereomeric products is inverted in favor of the anti-Cram product(s). In the case of benzil (83 Scheme 7) this can be accounted for by the unlikely formation of a cyclic intermediate such as (85), and thus the preferential intermediacy of the open chain intermediate (86) that leads to the threo compound (88). This view is substantiated by the fact that replacement of titanium with zirconium, which is characterized by longer M—O bonds, restores the possibility of having a cyclic intermediate and, as a consequence, leads to the erythro meso) compound (87) thus paralleling the action of Mg and Li complexes. 

Rac/meso-isomers of bridged bis(indenyl) zirconium dichlorides can interconvert photochemically.796 This interconversion has been utilized in the stereoselective synthesis of ansa-zirconoccnc binaphtholate stereoisomers.797 Specifically, the rac-meso-mixtures 1034 induced by irradiation in toluene react with 1 equiv. of the dilithium salt of racemic binaphthol to give the racemic binaphtholate complex 1035 (Scheme 253). Analogous reactions with 1 equiv. of the Z (+) enantiomer of dilithium binaphtholate afford the enantiomerically pure binaphtholate complex. The structure of the racemic binaphtholate complex, Me2Si(2-Me-4-But-C5H2)2Zr(binaphtholate), has been crystallographically determined. 

Zirconium and hafnium also give coloured Thoron I complexes, but these metals can be masked with tartaric acid (especially meso-tartaric acid) 1 ml of 5% tartaric acid solution in 25 ml of solution effectively masks 1 mg of zirconium. Uranium(IV) interferes in the determination of thorium, but does not interfere significantly when oxidized to U(VI). Up to 5 mg of Al, 5 mg of Fe(II), 5 mg of Ce, and 2 mg of Ti can be tolerated. Ascorbic acid is used to reduce iron(III). 

Highly enantioselective catalytic desymmetrization of meso-epoxides through nucleophihc ring opening was first effectively demonstrated by Nugent, who found that a zirconium trialkanolamine complex catalyzed the addition of azi-dosilanes to meso-epoxides (Scheme 1) [3]. Azide has been the most widely explored nitrogen nucleophile [4,5,6,7], in part due to its utiHty as an amine sur-... 

In this work the three chiral forms of the ligand have been prepared R,Rl S,Slmeso) and complexed to zirconium(iv). The chiral ligands bind to the metal centre to afford a-cis complexes. The chirality of the metal (either A or A) is predetermined by the chirality of the ligand - R,R affords the A and S,S the A isomer - and these are locked in solution as single stereoisomers. However, the meso ligand binds in a fac-fac (p-c/s) fashion to the zirconium(iv) centre and this is fluxional in solution - which has been assigned to the... 

In Special cases, this type of catalyst may oscillate, switching between the two stereoselective C2-symmetric racemic-like conformations and a nonstereoselective meso-Yikt conformation during the propagation of a single polymer chain. The result is a stereoblock polymer microstructure (see Chapter 8). The most-studied catalyst precursor of this type is the unbridged complex bis(2-phenylindenyl)zirconium dichloride (vide The oscillating stereocontrol mech-... 

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