Zirconocene chiral

Allyl aryl ethers undergo accelerated Claisen and [1,3] rearrangements in the presence of a mixture of trialkylalanes and water or aluminoxanes. The addition of stoichiometric quantities of water accelerates both the trimethylaluminum-mediated aromatic Claisen reaction and the chiral zirconocene-catalyzed asymmetric carboalumination of terminal alkenes. These two reactions occur in tandem and, after oxidative quenching of the intermediate trialkylalane, result in the selective formation of two new C-C bonds and one C-0 bond (Eq. 12.70).153 Antibodies have also been developed to catalyze Claisen154 and oxy-Cope155 rearrangements. 

An enantioselective variant of the diene cydization reaction has been developed by application of chiral zirconocene derivatives, such as Brintzinger s catalyst (12) [10]. Mori and co-workers demonstrated that substituted dial-lylbenzylamine 25 could be cyclized to pyrrolidines 26 and 27 in a 2 1 ratio using chiral complex 12 in up to 79% yield with up to 95% ee (Eq. 4) [ 17,18]. This reaction was similarly applied to 2-substituted 1,6-dienes, which provided the analogous cyclopentane derivatives in up to 99% ee with similar diastereoselectivities [19]. When cyclic, internal olefins were used, spirocyclic compounds were isolated. The enantioselection in these reactions is thought to derive from either the ate or the transmetallation step. The stereoselectivity of this reaction has been extended to the selective reaction of enantiotopic olefin compounds to form bicyclic products such as 28, in 24% yield and 59% ee after deprotection (Eq. 5) [20]. 

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

Asymmetric induction was observed with a chiral zirconocene catalyst (Equation (81))-... 

Subsequent to these studies by the author s group, Whitby and co-workers reported that enantioselective alkylations of the type illustrated in Scheme 6.6 can also be carried out with the non-bridged chiral zirconocene 31 [19]. Enantioselectivities are, however, notably lower when alkylations are carried out in the presence of 31. For example, this new chiral metallocene affords 29 and 30 (Scheme 6.5) with 82% and 78% ee, respectively. 

If the above research is an indication, the catalytic enantioselective variants of many of these exciting transformations will soon be disclosed in our leading journals. Another challenge in this area remains the difficulty encountered in preparing chiral zirconocene catalysts, particularly since many of the reactions promoted by this group of chiral catalysts cannot be effected by the non-metallocene variants. Thus, the development of more practical, but equally or even more selective and efficient variations of existing methods should not be viewed as any less significant. 

Many chiral, enantiomerically pure zirconocenes are known [20], In order to induce an asymmetric reaction, chiral zirconocenes have to be prepared, of which the most common are [(EBTHI)ZrCl2] EBTHI = r 10-ethylene-l,2-bis(tetrahydroindenyl), see Scheme 8.47 for the corresponding bis(triflate) and Erker s [(NMI)2ZrCl2] (NMI = r 5-neomenthyhn-dene) [21] (see Scheme 8.37). The [(EBTHI)ZrCl2] complex is commercially available as a racemate or in enantiomerically pure form (for a resolution procedure, see the supplementary material of [22]), and the precursor [(EBI)ZrCl2] is available as a racemate. 

The kinetic resolution using a chiral zirconocene-imido complex 286 took place with high enantioselectivity to result in chiral allenes 287 (up to 98% ee) (Scheme 4.74) [116]. However, a potential drawback of these methods is irreversible consumption of half of the allene even if complete recovery of the desired enantiomer is possible. Dynamic kinetic resolutions avoid this disadvantage in the enantiomer-differentiating reactions. Node et al. transformed a di-(-)-L-menthyl ester of racemic allene-l,3-dicarboxylate [(S)- and (RJ-288] to the corresponding chiral allene dicarbox-ylate (R)-288 by an epimerization-crystallization method with the assistance of a catalytic amount of Et3N (Scheme 4.75) [117]. 

Scheme 4.74 Kinetic resolution by using chiral zirconocene-imido complexes 286.

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

Spaleck et al. (154) reported a large number of chiral zirconocenes with different bridges and substitutions on the indenyl ligand (Table VII). Some C2-symmetric metallocenes give polypropylenes with a high melting point (162°C) and tacticities (mmmm pentades) of 97-99%, measured by 13C-NMR spectroscopy (155,156). 

Kaminsky W, Kulper K, Brintzinger HH, FRWP Wild, Polymerization of Propene and Butene with a Chiral Zirconocene and Methyl Aluminoxane as Cocatalyst, Angew Chem Int Ed Engl, 24,507 (1985)... 

Scheme 3. Synthesis of the chiral zirconocene catalyst 31 according to H.-H. Brintzinger.

The a-olefin insertions proceed regiospecrfically in the Zr-C bond, affording 1,2-insertion product, in which a stereogenic center is present at the -position (relatively to Zr) of the metallacycle. Starting from chiral zirconocene complexes (as rac-[(ebi)Zr( -pyridyl)]+), propene insertion occurs with a high level of diastereoselectivity in cases where the -pyridyl ligand contains a six-substituent. 

The catalytic cycle proposed for the enantioselective ethyhnagnesation involves the chiral zirconocene-ethylene complex (121), formed upon reaction of the dichloride with two equivalents of EtMgCl. Coupling of the alkene substrate leads to the formation of the metallacyclopentane intermediate, which gives the zirconate (122) with a... 

Enantioselective carboalnminations of inactivated alkenes are promoted by the nonbridged chiral zirconocene (125), neutral or in association with B(C6F5)3 (eqnation 69). An unexpected observation is that addition of water leads to substantial acceleration of the C-C bond-forming process. ... 

The regio- and stereoselective zirconocene-catalyzed addition of alkylmagnesium halides to alkenes, a process which has been described previously (see Section 7.5.2, Scheme 7-79) was investigated with ethylene-l,2-bis( M,5,6,7-tetrahydroind-l-enyl)zirconium dichloride [(EBTHI)ZrCl2l) [118] as chiral zirconocene. Thus, treatment of the latter with alkylmagnesium halides leads to the formation of the derived zirconocene-alkene complex 88, characterized by NMR [119], which reacts with cyclic ethers or amines to lead to the corresponding homoallylic alcohol or amine, respectively, in > 95% ee and good overall yield [120] (Scheme 7-103). 

Zirconocene dichloride was used as a catalyst for allylic displacement of allyl ethers with Grignard reagents [449]. An asymmetric version of this reaction by the use of a catalytic amount of a chiral zirconocene dichloride was independently reported, and it achieved a nearly complete chirality induction [Eq. (205) 450]. 

A separation of the racemic mixture of chiral zirconocene compounds into the optically active pure enantiomers is performed using 0-acetyl-(/ )-mandelic acid as chiral auxiliary (Structure 28). Using this enantiomerically pure metallocene in oligomerization experiments confirms the mechanistic hypothesis of stereospecificity predicting the topicity of insertion [69]. 

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

Kaminsky, W. Kulper, K. Brintzinger, H.H. Wild, F. Polymerization of propene and butane with a chiral zirconocene and MAO as co-cata-lyst. Angew. Chem. Int. Ed. Engl. 1985, 24, 507. 

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