Chiral Zr-complexes

Wipf and coworkers used a Claisen rearrangement of allyl phenyl ethers 4-309 followed by an enantioselective carboalumination using the chiral Zr-complex 4-310 and trimethyl aluminum (Scheme 4.67) [104]. After an oxidative work-up of the intermediate trialkylalane, the corresponding alcohols 4-311 were obtained with up to 80% ee and 78% yield. One can also transfer an ethyl group using triethyl aluminum with even better ee-values (up to 92%), but the yields were rather low (42%) due to a more sluggish oxidative cleavage of the Al-C bond. 

Scheme 6.12. Enantioselective carbo-aluminations of unactivated alkenes are promoted by neutral (37) and cationic (38 + B(C6F5)3) chiral Zr complexes.

Erker and co-workers reported in 1990 that in the presence of the chiral Zr complex 82, shown in Eq. 6.16, 1-naphthol adds to ethyl pyruvate with an appreciable level of enantio-... 

Erker and co-workers reported in 1990 that in the presence of the chiral Zr complex 82, shown in Eq. 6.16, 1-naphthol adds to ethyl pyruvate with an appreciable level of enantio-selectivity [81]. Higher optical purities were reported at lower temperatures, and interestingly, as later reported by Wipf for Zr-catalyzed carboaluminations of terminal alkenes (Scheme 6.14), addition of water leads to improvements in selectivity and reactivity [82],... 

Tagliavini and Umani-Ronchi found that chiral BINOL-Zr complex 9 as well as the BINOL-Ti complexes can catalyze the asymmetric allylation of aldehydes with allylic stannanes (Scheme 9) [27]. The chiral Zr catalyst 9 is prepared from (S)-BINOL and commercially available Zr(0 Pr)4 Pr0H. The reaction rate of the catalytic system is high in comparison with that of the BINOL-Ti catalyst 4, however, the Zr-catalyzed allylation reaction is sometimes accompanied by an undesired Meerwein-Ponndorf-Verley type reduction of aldehydes. The Zr complex 9 is appropriate for aromatic aldehydes to obtain high enantiomeric excess, while the Ti complex 4 is favored for aUphatic aldehydes. A chiral amplification phenomenon has, to a small extent, been observed for the chiral Zr complex-catalyzed allylation reaction of benzaldehyde. 

Scheme 2.28 Parallel kinetic resolution of racemic bicyclic cyclobutanones catalyzed by chiral Zr complex 116.

Ti-BINOL-catalyzed reactions have been well established. When the Ti is replaced by Zr,92 the resulting complex 140 can also catalyze the addition of allyl-tributyltin to aldehydes (aldehydes allyl-tributyltin 140 = 1 2 0.2 mol ratio) in the presence of 4 A MS. Product l-alken-4-ols are obtained in good yield and high ee. The, Sz-face of the aldehyde is attacked if (S)-BINOL is used, and Re-face attack takes place when (K)-BINOL is used as the chiral ligand. For Zr complex-catalyzed reactions, the reaction proceeds much faster, although the... 

The high levels of enantioselectivity obtained in the asymmetric catalytic carbomagnesa-tion reactions (Tables 6.1 and 6.2) imply an organized (ebthi)Zr—alkene complex interaction with the heterocyclic alkene substrates. When chiral unsaturated pyrans or furans are employed, the resident center of asymmetry may induce differential rates of reaction, such that after -50 % conversion one enantiomer of the chiral alkene can be recovered in high enantiomeric purity. As an example, molecular models indicate that with a 2-substituted pyran, as shown in Fig. 6.2, the mode of addition labeled as I should be significantly favored over II or III, where unfavorable steric interactions between the (ebthi)Zr complex and the olefmic substrate would lead to significant catalyst—substrate complex destabilization. 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

Metal complexes of pinene-fused boratabenzene ligands, analogous to chiral metallocenes that have found application in catalysis and enantioselective synthesis, have been prepared.122-124 With late transition metals such as Mn and Fe, the complexes are obtained as mixtures of diastereomers (e.g., 97) with the sterically less congested exo form predominating, but the bis(ligand) Zr complex 98 was obtained as the pure exo,exo product.124 A lithium... 

Chiral Zr(q2-6-Me-pyrid-2-yl) complexes 15 react readily with terminal alkenes to give the corresponding azazirconacyclopentenes 16 in high yields. Even internal alkenes react similarly with 15b [38]. Propene and 1-hexene react with 15a so as to place the alkyl substituent in the P position, whereas styrene and trimethylsilylethylene place the Ph and Me3Si groups in the a... 

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. 

Several other chiral Lewis acids have been developed for the addition of allyl and methallyl tributylin to aldehydes [28]. These additions usually proceed slowly with reaction times of days. Less reactive stannanes, for example crotyl tributyltin, require even longer times and diastereoselectivity is poor. The allyl and methallyl additions, however, afford products in high yield and ee. The most successful ligands are BINOL and BINAP as Ti or Zr complexes in the former case and an AgOTf complex in the latter. 

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]. 

One of the major aspects of these additions is the stereoselection which is achieved to create ( )- or (Z)-vinylmetals from alkynes, or to create erythro or threo structures from alkenes. The latest developments of this approach provide a route to non-racemic, chiral, metallated structures from common organometallics (RLi, RMgX), either by transmetallation with asymmetrically ligated transition metals (as Zr complexes) or simply... 

In the presence of a Zr complex of (J )-6,6 -dibromo-BINOL, A -(2-hydroxyphenyl)-aldimines participate in asymmetric hetero-Diels-Alder reactions It is an improvement to the reactions involving stoichiometric chiral boron reagents. 

Syndiotactic polymerization, shown in equation 11.31, occurs with the Cp-fluorenyl Zr complex 47.93 Note that 47 possesses a plane of symmetry bisecting the two cyclic n ligands, whereas 46 is a chiral molecule, possessing a C2 symmetry axis94 but no symmetry plane. 

Group 4 metals have also been used widely in conjunction with salen-type ligands (Figure 25). In 2006 Gregson et reported several chiral and achiral titanium salen alkoxide complexes for the ROP of lactide. All catalysts reported were modestly active and heteroselective (P 0.51-0.57). Several achiral Ti and Zr salan catalysts were reported by Gendler et for melt polymerization of lactide. While no stereoselectivity has been reported for either system, the Zr complexes were more active towards lactide ROP than the Ti analogs. 

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