Zirconium complex, asymmetric

Thus, a novel chiral zirconium complex for asymmetric aza Diels-Alder reactions has been developed by efficient catalyst optimization using both solid-phase and liquid-phase approaches. High yields, high selectivity, and low loading of the catalyst have been achieved, and the effectiveness of chiral catalyst optimization using a combination of solid-phase and liquid-phase methods has been demonstrated. 

Various titanium or zirconium complexes have been shown to catalyze the addition of allyl(tri-M-butyl)tin [29,30,31] or allyltrimethylsilane [32,33] to aldehydes, giving good enantioselectivities and some asymmetric amplification. In all these examples the chiral auxiliary is derived from (R)- or (S)-BINOL. 

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]. 

Following this report, Jordan et al. [74] described the catalytic ortho alkylation of 2-picoline with terminal olefins using a zirconium metallacyclic catalyst (Scheme 19.48). Interestingly, functionalization of one of the two ortho-C-H bonds was necessary to attain catalytic activity. An asymmetric variant of this process was later reported by the same group using a chiral tetrahydroindenyl-zirconium complex, affording an enantiomeric excess of up to 58% [74b]. 

In some cases, zirconium complexes were used for Sharpless epoxidation of allylic alcohols instead of titanium ones. Asymmetric epoxidation of allyl alcohol (176), which is an intermediate in synthesis of Celastraceae sesquiterpene core, was successfully carried out with Zr(Oi-Pr)4 as a promoter to afford epoxy alcohol (177) with excellent enantioselectivity (Equation 77) [82]. When the reaction was carried out with Ti(Oi-Pr)4, the enantioselectivity decreased into 14% ee. The reason is attributed to the longer Zr-O bond length than that of Ti-0 bond, because coordination of hindered tertiary alcohols such as (176) to zirconium seems to be more favored than to titanium. 

Yamashita Y, Ishitani H, Shimizu H, Kobayashi S. Highly anti-selective asymmetric aldol reactions using chiral zirconium catalysts. Improvement of activities, structure of the novel zirconium complexes, and effect of a small amount of water for the preparation of the catalysts. J. Am. Chem. Soc. 2002 124 3292-3302. 

Onaka and co-workers reported on a method for the asymmetric epoxidation of various homoallylic alcohols using chiral zirconium complexes as catalysts. For example, asymmetric epoxidation of olefin 32 using d-(—)-DlPT as a chiral ligand provided epoxide 33 in 93% yield and 89% ee. Compound 33 is the key intermediate for the synthesis of the potent tachykinin receptor antagonists 35 and 36 (Scheme 35.9). °... 

Table 19 Asymmetric hydroamination of aminopentenes catalyzed by zirconium complexes...

Zeijden [112] used chiral M-functionalized cyclopentadiene ligands to prepare a series of transition metal complexes. The zirconium derivative (82 in Scheme 46), as a moderate Lewis acid, catalyzed the Diels-Alder reaction between methacroleine and cyclopentadiene, with 72% de but no measurable enantiomeric excess. Nakagawa [113] reported l,T-(2,2 -bis-acylamino)binaphthalene (83 in Scheme 46) to be effective in the ytterbium-catalyzed asymmetric Diels-Alder reaction between cyclopentadiene and crotonyl-l,3-oxazolidin-2-one. The adduct was obtained with high yield and enantioselectivity (97% yield, endo/exo = 91/9, > 98% ee for the endo adduct). The addition of diisopropylethylamine was necessary to afford high enantioselectivities, since without this additive, the product was essentially... 

The exchange of zirconium in isostructural complexes leads to a new family of asymmetric metallocenes (Fig. 1) bearing a 2-methyl substituent and varied substituents in positions 5, 6, and 7 of the indenyl moiety. After borate activation all catalysts show an unexpected high and constant activity toward the polymerization of propylene and lead to significantly increased molecular weight products compared to the zirconocene species [9-11],... 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

---