Chiral alkoxide catalysts

Further work from Nakajima resulted in the development of an aldol reaction of aldehydes with trimetho g silyl enol ethers in the presence of chiral 

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Kobayashi and colleagues developed a catalytic enantioselective method for the allylation of imines 24 by substituted allylstannanes 25 with chiral zirconium catalysts 26 and 27 prepared from zirconium alkoxides and l,l -bi-2-naph-thol derivatives (Scheme 10) [19]. The allylation of aromatic imines 24 with 25 afforded the corresponding homoallylic amines 28 in good yields (71-85%) with high stereoselectivities (87-99% ee). 

The makeup of Mo-based complexes, represented by 1 [3], offers an attractive opportunity for the design, synthesis, and development of effective chiral metathesis catalysts. This claim is based on several factors 1) Mo-based catalysts such as 1 possess a modular structure [4] involving imido and alkoxide moieties that do not disassociate from the metal center in the course of the catalytic cycle. Any structural alteration of these ligands may thus lead to a notable effect on the reaction outcome and could be employed to control selectivity and reactivity. 2) Alkoxide moieties offer an excellent opportunity for incorporation of chirality within the catalyst structure through installment of non-racemic tethered chiral bis(hydroxy) ligands. 3) Mo-based complexes provide appreciable levels of activity and may be utilized to prepare highly substituted alkenes. 

During the last decade Shibasaki and co-workers focussed on the application of rare earth metal catalysts with special properties [2]. More recently, impressive studies by this group revealed the broad applicability of chiral heterobimetallic catalysts based on rare earth metal alkoxide complexes in asymmetric catalysis. Whereas initial... 

Development of molecularly imprinted enantioselective hydrogenation catalysts based on immobilised rhodium complexes was reported by Gamez et al. [29]. The imprinted catalysts were prepared by polymerising Rh(I)-(A,A -dimethyl-l,2-diphe-nylethanediamine) with di- and tri-isocyanates, using a chiral alkoxide as the template (9). The imprinted polymer, after removal of the template, was tested for the reduction of ketones to alcohols. An enhanced enantioselectivity was observed in the presence of the imprinted polymeric catalyst, in comparison to the control polymer. 

Epoxidation of the chiral allylic alcohol A with the achiral titanium alkoxide catalyst gives a 2.3 1 mixture of the a- and 3-epoxides B and C, indicating the chiral substrate s preference for a-attack. 

In a series of papers, the application of titanium alkoxide catalysts to the synthesis of sugars has been described. Asymmetric epoxidation and kinetic resolution of (48) afforded (+)-(49) (27% >95%e.e.) and (—)-(48) (33% 72%e.e.). The ring-opening reactions of the chiral epoxides that are produced, for example, from cis- and from trans- 50) provide new routes to saccharides. The reagents also find use in the synthesis of pheromones e.g., (+)-disparlure and (+)-2,6-dimethylhepta-l,5-dien-3-ol acetate via the epoxide (52), which was obtained from the dienol (51) by using D-(—)-... 

Asymmetric, borane-modified MPV reduction of a variety of aromatic ketones to their corresponding alcohols has recently (43) been reported using a chiral aluminum alkoxide catalyst shown in Figure 5. This compound was formed in situ from aluminum isopropoxide and (R)-l,r-binapthyl-diol in... 

Figure 5 Chiral aluminum alkoxide catalyst (From Ref 43).

Chmura et al. also prepared air and moisture resistant chiral imino phenoxide complexes of zirconium and titanium, 14 [16]. They envisioned to study the effect of supporting ligand chirality on the stereoselectivity of LA ROP reaction. But at the end, they did not gain acceptable evidence enable to support any relationship. They showed that all isolated polymers had similar and moderate heterotactic microstructure which implied simple chain end control mechanism and resulted to the selective racemic enchainment during the propagation process. First, they investigate polymerization in toluene at 80°C and ambient temperature in which titanium complexes were absolutely inactive and zirconium coxmterparts showed moderate activity after 2 and 24 hours, respectively. Then they checked out solvent free conditions at 130°C and received almost complete conversion after 30 minutes for both titanium and zirconium alkoxide complexes (Table 7.2, entry 33-36). In this condition, titanium coxmterpart, in contrast to zirconium, resulted to full atactic polymer. Their investigation also showed that zirconium complex retained its activity in moisture or with lactic acid impurity in crude monomer which is deleterious for most metal alkoxide catalysts. 

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