Heteropolymetalic catalyst

This newly developed heteropolymetallic catalyst system (LLB-ID was applied to a variety of direct cat-... 

Trost et al. [11] reported another impressive example of bimetallic catalysts in which a Zn-Zn homobimetallic complex (17, Scheme 7) serves as an effective catalyst for direct aldol reactions [11-13]. The proposed structure of the catalyst was verified by mass spectrometry and the best ratio of Et2Zn and the ligand. The chemical yield was moderate in the reaction of methyl ketones (1) (Scheme 7, top) [11,12], but a highly atom-economic system was achieved when a-hydroxylated ketones (10) were used as a substrate (Scheme 7, bottom) [13]. Excellent diastereo- and enantioselectivity were obtained under mild conditions. In contrast to the case of Shibasaki s heteropolymetallic catalyst, syn-1,2-diols (syn-11) were obtained as the major diastereomers. 

This newly developed heteropolymetallic catalyst system was applied to a variety of direct catalytic asymmetric aldol reactions, giving aldol products 48-64 in modest to good ee, as shown in Table 16. It is worthy of note that even 62 can be produced from hexanal 54 in 55% yield and 42% ee without the formation of the corresponding self-aldol product (-50 °C). This result can be imderstood by considering that aldehyde enolates are not usually generated by the catalyst at low temperatme, an assumption which was confirmed by several experimental results. It is also worthy of note that the direct catalytic asymmetric aldol reaction between 46 and cyclopenta-none 55 also proceeded smoothly to afford 64 in 95% yield synlanti = 93 7, syn = 76% ee, anti = 88% ee). 

The stereoselectivities seem to be kinetically controlled. In fact, the ee of the aldol product was constant during the course of the reaction. Thus, we have succeeded in performing the first catalytic asymmetric aldol reaction between aldehydes and unmodified ketones by using heterobimetallic or heteropolymetallic catalysts. Several reactions have already been synthetically useful especially for tertiary aldehydes, leading to the catalytic asymmetric synthesis of key intermediates en route to natural products [63]. Further studies are currently in progress. 

A direct catalytic asymmetric aldol reaction using aldehydes and unmodified ketones is promoted by an anhydrous lanthanide, LnLi3-tris-(/f)-binaphthoxide. Combination of this catalyst with potassium hydroxide and water yields a more active heteropolymetallic catalyst. The scope and limitations of this reaction have been explored, and ketone deprotonation is the rate-determining step. 

LLB, KHMDS (0.9 equiv to LLB) and H20 (1 equiv to LLB), which presumably forms a heteropolymetallic complex (LLB-ID, was found to be a superior catalyst for the direct catalytic asymmetric aldol reaction giving 49 in 89 % yield and 79 % ee (using 8 mol% of LLB). We employed this method to generate KOH in situ because of its insolubility in THE The use of KO-t-Bu instead of KHMDS gave a similar result, indicating that HMDS dose not play a key role. Interestingly, further addition of H20 (1 equiv with respect to LLB) resulted in the formation of 49 in 83 % yield and higher ee. The powder obtained from the cata-... 

We thus believe that the BINOL core of the active complex is essentially LLB. Therefore, the heteropolymetallic complex of LLB and KOH, with KOH axially coordinated to La, among other possible complexes, would be the most effective catalyst for this reaction. To clarify the reaction mechanism, we conducted kinetic studies. As a result, significant isotope effects 5) were observed, and the reaction rate... 

Catalysts based on RhMo6 heteropolymetallates. Bulk and supported preparation and characterization. 

In HYD, the heteropolymetallate systems also show a better performance, especially the catalyst based on RhMoe in which the activity is enhanced more than five times. 

Taking into account the specific activities a better performance of heteropolymetallate systems than commercial catalysts is clearly observed. The bifunctionality of the RhMoe system is an important feature. This advantage seems related to the adsorptive interaction process of the planar anion that offers a good site distribution and involves a synergic effect. 

The best practical HDS catalysts are those composed of sulfides of two different metals (e.g. Co-Mo, Ni-W, see Chapter 1) and therefore heterobimetallic complexes are to be considered as better model complexes for HDS-related reactions. Despite this obvious consideration, such chemistry is still rather underdeveloped, possibly due to the increased difficulty that is generally encountered when dealing with heterobimetallic or heteropolymetallic systems as compared to mononuclear complexes nevertheless, a few interesting examples of this type of cooperative chemistry have indeed appeared in tire literature. 

A direct catalytic asymmetric aldol reaction (30-93% ee) using aldehydes and unmodified ketones has been described for the first time. Deprotonation of the ketones by the heteropolymetallic asymmetric catalyst (a lanthanum lithium binaph-thol) is rate limiting, there being no dependence on the concentration of aldehyde. 

SCHEME 8.37. The Shibasaki direct catalytic asymmetric aldol reaction using heteropolymetallic asymmetric catalyst and following Baeyer-Villiger oxidation. 

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