Sharpless catalyst

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One of the most famous chiral titanium complexes is the Sharpless catalyst (16), based on a diisopropyl tartarate complex. Nmr studies suggest that the complex is dimeric ia nature (146). An excellent summary of chiral titanium complexes is available (147). 

In 1997, Shibuya showed that in reactions involving the Sharpless catalyst (see Scheme 5-26 above), ee depended strongly on the solvent. The highest ee (53%) was obtained in ether, while no ee was observed in methylene chloride (Scheme 5-30) [24],... 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

Other functionalized supports that are able to serve in the asymmetric dihydroxylation of alkenes were reported by the groups of Sharpless (catalyst 25) [88], Sal-vadori (catalyst 26) [89-91] and Cmdden (catalyst 27) (Scheme 4.13) [92]. Commonly, the oxidations were carried out using K3Fe(CN)g as secondary oxidant in acetone/water or tert-butyl alcohol/water as solvents. For reasons of comparison, the dihydroxylation of trons-stilbene is depicted in Scheme 4.13. The polymeric catalysts could be reused but had to be regenerated after each experiment by treatment with small amounts of osmium tetroxide. A systematic study on the role of the polymeric support and the influence of the alkoxy or aryloxy group in the C-9 position of the immobilized cinchona alkaloids was conducted by Salvadori and coworkers [89-91]. Co-polymerization of a dihydroquinidine phthalazine derivative with hydroxyethylmethacrylate and ethylene glycol dimethacrylate afforded a functionalized polymer (26) with better swelling properties in polar solvents and hence improved performance in the dihydroxylation process [90]. 

Complexes based on titanium excess tartrate combination (the Padova system). In 1984, the same year the Orsay group developed their system, a group in Padova, Italy, headed by Modena,47 developed a different system, able to oxidize sulfides to sulfoxides with high selectivity, also based on a modification of the Sharpless catalyst. The Padova group used TBHP in the presence of 1 mol equiv of Ti(0-/-Pr)4/(/ ,/ )-DET, 1/4 combination. The reactions were performed at -20... 

The use of Sharpless catalyst in this reaction gives the (/ )-adducts in yields up to 76% and with enantiomeric excesses up to 53 %60. 

Example 2 Sharpless catalysts grafted onto mesoporous MCM-41[71]... 

A close and well-researched homogeneous analogue of the epoxidation catalyst is the so-called Sharpless catalyst, a homogeneous titanium (TV) catalyst for fhe asymmefric epoxidation of allylic alcohols with f-butyl hydroperoxide (TBHP) [14]. A mechanism has been proposed for this type of epoxidation [15], which can be expressed in a generic form as shown in Fig. 13.4. 

The bottom line in Scheme 1-6 shows the eight aldohexoses of natural origin they all belong to the D-series. Their L-configured enantiomers have been synthesized by use of the abiotic Sharpless catalyst [38bj. 

Sharpless catalyst is comprised of titanium tetraisopropoxide and one enantiomer of diethyl tartrate (DET). 

Aldehydes and ketones are readily transformed into the corresponding cyanohydrin trimethylsilyl ethers when treated with cyanotrimethylsilane in the presence of Lewis acids (eq 1), triethylamine, or solid bases such as Cap2 or hydroxyapatite. The products can be readily hydrolyzed to the corresponding cyanohydrins. The cyanosilylation of aromatic aldehydes can be achieved with high enantioselectivity in the presence of catalytic amounts of a modified Sharpless catalyst consisting of titanium tetraisopropoxide and L-(+)-diisopropyl tartrate (eq 2). Catalysis with chiral titanium reagents yields aliphatic and aromatic cyanohydrins in high chemical and optical yields... 

Highly enantioselective trimethy Isilylcyanation of various aldehydes can be achieved by using Cyanotrimethykilane in the presence of a modified Sharpless catalyst consisting of Ti(0-i-Pr)4... 

Addition reaction with Sharpless catalyst (Ti tartrate) gave the highest value of 53% e.e. in diethylether solution for the reaction between aromatic aldehyde and diethylphosphite (Scheme 8.67) [178]. Strong dependence of enantiomeric excess from the solvent was reported for this transformation. 

Scheme 8.67 Hydrophosphorylation of aromatic aldehydes using Sharpless catalyst...

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