Oxidation unsaturated alcohols, titanium

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

In the case of alkenes with polar functional groups, two-site attachment of the substrate to a chiral oxidant is possible and has allowed spectacular enantioselection. Thus, both the hydroperoxide anion based epoxidation of a,/ -unsaturated carbonyls and the epoxidation of allylic alcohols by the titanium(IV)-based Sharpless method exhibit very high enantioselectivity on a wide variety of substrates. 

Hsu ER, Gebert MS, Becker NT, Gaermer AL. Effects of plasticizers and titanium dioxide on the properties of poly(vinyl alcohol) coatings. Pharm Dev Technol 2001 6(2) 277-284. Kakinoki K, Yamane K, Teraoka R, et al. Effect of relative humidity on the photocatalytic activity of titanium dioxide and photostability of famotidine. J Pharm Sci 2004 93(3) 582-589. Sayre RM, Dowdy JC. Titanium dioxide and zinc oxide induce photooxidation of unsaturated lipids. Cosmet Toilet 2000 115 75-80, 82. 

Because of this catalyst degradation, organometallic catalysts are currently the best synthetic reagents for enantioselective epoxidation of olefins. Chiral Mn(III)-salen complexes yield up to 99% ee for cw-disubstituted, tri- and tetra-substituted alkenes [62], but the best results require less desirable oxidants - iodosyl benzene or hypochlorite. Other catalysts accept a more limited substrate range the Sharpless-Katsuki titanium-tartrate ester [65] for allylic alcohols and the JuUa-Colonna epoxidation for a,P-unsaturated ketones [66]. 

Mittasch and Schneider. The first patent for the synthesis of methanol was granted in Germany to Mittasch and Schneider in 1913 (1 ). The catalysts which they described were oxides of cerium, chromium, manganese, molybdenum, titanium, and zinc which had been "activated" by incorporating alkalies such as sodium and potassium carbonates. The products were methanol, higher alcohols and saturated and unsaturated hydrocarbons. Pressures and temperatures were 100-200 atmospheres and 300 to 400 C. These were... 

The titanium- ate complexes of a-methoxy allylic phosphine oxides, generated in situ by reaction of the corresponding lithium anion and Ti(0-i-Pr)4, condense with aldehydes exclusively at the a-position to produce homoallylic alcohols in a diastereose-lective fashion. The overall result is the three-carbon homologation of the original aldehyde, and this protocol has been used in a synthesis of (-)-aplysin-20 from nerolidol. The titanium- ate complex produced by reaction of the chiral lithium anion of an ( )-crotyl carbamate with Ti(0- -Pr)4 affords -y-condensation products (homoaldols) on reaction with aldehydes. Allyl anions produced by the reductive metalation of allyl phenyl sulfides condense with a,p-unsaturated aldehydes in a 1,2-manner at the more substituted (a) allyl terminus in the presence of Ti(0-i-Pr)4. 1,2-Addition of dialky Izincs to a,p-unsaturated aldehydes can be achieved with useful levels of enantiocontrol when the reaction is conducted using a chiral titanium(IV) catalyst in the presence of Ti(0- -Pr)4 (eq 20). Higher ee values are observed when an a-substituent (e.g. bromine) is attached to the substrate aldehyde, but a -substituent cis-related to the carbonyl group has the opposite effect. 

Unsaturated chiral a-hydroxy esters are of interest due to possible reactions with the carbon double bond. They can easily be obtained from optically active cyanohydrins according to the Pinner method [35a]. Epoxidation of the chiral alcohols 9 with achiral oxidants, e.g., m-chloroperoxybenzoic acid (m-CPBA), yields a mixture of both possible epoxides 10a and 10b. With chiral (Sharpless titanium tartrate system) oxidants stereoselective epoxidation results. Using (+) dimethyl tartrate [(+) DMT] only the erythro isomer 10a is obtained (Scheme 6) [35b]. 

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