Dienes, catalytic hydrogenation epoxidation

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

TABLE 2.5 Catalytic asymmetric epoxidation of dienes with hydrogen peroxide mediated by Ih... 

Salmond of the Upjohn Company has reported several methods for the preparation of 25-hydroxycholesterol from the intermediate aldehyde (52). In a most efficient process the aldehyde was condensed with the Wittig reagent (59) obtained in three easy steps from isoprene. The product diene (61) could be selectively oxidized at the 24,25-double bond to give epoxide (62) as a mixture epimeric at C-24. Catalytic hydrogenation concomitantly reduced the 22,23-double bond and the epoxide to give the 25-hydroxylated cholesterol side chain. Reversal of the /-steroid ether... 

Isomerization of 1,3-diene epoxides. Noyori etalf have reported three types of isomerization of allylic oxides in the presence of a catalytic amount of this palladium(O) complex. One class, exemplified by the isomerization formulated in equation (I), involves transfer of a hydrogen atom from the Ca-alkyl group and results in a dienol of the type obtained in the ene reaction of singlet oxygen with 1,3-dienes. 

Cyclodextrins are the most used host compounds so far. Native cyclodextrins are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols [3], epoxidation [4], oxidation [5], or hydrosilylation [6] of olefins, reduction of a,y9-unsaturated acids [7], a-keto ester [8], conjugated dienes [9] or aryl alkyl ketones [10]. Interestingly, chemically modified cyclodextrins like the partially O-methylated y0-cyclodextrin (RAME-y9-CD) show a better catalytic activity than native cyclodextrins in numerous reactions such as the Wacker oxidation [11], hydrogenation of... 

The efQciency of these catalytic reactions of epoxides is ascribed to the eminent nucleophilic character of the Pd(0) catalyst and hydrogen-carrying ability of Pd(II) species. " In principle, the diene epoxide is susceptible to Pd(0) attack at C-2 (SN2-type reaction), C-4 (SN2 -type reaction), or C-3-C-4 double bond. /3,y-Unsaturated ketone would form via zwitterion and Pd hydride intermediate, while, alternatively, the epoxide can isomer-ize to dienols through the 7r-allyl complex (Scheme 7). The Tt-allyl complex is facilely scavenged with hydride, alkoxides, amides, and carboanion nucleophiles to enhance the synthetic utility (see Sect. V.2.1.5). 

Diene epoxides, under the influence of catalytic amounts of [Pd(PPh3)4], undergo an interesting isomerization, but the course of the reaction is highly dependent on the substitution pattern of the substrate. If the diene epoxide has a transferable hydrogen atom in the C-2 alkyl substituent then the rearrangement shown in equation (10) occurs. Open-chain substrates that lack alkyl substituents... 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

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