Oxidation of alkenyls

The substitution of aluminum by oxygen can be effected by various peroxide derivatives, such as di-r-butyl peroxide, benzoyl peroxide and r-butyl perbenzoate. The main reaction is accompanied by telltale free-radical side reactions, such as the formation of RR from R3AI, which become major pathways with aryl and vinyl compounds. Oxidation of alkenyl derivative (56) with r-butyl perbenzoate yields 45% of a mixture of cis and trans ethers (57 equation 44). As of yet there is no generally applicable, highly efficient method for oxidizing vinylaluminum compounds. 

Hydroxylation of aromatic rings in the presence of oxygen Oxidation of alkenyl stannates —copper mediated homocoupling [56] ... 

Allylic oxidation. Recycling of SeOj with r-BuOOH in situ is an old technique, and its extension to the oxidation of alkenyl fluorides is routine. The same reagent combination on silica under microwave irradiation shortens the reaction time for the allylic oxidation. ... 

The reactions of the second class are carried out by the reaction of oxidized forms[l] of alkenes and aromatic compounds (typically their halides) with Pd(0) complexes, and the reactions proceed catalytically. The oxidative addition of alkenyl and aryl halides to Pd(0) generates Pd(II)—C a-hondi (27 and 28), which undergo several further transformations. 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

Various electrophiles other than iodine have been used to induce alkenyl coupling (9). Alkyl haUdes and protic acids react with alkynylborates to yield mixtures of stereoisomeric alkenylboranes. Nevertheless, oxidation of these products is synthetically useful, providing single ketones (296—298). Alcohols are obtained from the corresponding alkenylborates. 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

An interesting application of the cydization of alkenyl thioacetals is the stereoselective preparation of olefmic diols. Thus, oxidative cleavage of the silicon—carbon bond [32] in the ring-closed metathesis products, i.e. cyclic allylsilanes such as 35 and 36, affords (Z)-alk-2-ene-1,5-diols 37 and 38 (Scheme 14.18) [33],... 

The rhodium-catalyzed hydroboration has opened the way to cyclization reactions starting from dienes [92], For instance, rhodium-catalyzed hydroboration of the terminal alkenyl group of an os/Tunsaturated lactone followed by reaction with the PTOC-OMe chain transfer reagent afforded the bicyclic a-S-pyridyl lactone in 63% yield (Scheme 39). After oxidation of the sulfide with m-CPBA, thermal elimination of the sulfoxide afforded the corresponding a-methylene lactone in 65% yield. Interestingly, such bicyclic a-methylenelactones are substructures that can be found in many natural products such as mirabolide [93]. 

Neutral aminyl radicals generated by anodic oxidation of lithium alkenyl amides undergo a stereoselective cyclization to cis-l-methyl-2,5-disubstituted pyrrolidines [249]. 

Stoichiometric Pd(II)-mediated oxidative cyclization of alkenyl anilines to indoles. Cf. Wacker oxidation. 

The oxidation of substituted pyridines to iV-oxides was reported by Sharpless and coworkers to proceed with yields between 78 and 99% (Scheme 154). A variety of substituents like electron donor as well as acceptor groups and alkenyl substituents are tolerated. In 1998, Sharpless and coworkers reported an alternative method for the preparation of pyridine-A-oxides in which the MTO/H2O2 catalyst could be replaced by cheaper inorganic rhenium derivatives (ReOs, Re207, HOReOs) in the presence of bis(trimethylsilyl) peroxide (equation 73). Yields of the prepared A-oxides after simple workup (filtration and bulb to bulb distillation) ranged from 70-98%. Molecular sieves slowed down the reaction while small amounts of water (0-15%) were essential for the reaction. Both electron-poor or electron-rich pyridines give high yields of their A-oxides and while para-... 

We have found that the secret to a successful synthesis of the alkenyl glycosides 64 lies in obtaining the pure -acetate 65 [47]. Condensation of the acetate 65 with the appropriate alkenol,in the presence of a small amount (5-20 mol% depending on the scale) of trimethylsilyl triflate, gave the alkenyl glycosides 64 in excellent yield. Oxidation of the alkene 64 with dimethyldioxirane, followed by deacetylation, then gave the putative enzyme inhibitors 60. 

Intramolecular cycloadditions of alkenyl-substituted nitrile oxides produce bicyclic isoxazolines. When monocyclic olehns are used, tricyclic structures are obtained. This approach was pioneered by both Kozikowski s and Curran s groups. A typical case involves the cycloaddition of nitro compound 191 [mixture of diastereomers derived from pentenose pyranoside 190], which produced a diaster-eomeric mixture of isoxazolines that contain cis-fused rings (i.e., 192) in near quantitative yield (326) (Scheme 6.85). Further elaboration of this mixture led to epoxycyclopentano-isoxazoline 193, which was then converted to the aldol product in the usual manner. The hydrogenation proceeded well only when rhodium on alumina was used as the catalyst, giving the required p-hydroxyketone 194. This... 

The use of alkenyl nitrile oxides is an effective method for the construction of bland polycyclic isoxazolines (2,4,200,236,237). Due to the rigid linear structure of the nitrile oxide, the reaction of alkenyl nitrile oxides almost always proceeds to give bicyclo[X,3,0] derivatives for X = 3-5. Most frequently, the diastereoselec-tivities are controlled by a chiral center on the link between the alkene and the dipole groups. 

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