Allylic C-H bonds oxidation

There are further subtle infiuences of structure on the strength of allylic C—H bonds. Oxidation reactions in which there is initial removal of allylic hydrogen proceed with probabilities governed in part by the relative strength of this bond. Detailed experimental data are beginning to appear, as noted later, on the more subtle effects of olefin structure, including substituent effects, steric effects, and charge effects. Theoretical treatments of such effects are not far advanced indeed, these will... 

Scheme 5.14 Pd-catal) ed asymmetric allylic C—H bond oxidation reported by White.

Another approach to create diverse C—H bond functionalization is to combine one C—H bond activation with a subsequent transformation which greatly enriches the diversity of the initial C—H bond functionalization products. Bearing this concept in mind, Sharma and Hartwig recently developed a one-pot process involving the linear selective Pd-catalyzed allylic C—H bond oxidation and subsequent enantioselective branched Ir-catalyzed allylic substitution to form products with new C—O, C—N, C—C, and C—S bonds (Scheme 5.67). The utility of this process was further demonstrated by an iterative sequence of C—H bond functionalization and homologations to prepare enantioenriched (l,n)-functionalized alkenes. 

Scheme 5.67 Pd-catalyzed allylic C—H bond oxidation and enantioselective Ir-cat-alyzed allylic substitution reported by Hartwig.

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

For the oxidation of alkanes, the reactivity order follows the sequence primary < secondary < tertiary < benzylic < allylic C—H bonds. The readily accessible and economical DMD is suitable for most substrates, although this oxyfunctionalization may... 

Pd(H) complexes with strongly electron-withdrawing ligands can insert into the allylic C—H bond (path c) to form directly the Jt-allyl complex via oxidative addi-tion.502,694,697 Pd(OOCCF3)2 in acetic acid, for example, ensures high yields of allylic acetoxylated products.698 The delicate balance between allylic and vinylic acetoxylation was observed to depend on substrate structure, too. For simple terminal alkenes the latter process seems to be the predominant pathway.571... 

DMD is suitable for the oxidation of most substrates with substances that are resistant to oxidation, however, the more reactive but also more expensive methyl (trifluoromethyl)dioxirane (TFD) is necessary. The oxidation is stereoselective for both dioxiranes and proceeds with complete retention of configuration at the oxidized carbon atom (Scheme 1) [20-22]. The reactivity follows the usual order of electrophilic oxidation-primary < secondary < tertiary < benzylic < allylic C-H bonds. Except for tertiary C-H bonds, which produce the oxidatively inert tertiary alcohols, further oxidation of the primary product (an alcohol) to a ketone or aldehyde (the latter is readily further oxidized to the corresponding acid) is possible, because the a-hydrogen of the alcohol is usually more reactive than that of the unactivated alkane, especially for allylic C-H bonds. 

The chemoselectivity of the dioxirane oxyfunctionalization usually follows the reactivity sequence heteroatom (lone-pair electrons) oxidation > JT-bond epoxida-tion > C-H insertion, as expected of an electrophilic oxidant. Because of this chemoselectivity order, heteroatoms in a substrate will be selectively oxidized in the presence of C-H bonds and even C-C double bonds. In allylic alcohols, however, C-H oxidation of the allylic C-H bond to a,/ -unsaturated ketones may compete efficaciously with epoxidation, especially when steric factors hinder the dioxirane attack on the Jt bond. To circumvent the preferred heteroatom oxidation and thereby alter the chemoselectivity order in favor of the C-H insertion, tedious protection methodology must be used. For example, amines may be protected in the form of amides [46], ammonium salts [50], or BF3 complexes [51] however, much work must still be expended on the development of effective procedures which avoid the oxidation of heteroatoms and C-C multiple bonds. 

Classical (metal-catalyzed) autoxidation of olefins is facile but not synthetically useful owing to competing oxidation of allylic C-H bonds and the olefinic double bond, leading to complex product mixtures [105]. Nonetheless, the synthetic chemist has a number of different tools for the allylic oxidation of olefins available. 

Unsaturated lipids are more easily oxidized than saturated ones because they contain weak allylic C-H bonds that are readily cleaved in Step [1] of this reaction, forming resonance-stabilized allylic radicals. Because saturated fats have no double bonds and thus no weak allylic C-H bonds, they are much less susceptible to air oxidation, resulting in increased shelf life of products containing them. 

Allylic C-H bonds are weaker than other C-H bonds and are thus susceptible to oxidation with molecular oxygen by a radical process. The hydroperoxide formed by this process is unstable, and it undergoes further oxidation to products that often have a disagreeable odor. This oxidation process turns an oil rancid. 

The two established pathways for transition metal-catalyzed alkene isomerization are the jr-allyl metal hydride and the metal hydride addition-elimination mechanisms. The metal hydride addition-elimination mechanism is the more common pathway for transition metal-catalyzed isomerization. In this mechanism, free alkene coordinates to a metal hydride species. Subsequent insertion into the metal-hydride bond yields a metal alkyl. Formation of a secondary metal alkyl followed by y3-elimination yields isomerized alkene and regenerates the metal hydride. The jr-allylhydride mechanism is the less commonly found pathway for alkene isomerization. Oxidative addition of an activated allylic C-H bond to the metal yields a jr-allyl metal hydride. Transfer of the coordinated hydride to the opposite end of the allyl group yields isomerized alkene. 

The preparative electrochemical oxidation of allylsilanes proceeds smoothly and the C-Si bond is cleaved selectively without affecting other allylic C-H bonds [110-113]. This selectivity is ascribed to the selective cleavage of the C-Si bond in the cation radical intermediate. The resulting allyl radical intermediate is further oxidized to give the allyl cation intermediate, which is trapped by nucleophiles such as alcohols, water, carbamates, and tosylamides to give the corresponding allylic substitution products as shown in Eq. (25). Usually, the nucleophiles are introduced to both ends of the allyl cation, and therefore a mixture of two regioisomeric products is formed. 

Alkenes possessing allylic C-H bonds are oxidized by SeOj either to allylic alcohols or esters or to a,p-unsaturated aldehydes or ketones, depending on the experimental conditions.The reaction involves an ene-type reaction (A) followed by a sigmatrop-ic [2,3]-shift (B) to give the selenium ester (C), which is converted to the corresponding allylic alcohol (D) on solvolysis. ... 

Other double-bond isomerizations (e.g., those that occur in the course of Pd-catalyzed hydrogenolyses or hydrogenations) proceed by insertion of the alkene into a M-H bond followed by /3-hydride elimination. Wilkinson s catalyst, though, lacks a Rh-H bond into which an alkene can insert. The reaction may proceed by oxidative addition to an allylic C-H bond, then reductive elimination at the other end of the allylic system. 

The induction period for the oxidation was inversely proportional to the vinyl content of the polymer, suggesting the importance of initiation by primary attack by oxygen of the allylic C-H bond. 

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