Allylic Oxidation of Olefins

It has been reported that molecnlar oxygen plays an important role in the allylic oxidation of olefins with TBHP (25, 26). Rothenberg and coworkers (25) proposed the formation of an alcoxy radical via one-electron transfer to hydroperoxide, Equation 4, as the initiation step of the allylic oxidation of cyclohexene in the presence of molecnlar oxygen. Then, the alcoxy radical abstracts an allylic hydrogen from the cyclohexene molecnle. Equation 5. The allylic radical (8) formed reacts with molecular oxygen to yield 2-cyclohexenyl hydroperoxide... 

Scheme 39 Allylic oxidation of olefins with an oxoferryl porphyrin complex.

Fe203-Sb204 has been shown to be an excellent catalyst for the allylic oxidation of olefins. The surface oxygen content and the ratio of Fe/Sb are believed to be important for catalysis (Aso et al 1980). The active surface is shown to be an Sb-enriched surface layer, as found in the Sb-Sn oxide systems. 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

The redox mechanism applies not only to allylic oxidation of olefins and to the oxidation of aromatic hydrocarbons, but also to the oxidation of methanol and sulphur dioxide, as well as the oxidation of ammonia to nitrogen. Only in the case of ethylene oxidation and oxyhydration of olefins do catalysts act according to another mechanism. The latter processes seem to be always low temperature reactions, occurring below 300° C, whereas redox mechanisms are possible above this temperature (e.g. 400—500°C). 

Several authors have suggested that the allylic oxidation of olefins to aldehydes requires a bifunctional catalyst. The two functions then concern the formation of an allylic radical and the coupling of such a radical with lattice oxygen. This idea is primarily based on the fact that several single oxides (e.g. Bi203, Sn02, T1203) catalyze the formation of allyl... 

Solid heteropoly compounds are suitable oxidation catalysts for various reactions such as dehydrogenation of O- and N-containing compounds (aldehydes, carboxylic acids, ketones, nitriles, and alcohols) as well as oxidation of aldehydes. Heteropoly catalysts are inferior to Mo-Bi oxide-based catalysts for the allylic oxidation of olefins, but they are much better than these for oxidation of methacrolein (5). Mo-V mixed-oxide catalysts used commercially for the oxidation of acrolein are not good catalysts for methacrolein oxidation. The presence of an a-methyl group in methacrolein makes the oxidation difficult (12). The oxidation of lower paraffins such as propane, butanes, and pentanes has been attempted (324). Typical oxidation reactions are listed in Table XXXI and described in more detail in the following sections. 

Recently, iron catalysis gained general importance. Its catalytic chemistry has been summarized ([2] recent reviews [3, 4]). Iron(II) and iron(III) salts have a long history in radical chemistry. The former are moderately active in atom-transfer reactions as well as initiators for the Fenton reaction with hydrogen peroxide or hydroperoxides (reviews [5-12]). Important applications of this principle are the Kharasch-Sosnovsky reaction (the allylic oxidation of olefins) [13], which often... 

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. 

Fig. 4.40 The Kharasch-Sosnovsky reaction for allylic oxidation of olefins.

Allylic oxidation of olefins is a reaction of considerable value in organic synthesis [18] and selenium dioxide itself or in combination with other cooxidants remains a highly predictable and reliable reagent to perform these reactions. Thus, selenium dioxide oxidation of (Z)-tributyltin 1-alkenylcarba-mates 46 constitutes the first successful example of such a conversion ever reported with an element other than hydrogen [19]. Namely, it was found that with the allylic stannanes 46a or 46b oxidation occurred smoothly within 15 min to deliver in good yields the expected corresponding allylic alcohols 47a and 47b, respectively (Eqs. 6 and 7). 

This may be attributed to the greater electron-withdrawing effect of the pyridine nucleus. Consequently, pentafluoroseleninic acid (95) and 2-(AT-oxi-do)pyridineseleninic anhydride (96) were introduced [47] as the most efficient reagents in the oxidation of alcohols and in the allylic oxidation of olefins. 

Allylic oxidation of olefins.11 Oxidation of allylbenzene (1) with Hg(OAc)2 in HOAc at reflux for 50 hr. gives metallic mercury (70%) and 72 % of organic products. 

The oxidation proceeds via sequential ene reaction, dehydration, [2,3]sigmatropic rearrangement, and hydrolysis (or solvolysis) (Scheme 15.92) [180]. Oxidation of trisubstituted olefins occurs at the a position of the more substituted vinylic carbon in the order CH2>CH3>CH. On combination with a slight excess, or more, of t-butyl hydroperoxide ( BuOOH) allylic oxidation of olefins proceeds as a catalytic reaction of SeO2. 

The general trend is that metals which react via an oxometal pathway show a very similar behaviour using these two hydroperoxides as oxidant, e.g. the selenium catalyzed allylic oxidation of olefins to the corresponding a, p-unsaturated alcohols. Reactions which involve a peroxometal pathway, e.g. the molybdenum catalyzed epoxidations of olefins, show a completely different behaviour using these two hydroperoxides, namely virtually no reaction is observed with the bulky PHP. We conclude that PHP is a suitable mechanistic probe for distinguishing between oxometal and peroxometal pathways in catalytic oxidation. 

Selective allylic oxidation of olefins like isophorone is a very demanding chemical reaction [135].The keto-isophorone obtained by oxidation of (3-isophor-one (Fig. 14) is an important intermediate for the preparation of flavors and fragrances. In homogeneous catalysis, yields superior to 90% are reached [136, 137]. The heterogeneous analogs were always worse catalysts with, for example, a drop of the yield from 67% to rally 6% when the Co(salen) complex was immobilized in silica [138]. 

Indeed, one can easily conclude that selective allylic oxidation of olefins, in the context of fine chemicals, is a largely underdeveloped area of catalysis. Selenium dioxide catalyzes the allylic oxidation of a variety of olefins with TBHP, affording the corresponding allylic alcohols, but the system is homogeneous [3] and, hence, falls outside the scope of this book. The only heterogeneous catalysts for allylic oxidation which seem to have synthetic utility are palladium-based [4]. 

Moiseev and coworkers showed [10,13] that giant palladium clusters with an idealized formula Pd56iL5o(OAc)igo (L = phenanthroline or bipyridine) are highly active catalysts for allylic oxidation of olefins. The catalytically active solution was prepared by reduction of Pd(OAc)2, e. g. with H2, in the presence of the ligand, L, followed by oxidation with O2. The giant palladium cluster catalyzed the oxidation of propylene to allyl acetate under mild conditions. Even in 10% aqueous acetic acid, allyl acetate selectivity was 95-98 % [10]. Oxidation catalyzed by Pd-561 in water afforded a mixture of allylic alcohol (14%), acrolein (2%), and acrylic acid (60%), and only 5% acetone [10]. 

As with the allylic oxidation of olefins (see above) the giant Pd-561 cluster was also found to catalyze benzylic acetoxylation under mild conditions in acetic acid [10]. 

Soluble chromium compounds are known to catalyze the allylic oxidation of olefins [22,23] and benzylic oxidations of alkyl aromatics [22,24] using tert-butyl-hydroperoxide as the primary oxidant. Chromium-substituted aluminophosphates, e. g. CrAPO-5, were shown to catalyze the allylic oxidation of a variety of terpene substrates with TBHP to give the corresponding enones [25,26]. For example, a-pinene afforded verbenone with 77% selectivity (Eq. 6) and 13% of the corresponding alcohol. 

Iron phthalocyanine encapsulated in zeolites was used as oxygen activating catalysts in the triple catalytic aerobic oxidation of hydroquinone to benzoquinone, in the allylic oxidation of olefins and in the selective oxidation of terminal olefins to ketones. The catalyst proved active in the above reactions. It is stable towards self-oxidation and can be recovered and reused. 

Allylic oxidation. Selenium dioxide has been widely used for allylic oxidation of olefins, but this reaction is often troublesome because of formation of selenium-containing by-products and colloidal selenium, which are not easily eliminated. The combination of Se02 with H2O2 to reoxidize selenium species to Set) , has been used to convert olefins into diols and epoxides (2, 362), but this iiicIIuhI is not general for allylic oxidations. Umbreit and Sharpless have effected allylic oxidations with an excess of /-butyl hydroperoxide (90"i.) in combination... 

In the presence of AcOH, benzoquinone (as oxidarrt) and o-methoxyacetophenone or Ph3P as ligands, Pd(tfa)3 catalyses selective allylic oxidation of olefins into their allyl acetates [McMurry Kocovsky Tetrahedron Lett 25 4187 1984],... 

Allylic Oxidation of Olefins 9.6.1. Transition-Metal Oxidants... 

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