Alkenes, allylic oxidation

Enhancing the utility of the allq lation-rearrangenient sequence in synthesis, the Evans group addressed the problems of a-versus-y alkylation as well as low anion reactivity by employing heterocyclic sulfides as the alleviation substrates tScheme IR.IfiE For instance, allylic imidazolyl sulfide 56 could be allqvlated efficiently, reaction at the a-position being favored by a chelated but reactive allyl lithium intermediate. Oxidation of 57 to the allylic sulfoxide and treatment with a secondary amine thiophile provided allylic alcohol 58 in high yield and with excellent stereoselectivity at the trisubstituted alkene. Allylic oxidation with manganese dioxide completed a synthesis of the sesquiterpene nuciferal (59). ... 

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

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]. 

Alkenes can be aminated in the allylic position by treatment with solutions of imido selenium compounds R—N—Se=N—R. The reaction, which is similar to the allylic oxidation of alkenes with Se02 (see 14-4), has been performed with R = t-Bu and R=Ts. The imido sulfur compound TsN=S=NTs has also been used... 

Selenium dioxide is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

Tertiary butylhydroperoxide (TBHP) is a popular oxidizing agent used with certain catalysts. Because of its size, TBHP is most effective with catalysts containing large pores however, it can also be used with small-pore catalysts. Using first-row transition metals, Cr and V, impregnated into pillared clays, TBHP converts alcohols to ketones, epoxidizes alkenes, and oxidizes allylic and benzylic positions to ketones.83-87... 

It has been pointed out earlier that the anti/syn ratio of ethyl bicyclo[4.1,0]heptane-7-carboxylate, which arises from cyclohexene and ethyl diazoacetate, in the presence of Cul P(OMe)3 depends on the concentration of the catalyst57). Doyle reported, however, that for most combinations of alkene and catalyst (see Tables 2 and 7) neither concentration of the catalyst (G.5-4.0 mol- %) nor the rate of addition of the diazo ester nor the molar ratio of olefin to diazo ester affected the stereoselectivity. Thus, cyclopropanation of cyclohexene in the presence of copper catalysts seems to be a particular case, and it has been stated that the most appreciable variations of the anti/syn ratio occur in the presence of air, when allylic oxidation of cyclohexene becomes a competing process S9). As the yields for cyclohexene cyclopropanation with copper catalysts [except Cu(OTf)2] are low (Table 2), such variations in stereoselectivity are not very significant in terms of absolute yields anyway. 

Scheme 6.97 Copper-catalyzed asymmetric allylic oxidation of bridged bicyclic alkenes.

When the reactant is cyclohexene, in the first step of Scheme 26, the direct hydrogen abstraction for the allylic oxidation (path 1) competes with the electron transfer (from the alkene to the M-oxo complex) for the epoxidation (path 2). Because the manganese complex is more readily reduced than the chromium... 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

In 1965, Denney et al. (98) reported the reaction of a number of alkenes with ferf-butyl hydroperoxide (TBHP) and cupric salts of chiral acids. The use of ethyl camphorate copper complex 144 in the allylic oxidation of cyclopentene provides, upon reduction of the camphorate ester, the allylic alcohol in low yield and low selectivity, Eq. 82. The initial publication only provided the observed rotation of cyclopentenol, but comparison to subsequent literature values (99) reveals that this reaction proceeds in 12% ee and 43% yield (based on the metal complex). 

Scheme 8. General mechanism of the copper-catalyzed allylic oxidation of alkenes (Kharasch-Sosnovsky reaction).

Figure 13. Stereochemical model proposed by Andrus for the allylic oxidation of alkenes using 55c Cu complexes. [Adapted from (109).]...

Scheme 11. Results and stereochemical model proposed by Singh for the allylic oxidation of alkenes using 157-Cu complexes [Adaptedfrom (111).]...

The Effect of Hydrazine on the Allylic Oxidation of Various Alkenes Using 156 CuOTf... 

Scheme 12. Proposed mechanism leading to the allylic imide observed as a side product in the allylic oxidation of alkenes in nitrile solvents. [Adapted from (120).]...

Scheme 48 Allylic oxidation of alkene with Ru(IV)-complexes.

A catalytic method for the allylic oxidation of alkenes was first reported by Umbreit and Sharpless in 1977, who utilized TBFIP as oxidant and Se02 as catalyst for selective aUylic oxidation. Yields were moderate providing aUylic alcohols or ketones with 54-86% yield. The reaction did not proceed under strictly anhydrous conditions but with one equivalent of water present the oxidation proceeds smoothly at room temperature. In... 

These multicomponent catalyst systems have been employed in a variety of aerobic oxidation reactions [27]. For example, use of the Co(salophen) cocatalyst, 1, enables selective allylic acetoxylation of cyclic alkenes (Eq. 6). Cyclo-hexadiene undergoes diacetoxylation under mild conditions with Co(TPP), 2 (Eq. 7), and terminal alkenes are oxidized to the corresponding methyl ketones with Fe(Pc), 3, as the cocatalyst (Eq. 8). 

The related dirhodium(II) a-caprolactamate (cap) complex [Rh2(p--cap)4] undergoes a one-electron oxidation process at quite a lower potential (11 mV) than the acetate complex (1170 mV). In agreement with the Kochi hypothesis, the a-caprolactamate complex has recently been found to be an exceptional catalyst for the allylic oxidation of alkenes under mild conditions. A wide range of cyclohexenes, cycloheptenes, and 2-cycloheptenone (Eq. 5) are rapidly converted to enones and enediones in 1 h with only 0.1 mol % of [Rh2( x-cap)4] and yields ranging from 60 to 90%, in the presence of potassium carbonate [34]. 

In addition to the examples outlined above, various funetionalized olefins are also known to undergo CM with high E-selectivity. Functional groups that contribute to high E-olefin formation will be discussed in Section 11.06.4. In these cases, CM provides an orthogonal route to products that are typically generated via either the selective C-H activation of alkenes or allylic oxidation. 

---