Oxidation of cycloalkenes

Alkenes may also react with certain oxidizing agents to result in anti hydroxyla-tion. Treatment with peroxycarboxylic acids435 leads initially to an epoxide. Ring scission of the latter via an SN2 reaction in an anti manner with the corresponding carboxylic acid or water gives the trans monoester or tram diol, respectively. Complete anti stereoselectivity and high yields in the oxidation of cycloalkenes are... 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

The nitric acid oxidation of cycloalkenes yields, among other products, a,(0-dicarboxylic acids.663,664 The reaction is catalyzed by vanadium(V) ion.665,666 Oxidations with H202,284,667 PhIO,650 or KHSO5650 catalyzed by heteropoly acids284,650 or tungstic acid667 have been recently described. 

Allylic acetoxylation.1 The combination of r-butyl hydroperoxide and Se02 has been used for allylic hydroxylation of alkenes (8, 64-65), but this system is not useful for oxidation of cycloalkenes. Allylic acetoxylation of cycloalkcnes is possible, but in modest yield, with PdCl2 and AgOAc, which probably form a reactive species such as [PdCl(OAc)] . This system can be used in catalytic amounts in the presence of t-butyl hydroperoxide for a reoxidation step. The yield is improved by addition of TcO, which seems to accelerate the oxidation. The most satisfactory ratios of... 

The rate constants for oxidation of a series of cycloalkenes with ozone have been determined using a relative rate method. The effect of methyl substitution on the oxidation of cycloalkenes and formation of secondary organic aerosols has been analysed.155 Butadiene, styrene, cyclohexene, allyl acetate, methyl methacrylate, and allyl alcohol were epoxidized in a gas-phase reaction with ozone in the absence of a catalyst. With the exception of allyl alcohol, the yield of the corresponding epoxide ranged from 88 to 97%.156 Kinetic control of distereoselection in ozonolytic lactonization has been (g) reported in the reaction of prochiral alkenes.157... 

Epoxide -> aliylic alcohol. Treatment of an oxirane with equimolar amounts of 1 and DBU in an aromatic solvent affords aliylic trimethylsilyl ethers in moderate yield. 2,2-Di-, tri-, and tetrasubstituted oxiranes, as well as oxides of cycloalkenes, react at 23° or below. 2,3-Di- and monosubstituted oxiranes do not react at this temperature these species react with 1 and DBU at 70-80° to give trimethylsilyl enol ethers. The reaction of epoxycyclooctane gives a product of transannular cyclization. In the case of epoxycyclohexane, the intermediate 2 has been isolated. 

Oxidations. A widely used method for allylic oxidation is the Kharash-Sosnovsky reaction using a peroxide and a copper(I) salt system. Enantioselective allylic oxidations of cycloalkenes such as cyclopentene, cyclohexene and cycloheptene with tert-butyl peibenzoate were investigated with a variety of catalysts derived from bis(oxazoline) ligands and copper(I) triflate complexes (eq 18). The ligand-copper(I) complexes from the /-Bu-... 

Hydroxymercuration and thallium(III) oxidation of cycloalkenes, 1-methylcycloalkenes and methylenecycloalkanes have been reported to yield ring-contracted aldehydes, ring-contracted ketones and ring-expanded cycloalkanones, respectively. 

Oxidation of cycloalkenes to dialdehydesThis reaction is usually effected with >zone, but can also be effected with H202 catalyzed by tungstic acid in /-butyl alcohol... 

On the other hand, DattaGupta and Singh reported that oxidation of cycloalkenes is effeeted by using the copper(I) complex bearing a modified Nishiyama ligand 19 as the catalyst (Scheme 13) [23]. The reactions of cyclohexene and cyclo-octene in the presence of moleeular sieves proceeds with high enantioselectivity of 86 and 81% ee, while that of eyclopentene proceeds with moderate selectivity of 54% ee [23bj. 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

The oxidation of cycloalkenes to cyclic ketones with PdCl2 is difficult under usual conditions, but allylic oxidation proceeds smoothly. Reaction of cyclohexene with Pd(OAc)2 gives 3-acetoxycyclohexene (81) 1-acetoxycyclohexene (82) is not formed, because no /3-H syn to Pd is available on the acetoxy-bearing carbon, and syn elimination yields the allylic acetate 81 as shown by 80 [39]. 

The oxidation of cycloalkenes to cycloalkanones in fair yields by the Jones reagent is catalysed by mercuric ions and is less expensive than the palladium-based Wacker method. It can also be applied to unprotected olefin carboxylic acids. 

A photoreaction that has been studied extensively by Satos [41] group is the oxidation of cycloalkenes in the presence of FeCL and oxygen to yield, depending on the substrate, a-chloro- or o)-dichloroketones. Scheme 15. Since the chloride ion is incorporated into the organic photoproduct the reaction as it was conducted is not catalytic but requires stoichiometric... 

Melon G, Selby TM, Osborn DL, Taatjes CA (2008) Enol formation and ring-opening in OH-initiated oxidation of cycloalkenes. J Phys Chem A 112 13444—13451... 

A similar situation exists in the hydroxymercuration and thallium(m) oxidation of cycloalkenes and cycloalkanes, the rate laws showing strict second-order behaviour for both metal ions. In the thallium(ni) reactions two products have been observed, a 1,2-diol and a ketone or aldehyde. Two-electron coupling with trifluoroacetate has been shown to promote oxidative phenol coupling. The mechanism of oxidation of olefins by palladium(n) has been investigated. In the presence of PdClg in aqueous media, the data conform to the rate expression... 

---