Cycloalkenes allylic oxidation

Tris(oxazoline) complexes have also been investigated as ligands in the allylic oxidation reaction. Katsuki and co-workers (116) observed that Cu(OTf)2 com-plexed to the tris(oxazoline) 160 is a more selective catalyst than one derived from CuOTf, Eq. 99, in direct contrast to results observed with bis(oxazohnes) or pyridyl bis(oxazohnes) as ligands (cf. Section III.A.3). When the reaction is conducted at -20°C, the cyclopentenyl benzoate is delivered in 88% ee albeit in only 11% yield after 111 h. Larger cycloalkenes are less selective (cyclohexene 56% ee, cyclohep-tene 14% ee, cyclooctene 54% ee). 

Copper-catalyzed allylic oxidation allows the functionalization of unactivated alkenes into chiral allylic carboxylates.1347 The use of oxazoline-containing ligands give good enantioselectivities, but the reaction is extremely slow.1348-1351 Chiral bipyridine complexes, in turn, are much more active and give products in good yields and enantioselectivities up to 70% when applied in benzoyloxylation of cycloalkenes with rm-butyl perbenzoate.1352,1353... 

Allylic oxidation.2 t-Butyl hydroperoxide (0.5 equiv.) in acetic acid in the presence of catalytic amounts of this Rh30 complex (1) oxidizes cycloalkenes to a,/ -enones and the corresponding allylic acetates in the ratio of 6-7 1. Other rhodium complexes are less effective. Allylic alcohols (but not the acetates) are oxidized by this reaction to a,/ -enones. 

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 allylic oxidation of 1,3-dienes catalyzed by Pd(II) complexes in the presence of BQ (added in stoichiometric quantities or used catalytically with a stoichiometric amount of a co-oxidant) leads to the formation of 1,4-oxidation products under mild reaction conditions (at room temperature in AcOH) [42], A very interesting stereochemical aspect arises in 1,4-diacyloxylation reactions of cycloalkenes when carboxylate salts are used as nucleophiles. Although the relative stereochemistry... 

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-... 

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 growing interest in enantioselective isomerization of meso oxiranes to allylic alcohols arises from the ready availabihty of starting materials and the synthetic value of the homochiral products. First apphed to simple meso cycloalkene oxides, this methodology has been successfully exteuded to fuuctioualized meso oxiranes, and even to the kinetic resolution of racemic oxiranes, demonstrating its potential in accessing highly advanced synthons. 

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 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... 

The other possibility is the nucleophilic substitution or addition to the radical cation or the dication (path b) in the presence of external or internal nucleophiles. Thus, electrochemical oxidation of morpholino cycloalkenes in methanol/methoxide leads to the formation of a compound with an allyl or vinyl methoxy substituent [Eq. (26)] [152]. 

The Wacker reaction has also been applied to numerous simple alkenes such a-alkenes or cycloalkenes, or to functionalized alkenes such nitroethylene, acrylonitrile, styrene, allyl alcohol, or maleic acid [3]. The carbonyl group is formed at the carbon atom of the double bond where the nucleophile would add in a Markovni-kov addition (Eq. 5). Among these different alkenes, the oxidation of propene to acetone is the only oxidation which has been developed to an industrial scale. 

An allylic trichloroacetamino group in cycloalkenes directs dihydroxylation with OSO4 and quinuclidine A-oxide as the oxidizing system. An interesting change in the diastereoselectivity by variation of the oxidant composition has been observed."... 

A cascade Heck reaction with termination by nucleophiles is considered to start with an oxidative addition of a heteroatom-carbon bond (starter) onto a palladium(O) species (startup reaction), followed by carbopalladation of a nonaromatic carbon-carbon double or triple bond without subsequent dehydropalladation (relay), a second and possibly further carbopalladation of a carbon-carbon double or triple bond (second etc. relay). The terminating step is a displacement of the palladium residue by an appropriate nucleophile. It is crucial for a successful cascade carbopalladation that no premature dehydropalladation takes place, and that can be prevented by using alkynes and 1,1-disubstituted alkenes (or certain cycloalkenes) as relay stations since they give kinetically stable alkenyl- or neopentylpalladium intermediates, respectively. In addition, reaction of haloalkenes with alkenes in certain cases may form rr-allyl complexes, which are then trapped by various nucleophiles. 

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