Allylic hydroperoxides isomerization

A closer examination by ex situ analysis using NMR or gas chromatography illustrates that intrazeolite reaction mixtures can get complex. For example photooxygenation of 1-pentene leads to three major carbonyl products plus a mixture of saturated aldehydes (valeraldehyde, propionaldehyde, butyraldehyde, acetaldehyde)38 (Fig. 33). Ethyl vinyl ketone and 2-pentenal arise from addition of the hydroperoxy radical to the two different ends of the allylic radical (Fig. 33). The ketone, /i-3-penten-2-one, is formed by intrazeolite isomerization of 1-pentene followed by CT mediated photooxygenation of the 2-pentene isomer. Dioxetane cleavage, epoxide rearrangement, or presumably even Floch cleavage130,131 of the allylic hydroperoxides can lead to the mixture of saturated aldehydes. 

The preparation of acyclic allylic hydroperoxides has been described before (3, 7, 9), but it is not clear how the reactivities differ from the better known saturated hydroperoxides and cyclic allylic hydroperoxides. Dykstra and Mosher prepared allyl hydroperoxide by the reaction of allyl methanesulfonate with hydrogen peroxide and alcpholic potassium hydroxide and purified the hydroperoxide by gas chromatography. It detonated on heating and decomposed on exposure to light but was relatively stable in the cold and dark. The isomeric allylic hydroperoxides formed from the autoxidation of the branched olefin, 4-methyl-2-pentene, have also been isolated and were not abnormally reactive (3). In the present study, cis- and trans-2-butene were photooxidized in the presence of methylene blue as a sensitizer (14), and the product, l-butene-3-hydro-peroxide, was isolated by preparative chromatography. 1-Butene proved unreactive and 2-butene-l-hydroperoxide could be formed only by isomerization of the secondary hydroperoxide. 

The free radical isomerization of allylic hydroperoxides (3) in dilute nonpolar solvents appears to give a reliable measure of their relative... 

AUylic hydroperoxides. The product ratios of the allylic hydroperoxides obtained on oxidation of alkenes with singlet oxygen differ significantly from those obtained by base-catalyzed isomerization of /3-halo hydroperoxides, which involves perepoxide intermediates. A third mechanism must be operating in the reaction of triphenyl phosphite ozonide (3, 323-324 4, 559). This last reaction presumably proceeds by an ionic mechanism, since singlet oxygen is not formed at — 70° from the ozonide. ... 

Stereochemical studies based on C-nuclear magnetic resonance spectroscopy ( C-NMR) showed the presence of eight cis and trans allylic hydroperoxides (Table 2.1). To determine the isomeric distribution of allylic hydroxyooctadecenoate derivatives, cis and trans fractions were separated by silver nitrate-thin layer chromatography (TLC), a procedure that separates according to the number, position and geometry of double bonds, and they were hydrogenated prior to GC-MS analyses of the TMS ether derivatives. More recently, the six major hydroperoxide isomers of methyl oleate were partially separated by silica HPLC, and identified by chemical-ionization mass spectrometry and IH NMR (Table 2.1). These hydroperoxide isomers were better separated as the hydroxy octadecenoate derivatives by the same silica HPLC method and re-analysed by GC-MS. 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

Hydroperoxide formation is characteristic of alkenes possessing tertiary allylic hydrogen. Allylic rearrangement resulting in the formation of isomeric products is common. Secondary products (alcohols, carbonyl compounds, carboxylic acids) may arise from the decomposition of alkenyl hydroperoxide at higher temperature. 

In some instances the primary product of alkene photooxidatitxi is not the allylically rearranged hydroperoxide, but the dioxetane addition product, e.g. (56), which may or may not formed by concerted [2 + 2] cycloaddition. Some of these dioxetanes, e.g. (57), ate relatively stable, although most suffer cleavage to produce carbonyl compounds or other materials. For example, photooxidation of indene gives homophthaldehyde (58) which was not produced under identical reaction conditions from hydroperoxide (59). Isomeric hydroperoxides (60) and (61) were also isolated when the oxidation was carried out in methanolic solution (Scheme 14). 

Chromium(VI) oxide can be used as a catalytic oxidant for alcohols with r-butyl hydroperoxide as the cooxidant. This reagent appears to be selective for allylic and benzylic over saturated alcohols, though ( )/(Z)-isomerization has been observed during the preparation of a,3-unsaturated aldehydes. This reagent is also a good oxidant for allylic and ben lic C—bonds these may be competing pathways in more sophisticated substrates. ... 

Reactions with steroidal monoolefms Photooxygenation of A -3-methyl-5a-cholestene (1) with hcmatoporphyrin as sensitizer, followed by reduction of the initially formed hydroperoxides with methanolic sodium iodide, gives the two allylic alcohols (2) and (3). Similar oxidation of the isomeric A -2-methyl-5a -cholestene yields the... 

From the oil of Santalum spicatum (R.Br.) A.DC., Birch et al have isolated the two isomeric dihydrofarnesenes (1) and (2). These two compounds, although not identical with the trail pheromone of the Nasutitermes species of termites, both exhibit similar specific trail activities. Cavill and Coggiola have reported that the photosensitized oxygenation of a-farnesene (3) yields the five allylic alcohols (4)—(8) after reduction of the corresponding hydroperoxides. A new... 

The formation of 3-methyl-2-buten-l-ol, 4, and of 3-methyl-2-butenal, 5, in some experiments can be related to the isomerization of la and subsequent oxidation of product 4, thus formed, to 5. We observed that the vanadium oxo-alkoxide, [OV(OC3H7)3], is a good precursor for the catalytic epoxidation of substrates la, lb and Ic with systems involving tert-butyl hydroperoxide and tertiary allylic alcohols. These systems proved to be more active than those involving [VO(acac)2] usually associated with anhydrous r-BuOOH in benzene or toluene (Figure 1 and Table 2). 

Together with hydrogenation and isomerization, epoxidation completes the trio of commercially significant applications of enantioselective homogeneously catalyzed reactions. Stereospecific olefin epoxidation is distinctive in that it creates two chiral centers simultaneously. The enantioselective epoxidation method developed by Sharpless and co-workers is an important asymmetric transformation known today. This method involves the epoxidation of allylic alcohols with tert.-butyl hydroperoxide and titanimn isopropoxide in the presence of optically active pure tartrate esters (Eq. 3-14). 

The possible formation of the nonconjugated bis-allylic 11-hydroperoxide from linoleate after hydrogen abstraction from carbon-11 has been debated for a long time in the literature. Whether or not this proposed bis-allylic 11-hydroperoxide is unstable and immediately isomerized to the more thermally stable conjugated 9- and 13-hydroperoxides is not clear. Recently, the bis-allyl 11-hydroperoxide was produced in about 5-10% relative yield by kinetically... 

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