Allyl hydroperoxides

Alkenes slowly undergo a reaction in air called autoxidation in which allylic hydroperoxides are formed. 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

The allylic hydroperoxides generated by singlet oxygen oxidation are normally reduced to the corresponding allylic alcohol. The net synthetic transformation is then formation of an allylic alcohol with transposition of the double bond. 

Olefins containing at least one allylic hydrogen atom react with 02 to form an allylic hydroperoxide. The analogous formal reaction of such alkenes with ethylene is known as ene-reaction. The tricyclic lactone peroxypartheno-lide has been prepared by such a reaction (6.13)619). 

Alternatively the allylic hydroperoxide can undergo the so called Hock-type fragmentation621 under acid-catalyzed hydrolysis with formation of two carbonyl compounds (or moieties). The sesquiterpene (—)-geijerone has been obtained from the triene (+)-y-elemene by such a sequence (6.15) 622>. 

In special cases allylic hydroperoxides decompose directly to a,P-un-saturated ketones. The allylic hydroperoxide formed in the photooxydation of calarene gives the three types of products discussed above (6.16) 623). 

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. 

Hydroxy epoxidation of dienes.2 Photosensitized oxygenation of dienes when catalyzed by titanium(IV) isopropoxide results in an epoxy alcohol, formed by an oxygen transfer from an allylic hydroperoxide. 

Allylic hydroperoxides, 13 435 Allylic organoboranes, 13 661-662 Allylic polymers, as flocculating agents, 11 631... 

Allylations of hydroquinone derivatives, 32 459-461 Allyl chloride oxidation, 41 305 thermal decomposition, 41 80 tl-Allyl complexes, 25 129-134 Allyl hydroperoxide, 27 187-189 Allylic alcohols, olefinic substitution of, 26 343-345... 

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

The processes that occur when cyclohexene reacts with oxygen in the presence of an initiator to give the allylic hydroperoxide exemplify this nicely. 

AUyl transfer reactions, 73, 1 Allylic alcohols, synthesis from epoxides, 29, 3 by Wittig rearrangement, 46, 2 Allylic and benzylic carbanions, heteroatom-substituted, 27, 1 Allylic hydroperoxides, in... 

Also among the oxidants that add oxygen at carbon-carbon double bonds is singlet oxygen.121 For most alkenes, this reaction proceeds with the specific removal of an allylic hydrogen and shift of the double bond to provide an allylic hydroperoxide as the initial product. 

The crosslinking process is different depending on whether the unsaturation is an unconjugated double bond as in oleic acid or a conjugated double bond as in linoleic acid. Unconjugated double bonds undergo crosslinking by the initial formation of an allylic hydroperoxide... 

Danishefsky dienes [98] cycloadd to Cjq in refluxing toluene or benzene [5, 38, 99-101]. The diene 103 adds in 60% yield to Cjq to give the desilylated ketone 104 [5,101]. Acid-catalyzed methanol elimination then furnishes the enone 105 in 82% yield (Scheme 4.17). As already described, this enone can be reduced by DIBAL-H to the corresponding alcohol for further functionalization. The same a,(3-un-saturated alcohol can also be obtained in better yield by Diels-Alder reaction of Cg0 with butadiene, followed by oxidation with singlet oxygen to the allylic hydroperoxide and PPhj reduction to the desired alcohol [101]. This sequence yields the allylic alcohol in 53%, starting from Cjq without the need of isolating intermediates. 

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