4-hydroperoxy-2-alkenals

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

The reactivity order of alkenes is that expected for attack by an electrophilic reagent. Reactivity increases with the number of alkyl substituents.163 Terminal alkenes are relatively inert. The reaction has a low AHl and relative reactivity is dominated by entropic factors.164 Steric effects govern the direction of approach of the oxygen, so the hydroperoxy group is usually introduced on the less hindered face of the double bond. A key mechanistic issue in singlet oxygen oxidations is whether it is a concerted process or involves an intermediate formulated as a pcrcpoxide. Most of the available evidence points to the perepoxide mechanism.165... 

The hydroperoxy functionality can be introduced into an alkene by a singlet oxygen ene reaction and subsequently reduced quantitatively to an allylic alcohol, by addition of reducing agents such as PPhs, Me2S or NaBHj ". In addition, the allylic hydroperoxides can be transformed stereospecilically in the presence of Ti(OPr-i)4 to an epoxy allylic alcohol, where epoxide and hydroxyl functionalities are cis to each other (e.g. substrate 160, Scheme 58) - . ... 

The literature on liquid-phase olefin oxidation has been well reviewed (1, 2, 3, 5, 6, 8,12,14,15, 16,17, 18,19,20). Recent attention has been focused on the effects of structure and reaction conditions on the proportions of alkenyl hydroperoxy radical reaction by the abstraction and addition mechanisms at lower temperatures and conversions. The lower molecular weight cyclic and acyclic olefins have been extensively studied by Van Sickle and co-workers (17, 18, 19, 20). These studies have recently been extended to include higher molecular weight alkenes (16). 

Reaction 8 may, therefore, be the major chain-propagating reaction of H02 between 250° and 400°C. The radicals produced will, of course, undergo the same fates as those produced in Reaction 4, regenerating (eventually) alkyl radicals. The main difference between the alkene-H02 addition route and the alkylperoxy radical isomerization route is that in the former case the hydroperoxyalkyl radicals formed are necessarily a-radicals—i.e., radicals in which the unpaired electron is borne by a carbon atom adjacent to that bearing the hydroperoxy group, such as... 

Since Reactions 4 and 8 have a common product, the scheme also incorporates, and hence in principle reconciles, the apparently contradictory alkylperoxy radical isomerization and alkene-hydroperoxy radical addition schemes for propagating chains during the oxidation of alkyl radicals. If this reconciliation is to hold in practice as well as in principle, however, two conditions are necessary. 

First, hydroperoxy radicals must react predominantly via Reaction 8 and not via Reaction 7. It is difficult to assess this competition because of the uncertain energetics of these reactions. Assuming that k7 — k3, Reaction 8 is faster than Reaction 7 when the hydrogen abstracted in 7 is primary, the rate constants are approximately equal when it is secondary, and 7 predominates when it is tertiary. Only under conditions where the yields of alkenes are considerable and the alkane has no tertiary C—H bonds will Reaction 8 be important. Even then, abstraction of allylic hydrogen from the alkene by HO2 will compete strongly with Reaction 8. 

It appears, then, that alkylperoxy radical isomerization is capable of producing hydroperoxyalkyl radicals during the oxidation of all alkanes and that alkene-hydroperoxy radical addition will serve a similar function during the oxidation of those alkanes which contain a high proportion of primary C—H bonds. It remains to determine the proportion of hydroperoxy alkyl radicals arriving by each route as equilibrium is approached. 

Small alkyl radicals, such as ethyl, are oxidized predominantly to the corresponding alkene because the isomerization of an a-hydroperoxy-alkyl radical to the corresponding alkylperoxy radical competes successfully with its decomposition to an oxiran and a hydroxyl radical. The formation of alkenes via a-hydroperoxyalkyl radicals (and not vice-versa) cannot be excluded, however. 

Here Q denotes an alkyl radical with two unpaired electrons (in QOOH and QO) which may rearrange to form a stable alkene. The compound QO is a cyclic ether3 (which may break down to form an aldehyde4 and a smaller alkene). The sequence of reactions (R64) to (R67) is chain propagating, in that the initial alkyl radical has produced one HO2 or OH radical in addition to one or more stable components. However, it is also possible that a second oxygen molecule may add to QOOH to form a peroxy alkyl hydroperoxy radical,... 

The conversion of 3-chloropentafluoropropene to 2-(chlorodifluoromethyI)-2,3,3-trifluoro-oxirane (33) can be carried out64 by heating the mixture of the alkene and oxygen in 1,1,2-trichlorotrifluoroethane (CFC-113) in an autoclave.64 The oxidation with hydrogen peroxide in alkaline solution is negatively influenced by the high nucleophilic reactivity of allylic chlorine.65 66 The reaction is performed at very low temperatures that favor the attack of the hydroperoxy anion in competition with the hydroxy anion. Acceptable yields of 31 -38 % are obtained in the presence of a phase-transfer catalyst.66... 

Dimethyl-l,3-dioxolan-4-yl)-2-(7 ,5)-methyl- -methylester E19a, 934 (R — I/Redm. + Alken-saure-ester), 937 (R — X + En) Cyclodecan 1-Hydroperoxy-l-methoxy-6-oxo- IV/la, 31 (Ozonisier.) VIII, 32 (Ozoni-sier.)... 

The most widely used and, presumably, the most chemoselective reagents for the epoxidation of nucleophilic C—C double bonds are the peroxycarboxylic acids (see Houben-Weyl, Vol. IV/ 1 a, p 184, Vol. Vl/3, p 385, Vol. E13/2, p 1258). Using chloroform as solvent, epoxidation rates are particularly high79. Reactive or acid/base sensitive epoxides can often be obtained with dimethyldioxirane (see Houben-Weyl, Vol. R13/2, p 1256 and references 15, 16, 87-90), peracid imides (see Houben-Weyl, Vol. IV/1 a, p 205, Vol. VI/3, p 401, Vol. E13/2, p 1276) (prepared in situ from nitriles and hydrogen peroxide), hydroperoxy acetals (see Houben-Weyl, Vol. El3/2, p 1253) or peroxycarbonic acid derivatives (see Houben-Weyl, Vol. IV/la, p 209 and references 17-19) as oxidants. For less reactive alkenes, potassium hydrogen persulfate is a readily available reagent for direct epoxidation20. 

Salooja [21] carried out extensive studies of this phenomenon and considered that it was due to the inhibiting effect of alkenes produced as initial products and that the decrease with temperature in the concentration of hydroxyl radicals was accompanied by a corresponding increase in the concentration of the less reactive hydroperoxy radicals. Norrish s mechanism concurs with this reasoning. Enikolopyan [22] put forward two reaction schemes in an attempt to explain the negative temperature coefficient. In the first he considered the reactions... 

The production of carbonyl compounds and lower alkenes in pairs may be accounted for qualitatively in this way [77]. C-tracer studies [113,114] leave little doubt that these reactions occur to an appreciable extent. Even so they have received little attention from the thermokinetic point of view. The likely order of magnitude of 1 may be assessed, however, from consideration of the strengths of the bonds broken and of those formed. Thus, for the decomposition of the 2-hydroperoxy-3-ethylpent-4-yl radical... 

Above ca. 400—450 °C abstraction of a hydrogen atom from alkyl radicals by oxygen to yield the conjugate alkene and hydroperoxy radical... 

Methoxy-2,2,3,3-tetramethyl-l-phenylcyclopropane [the addition product of methoxy-(phenyl)carbene, generated thermally or photolytically from 3-methoxy-3-phenyl-3//-diazirine, to 2,3-dimethylbut-2-ene] can be prepared in a low yield (in addition to a significant amount of azine) provided that the freshly purified alkene is used. The use of the commercial alkene leads exclusively to the formation of peroxyacetal, the product of the carbene reaction with 3-hydroperoxy-2,3-dimethylbut-l-ene which in turn results from oxidation of 2,3-dimethylbut-... 

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