Alkanals, secondary oxidation products

Alkanals, secondary oxidation products, 665 n-Alkanals, lipid peroxidation, 614 Alkanes... 

The large number of precursors of volatile decomposition products affecting the flavor of oils has been discussed in Chapter 4. Only qualitative information is available on the relative oxidative stability of hydroperoxides, aldehydes and secondary oxidation products. As observed with the unsaturated fatty ester precursors, the stability of hydroperoxides and unsaturated aldehydes decreases with higher unsaturation. Different hydroperoxides of unsaturated lipids, acting as precursors of volatile flavor compounds, decompose at different temperatures. Hydroperoxides of linolenate and long-chain n-3 PUFA decompose more readily and at lower temperatures than hydroperoxides of linoleate and oleate. Similarly, the alkadienals are less stable than alkenals, which in turn are less stable than alkanals. The short-chain fatty acids produced by oxidation of unsaturated aldehydes will further decrease the oxidative stability of polyunsaturated oils. For secondary products, dimers are less stable than dihydroperoxides, which are less stable than cyclic peroxides. 

In spite of significant fundamental studies and its significant economic potential as an alternate route to alkenes, the oxidative dehydrogenation of alkanes to alkenes is not currently practiced.383 The main reason is that the secondary oxidation of the primary alkene products limits severely alkene yields, which becomes more significant with increasing conversion. This is due mainly to the higher energies of the C—H bonds in the reactant alkanes compared to those of the product alkenes. This leads to the rapid combustion of alkenes, that is, the formation of carbon oxides, at the temperatures required for C—H bond activation in alkanes. 

Oxidation of alkanes to alcohols anil or ketones.1 This dioxirane oxidizes hydrocarbons in CH2Cl2/l,l,l-trifluoro-2-propanone (TFP) at -22 to 0° to alcohols or further oxidation products in high yield. Tertiary C-H bonds are attacked more rapidly than secondary ones, and primary C-H bonds are scarcely affected. The oxidation apparently involves insertion of O-atom. Oxidations can be stereospecific, as in the case of cis- and trans- 1,2-dimethyIcyclohexane. 

An important goal is, therefore, to develop effective methods for catalytic oxidations with dioxygen, under mild conditions in the liquid phase. Two substrates which are often chosen as models for alkane oxidations are cyclohexane and adamantane. Cyclohexane is of immense industrial importance as its oxidation products - cyclohexanone and adipic acid - are the raw materials for the manufacture of nylon-6 and nylon-6,6. Adamantane is an interesting substrate as the ratio of oxidation at the secondary versus the tertiary C-H bonds is used as a measure of radical versus nonradical oxidation pathways. Industrial processes for the oxidation of cyclohexane, to a mixture of cyclohexanol and cyclohexanone, generally involve low conversions (under 10%). Even at such low conversions, selectivities are modest (70-80%) and substantial amounts of overoxidation products, mostly dicarboxylic acids, are formed. 

Oxidation of nitrogenous compounds. Both ketoximes and secondary nitro-alkanes are oxidized to ketones. Aldoximes are converted to nitroalkanes, but nitriles are formed under different conditions. In the presence of a chlorine source (NaCl, HQ) ketoximes give a-chloronitroaUcanes and nitroalkenes give the a-chlorinated products. ... 

The unusual reactivity of dioxiranes is impressively exhibited in their ability to insert into C — H bonds (Scheme 7) [28]. Thus, tertiary alkanes are oxidized to their respective alcohols [29]. In the example shown, the insertion took place with complete retention of configuration at the chirality center. 1,3-Dicarbonyl derivatives [30] are hydroxylated with high efficiency, but more than likely the intermediary enol is being oxyfunctionalized. Secondary alcohols are transformed into ketones, a specific example is the oxidation of the epoxy alcohol in the rosette [31], In an attempt to epoxidize the hydroxy acrylic ester [22], the epoxy 1,3-dicarbonyl product was obtained, although in low yield in accord with its rather reluctant nature towards oxidation. 

Selectivity among various sp3 hybrid C—H bonds is more difficult to rationalize. For example, the reaction of in situ generated Cp Rh(PMe3) with alkanes generates the product of activation of the stronger primary C—H bonds in preference to weaker secondary (or tertiary) C—H bonds. Other systems have been demonstrated to possess similar selectivity. A possible explanation of such selectivity is a steric inhibition against activation of internal C—H bonds of a linear alkane. That is, on the timescale of the C—H oxidative addition event, the metal center might access only the terminal C—H bonds. However, recent detailed mechanistic studies have revealed this notion to be incorrect, at least for some systems. 

Shilov reported some of the earliest evidence that transition metal complexes could selectively cleave the C-H bonds of alkanes in a catalytic fashion. Shilov showed that H/D exchange would occur between alkanes and deuterated acid in the presence of platinum complexes (Equation 18.5 and Table 18.1). In addition, Shilov showed that the oxidation of alkanes occurred in the presence of a platinum(II) catalyst, although a platinum(IV) complex was needed as the oxidant. These reactions led to a mixture of alkyl halides formed from the halide of the Pt(IV) oxidant (Equation 18.6) and trifluoroacetate from the trifluoroacetic acid solvent. The cost of platinum(IV) as an oxidant makes this reaction impractical. However, these results provided hope that selective alkane functionalization could be developed because H/D exchange occurred faster at primary C-H bonds than at secondary C-H bonds (Table 18.1), and some selectivity for oxidations of primary C-H bonds over secondary C-H bonds was observed. As noted in Chapter 6, these results motivated a large number of groups to seek transition metal complexes that would insert into, or by other means selectively cleave, the C-H bond of alkanes and create products from this bond cleavage that could be observed directly. 

Saturated and unsaturated hydrocarbons with odd and even numbers of carbon atoms in the molecule (about C11-C35) are present as the primary substances in all vegetable oils and animal fats. Alkanes, alkenes, alkadienes and alkatrienes also arise as oxidation products of unsaturated fatty acids, catalysed by lipoxygenases or by autoxidation of fatty acids during food storage and processing. Only the lower hydrocarbons can play a role as odour-active substances. The main hydrocarbons resulting from oxidation of unsaturated fatty acids are ethane from Hnolenic acid, pentane and butane from Hnoleic acid and hexane and octane from oleic acid. The immediate precursors of hydrocarbons are the fatty acid hydroperoxides (Table 8.4). The unsaturated hydrocarbons are predominantly (Z)-isomers. Numerous other hydrocarbons, including ahcycHc hydrocarbons, appear as secondary hpid oxidation products. 

The above mechanism, implying a bis-alkyl species, is the best way to explain (i) the observed selectivities and (ii) the primary and secondary products. It also takes into account important facts (i) the minor ](=SiO)2ZrH2] supported species (20-35% depending on the oxide support) is more electrophilic and activates C-H bonds of alkane faster than its homolog ](=SiO)3ZrH] (65-80%) [48] (ii) the higher selectivity in isobutene compared to that of methyl-branched pentenes is not consistent with a simple insertion of adsorbed propene into Zr-alkyl species because the concentration of Zr-propyl has to be greater than Zr-Me species (Scheme 3.12). 

The data in Table 7 show that the selectivity for 2-oxygenated products in the oxidation of alkanes on TS-1 is somewhat higher than could be expected on statistical grounds. Only for 3-methylpentane, this selectivity becomes overcompensated by the higher reactivity of tertiary C-H compared to secondary C-H positions. This indicates that the first step of the oxidation, i.e. the formation of alcohols from alkanes is slightly regioselective. Within the ketone fraction, the selectivity for 2-ketones is even more pronounced, indicating that 2-alcohols are selectively oxidized to 2-ketones in the... 

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