Alkenes oxidation level

Nitronates derived from primary nitroalkanes can be regarded as a synthetic equivalent of nitrile oxides since the elimination of an alcohol molecule from nitronates adds one higher oxidation level leading to nitrile oxides. This direct / -elimination of nitronates is known to be facilitated in the presence of a Lewis acid or a base catalyst [66, 72, 73]. On the other hand, cycloaddition reactions of nitronates to alkene dipolarophiles produce N-alkoxy-substituted isoxazolidines as cycloadducts. Under acid-catalyzed conditions, these isoxazolidines can be transformed into 2-isoxazolines through a ready / -elimination, and 2-isoxazolines correspond to the cycloadducts of nitrile oxide cycloadditions to alkenes [74]. 

For alkenes, several carbon oxidation levels are again possible. Furthermore, both carbon atoms must be considered as part of the same alkene functional group. While the total oxidation level can go from —4 for ethylene (as the sum of the oxidation level of both carbon atoms in the functional group) to 0 for a tetrasub-stituted alkene, we again recognize that all are of the same functional class. 

Furthermore it is evident that because the lowest possible oxidation level of a single carbon atom in an alkene is —2 while the lowest possible oxidation level of a carbon atom in an alkane is —4, alkenes are thus oxidized relative to alkanes. 

The same process can be carried out to determine the oxidation levels of carbon atoms in several common functional types. It is clear that by using these procedures we can assign oxidation levels to carbon atoms in a wide variety of compounds. It is also clear that knowing the oxidation level is insufficient to assign the functional group present. For example, the alkane neopentane, the alkene isobutylene, the alkyne propyne, the alcohol isopropanol, and formaldehyde all have a carbon with an oxidation level of 0 yet all belong to completely different functional classes and have different physical and chemical characteristics. 

Consideration of the oxidation level reveals diat while one carbon is reduced (the one to which hydrogen adds), die other is oxidized (die one to which the oxygen adds). There is no net change in oxidation level of the alkene functional group. Likewise die reverse processes of these addition reactions, namely, elimination of HX from alkyl halides and dehydration of alcohols to give alkenes, are not redox processes. Additions of water to alkynes is analogous. In this case, however, the product is a ketone, the oxidation level of the ketone is seen to be the same as the alkyne, and so no net change in oxidation level has occurred. 

The conversion of alkenes to 1,2-diols by osmium tetroxide is also an olefin addition reaction. In this case a hydroxy group is added to each carbon of the olefin group, and the addition is termed an oxidative addition since the diol product is at a higher oxidation level than the alkene reactant. Oxidation of the carbon atoms of the alkene takes place in the first step, which is the reaction with 0s04 to produce the intermediate osmate ester. 

Alkenes are relatively low oxidation level hydrocarbons. The most common way to prepare alkenes is to cany out the elimination of a small molecule from between vicinal carbon atoms. However, this is only a viable strategy if the regiochemistry of elimination can be controlled. That is, traditional dehydrohalo-genations or dehydrations often are regioselective but not regiospecific, so that mixtures of structurally isomeric olefins are formed. For example,... 

The reactivity of a transition metal catalyst can be modified by redox modification of its oxidation level. In a remarkable example the complex 2, which is formed by the hydrogenation of a precursor complex, may be switched between the +1 and +2 from by the use of an electrode. The +2 form is highly effective at the hydrogenation of alkenes, whilst the +1 complex is more effective for other applications such as the hydrosilylation of ketones526. 

If the sulfur-based anion-stabilizing group is at a higher oxidation level, it is not usually necessary to provide chelating groups to ensure reaction next to sulfur. The allylic sulfone we made earlier in the chapter (p. 1250) reacts in this wayVith an unsaturated ester to give a cyclopropane. Notice how much weaker a base (MeO-) is needed here, as the anion (and it is an anion if the counterion is Na+ or K+) is stabilized by sulfone and alkene. 

Oxidation of ]V-MeTTPFenCl (46, 52). Catalytic alkene oxidation by iron N-alkylporphyrins requires that the modified heme center can form an active oxidant, presumably at the HRP compound I level of oxidation. To show that iron N-alkyl porphyrins could form highly oxidized complexes, these reactive species were generated by chemical oxidation and examined by NMR spectroscopy. Reaction of the (N-MeTTP)FenCl with chlorine or bromine at low temperatures results in formation of the corresponding iron(III)-halide complex. Addition of ethyl- or t-butyl-hydroperoxide, or iodosylbenzene, to a solution of N-MeTTPFenCl at low temperatures has no effect on the NMR spectrum. However, addition of m-chloroperoxybenzoic acid (m-CPBA) results in the formation of iron(III) and iron(IV) products as well as porphyrin radical compounds that retain the N-substituent. 

Consider the C-H bond in alkanes. Carbon is a more electronegative element than hydrogen. Consequently, the electron pair that forms this bond is shifted towards the carbon atom. In the extreme, an ionic representation of this bond can be given as pictured in 122 (Scheme 2.45). Within these conventions the carbon atom in an alkane can be approximated as a carbanion (oxidation level 0 by definition). Using this definition it becomes possible to apply oxidation-reduction terminology to the processes as if they occurred to ion pair 122. Thus, oxidation of 122 with the loss of one electron leads to the radical 123. With the loss of two electrons, the oxidation leads to carbocation 124. Similarly, the conversion of an alkane to an alcohol and the alcohol into an aldehyde and the aldehyde eventually to a carboxylic acid can unambiguously be classified as an oxidation sequence with the loss of two, four, and six electrons. The oxidation levels 1, 2, and 3 are ascribed respectively to these functional derivatives. The conversion of an alkane to an alkene or alkyne can be interpreted in an analogous fashion. 

Thus, for example, the direct conversion of an ether into an acetal or ketal is difficult to achieve whereas the oxidation of an alcohol to an aldehyde or ketone (or the reverse process) is a trivial transformation. Similarly, the transition from an oxidation level of 2 to level 1 is problematic in the case when one tries to convert dihalides into monohalides while the transformation of alkynes into alkenes may be safely considered a viable route to carry out this transition. 

The conversion of alkenes into epoxides is important not only because it is one of the most reliable routes leading from oxidation level 1 to level 2, but also because reactions of non-symmetrical epoxides with nucleophiles invariably proceed as an attack at the less substituted carbon with inversion of configuration. Thus, hydride reduction of epoxides represents an additional option for the preparation of alcohols (Scheme 2.62), especially valuable for the synthesis of optically pure isomers from epoxides obtained by the Sharpless oxidation. It is also of merit that as a result of alkene-epoxide conversion, a nucleophilic moiety (double bond) is transformed into an electrophilic epoxy ring. The latter... 

Dehydrogenation of alkanes such as ethane (1.38) relates them to alkenessuch as ethene (ethylene, 1.39). The same functional group may be obtained by dehydration of ethanol (1.40). Further dehydrogenation of ethene would generate an alkyne, ethyne (acetylene, 1.41). In terms of oxidation level, the alkene is related to the alcohol and the alkyne is related to the ketone. 

Carbonium ions can be generated at a variety of oxidation levels. The alkyl carbocation can be generated from alkyl halides by reaction with a Lewis acid (RCl + AICI3) or by protonation of alcohols or alkenes. The reaction of an alkyl halide and aluminium trichloride with an aromatic ring is known as the Friedel-Crafts alkylation. The order of stability of a carbocation is tertiary > secondary > primary. Since many alkylation processes are slower than rearrangements, a secondary or tertiary carbocation may be formed before aromatic substitution occurs. Alkylation of benzene with 1-chloropropane in the presence of aluminium trichloride at 35 °C for 5 hours gave a 2 3 mixture of n- and isopropylbenzene (Scheme 4.5). Since the alkylbenzenes such as toluene and the xylenes (dimethylbenzenes) are more electron rich than benzene itself, it is difficult to prevent polysubsiitution and consequently mixtures of polyalkylated benzenes may be obtained. On the other hand, nitro compounds are sufficiently deactivated for the reaction to be unsuccessful. 

In these cases we have examples of all oxidation levels. Check the answer against yours and the aUc-Inthe case of the alkene, formally a dehydration product from an alcohol, either but not both eftlieC atoms is at the alcohol oxidation level. 

The fust important test of this methodology came in Hanessian s investigation of the spiroketal portion of avermectin Bu- This highly convergent approach incorporates all the oxidation levels and functionality required for carbons C(15)-C(28), except for the necessity of alkyne to alkene conversion. The lithium alkynide was prepared at -78 C and then mixed with boron trifluoride etherate under the conditions of Yamaguchi (Scheme 19). (Direct condensation of the lithium salt and lactone lead to substantial amounts of a, -unsaturated lactone.) Addition of the lactone in stoichiometric amounts to the solution of the modified alkynide led to the formation of the desired hemiketal in acceptable yield. Further improvements could be obtained by the recycling of starting material. ... 

The reactions of (39 R = H) with aliphatic aldehydes give alkenes in yields that are strongly temperature dependent. If the reaction is carried out in the presence of TFAA, then excellent yields of the methylene compounds result. However, reaction of iphatic aldehydes with (39 R = (TtHis) in the same conditions proceeds by a most unexpected redox process to yield ketones in good yields after aqueous work-up. The appearance of ketones as products is unique in any Wittig-type of reaction at tlds oxidation level. Alkenyloxyboranes appear to be the intermediates in the reactions, and these react with excess TFAA to give enol trifluoroacetates, which can be isolated and characterized. A possible sequence is shown in Scheme 7. ... 

The electrophilic addition of bromine to aikenes is an oxidation. The starting alkene is at the alcohol oxidation level, but the product has two carbons at the alcohol oxidation level—the elimination reactions of dibromides to give alkynes that you met in the last chapter (p. 000) should convince you of this. There are a number of other oxidants containing electrophilic oxygen atoms that react with nucleophilic aikenes to produce epoxides (oxiranes). You can view epoxides as the oxygen analogues of bromonium ions, but unlike bromonium ions they are quite stable. 

This type of reaction is available only when the alkene is conjugated to an electron-withdrawing group—usually carbonyl (Chapter 10) but it can be nitro, cyanide, etc. (Chapter 23). This disconnection is available only at this oxidation level unlike the last. We can do a two-group 1,3-disconnection on this sulfide, for example. 

Use of Halide Ions to Improve Selectivity. Earlier work has claimed that enhanced selectivities for alkene oxidation can be achieved by the inclusion of electronegative elements such as S, Se, or halogens. This has been reviewed elsewhere. " More recent work has demonstrated substantial improvements in selectivity for propene (25—70%) and isobutene (35—80%) oxidation when either chloride or bromide is present. Both elements are added to the catalyst in the form of trace levels of organo-halide in the process gas stream. The selectivity increase is the result of a decrease in the rate of complete oxidation rather than an increase in the partial oxidation rate. Since the reaction is first order in oxygen pressure and zero order with respect to alkene in the presence and absence of halide, the reaction mechanism is probably similar in both cases. In the light of Anshits recent work, the effect of the halide is presumably to reduce the relative number and/or reactivity of surface lattice oxygen species and thus reduce the amount of irreversibly adsorbed alkene. 

Dipolar cycloadditions can, of course, only produce five-membered rings. Addition of dipolarophiles can generate tetrahydro, dihydro or aromatic oxidation level heterocycles, as illustrated above. Alkene dipolarophiles, with a group that can be eliminated following cycloaddition, give the same result as equivalent alkyne dipolarophiles, for example enamines as the dipolarophile, interact with azides, as the 1,3-dipole, with subsequent elimination of the amine, affording 1,2,3-triazoles. ... 

A final solution to the homologation problem involves the geminally disubstituted terminal alkene trap shown in Scheme 35, which is already at the correct oxidation level for alkaline hydrolysis to the desired acid [36]. 

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