Oxidation of Alkenes and Alkynes

Alkenes and alkynes can be oxidatively functionalized by electrophilic X -iodanes, such as iodosylbenzene, [bis(acyloxy)iodo]arenes and organoiodine(lll) derivatives of strong acids. Iodosylbenzene itself has a low reactivity to alkenes due to the polymeric structure. However, the relatively weak electrophilic reactivity of (PhlO) can be increased considerably in the presence of BF3-Et20 or other Lewis acids. This activation 

R = C4H9, C5H11, CgHig, C7H15, CsHi7, cydohexyl 

Mechanistic studies of the diacetoxylation of alkenes using (diacetoxyiodo)benzene have demonstrated a protio-catalytic nature of this reaction [255]. Systematic studies into the catalytic activity in the presence of proton-trapping and metal-complexing additives indicate that strong acids act as catalysts in the reaction. When trifluoromethanesulfonic acid is used as catalyst, the selectivity and reaction rate of the conversion is similar or superior to most efficient metal-based catalysts, such as Pd(II) and Cu(II) metal cations. Based on a kinetic study as well as in situ mass spectrometry, a mechanistic cycle for the proton-catalyzed reaction was proposed in this work [255]. 

Likewise, [bis(acyloxy)iodo]arenes can be used as the oxidants in organocatalytic, asymmetric epoxidation of a,p-unsaturated aldehydes using chiral imidazolidinone catalyst 207 [266]. In a specific example, the 

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The Phase-Transfer-Assisted Permanganate Oxidation of Alkenes and Alkynes... 

Oxidations of alkenes and alkynes have been reviewed, including mechanistic information in some cases. They include treatment of epoxidations [1-9], ketohydroxylations [7-9] and alkene cleavage [4, 6,10-14]. Oxidations of alkynes have been reviewed in [4, 12, 14, 15]. 

SCHEME 126. AUyUc oxidation of alkenes and alkynes with Se02/TBHP... 

Oxidation of alkenes and alkynes—sometimes with oxidative breakage of the carbon-carbon multiple bond—affording 1,2-diols,46,47 a-hydroxyketones 46 diketones,46 aldehydes,311,46 ketones46 or carboxylic acids31,44,46,29... 

Another synthetic route to monoorganothallium compounds is the reaction of aryl or vinyl derivatives of mercury(II) or tin(IV) with TIX3 (X = halide, carboxy-late). Monoalkylthallium derivatives are intermediates m the oxidation of alkenes and alkynes by thallium(III) salts (oxythallation) (see Section 7). 

Oxidation of Alkenes to Oxirans, using Peroxides. Two reviews in this area have been published, one dealing with new methods for the catalytic epoxidation of alkenes using hydrogen peroxide and the other with selective oxidation of alkenes and alkynes with t-butyl hydroperoxide. ... 

Dia ene deductions. Olefins, acetylenes, and azo-compounds are reduced by hydrazine in the presence of an oxidizing agent. Stereochemical studies of alkene and alkyne reductions suggest that hydrazine is partially oxidized to the transient diazene [3618-05-1] (diimide, diimine) (9) and that the cis-isomer of diazene is the actual hydrogenating agent, acting by a concerted attack on the unsaturated bond ... 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

Ammonium cerium(IV) nitrate on reaction with acetone or acetophenone generates acetyl- or benzoylformonitrile oxides, respectively (99). These nitrile oxides dimerize to furoxans and give, in the presence of alkenes and alkynes, 3-acetyl- or 3-benzoyl-4,5-dihydroisoxazoles and 3-acetyl- or 3-benzoylisoxazoles, respectively the yield of the isoxazole derivatives was improved on using ammonium cerium(III) nitrate tetrahydrate-formic acid (99). 

In a related smdy, the reactions of a bicyclic dithioether dication (58) (generated from 1,4-dithiane 1-oxide) with alkenes and alkynes has been found to proceed as conjugate addition of two sulfonium groups, giving rise to derivatives of dithioniabicyclo[2.2.2]octane (56) and (57), respectively (Scheme 10). The reaction is sensitive to electronic and steric factors and appears invariably to proceed with retention of the relative arrangement of substiments at the double bond of the original alkene (58). " ... 

By far the most commonly used - though not the most environmentally friendly -solvent is CCl (or more usually water-CCl ). In a classic paper Sharpless et al. showed that oxidation reactions of RuO (and other some Ru-based oxidants) were accelerated by addition of a little acetonitrile to the conventional water-CCl biphasic mixture. It was suggested that the CH3CN might function as a mild donor stabilising a lower oxidation state carboxylato Ru species which could be involved in the catalytic process [260]. A comparative study of CCl, acetone, ethyl acetate, cyclohexane and acetone for cleavage of alkenes and alkynes by RuClg/aq. IO(OH)3/solvent showed that cyclohexane was the most effective [216]. Other solvents sometimes... 

There is a rich chemistry of alkene and alkyne oxidation by RuO. The main application lies in alkene cleavage, bnt there is growing interest in cw-dihydroxylation by the reagent. In the sections below we first consider oxidations which do not sever the C=C bond (epoxidation, ctT-dihydroxylation, ketohydroxylation), and then alkene cleavage reactions. 

This is one of the most important applications for RuO. Oxidative cleavage of alkenes and alkynes by a variety of reagents has been reviewed [30, 35, 50, 60, 68-71]. The gentler cleavage reactions of alkenes to aldehydes or ketones are considered first (Table 3.3), then the commoner cases of cleavage to carboxylic acids (Table 3.6). 

Benzodioxaboroles (79) are easily hydrolyzed by water at room temperature. Attention has been paid to the parent 1,3,2-benzodioxaborole (79 R = H) as a hydroborat-ing agent of alkenes and alkynes to give, after hydrolysis, alkyl- and alkenyl-boronic acids. Compound (79 R = H) is oxidized by dry air at room temperature (75JA5249). 

Isoxazoles and their partially or fully saturated analogs have mainly been prepared, both in solution and on insoluble supports, by 1,3-dipolar cycloadditions of nitrile oxides or nitrones to alkenes or alkynes (Figure 15.10). Nitrile oxides can be generated in situ on insoluble supports by dehydration of nitroalkanes with isocyanates, or by conversion of aldehyde-derived oximes into a-chlorooximes and dehydrohalogenation of the latter. Nitrile oxides react smoothly with a wide variety of alkenes and alkynes to yield the corresponding isoxazoles. A less convergent approach to isoxazoles is the cyclocondensation of hydroxylamine with 1,3-dicarbonyl compounds or a,[3-unsatu-rated ketones. 

Further chemistry of alkenes and alkynes is described in this chapter, with emphasis on addition reactions that lead to reduction and oxidation of carbon-carbon multiple bonds. First we explain what is meant by the terms reduction and oxidation as applied to carbon compounds. Then we emphasize hydrogenation, which is reduction through addition of hydrogen, and oxidative addition reactions with reagents such as ozone, peroxides, permanganate, and osmium tetroxide. We conclude with a section on the special nature of 1-alkynes— their acidic behavior and how the conjugate bases of alkynes can be used in synthesis to form carbon-carbon bonds. 

Alkenes and alkynes coordinate to transition metals and undergo a variety of reactions, and are very important substrates for transition metal-catalysed reactions. Their reactions with halides, cyclization via carbene complexes, hydrogenation, and oxidative reactions with Pd(II) are treated in Sections 3.2, 8.2, 10.1 and 11.1, respectively. The many other reactions of alkenes and alkynes as main reactants are treated in this chapter. 

Addition of H and CO to alkenes and alkynes catalysed by transition metal complexes is called hydrocarbonylation, and is useful for the syntheses of carboxylic acids, their esters, aldehydes and ketones [1]. Oxidative carbonylation of alkenes and alkynes with Pd(II), treated in Section 11.1.5, differs mechanistically from hydrocarbonylation. Some carbonylation reactions occur at under 1 atm or low pressures, without using a high-pressure laboratory apparatus. Several commercial processes based on hydrocarbonylation have been developed. 

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