Oxidation with Stoichiometric Oxidants

Direct Oxidation with Stoichiometric Oxidants. Discovered by Prilezhaev in 1909,211 the typical epoxidation reaction of alkenes is their oxidation with organic peracids. Of the large number of different peroxycarboxylic acids used in 

A simple generally accepted mechanism known as the butterfly mechanism first suggested by Bartlett79 and Lynch and Pausacker223 involves the nearly nonpolar cyclic transition state 22  

in accord with the mechanism is the increasing reaction rate with increasing dielectric constant of the solvent and the complete syn stereoselectivity.224 

A closely related 1,3-dipolar cycloaddition mechanism with an 1,2-dioxolane intermediate227 could not be experimentally proved 228 Further studies concerning details of the mechanism, including molecular-orbital calculations229,230 and solvent effects,231-234 have been carried out, leading, for instance, to the suggestion of the formation of a nonsymmetric transition state230 and an electron donor- 

The more energetic cis double bond in acyclic alkenes is epoxidized faster than the trans double bond.215 In contrast, opposite reactivity is observed for the stereoisomers of cycloalkenes, where the trans isomers are more reactive as a result of higher ring strain. This was demonstrated in the selective monoepoxidation of cis, trans-1,5-cyclodecadiene 235 

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Oxidative cleavage of alkenes to aldehydes, ketones or carboxylic acids is an important transformation usually carried out by ozonolysis or oxidation with stoichiometric oxidants, i.e. OSO4, MnO 1 etc.199. The serious drawbacks of most of these reagents,... 

Oxidation with Stoichiometric Oxidants. Certain peracids reacting with alkanes yield alcohols. Peracetic acid,68 69 perbenzoic acid,70 m-CPBA,71,72 and nitroper-benzoic acids may be used. Alcohols or an equilibrium mixture of the alcohol and the trifluoroacetate77 are formed on the action of pertrifluoroacetic acid. A high degree of regioselectivity (better than 97%), specifically, preferential attack at the tertiary C—H bonds, is usually observed ... 

The stoichiometric and the catalytic reactions occur simultaneously, but the catalytic reaction predominates. The process is started with stoichiometric amounts, but afterward, carbon monoxide, acetylene, and excess alcohol give most of the acrylate ester by the catalytic reaction. The nickel chloride is recovered and recycled to the nickel carbonyl synthesis step. The main by-product is ethyl propionate, which is difficult to separate from ethyl acrylate. However, by proper control of the feeds and reaction conditions, it is possible to keep the ethyl propionate content below 1%. Even so, this is significantly higher than the propionate content of the esters from the propylene oxidation route. 

Rhodium(III) forms a wide range of complexes with tertiary phosphines and arsines [108, 109], though in some cases other oxidation states are possible. Table 2.5 summarizes the complexes produced from reaction of RhCl3 with stoichiometric quantities of the phosphine. 

The most widely employed methods for the synthesis of nitrones are the condensation of carbonyl compounds with A-hydroxylamines5 and the oxidation of A+V-di substituted hydroxylamines.5 9 Practical and reliable methods for the oxidation of more easily available secondary amines have become available only recently.10 11 12 13. These include reactions with stoichiometric oxidants not readily available, such as dimethyldioxirane10 or A-phenylsulfonyl-C-phenyloxaziridine,11 and oxidations with hydrogen peroxide catalyzed by Na2W044 12 or Se02.13 All these methods suffer from limitations in scope and substrate tolerance. For example, oxidations with dimethyldioxirane seem to be limited to arylmethanamines and the above mentioned catalytic oxidations have been reported (and we have experienced as well) to give... 

Later, Torii et al. found that the tin-aluminum-mediated allylation can be carried out with the less expensive allyl chloride, instead of allyl bromide, when a mixture of alcohol-water-acetic acid was used as the solvent.77 When combined with stoichiometric amounts of aluminum powder, both stoichiometric and catalytic amounts of tin are effective. As reported by Wu et al., higher temperatures can be used instead of aluminum powder.78 Under such a reaction condition, allyl quinones were obtained from 1,4-quinones, followed by oxidation with ferric chloride. Allylation reactions in water/organic solvent mixtures were also carried out electrochemically, with the advantage that the allyltin reagent could be recycled.79... 

Cu(0) species. Alternatively, the Cu(n) species may first undergo oxidation by an external oxidant (or internal redox process) to a Cu(m) intermediate, and then undergo reductive elimination to provide the product and a Cu(i) species. Re-oxidation to Cu(n) would then, in theory, complete the catalytic cycle, but in practice, most reactions of this type have been performed with stoichiometric amounts of the copper reagent. 

Nickel oxide, NiO, which adopts the sodium chloride structure (Fig. 1.14), can readily be made slightly oxygen rich, and, because the solid then contains more oxygen than nickel, the crystal must also contain a population of point defects. This situation can formally be considered as a reaction of oxygen gas with stoichiometric NiO, and the simplest assumption is to suppose that the extra oxygen extends the crystal by adding extra oxygen sites. Atoms are added as neutral atoms, and... 

Allenylcopper reagents can be generated from allenyllithium precursors by treatment with stoichiometric amounts of CuBr (Table 9.6) [12]. These intermediates were not characterized, per se, but subsequent reaction with alkenyl iodides led to allenynes in high yield. Thus it is assumed that the reagents are allenic rather than propargylic. The same intermediates afford 2-alkynylsulfmamides on treatment with N-sulfmylaniline (Table 9.7) [13], Cyclization to the N-phenyldihydroisothiazole S -oxides proceeds in nearly quantitative yield on treatment with base. 

Figure 9.1 compares the synthesis of acetophenone by classic oxidation of 1-phenylethanol with stoichiometric amounts of chromium oxide and sulphuric acid, with an atom efficiency of 42%, with the heterogeneous catalytic oxidation with O2, with an atom efficiency of 87%, and with water as the only by-product. This is especially important if we consider the environmental unfriendliness of chromium salts the potential environmental impact of reactions can be expressed by the environmental quotient (EQ), where E is the E-factor (kg waste/kg product) and Q is the environmental unfriendliness quotient of the waste. If Q is... 

While most of the iminium salts studied are cyclic, several acyclic iminium salts have also been investigated. In 1997, Armstrong and coworkers reported the use of acyclic iminium salt 83 as chiral epoxidation promoter (Fig. 27) [156, 157]. 1-Phenylcyclohexene oxide could be obtained in 100% conversion and 22% ee with stoichiometric amounts of 83. In 2002 acyclic iminium salt 84, prepared from L-prolinol, was investigated by Komatsu and coworkers, and cinnamyl alcohol was epoxidized in 70% yield and 39% ee (Fig. 27) [158]. 

Furukawa et al. reported the total synthesis of murrayaquinone A (107) by a palladium(II)-mediated oxidative cyclization of the corresponding 2-arylamino-5-methyl-l,4-benzoquinones. 2-Anilino-5-methyl-l,4-benzoquinone (842) was prepared starting from 2-methyl-l,4-benzoquinone 841 and aniline 839, along with the regio-isomeric 2-anilino-6-methyl-l,4-benzoquinone (844). The oxidative cyclization of 2-anilino-5-methyl-l,4-benzoquinone (842) with stoichiometric amounts of palla-dium(ll) acetate provided murrayaquinone A (107) in 64% yield. This method was also applied to the synthesis of 7-methoxy-3-methylcarbazole-l,4-quinone (113) starting from 3-methoxyaniline (840) (623). Seven years later, Chowdhury et al. reported the isolation of 7-methoxy-3-methylcarbazole-l,4-quinone (113) from the stem bark of Murraya koenigii and named it koeniginequinone A (113) (49) (Scheme 5.101). 

The well-known Wacker oxidation of terminal alkenes to methylketones has been used for many years on a large scale. It requires a catalytic amount of Pd(II) together with stoichiometric CuCl2 under aerobic conditions. But it is hmited by palladiiun decomposition and chlorinated byproducts. Therefore, a lot of research has been devoted to modifying the reaction, but most of the time copper cocatalysts were necessary. Another problem is the often observed cleavage of the double bond and the production of aldehydes. 

Finally, we are not saying that anion... anion repulsion is always negligible, and that cation... cation repulsion alone determines the nature of an oxide structure see, for example, Sect. 3.5. (But neither would we agree with the reverse statement.) However, for compounds with stoichiometric ratios anion/cation 2 and with first row anions, cation... cation repulsion is likely to dominate. 

The use of cyclic sulfates in synthetic applications has been limited in the past because, although cyclic sulfites are easily prepared from diols, a convenient method for oxidation of the cyclic sulfites to cyclic sulfates had not been developed. The experiments of Denmark [70] and of Lowe and co-workers [71 ] with stoichiometric ruthenium tetroxide oxidations and of Brandes and Katzenellenbogen [72a] and Gao and Sharpless [68] with catalytic ruthenium tetroxide and sodium periodate as cooxidant have led to an efficient method for this oxidation step. Examples of the conversion of several diols (67) to cyclic sulfites (68) followed by oxidation to cyclic sulfates (69) are listed in Table 6D.7. The cyclic sulfite/cyclic sulfate sequence has been applied to 1,2-, 1,3-, and 1,4-diols with equal success. Cyclic sulfates, like epoxides, are excellent electrophiles and, as a consequence of their stereoelectronic makeup, are less susceptible to the elimination reactions that usually accompany attack by nucleophiles at a secondary carbon. With the development of convenient methods for their syntheses, the reactions of cyclic sulfates have been explored, Most of the reactions have been nucleophilic displacements with opening of the cyclic sulfate ring. The variety of nucleophiles used in this way is already extensive and includes H [68], [68,73-76], F" [68,72,74], PhCOCT [68,73,74], NOJ [68], SCN [68],... 

General Procedure for Oxidation of Secondary Alcohols with Stoichiometric Ru04... 

Selective oxidations with stoichiometric RuCl2(PPh3)3 are normally carried out simply by stirring a solution of the alcohol in benzene at room temperature in the presence of the oxidant. The addition of 2 equivalents of K2C03 may improve the reaction.228... 

In 1986, Minami and his coworkers described the optical resolution of the oxide of tram-his-1,2-(diphcnylphosphino)cyclobutanc (20, Scheme 10.) with stoichiometric amount of DBTA in methanol solution. [30] In the same year, Noyori and his coworkers published the synthesis of optically active BINAP via resolution of BINAP oxide (21) with DBTA. [31] In this process, hot chloroformic solution of racemic 21 was mixed with a molar equivalent amount of DBTA previously solved in ethyl acetate. Optically pure -21 was isolated in 79 % yield from the recrystallized diastereoisomeric complex by... 

Thus, in the fine chemicals industry, reduction of ketones and aldehydes relies mainly on the use of complex metal hydrides that require time-consuming workup of reaction mixtures and produce significant amounts of inorganic and organic wastes. Similarly, the oxidation of alcohols into carbonyls is traditionally performed with stoichiometric inorganic oxidants, notably Cr(VI) reagents or a catalyst in combination with a stoichiometric oxidant [1]. 

Fig. 17.19. cis-wc-Dihydroxy-lation of alkenes with catalytic amounts of Os(VIII)/stoichio-metric amounts of methylmor-pholin-/V-oxide [NMO] top) and with stoichiometric amounts of Os(VIII) (bottom), respectively, and pertinent mechanistic insights obtained thus far. 

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