Halogenated alkenes, epoxidation

The N-alkylation reaction represents a bifurcation of the normal alkene epoxidation reaction cycle and, therefore, N-alkylation is a suicide event that leads to catalytic inhibition in the native system. With synthetic tetraarylporphyrins that mimic the N-alkylation reaction, the use of halogen-substituted catalysts that are stable toward oxidative degradation (26, 27) provide the most useful model systems because the heme model remains intact for a significantly greater number of turnovers than the partition number. The partition number is the ratio of epoxidation cycles to N-alkylation cycles, i.e., N-alkyl porphyrins are formed before the heme is oxidatively destroyed. 

Halogenated alkenes are transformed into halogenated epoxides by oxygen or hydrogen peroxide (equation 58) [1100],... 

Seleninic acids Seleninic adds of the type used for alkene epoxidation (c Section 4.3.2.3) have also been employed as catalysts for the Baeyer-VUhger oxidation of ketones with hydrogen peroxide, mainly by Syper [57] and by Sheldon et al. [58,59]. In most cases, halogenated solvents such as dichloromethane or l,2-didiloroethane were used. In a study of solvent effects, Sheldon et cd. observed that, once again, TFE and in particular 1,1,1,3,3,3-hexafluoro-2-propanol are superior to dichloromethane with regard to selectivity and rate [58]. However, the effects are not as pronounced as in the case of alkene epoxidation (e.g., a factor of about 2 in rate between dichloromethane and HFIP, and 1.3 for TFE). The Baeyer-Villiger oxidation of a series of ketones and aldehydes was studied in TFE, and the results are summarized in Table 4.3 [58]. 

A number of chiral ketones have been developed that are capable of enantiose-lective epoxidation via dioxirane intermediates.104 Scheme 12.13 shows the structures of some chiral ketones that have been used as catalysts for enantioselective epoxidation. The BINAP-derived ketone shown in Entry 1, as well as its halogenated derivatives, have shown good enantioselectivity toward di- and trisubstituted alkenes. 

Oxidation is the first step for producing molecules with a very wide range of functional groups because oxygenated compounds are precursors to many other products. For example, alcohols may be converted to ethers, esters, alkenes, and, via nucleophilic substitution, to halogenated or amine products. Ketones and aldehydes may be used in condensation reactions to form new C-C double bonds, epoxides may be ring opened to form diols and polymers, and, finally, carboxylic acids are routinely converted to esters, amides, acid chlorides and acid anhydrides. Oxidation reactions are some of the largest scale industrial processes in synthetic chemistry, and the production of alcohols, ketones, aldehydes, epoxides and carboxylic acids is performed on a mammoth scale. For example, world production of ethylene oxide is estimated at 58 million tonnes, 2 million tonnes of adipic acid are made, mainly as a precursor in the synthesis of nylons, and 8 million tonnes of terephthalic acid are produced each year, mainly for the production of polyethylene terephthalate) [1]. 

Divalent chromium reduces triple bonds to double bonds (trans where applicable) [195], enediones to diones [196], epoxides to alkenes [192] and aromatic nitroso, nitro and azoxy compounds to amines [190], deoxygenates amine oxides [191], and replaces halogens by hydrogen [197,198],... 

Solutions of low-valence titanium chloride (titanium dichloride) are prepared in situ by reduction of solutions of titanium trichloride in tetrahydrofuran or 1,2-dimethoxyethane with lithium aluminum hydride [204, 205], with lithium or potassium [206], with magnesium [207, 208] or with a zinc-copper couple [209,210]. Such solutions effect hydrogenolysis of halogens [208], deoxygenation of epoxides [204] and reduction of aldehydes and ketones to alkenes [205,... 

Halohydrins are useful intermediates especially in the synthesis of epoxides. The main reaction is usually accompanied by the formation of a dihalide. When the reactions are performed in the presence of acetic acid, then acetates of the hydrins can be the predominant products. With several exceptions, alkenes with a nonfluorinated C = C bond have been subjected to halohydrinations. Halogen cations usually undergo addition to the substituted carbon of the C = C bond in (fluoroalkyl)ethenes. 

Ketones from halohydrins. Palladium acetate complexed with a triarylphos-phine, particularly tri-o-tolylphosphine, converts halohydrins into ketones in the presence of K2C03. Yields are about 70-85% for substrates in which the halogen is secondary or tertiary, but less than 50% when the halogen is primary because of epoxide formation. The reaction is useful for conversion of alkenes to ketones in those instances in which halohydrins are formed regioselectively. 

The obtained results are of great interest in fine chemistry and organic synthesis, since epoxides are very versatile building blocks and halogenation of alkenes is still carried out using hazardous reagents and drastic conditions. 

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