Alkenes, reactions with oxygen ions

Alkanes—Continued reactions—Continued with ozonide ions, 135 with superoxide ions, 134-35 role of oxygen ions in oxidation. 138-41 Alkenes, reactions with oxygen ions, 134 with ozonide ions, 135 with superoxide ions, 134-35 Aluminosilicate gels, alkali cations, 241... 

A variety of double bonds give reactions corresponding to the pattern of the ene reaction. Those that have been studied from a mechanistic and synthetic perspective include alkenes, aldehydes and ketones, imines and iminium ions, triazoline-2,5-diones, nitroso compounds, and singlet oxygen, 10=0. After a mechanistic overview of the reaction, we concentrate on the carbon-carbon bond-forming reactions. The important and well-studied reaction with 10=0 is discussed in Section 12.3.2. 

The Mo+ ion was generally less reactive to alkenes than the other ions. Additions of the alkene to the Mo+ and [MoO]+ with loss of H2 or 2H2 were the major reaction products similar to the alkanes. The reactions of [Mo02]+ with alkenes gave a variety of products, the major reactions being reduction by loss of an oxygen atom. 

The paramagnetic oxygen ions 0 , 01> and 0J have been formed on magnesium oxide and studied by EPR spectroscopy. The reactivity of these ions with hydrocarbons follows the sequence 0 >>03> >02. Both with alkanes and alkenes the initial reaction is thought to be hydrogen atom abstraction. 

The proposed surface intermediates are summarized in Table II. The intermediates in each reaction of alkanes includes alkox-ide ions, regardless of the type of active oxygen species. Although carboxylate ions are believed to be the intermediate in the reactions of C2 and C3 alkenes, the type of carboxylate formed with 0 as a reactant is different from the type of carboxylate ion formed when 07 or 07 was a reactant. With 0 ions the carbon number of the carboxylate ions is the same as that of the hydrocarbon reactant, but with 07 or 07 the carboxylate ions have carbon numbers smaller than the parent hydrocarbon. The reaction schemes of Ci alkenes are somewhat complicated, yet it appears that they react in a manner more similar to C2-C1 alkanes than to C2 or C3 alkenes. 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

In the presence of freeze-dried potassium fluoride, perfluoro-2-methylpent-2-ene reacts with activated methylene compounds to yield pyrans (81MI22400). The fluoride ion abstracts a proton from the methylene group and subsequent condensation of the carbanion with the perfluoroalkene affords a dienone (19) which ring closes to the pyran (20 Scheme 3). In the case of pentane-2,4-dione a divinyl ether (21) is also formed. This product is considered to arise from reaction of the alkene at the oxygen of the enolate ion. 

Oxygen may also be used as the oxidant for epoxidation. For example, a selective epoxidation, in the absence of metal ions, has been performed in excellent yield by treatment of alkenes with oxygen in the presence of a large excess of benzaldehyde36. Photooxygenation of alkenes, in the presence of transition metal catalysts, is a very general and synthetically useful reaction which leads to hydroxy-epoxidation (equation 4)37-39. 

Fluorinated -alkenes and -cycloalkenes have a special relationship with their hydrocarbon analogues, usually exhibiting a chemistry that is complementary. For example, the fluorinated systems are frequently susceptible to nucleophilic attack, in some cases dramatically so, and therefore reactions of nucleophiles with fluorinated alkenes often reveal unique new chemistry. This chapter covers electrochemical reduction, principles governing orientation and reactivity of fluorinated alkenes towards nucleophiles, fluoride ion as a nucleophile and the mirror-image relationship of this chemistry with that of proton-induced reactions, reactions with nitrogen-, oxygen-, carbon- centred nucleophiles etc., and, finally, chemistry of some oligomers of fluorinated -alkenes and -cycloalkenes. 

The normal synthetic pathway for hydroboration is reaction with an ambiphilic nucleophile of which the simplest example is hydroperoxide ion. This elicits a 1,2-migration of an alkyl group from boron to oxygen with concurrent loss of hydroxide ion. The step occurs with essentially complete retention of configuration. In similar vein, ambiphilic species with the structure NH2X may be used in amination, so that the overall reaction is an addition of ammonia to the alkene with the regio- and chemoselectivity driven by the hydroboration step. A majority of reactions of organoboranes can be rationalized in terms of these ionic mechanistic pathways, or closely related protocols (Scheme 2). 

Since this is the first time the radical reactions of enamines are being reviewed, an attempt has been made to cover all the relevant literature. In Section II of this chapter, a brief discussion of the general principles of radical additions to. alkenes with special emphasis on the modifying effect due to the presence of amino group (as is the case in enamines) is included. Section III deals with the reactions of enamines in which the radical addition takes place on the C=C bond of enamines while Section IV describes the reactions with one-electron oxidants such as metal ions and oxygen. 

Epoxidation. NaOCl provides the active oxygen for delivery to alkenes through interaction with metal-porphyrin complexes. Typically, 5,10,15,20-tetrakisaryl-porphyrins and their 2,3,7,8,12,13,17,18-octahalo derivatives form active catalysts with metal ions such as Mn. Another heterocyclic ligand (e.g., 4-methylpyridine) and a phase-transfer agent are also added to the reaction medium. 

The behavior of alcohol as a base is evident in the chemical reaction in which alkenes are formed. In a strong acid medium, alcohol is protonated at the oxygen atom, forming the oxoniun ion intermediate. The reaction ends with the formation of an alkene by the removal of the water molecule from this oxonium ion. Such reactions, in which some atoms or groups depart from the molecule, are called elimination reactions. In most cases the preparation of alkenes from alcohols includes a reaction with concentrated sulfuric acid, which is a sufficiently strong protonating agent. 

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