Electrophilic oxygen

The superacid-catalyzed electrophile oxygenation of saturated hydrocarbons, including methane with hydrogen peroxide (via H302 ) or ozone (via HOs ), allowed the efficient preparation of oxygenated derivatives. 

You have just seen that cyclic halonmm ion intermediates are formed when sources of electrophilic halogen attack a double bond Likewise three membered oxygen containing rings are formed by the reaction of alkenes with sources of electrophilic oxygen... 

The hydroxy oxygen of a peracid has a higher electrophilicity as compared to a carboxylic acid. A peracid 2 can react with an alkene 1 by transfer of that particular oxygen atom to yield an oxirane (an epoxide) 3 and a carboxylic acid 4. The reaction is likely to proceed via a transition state as shown in 5 (butterfly mechanism), where the electrophilic oxygen adds to the carbon-carbon n-hond and the proton simultaneously migrates to the carbonyl oxygen of the acid ... 

Figure 2,40. Schematic of the two extreme conformations of adsorbed atomic oxygen on Ag covalently bonded electrophilic oxygen (a-) and ionically bonded oxygen (P-).98 Reprinted with permission from Academic Press.

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

A tentative mechanism involving the heterolytic cleavage of the 0-0 bond along with electron transfer from the alkene to the electrophilic oxygen of the Ti(02H) complex is shown in Scheme 27. 

Alkenes can be transformed into epoxides by hydroperoxides and a catalyst, which often is a high-valent titanium or molybdenum complex acting as a Lewis acid. The mechanism is not clear in great detail in Figure 2.34 a suggested mechanism is given. Coordination of the alkene to the metal prior to attack of the electrophilic oxygen is not considered as a necessary step. 

Cinnamyl alcohols substituted at the para carbon atom lend support to the mechanism in which the rate-determining step is the attack of the electrophilic oxygen atom the rate increases with the electron donicity of X, see Figure 14.7 [2]. 

The transition structures for the epoxidation of ethylene and propylene with peroxyformic acid and of ethylene with dioxirane and dimethyldioxirane calculated at the B3LYP, QCISD and CCSD levels are symmetrical with a spiro orientation of the electrophilic oxygen, whereas the MP2 calculations favor unsymmetrical transition structures. The geometries of the transition structures calculated using the B3LYP functional are close to those found at QCISD, CCSD, CCSD(T) levels as well as those found at the CASSCF(10,9) and CASSCF(10,10) levels for the transition structure of the epoxidation of ethylene. 

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