Butterfly mechanism

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 ... 

Intramolecular Folding The Excimer/Exciplex Mechanism and Dewar Isomerization (Butterfly Mechanism)... 

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 ... 

A) Nucleophilic attack of the alkene on the electrophilic oxygen atom covalently bound to the metal, which is reminiscent of Bartlett s butterfly mechanism for epoxidation of alkenes by percarboxylic acids.229... 

The transition state, in which oxygen is added and the proton is shifted simultaneously, resembles a butterfly and is known as the Butterfly Mechanism ... 

The reaction of alkenes with peroxycarboxylic acids to produce epoxides was discovered by Prilezhaev over 80 years ago.14 It is still the most widely used method for epoxidation, and considerable work has been carried out to elucidate the mechanism. The commonly accepted explanation for oxirane formation involves a cyclic polar process where the proton is transferred intramolecularly to the carbonyl oxygen, with simultaneous attack by the alkene rc-bond. This concerted process was suggested by Bartlett,15 and because of the unique planar transition structure it is referred to as the butterfly mechanism (Figure 3.2). 

Figure 3.2 Bartlett butterfly mechanism for the epoxidation of alkenes with peroxycarboxylic acids.

Methyl(trifluoromethyl)dioxiiane (TFDO), prepared from 1,1,1-trifluoroacetone and KHSOg, is more reactive than DMDO by a factor of 600. In addition to facile epoxidation of alkenes, TFDO can be used to regioselectively oxidize tertiary over secondary C-H bonds via an oxenoid (butterfly) mechanism. ... 

The butterfly mechanism (usual representation) is illustrated in 5 and was described by Bartlett. The representation has been refined by Houk to a trans antiperiplanar arrangement of the O—O bond and reacting alkene, with n-p stabilization by reacting lone pair in plane 7.6 The synchronicity of epoxide C—O bond formation and an overall transition state structure was postulated using ab initio calculations and experimental kinetic isotope effects.7-9... 

In spite of intensive efforts, the knowledge accumulated to date about polar oxygen-transfer reactions on peroxo, hydroperoxo, and alkylperoxo complexes is not yet sufficient to decide whether to agree with the direct attack of the olefin at the positively polarized oxygen center [9] ( butterfly mechanism without an organometallic intermediate, step A in Scheme 1) discussed by Chong and Sharpless, or with the mechanism postulated by Mimoun based on an olefin coordination. 

Young, A. M. and Moffett, M. W. (1979) Studies on the population biology of the tropical butterfly Mechanitis isthmia in Costa Rica. Am. Midi. Nat., 101, 309-19. 

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