Butterfly transition state

Concerted mechanisms have been proposed on the basis of work carried out with soluble Movl, Wvl and TiIV peroxo compounds. The experimental evidence is consistent with the hypothesis that these compounds act as oxidants in stoichiometric epoxidations and that the reactions involve electrophilic attack of the peroxo compound on the organic molecule or, what is equivalent, a nucleophilic attack of the organic molecule on the peroxidic oxygen, in a butterfly transition state. The reaction product is formed and, after desorption, the peroxo compound is regenerated by reaction of TiIV with H202 this accounts for the catalytic nature of the reaction (Amato et al, 1986). The same type of mechanism... 

Alkenes can be epoxidized with a variety of peroxy acids benzoic acid (MCPBA) is the most commonly used. Woods and Beak have provided experimental evidence for the butterfly transition state (A) in Scheme 61 (Bartlett mechanism <50RCP47 involved in peroxy acid epoxidations <9iJA628i>. 

The previous section amply demonstrates the advantages of dimethyldioxirane as an oxygen transfer agent for preparative oxidation chemistry compared to other oxidants such as peroxy acids and alkaline hydrogen peroxide the synthetic applications are literally endless. Nonetheless, the mechanistic details of the oxygen transfer are to date still inadequately understood. Presently available experimental data [3] such as sterochemistry, kinetics, activation parameters, isotope effects, reactivity patterns, etc. are consistent with the complex butterfly transition state A, initially proposed for peroxy acids as oxygen atom donors, and the novel diradical-like transition state B (Scheme 9). 

Many mechanisms have been proposed for this reaction, such as the epoxidation via initial attack of a hydroxyl cation, 1,3-Dipolar Cycloaddition of a hydroxycarbonyl oxide to an olefinic double bond, and the commonly accepted planar butterfly transition state, " by which the n HOMO orbital of the olefin approaches the terminal oxygen of perbenzoic acid and interacts with the a LUMO of the 0-0 bond at 180". The planar butterfly transition state is further extended by Sharpless to a spiro-trunsition state, which has been consolidated by many other investigators. An illustrative mechanism from mCPBA epoxidation is provided here. 

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 ... 

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 ... 

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

Stereochemical analyses of directed peracid epoxidations (see Section 4.5.1.2.) are incompatible with this transition-state geometry and instead support an in plane transition state ( butterfly or Bartlett-Lynch mechanism46). 

The Simmons-Smith cyclopropanation is a concerted process, and it proceeds via a three-centered "butterfly-type" transition state. This is in agreement with the result of theoretical studies as well as the stereochemical outcome of a large number of reactions. 

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... 

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