Bartlett ‘butterfly’ mechanism

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

For more than forty years the Bartlett butterfly TS was the generally accepted mechanism for peracid epoxidation and numerous experimental studies supported this transition structure" " . During these formative years theoretical calculations did not play a major role due to limitations of available methods that could adequately treat the peroxide functional group. Theoretical contributions in 1978 were at the Hartree-Fock (HF) level since... 

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 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). 

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... 

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>. 

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