Bartlett butterfly epoxidation transition state

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). 

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 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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