EPR Support for the Ketyl Radical-Anion “Triggered” [3,3]-Sigmatropic Rearrangement

Abstract

New physical evidence to support a ketyl radical-anion mechanism for the [3,3]-sigmatropic rearrangement is presented. With use of EPR spectroscopy, spectra are observed that can be attributed to 8, an acyl radical-anion species resulting from a [3,3]-rearrangement; this also functions as a key intermediate in the process. The spectrum of an additional paramagnetic species resulting from further addition of tin-centered radicals to the reaction product was also observed.

10 Reaction mechanisms : Part (iii) Pericyclic reactions

“…The ketyl radical anion Claisen rearrangement of 11 (Scheme 4) was reported by Enholm et al 39 The reaction to form 12 could conceivably occur either through an anionic or a radical-anionic Claisen mechanism, depending on whether [3,3] rearrangement precedes or comes after reaction with Bu 3 SnH. The authors carried out the rearrangement of 11 in an EPR spectrometer at ≥353 K, and the main signal recorded is consistent with the cyclization occurring first.…”

Captodative Stabilization in Geminally Substituted Allylic Radicals Assessed by Means of the EPR‐Spectral D Parameter of 1,3‐Cyclopentanediyl Triplet Diradicals

“…The higher affinity of the methoxycarbonyl versus the cyano group to interact synergistically with a methoxy substituent means that the captodative effect does not necessarily obey the radical-stabilizing trend displayed by the corresponding monosubstituted allylic radicals. Evidently, the ketyl-type [14] mesomeric structure M12d contributes more to the spin delocalization than the iminyl-type M11d structure.…”

10 Reaction mechanisms : Part (i) Radical and radical ion reactions

“…The major evidence in support of this mechanism was an EPR spectrum which was attributed to intermediate 38. 57 In other mechanistic studies, Yus, Herrera, Guijarro clarified the mechanism of alkyllithium formation from alkyl halides, Li, and a catalytic quantity of an arene. Based upon the kinetics of electron transfer to organic chlorides and its more negative redox potential, the dianion of naphthalene appears more likely to transfer electrons to alkyl halides which are difficult to reduce (as opposed to the radical anion).…”