Electron spin resonance studies of cycles and bicycles

Abstract

The e.s.r. spectra of cyclohexylmethyl radicals have shown that two conformations, one with the CH₂^˙ group equatorial and one with the CH₂^˙ group axial, can be distinguished. The H_β h.f.s. of the axial conformer is large because rotation about the C_β—C_α^˙ bond is hindered by axial hydrogens at C(3) and C(5) in the ring. This proved to be a very useful property enabling the conformations and ring-inversion barriers of cyclohexanes, cyclohexenes and related radicals to be studied by e.s.r. spectroscopy. In addition the various conformers of larger rings (up to 15-membered) also show different spectra. Their preferred conformations and the dynamics of ‘corner migration’ in the medium rings have been investigated. The e.s.r. spectra of cycloheptenylmethyl radicals showed the presence of a minor conformer which suggested that transannular cyclisation might be important. Product analysis confirmed that bicyclo[3.2.1]octane can be obtained in good yield.

The stabilisation energy of cyclopropylmethyl radicals was determined from exchange-broadened spectra; several cyclic homoallyl-type radicals were shown to have essentially zero stabilisation.

Hydrogen abstraction from small strained bicycloalkanes, including bicyclo[n. 1. 0]alkanes, spiro[2.n]alkanes and spiro[3.n]alkanes yields the corresponding strained bicycloalkyl radicals, and their rearrangements have been followed by e.s.r. spectroscopy. Bicyclo[1. 1. 1]pentane and bicyclo[2. 1. 1]hexane are unusual in that bridgehead radicals are formed. Bicyclo[2. 2. 0]hexane also shows significant bridgehead reactivity and provides the first example of an S_H2 reaction involving a four-membered ring. Bicyclo[3. 2. 0]heptane, in which four- and five-membered rings are fused together, does not undergo this S_H2 reaction with halogens. The rates of rearrangement of several cycloalkyl and bicycloalkyl radicals have been determined by kinetic e.s.r. spectroscopy.