Electron Spin Resonance of α‐(Alkoxycarbonyl)alkyl Radicals in Solution

Abstract

2. Experimental. -The ESR. spectra and optical arrangements were as described previously [3]. Coupling constants were determined by NMR. and are accurate to k0.02 G. g-Factors are relative to DPPH with absolute accuracies of L-1 . and relative accuracies of f4. lo-? Three conventional methods for radical generation were used: a) photochemical C1-elimination from a-chloroesters 20 vol-% in CH30H [9]. Besides R1R2C'COOR3 the spectra showed the formation of CH20H from the reaction of C1-atoms with the solvent; b) photolysis of di(t-butyl) peroxide 30 vol-% in t-butyl propionate or isobutyrates; c) photolysis of ethylene oxide solutions containing 20 vol-% di(i-butyl) peroxide, 10 vol-% triethylsilane and 1 vol-% of a-bromo-esters, particularly for studies at various temperatures.The isopropyl esters of a-chloroacetic, propionic and isobutyric acids and t-butyl isobutyrate were prepared by standard procedures to purities larger than 95% as revealed by NMR. spectra. All other chemicals were commercial samples of the purest available forms.

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“…The ESR spectra of these radicals are quite complex, because there is a hindered rotation about the C . ÀCO bond [10] which renders the two Me groups inequivalent, and because of the appreciable second-order splittings. However, for moderate resolution 2-(methoxycarbonyl)propan-2-yl (Me 2 C…”

“…4. In trace a, the decay is dominated by the second-order self-termination of the radical (t 2 500 ms), which is again slightly perturbed by a slow first-or pseudo-first-order process (t 10 14.5 ms). The latter is attributed to a minor reaction of CF 3 COC .…”

N,2,3,4‐Tetrasubstituted Pyrrolidines through Tandem Lithium Amide Conjugate Addition/Radical Cyclization/Oxygenation Reactions

“…These enolates are also efficiently oxidized, but should only form nonchelated radicals 30, since radicals with an ester group in the α position have been shown to have a significant rotational barrier, which should preserve the configuration of the precursor under the very mild reaction conditions. [39] Irrespective of the pathway by which they were generated, radicals 29 and/or 30 undergo efficient 5-exo cyclization to give diastereomeric radicals 32 and 33. The radical cyclization must be very fast, since the formation of acyclic coupling products 31, which is also fast, [40] does not compete under the reaction conditions.…”

EPR and Preparative Studies of 5-endo Cyclizations of Radicals Derived from Alkenyl NHC-Boranes Bearing tert-Butyl Ester Substituents

“…12 Furthermore, the energy barriers to internal rotation of these -carbonylalkyl radicals were all determined to be in the range 10-12 kcal/mol. [10][11][12] These structures and barrier heights show 12T and 12C to be better models for the oxaborole radicals. The comparatively large energy barriers to internal rotation, about what are formally single bonds in radicals 6, can therefore be attributed to partial -electron delocalization.…”