The Only Stable Organic Sigma Radicals: Di-tert-Alkyliminoxyls - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 231

6

The Only Stable Organic Sigma
Radicals: Di- tert -Alkyliminoxyls

Keith U. Ingold
National Research Council, Ottawa, Canada

6.1 Introduction

Radicals can be divided into classes both on the basis of the kinetics of their bimolecular self-reactions and
on the basis of their structural/electronic properties. Three kinetic classes are recognized: 1 (i) Transient rad-
icals undergo their bimolecular self-reactions at, or close to, the diffusion-controlled limit; (ii) Persistent
radicals undergo much slower bimolecular self-reactions and slow, or no, unimolecular decay reactions
(such as

-scissions); (iii) Stable radicals do not undergo such reactions at ambient temperatures (or, if
they do, the rates are negligible). The present author applies the following pragmatic test as to whether a
radical should be described as stable or persistent . Only if the radical can be isolated, handled, and stored
(for prolonged periods) as the pure compound under normal laboratory conditions, should the radical be
described as stable . This means that the adjective stable should be confined to those radicals that do not
react with oxygen or moisture and, from a practical perspective, can be “put in a bottle” and kept on a
lab bench. 1
There are two main structural/electronic classes of radicals. The vast majority of organic radicals have
their unpaired electron occupying an orbital perpendicular to the local molecular framework. Such species
are classified as

β

radicals . A few families of organic radicals have their unpaired electron occupying an
orbital lying in the plane of the local molecular framework. Such species are classified as

π

σ radicals .

radicals. Examples include, amongst many others 2 : perchloro-
triphenylmethyl; Koelsch's radical; N , N -diphenyl- N -picrylhydrazyl (DPPH • ); di- tert -alkyl nitroxides
(e.g., TEMPO, 2,2,6,6-tetramethylpiperidin-N-oxy); and pyridinyl. 3

Not surprisingly, most stable radicals are π

radicals have
been rendered persistent by steric protection of the radical center. Examples include: various phenyls 1,4 ;
various vinyls 5 ; a cyclopropyl 6 ; and various iminyls 7 ; but none of these radicals could be rendered stable .
The only stable organic

A few types of σ

radicals are certain iminoxyls, R,R C

σ =

NO • , the subject of this chapter.