Nitroxide Radicals: Properties, Synthesis and Applications - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 173

5

Nitroxide Radicals: Properties,
Synthesis and Applications

Hakim Karoui, Fran¸ois Le Moigne, Olivier Ouari and Paul Tordo
Laboratoire Chimie Provence, Aix-Marseille Universite and CNRS, Marseille, France

5.1 Introduction

Since 1990, more than 4000 papers have appeared dealing with the synthesis, the physicochemical proper-
ties, and the applications of stable nitroxides and molecules composed of two or more nitroxide moieties.
Within the limited pages of this chapter, only some relevant contributions that are not tackled in the other
parts of this topic are commented on or mentioned. Although the term “stable” nitroxide is used here in the
Ingold sense, 1 the review also mentions results from the interesting branch of nitroxide chemistry relating
to the “spin trapping” method 2 that involves persistent nitroxides.
The first organic nitroxide, porphyrexide 1 (Scheme 5.1), was prepared and named by Piloty and
Schwerin in 1901, 3 and half a century after its radical character was elucidated by Holden et al . 4 by means
of Electron Paramagnetic Resonance (EPR) spectroscopy. The next important contribution to the develop-
ment of nitroxide chemistry came from Wieland et al . 5 and Meyer et al ., 6 who prepared diarylnitroxides
as shown in Scheme 5.1. The first synthesis of 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (4-oxo-TEMPO),
2 , by Lebedev et al . 7 in 1959 marked the beginning of the development of TEMPO derivatives a widely
used class of nitroxides.
Rozantsev et al . 8 greatly contributed to this development showing that nitroxides can be involved in
various organic reactions without direct involvement of the aminoxyl group. Over the period 1960 - 1980,
the fundamental chemical and physical properties of stable nitroxides were established, largely through the
pioneering research of the groups of Rozantsev 9 and Rassat. 10 Since then, the interest raised by nitroxides
has continued to grow and now they find applications in many important fields of chemical and biological
research, as illustrated by the number of topics 11 summarizing the many contributions in this area. Together
with the development of nitroxides, the interest of the scientific community in molecular magnetism and
organic magnets favored the development of nitroxide multiradicals (mainly dinitroxides) and nitronyl
nitroxides 12 (Scheme 5.2).