R. Ziessel et A. El-ghayoury, Facile and multistep synthesis of functionalized nitroxide radicals containing a bipyridine frame, SYNTHESIS-S, (14), 2000, pp. 2137-2140
The synthesis of stable bipyridine substituted nitronyl-nitroxide (NIT) or
imino-nitroxide (IM) is reported. These methyl or ethynyl functionalized co
mpounds have been prepared from the bromo-substituted analogues by a carboa
lkoxylation reaction catalyzed by low valent palladium(0) using carbon mono
xide in the presence of a primary alcohol as nucleophile and a tertiary ami
ne as base. In the case of the 6,6'-dibromo-2,2'-bipyridine, a quite select
ive mono-carboethoxylation reaction is achieved under mild conditions (70 d
egreesC, 1 atmosphere CO). Stepwise reduction of the resulting esters with
sodium borohydride, followed by Swern oxidation, affords the corresponding
carbaldehydes in good yield. Furthermore, these derivatives were, used in a
multistep protocol to prepare hybrid molecules bearing both an ethynyl gro
up and a nitroxide free radical. The synthetic methods reported herein prov
ide a practical methodology to the rational design of ligands bearing diffe
rent kinds of functionalities.