CMDITR Review of Undergraduate Research - Pluto - University of ...

unchanged (see 21). The inhibition of the second
condensation reaction is most likely due to the
precipitation of (21) immediately after a single
benzimidazole-3-oxyl-1-oxide moiety is formed;
since the reaction of benzofuroxan with phenyl
nitrone can only occur in solution, the
precipitation of (21) would, therefore, obstruct
any subsequent reactions at the furoxan half of
this molecule.
In a separate study conducted in our lab 10 , it
was found that the use of tertiary amine
hydroxides, such as tetramethylammonium
hydroxide or tetrabutylammonium hydroxide,
allowed for solubilization of benzimidazole-3-
oxyl-1-oxides by polar solvents, such as DMF.
Therefore, the condensation of (20) with phenyl
nitrone was attempted in the presence of
tetramethylammonium hydroxide; however, it
was found that the nucleophile-susceptible bisbenzofuroxan
(and other benzofuroxan
derivatives) reacted irreversibly with the
tetramethylammonium hydroxide, thus
counteracting the purpose of the ammonium salt.
An analysis of these observations,
fortunately, revealed a possible synthetic route to
the desired radical dimer (4) via (21): depending
on the relative reactivity of the furoxan half of
(21), the benzimidazole-3-oxyl-1-oxide moiety
should, in theory, preferentially react with
tetramethylammonium hydroxide, hence
solubilizing (21) while suppressing the side
reaction of the furoxan moiety with the phase
transfer catalyst. Thus, a subsequent
condensation reaction can lead to the formation
of a bis-benzimidazole-3-oxyl-1-oxide (22) that
can be oxidized with excess lead dioxide to form
the desired “monomer” (4). The results of this
proposed synthetic route will be reported in the
future.
Conclusions
5-Thienyl derivatives of benzimidazole-3-
oxyl-1-oxides (13 and 3) can be synthesized by
transition-metal catalyzed coupling reactions in
high yield; however, 5,6-dithienyl derivatives of
benzimidazole-3-oxyl-1-oxides cannot be
synthesized via similar coupling reactions due to
steric hinderance. The characteristics of the 6-
bromo-5-(2-thienyl)–benzimidazole–3–oxyl–1-
oxide radical (13) are dominated by the bromine
atom, and therefore, the thiophene moiety of (13)
does not significantly alter the electronic
properties of this radical. The presence of a
bromine atom in (13) bathochromically shifts the
n→pi* transition of this radical, in addition to
rendering it less stable than the parent
benzimidazole-3-oxyl-1-oxide radical. This
observation is in accordance with the trend of
general loss in radical stability with an increase
in the electron deficiency of the annelated pisystem.
Dimers of benzimidazole-3-oxyl-1-oxides
cannot be synthesized by straight-forward
condensation of a benzofuroxan dimer and
phenyl nitrone and, instead, require stepwise
treatment of each half of the dimer and the use of
a phase transfer catalyst, such as
tetramethylammonium hydroxide.
Overall, the observation that benzimidazole-
3-oxyl-1-oxides cannot be functionalized with
two ortho thiophene molecules via metalcatalyzed
coupling techniques leads to the
conclusion that either alternate functionalization
methodologies need to be pursued, or that the
structure of the proposed polymer (1) needs to be
amended. Other candidates for the radical
component of (1) are also currently being
investigated to determine if a different class of
organic radicals will prove less of a synthetic
challenge than the benzonitronyl nitroxide
radicals.
20 CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004