Ionic Liquids/[bmim][N3] Mixtures
CHART 1. Structures of Used Substrates
their peculiar characteristic is the ionic nature. The high number
of possible cation-anion combinations allows us to modulate
the properties of these solvents, as a function of the target
reaction. As a consequence, ILs frequently affect the outcome
of a reaction, giving yields and selectivities higher than
molecular solvents, under mild conditions too.5
A particular class of ILs is constituted by the ones whose
anion may promote the target reaction.6 Among these ILs, we
focused our attention on 1-butyl-3-methylimidazolium azide
([bmim][N3]) and its reactivity. Only few reports have consid-
ered untill now the effect that non-nucleophilic ionic liquids
may exert on the anion activation.7,8 To the best of our
knowledge, ILs such as [alkmim][N3] have been used to study
the nucleophilic substitution on some alkyl sulfonates in solution
of non-nucleophilic ILs.8 The [bmim][N3], having a reactive
anion, could promote the formation of aryl azides starting from
aryl halides. On the other hand, some recent reports have
outlined that ILs are solvent media capable of catalyzing
nucleophilic aromatic substitution (SNAr).9
their different aromatic ring structures. In particular, 6π electron
(five- and six-membered rings) and 10π electron (bicyclic ring)
derivatives were considered.
To get information about the effect of the ionic liquid
structures on this nucleophilic aromatic substitution, the 2-bromo-
5-nitrothiophene was chosen as a model substrate, and its
reactivity was analyzed in the presence of five ILs, namely,
[bmim][X] (where X ) BF4-, PF6-, and NTf2-, NTf2
)
bis(trifluoromethylsulfonylimide)), [bmpyrr][NTf2], and [bm2im]-
[NTf2] (where bmpyrr ) butyl-methylpyrrolidinium and bm2im
) 1-butyl-2,3-dimethylimidazolium). The used ILs are different
for symmetry, size, and coordination ability of the anion but
also for cation ability to give H-bond, π-π, and cation-π or
more generally ion-quadrupole type interactions.10
Finally, to have a comparison with azide anion reactivity in
conventional organic solvents, the target reaction has also been
carried out in methanol solution, using NaN3 as a nucleophile
source.
Here, we report a systematic study of reactivity of some
activated aryl or heteroaryl halides (Chart 1) in the presence of
[bmim][N3], in 1-butyl-3-methylimidazolium tetrafluoroborate
([bmim][BF4]) solution at 298 K. Substrates were chosen for
Results and Discussion
IL Effect. First, to evaluate the nucleophilicity of [bmim][N3]
under our experimental conditions, we analyzed the reactivity
of benzyl bromide. The study of this nucleophilic aliphatic
substitution allowed us to have a comparison with some
previously reported data about alkyl azide synthesis. Data show
that [bmim][N3] in [bmim][BF4] solution (yield ) 96.1%,
conversion ) 96.1%)11 is a better nucleophile than NaN3 in
MeOH solution (yield ) 39.5%, conversion ) 39.5% for the
samereactiontime).Furthermore,theyieldobtainedin[bmim][N3]/
[bmim][BF4] mixture is higher than the one previously reported
by using the [bmim][PF6]/H2O/NaN3 mixture.12
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In light of this result, we used 1 as a model substrate to study
the effect of IL nature on the target reaction. In Table 1, yield
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J. Org. Chem. Vol. 73, No. 16, 2008 6225