nucleophilicity of Br2, but not of [Tf2N]2, is drastically reduced by
the interaction with [bmim]+ cation.
5 D. J. Moody and A. Noel, PCT Int. Appl. (2004). WO 2004-GB2339
20040603.
6 In a typical experiment 3?1023 mol of [bmim][Tf2N] or [bmim][PF6] and
the same amount of [bmim][X] (X = Cl, Br) were mixed in a Schlenk
tube under nitrogen atmosphere and gently warmed until the
solubilization of the halogenide. To these mixtures 3?1024 mol of
diazonium salt were then added at room temperature. The mixtures
were stirred for 24 hours at 298 K. The same procedure was followed
also in the case of the mixture [emim][Tf2N]/[emim][I]. The products
were extracted with ethyl ether (4 6 3 ml), the organic layers were dried
over MgSO4 and evaporated at reduced pressure. The residues were
analyzed by NMR and GC (using a 30 m ECONOCAP EC-5 column)
after resolubilization in CH2Cl2 and addition of an appropriate amount
of PhCH2Br as internal standard. GC-MS spectra were carried out on a
30 m DB5 capillary column using an instrument equipped with an ion
trap detector. Some reaction mixtures were also analyzed directly,
avoiding the extraction procedure. In this case small quantities (50 ml) of
the raw reaction mixtures were dissolved in 0.5 ml of CH2Cl2 and
immediately analysed by GC after addition of the internal standard.
7 Slightly higher amounts of bromobenzene (around 10%) were detected
analysing directly the reaction mixture. In this case however we cannot
exclude the formation of this product after the dilution with the organic
solvent (CH2Cl2) or inside the ionization chamber.
8 S.-Z. Zhu and D. D. DesMarteau, Inorg. Chem., 1993, 32, 223.
9 In a typical experiment 3?1024 mol of diazonium salt were dissolved in
0.6 mL (3 6 1022 mol) of water containing 3?1023 mol of [bmim][Tf2N]
and the same amount of [bmim][Br]. The mixture was stirred for
24 hours at 298 K then extracted with ethyl ether (4 6 3 ml). The
organic phase was analyzed by GC after addition of PhCH2Br as
internal standard.
10 I. M. Cuccovia, M. A. da Silva, H. M. C. Ferraz, J. R. Pliego,
J. M. Riveros and J. Chaimovic, J. Chem. Soc., Perkin Trans. 2, 2000,
1896; R. U. Bryson and D. A. Singleton, J. Am. Chem. Soc., 2005, 127,
2888.
Finally, to obtain information on the ability of other
nucleophiles to compete with [Tf2N]2, we performed the reaction
+
2
of PhN2 BF4 in 1 : 1 mixtures [bmim][Cl]–[bmim][Tf2N] and
[emim][I]–[emim][Tf2N]. The two adducts incorporating the
bis(trifluoromethanesulfonyl)amide anion were the main products
of the reaction in [bmim][Cl]–[bmim][Tf2N], the amount of
chorobenzene being around 20%, whereas only iodobenzene was
detected in the reaction carried out in the presence of [emim][I].
Iodide is therefore the sole halide able to compete significantly with
[Tf2N]2. The behaviour of iodide is in agreement with the ESI-MS
data showing a reduced ability of this anion with respect to
bromide to interact with the imidazolium cation.
In conclusion, it is evident from these preliminary data that the
nucleophilicity scale of common ions in ILs towards highly reactive
short lived-intermediates, such as those characterizing the dediazo-
niation reaction, is significantly different from those determined in
molecular solvents. The reaction presently investigated may
become a probe to develop nucleophilicity scales in ILs. Finally,
the fact that bis(trifluoromethanesulfonyl)amide anion may
compete with other nucleophiles in substitution processes may be
important, because many reactions carried out in ionic liquids
might be crucially influenced by the choice of IL counteranion.
We thank MIUR (PRIN 2003035403_002) and the University
of Pisa for financial support.
11 This peculiarity of the dediazoniation process might explain the fact that
the unusual nucleophilicity of [Tf2N]2 was not previously observed in
other nucleophilic substitution reactions in ILs. L. Crowhust,
N. L. Lancaster, J. M. P. Arlandis and T. Welton, J. Am. Chem.
Soc., 2004, 126, 11549; N. L. Lancaster, T. Welton and G. B. Young,
J. Chem. Soc., Perkin Trans. 2, 2001, 2267; N. L. Lancaster, P. A. Salter,
T. Welton and G. B. Young, J. Org. Chem., 2002, 67, 8855;
N. L. Lancaster and T. Welton, J. Org. Chem., 2004, 69, 5986;
C. Chiappe and D. Pieraccini, J. Org. Chem., 2004, 69, 6059; R. Bini,
C. Chiappe, D. Pieraccini, P. Piccioli and C. S. Pomelli, Tetrahedron
Lett., 2005, 46, 6675.
Notes and references
1 Ionic Liquids in Synthesis, P. Wasserscheid and T. Welton (Eds), Wiley-
VCH, Weinheim, 2003; Ionic Liquids as Green Solvents: progress and
prospects, R. D. Rogers and K. R. Seddon (Eds), ACS Symposium
Series, Washington, 2003; Ionic Liquids IIIA: Fundamentals, Progress,
Challenges, and Opportunities, R. D. Rogers and K. R. Seddon (Eds),
ACS Symposium Series, Washington, 2005.
2 C. Chiappe and D. Pieraccini, J. Phys. Org. Chem., 2005, 18, 275.
3 H. Zollinger, Diazo Chemistry I: Aromatic and Heteroaromatic
Compounds; VCH: Weinheim, Germany, 1994; H. Zollinger, The
Chemistry of Triple Bonded Functional Groups; S. Patai and
Z. Rappoport (Eds), Wiley: Chichester, UK, 1983; C. Galli, Chem.
Rev., 1988, 88, 765.
12 A. Aggarwal, N. L. Lancaster, A. R. Sethi and T. Welton, Green Chem.,
2002, 4, 517–520.
13 F. G. Gozzo, L. S. Santos, R. Augusti, C. S. Consorti, J. Dupont and
M. N. Eberlin, Chem. Eur. J., 2004, 10, 6187.
4 K. K. Laali and V. J. Gettwert, J. Fluorine Chem., 2001, 107, 31.
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