Paper
NJC
give Meisenheimer complex. In the second step, chlorine leaves the
Meisenheimer complex to give fluorinated product. Formation of
Meisenheimer complex is assisted by the addition of DMSO to the
3 D. Cantillo and C. O. Kappe, React. Chem. Eng., 2017, 2,
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7,28
reaction mixture.
dried and heated in the presence of dry acetonitrile to yield
It can be isolated from the reaction system,
29
fluorinated product. The higher yields of substrates with two
electron withdrawing groups is consistent with the formation of
isolable Meisenheimer complex. 4-FNB and 4-FBn were obtained in
moderate yields which is due to the presence of only one electron
withdrawing group which is not sufficient for activation of sub-
strate required for Halex reaction.
8 G. C. Finger and C. W. Kruse, J. Am. Chem. Soc., 1956, 78,
6034–6037.
3.2.1 Characterization of Meisenheimer complex. Formation
of the Meisenheimer complex was confirmed by HPLC, IR and
H and F NMR spectroscopy. The Meisenheimer complex loses
9 P. LaBeaume, M. Placzek, M. Daniels, I. Kendrick, P. Ng,
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1
19
aromatic character and behaves like an alkene which is evident
1
from the H NMR spectra. It shows one of the protons has a d value 10 C. L. Liotta and H. P. Harris, J. Am. Chem. Soc., 1974, 96,
of 6.27 ppm which is typical of alkenes with a J value of 12 Hz
2250–2252.
verifying cis coupling with adjacent proton. The F NMR spectra 11 N. F. Carvalho and J. R. Pliego, J. Org. Chem., 2016, 81,
8455–8463.
19
shows a shift in d values to À69 which is also indicative of loss of
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with lmax 362 nm and 470 nm which is consistent with the 13 A. Takashi, J. H. Clark, D. G. Cork and K. Takahide, Bull.
structure of Meisenheimer complex.
Chem. Soc. Jpn., 1986, 59, 3281–3282.
1
1
1
1
4 J. H. Clark, D. G. Cork and H. W. Gibbs, J. Chem. Soc., Perkin
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4
. Conclusion
29, 69–74.
Silica–KF for fluorination of activated aromatic substrates has
been successfully synthesized and characterized. It possesses
reactive fluoride anions on the surface of silica which aids in
fluorination of activated chlorinated substrates. Fluorination of
substrates with two activating groups undergoes via formation of
the Meisenheimer complex in good yields which corresponds to
the formation of Meisenheimer complex in the presence of this
reagent. While fluorination of substrates with only one activating
group directly gives fluorinated products in low yields.
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Conflicts of interest
The authors have no conflicts to declare.
2
Acknowledgements
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Authors are thankful to the Chemical Engineering and Chem-
istry department at ICT for XRD and SEM analyses, IIT-SAIF for
1
9
recording F NMR spectra and Dr Krishnan Ravi Kumar from
IICT, Hyderabad for performing XRD analysis. Authors are also
thankful to TEQIP-PHASE II for providing funding for the
project.
25 C. R. Ranucci, L. M. S. Colpini, M. R. Monteiro, V. Kothe,
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