Full Paper
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maintaining the regioselectivity of the uncatalysed reaction,
whereas CuII acetate seems to be a precursor of the reactive
CuI acetylide, which leads to the formation of the opposite re-
gioisomer through a productive Cu–N interaction during the
rate-determining transition state.
Experimental Section
General procedure for the Cu(OAc)2-mediated cycloaddition
of sydnones and alkynes: Synthesis of 2b
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Phenylacetylene (270 mL, 2.47 mmol) was added to a stirring solu-
tion of N-phenylsydnone 1 (200 mg, 1.23 mmol) and Cu(OAc)2
monohydrate (432 mg, 1.23 mmol) in o-dichlorobenzene (0.21–
0.25m) under N2 at room temperature. The mixture was left stirring
at room temperature for 15–20 min and afterwards, heated at
1508C for 3.5 h. After cooling, the reaction mixture was quenched
with 10% HCl (aq.) and water. The organic layer was extracted
with CH2Cl2, neutralized with saturated NaHCO3 (aq.), washed with
NaOH (1m) and brine, dried over MgSO4, and filtered. Finally, the
volatiles were removed in vacuo. The crude product was purified
by flash chromatography on silica gel (eluting solvent: 5% ethyl
acetate in 60–40 petroleum ether). 1,4-Diphenyl-1H-pyrazole 2b
was isolated as a yellow solid after recrystallisation from CH2Cl2/pe-
troleum ether (60:40) (199 mg, 73% yield, regioselectivity >95:5).
Melting point: 92–948C (lit: 95–968C4). 1H NMR (CDCl3, 400 MHz):
d=8.17 (s, 1H; PyzlH), 8.01 (s, 1H; PyzlH), 7.80–7.71 (m, 2H; PhH),
7.61–7.54 (m, 2H; PhH), 7.53–7.45 (m, 2H; PhH), 7.44–7.36 (m, 2H;
PhH), 7.36–7.27 ppm (m, 2H; PhH); 13C NMR (CDCl3, 100.6 MHz):
d=140.2, 138.9, 132.2, 129.6, 129.1, 127.0, 126.7, 125.8, 125.0,
123.4, 119.2 ppm.
[17] G. C. Kuang, P. M. Guha, W. S. Brotherton, J. T. Simmons, L. A. Stankee,
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See also reference 7 for further details on electrophilicity indexes in the
context of cycloadditions of sydnones.
[30] F. Himo, T. Lovell, R. Hilgraf, V. V. Rostovtsev, L. Noodleman, K. B. Sharp-
[31] 3D-structures have been represented with CyLview software: CYLview,
1.0b; Legault, C. Y., UniversitØ de Sherbrooke, 2009 (http://www.cylvie-
w.org).
Acknowledgements
This work was supported by The University of Sheffield and
the FP7 Marie Curie Actions of the European Commission via
the ITN ECHONET Network (MCITN-2012–316379). We also
thank SGIker (UPV/EHU) for providing computational resources.
Keywords: cycloadditions · density functional calculations ·
pyrazole · regioselective reactions · sydnones
[3] A. M. Gonzµlez-Nogal, M. Calle, P. Cuadrado, R. Valero, Tetrahedron
[7] D. L. Browne, J. F. Vivat, A. Plant, E. Gomez-Bengoa, J. P. A. Harrity, J. Am.
[9] a) A. Rodriguez, W. J. Moran, Synthesis 2009, 650–654; b) A. Rodriguez,
Received: November 17, 2014
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