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In the course of our studies on the reactivity of pyridinium N-aminides1 (i.e 1, Scheme 1), it has been shown that it is possible to generate the pyrazolopyridine nucleus 2 and 3 from this kind of compounds, using both a radical or a Pd-direct arylation pathway. Moreover, we will report a preliminary study of their fluorescent properties.
The slow dropwise addition (32 h, syringe pump) of a solution of azobisisobutyronitrile and tris(trimethylsilyl)silane as initiator and mediator of the radical process2 to a solution of the corresponding N-aminide, furnishes only poor yields of tricyclic derivatives 2 or 3. Better results are obtained through a Pd-direct arylation metodology, from N-aminides 4 or 5, or from N-pyridinium sals 6, having in mind the work reported by Dominguez and col.3 concerning to Heck-like process and the Fagnou’s experiments,4 and using microwave irradiation, as shown in the Table 1.
Figure 1 depicts the emission spectra of derivates 2 and 3 obtained in THF at 25ºC, upon excitation to the low energy band of the absorption spectra. The presence of an aryl containing -electron moiety on the dipyridopyrazole nucleus contributes to extending the -delocalization by stabilizing the system, and thus inducing a shift on the *- transitions to a higher wavelength. Table 2 collects the fluorescence quantum yields (f) and lifetimes (τ) for all the systems studied in THF at 25 ºC upon excitation at the selected wavelengths.
This study represents the first example of an intramolecular addition of an aryl radical to a -deficient pyridinium fragment linked to a -excessive 2-azinyliminopyridine moiety to furnish dipyridopyrazole and pyridopyrazolopyrazine derivatives 3. In all the cases, Pd-direct arylation, using microwave irradiation, furnishes better results. Our approach permits the preparation of the fully aromatic nucleus, which exhibit a high degree of conjugation and consequently possess multiple pathways for intramolecular electronic and photonic transfer and display a bright-yellow strongly fluorescent properties.
1. Nuñez, A.; Sanchez, A.; Burgos, C.; Alvarez-Builla, J. Tetrahedron2007, 63, 6774-6783.
2. Núñez, A.; Garcia de Viedma, A.; Martinez-Barrasa, V.; Burgos, C.; Alvarez-Builla, J. Synlett2002, 1093-1096.
3. Hernandez, S.; SanMartin, R.; Tellitu, I.; Domınguez, E. Org. Lett. 2003, 5, 1095-1098.
4. Campeau, L. C.; Rousseaux, S.; Fagnou, K. J. Am. Chem. Soc. 2005, 127, 18020-18021.
The authors acknowledge support for this work from the MCYT through project CTQ2005-08902/BQU and grants from the MEC (A.V. and A.V).
Abet, Valentina; Nuñez, Araceli; Mendicuti, Francisco; Burgos, Carolina and Alvarez-Builla, Julio
Departamento de Química Orgánica and Departamento de Química Fisíca, Universidad de Alcalá, 28871-Alcalá de Henares, Madrid, Spain
aTTMSS (2 equiv), AIBN (2 equiv) in MeCN/PhH are added during 32 h, K2CO3 (2 equiv) and the N-aminide (1 equiv) in MeCN, 80 ºC, 24 h.
bN-amidine (1 equiv), Pd(OAc)2 (20% mol), K2CO3 (5 equiv), LiCl (1.5 equiv), n-Bu4NBr (1 equiv) in DMF, MW (170ºC, 10 min).
cN-amidine (1 equiv), Pd(OAc)2 (20% mol), K2CO3 (5 equiv), LiCl (1.5 equiv), n-Bu4NBr (1 equiv) in DMF, MW (150ºC, 80 min).
dN- Pyridinium salt 6 (1 equiv), Pd(OAc)2 (20% mol), K2CO3 (5 equiv), LiCl (1.5 equiv), n-Bu4NBr (1 equiv) in DMF, MW (170ºC, 10 min).
a Fluorescence quantum yields (f) were performed using quinine sulphate in 0.1 M sulfuric acid as standard.