A facile , metal-free , oxidative coupling of new 6-( hetero ) aryl-[ 1 , 2 , 5 ]-oxadiazolo [ 3 , 4-b ] pyrazines with pyrroles , indoles and carbazoles

A facile, transition metal free, one-pot oxidative coupling reaction between 6-(hetero)aryl[1,2,5]oxadiazolo[3,4-b]pyrazines and pyrroles, indoles or carbazoles is reported. This atomeconomic C-H functionalization procedure requires only stoichiometric amounts of reacting heterocycles and an appropriate Lewis acid, as catalyst. The structures of representative new 5,6di(hetero)aryl-[1,2,5]oxadiazolo[3,4-b]pyrazines were confirmed by X-ray crystallography. The redox and optical measurements for new compounds, bearing carbazole units, have also been performed.

In the crystals of 9a and 10d the conformations of these molecules are ordered by intramolecular H-bonds between NH-group of the pyrrole ring and N-atom of the pyrazine ring.As a result, pyrrole and pyrazine fragments proved to form a planar system, and the second (hetero)aryl substituent in the azine ring is turned toward the plane of the pyrazine ring.In particular, in the crystals of 10d the phenyl, pyrrole-2-yl and pyrazine moieties are placed in the plane with deviations of atoms from this plane in the limits of 0.24 Å.The thien-2-yl substituent is turned toward the pyrazine ring with the angle 56 o .The crystal packing is characterized by π-π interaction between furazano and pyrrole moieties with interatomic distance of 3.35-3.17Å.
The formation of the intramolecular H-bond in 14a is not possible due to a strong sterical interaction between the benzene ring of indole and the aryl substituent in the pyrazine ring.As a result, only weak intermolecular H-bonds between N-atoms of the pyrazine ring and H-donating groups are observed in the crystals.The presence of many intermolecular H-bonds does not permit one to assume any efficient π-π-interactions.

Optical and electrochemical properties
0][21][22][23][24][25][26] On the basis of these  2 and Figures 7 and S4 2) by 2-5 nm compared with those for the corresponding dyes in CH 3CN.For D-π-A fluorescent dyes, in general, the redshifts of absorption or fluorescence maxima by changing from solution to the solid state are quite common and explained in terms of the formation of intermolecular π-π interactions 27 or continuous intermolecular hydrogen bonding 28 in the crystalline state leading to delocalization of excitons or eximers.
The most impressive solid state fluorescence for powder of dyes 20d and 21d under daylight and UV irradiation are shown in Figure 8.In order to determine electrochemical behavior and calculate HOMO-LUMO energy gap of any organic compound, cyclic voltammetry (CV) is one of the useful methods.In the cathodic scan regime of the cyclic voltammogram of compound 20 and 21, it exhibits one characteristic irreversible reduction peak with a onset wave potential (Ered onset ) in a range from −0.70 to −0.80 V, which probably is attributed to furazano moiety reduction (Fig. S12-S19).Similarly, two consistent oxidation peaks with onset wave potential in a range from 0.90 to 0.98 V were observed for radical anion formation (Fig. S3-S10) emanating from carbazole moiety.The electrochemical HOMO-LUMO band gap of all molecules were calculated from the onset potentials of their oxidation and reduction peaks and these values are presented in Table 2.
This finding suggests that the further tuning of these compounds could lead to very narrow band gap and may be a useful strategy for the design of electron-donating (p-type) semiconductors and chromophores.

Conclusions
The SN H methodology has proven to be an effective approach to modify 6-(hetero)aryl-[1,2,5]oxadiazolo [3,4-b]pyrazines, derivatives of a highly reactive electron-deficient heterocyclic system, through nucleophilic displacement of hydrogen by aromatic C-nucleophiles, such as pyrroles, indoles or carbazoles, proceeding via the intermediacy of the C 5 -adducts.The X-ray crystallography data for a number of new 5,6-di(hetero)aryl-[1,2,5]oxadiazolo [3,4-b]pyrazines have been presented.On a base of the redox and optical studies for derivatives bearing carbazole units it is legitimate to say that these compounds can be potentially used for organic electronic applications, but additional modifications are required in order to do so.It should be noted that the N-ethyl derivatives 21 and 22 are more realistic prospects for organic electronic applications than the other products with free N-H moieties which are redox liabilities.

Experimental Section
General.All reagents and solvents were obtained from commercial sources and dried by using standard procedures before use.5-Phenyl-[1,2,5]oxadiazolo [3,4-b]pyrazine (1a) and 5-(4bromophenyl)-[1,2,5]oxadiazolo [3,4-b]pyrazine (1c) were prepared according to the earlier reported method. 10,29 1H and 13 C NMR spectra were recorded on a AVANCE-500 instruments using Me4Si as an internal standard.Elemental analysis was carried on a Eurovector EA 3000 automated analyzer.Melting points were determined on Boetius combined heating stages and were not corrected.Flash-column chromatography was carried out using Alfa Aesar silica gel 0.040-0.063mm (230-400 mesh), eluting with ethyl acetate-hexane.The progress of reactions and the purity of compounds were checked by TLC on Sorbfil plates (Russia), in which the spots were visualized with UV light (λ 254 or 365 nm).Single crystals were investigated on a Xcalibur E diffractometer on standard procedure (graphitemonochromated MoK-irradiation, ω-scans with 1 o step).Empirical absorption correction vas applied.Using SHELXTL, the structures were solved with the ShelXS structure solution program by direct methods and refined with the ShelXL refinement package using Least Squares minimization in anisotropic approximation for non-hydrogen atoms.The mean crystallographic data and results of the refinements are presented in the Table S1.The X-ray crystallography data for structures reported in this paper have been deposited with Cambridge Crystallography Data Centre as supplementary publications CCDC 1498293 for 9a, CCDC 1498294 for 10d, CCDC 1498295 for 14a, CCDC 1498296 for 21a, CCDC 1498297 for 21c and CCDC 1498298 for 22c.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Cyclic voltammetry was carried out on a Metrohm μAutolab type III potentiostat with a standard three-electrode configuration.Typically, a three electrodes cell equipped with a platinum working electrode, a Ag/AgNO3 (0.01 M AgNO3 in acetonitrile) reference electrode and a glassy carbon rod counter electrode were employed.The measurements were performed in anhydrous CH3CN solution containing the present compound (5×10 -3 M) and LiClO4 (0.1 M) as the supporting electrolyte at a scan rate of 100 mV/s.The potential of reference electrode was calibrated by using the ferrocene/ferrocenium redox couple (Fc/Fc + ), which has a known oxidation potential of +4.8 eV.The electrochemical energy gap was determined as the difference between the onsets of the oxidation and the reduction potentials (Eg elc = Eox onset -Ered onset ).The HOMO and LUMO energy values were estimated from the onset potentials of the first independent oxidation and reduction process, respectively.After calibration of the measurements against Fc/Fc + , the HOMO and LUMO energy levels were calculated according to the following equations: EHOMO (eV) = -[Eox onset -E1/2(Fc/Fc + ) + 4.8] ELUMO (eV) = -[Ered onset -E1/2(Fc/Fc + ) + 4.8] where E1/2(Fc/Fc + ) is the half-wave potential of the Fc/Fc + couple (the oxidation potential of which is assumed at 4.8 eV) against the Ag/Ag+ electrode.
UV-vis spectra were recorded for a 1×10 -5 M acetonitrile solution with Shimadzu UV-2401PC spectrophotometer.Fluorescence spectra measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer at room temperature.
Preparation of polymeric-fluorescent dye thin film.PMMA thin films were prepared as follows: 20a-d and 21a-d (0.20 mg) was dissolved in 200 μL of a PMMA-CHCl3 solution (10%, w/w).A film was obtained by spin coating a 0.2 mL PMMA-CHCl3 solution on quartz glass (size: 2.5 cm × 1.2 cm), and dried in air at room temperature.The concentration of 20a-d and 21a-d in PMMA thin film is ca 0.1%, 0.2% and 0.4% (w/w).

Figure 6 .
Figure 6.The tetrameric π-stacked "sandwiches" in the molecular packing for 22c.The interfacial distances are also given.Distances are presented in Å.
considerations, we have decided to investigate the photophysical and electrochemical properties of carbazole-substituted furazano[3,4-b]pyrazines 20a-d and 21a-d.UV-vis absorption spectra of 20a-d and 21a-d have been carried out, and the results obtained are summarized in Table .