a Reaction conditions: 1a (0.20 mmol, 1 equiv), 2a (0.4 mmol, 2 equiv), catalyst (0.5 euqiv), BF2 source (2 equiv), solvent (3 mL), 110oC; b Isolated yields;c The reaction was performed at 80oC; c The reaction was performed at 60 oC
With the optimized reaction conditions established, we explored the scope and generality of the present process (Table 2). At the outset, a series of ketones including aryl and alkyl ketones were examined. The electromeric effect of the substituents at the C4-position of the phenyl ring of acetophenone has negligible influence on the yield of the reaction, substrates with simple electron-donating groups substituents (eg., -Me, -OMe, -NH2) and electron-withdrawing groups (eg., -Br, -I) offer corresponding products (3ab -3ag ) in the yield of about 68%. When the substitutant was changed to bigger morpholine, the product was also synthesized in 62% (3ah ) yield respectively. Next, acetophenone with substituent at other position and multi-substituted samples were tested, and the products were obtained in the yields of 57% to 69% (3ai -3al ). When the phenyl group of 2 was changed with naphthyl, anthryl, and N-protected car-
Table 2 Scope of the reaction. a, b
a Standard conditions A : substrates1 (0.2 mmol), substrates 2 (0.4 mmol, 2.0 equiv.), Cu(BF4)·6H2O (0.1 mmol, 50 mol%), and HBF4 (0.4 mmol, 2.0 equiv.) in DCE (3.0 mL), at 110oC for 48 h. Standard conditions B :1 (0.2 mmol), 2 (0.4 mmol, 2.0 equiv.), and HBF4 (0.4 mmol, 2.0 equiv.) in DCE (3.0 mL), at 80oC for 48 h. b Isolated yields, the yields of the relevant products obtained under Condition B are marked in blue. c Absorption maxima of the complexes in DCM (2×10-5 M). d Emssion maxima of the complexes in DCM (10-7 M).
bazole, the desired products (3am -3ao ) were prepared in59%-65% yields. Except for these aromatic ketones, acetone can also react smoothly to give 3ap in 53% yield. Next, we sought to examine the limitation of quinoxalin-2(1H)-ones. In the beginning, N-benzyl and N-acetate group were compatible with the standard conditions, offering the desired products (3ea , 3fa ) in 76% and 70% respectively. Next, the N-protecting group were displaced with a range of aryl groups, the products were synthesized in 59% to 72% yields (3ba -3da ). To our delight, when the N-protecting group was removed, the reaction performed smoothly offering corresponding product in over 50% yields (3ga -3ha ). To demonstrate the efficiency of this procedure further, the reaction was carried out at 10 mmol scale, and3aa was prepared in 65% yield at the gram scale. In consideration of the cost and environment factors, the reactions were also evaluated under conditions B without Cu(BF4)2·6H2O, and the yields of the products were provided in the Table 2 marked in blue.
Just as documented, organic difluoroboron complexes display marvelous photoluminescence. When the synthesized products were put under UV light irradiation (365 nm), bright fluorescence emission from green to red were observed not only in solution but also in solid state (Figure 1a, b). Next, a series of studies were carried out to get more understanding about the photophysical properties of these compounds. According to the Uv-vis and fluorescence spectra of 3aa in multiple solvents (Figure 2c), the dichloromethane solution of3aa exhibit the highest quantum yield of up to 0.548 (using naphthacene [ΦF = 0.6 (λex = 443 nm)] as the standard).