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).