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Different solvent mixtures (e.g.,
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be less effective and provided poor yield (15-57%)
of product 4a. When Cs2CO3 was
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being identical. Comparable results in terms of yield and reaction
time were also obtained by employing up to 6 equiv of K2CO3.
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Similarly, the tetraazaterphenyl derivative 4o revealed an absorption at λmax = 279
(DMSO) and fluorescence emission maximum at λmax = 428
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In contrast to what was observed in
the one-pot coupling reaction of N-Boc-2,5-dibromopyrrole
with two different boronic acids (see ref. 25), in our case, the
addition up to 3 equiv of LiCl had a detrimental effect on the yield
of 4s which, indeed, dramatically decreased
to 15%. Further investigations are currently underway to
find more general conditions for effecting unsymmetrical double
SM couplings employing salt 2 or different
bifunctional derivatives and results will be reported in due course.
<A NAME="RS02911ST-27">27</A>
General Procedure
for the Synthesis of Compounds 4a-s: To
a suspension of dipotassium phenylene-1,4-bis-(trifluoroborate)
(2; 1.0 mmol) in THF-H2O
(5.0 mL + 1.0 mL), K2CO3 (3 mmol),
aryl(heteroaryl) bromide (2.1 mmol) (note: for the synthesis of
compound 4s, 1.0 mmol of both 3s and 3b were
employed instead) and PdCl2(dppf)˙CH2Cl2 (5 mol%)
were sequentially added under an argon atmosphere. After the mixture
was stirred at 50 ˚C for 24 h in a closed reactor, the
resulting solution was cooled to r.t., diluted with brine (10 mL)
and extracted with Et2O (3 × 10 mL). The solvent
was finally stripped off in vacuo and the crude product so obtained
was purified by silica gel column chromatography (see the Supporting
Information for details) to provide the desired tri(hetero)aryl
derivative.