A thorough investigation has enabled
the optimization of the synthesis
of 1,4-dihydro-pyrrolo[3,2-
b
]pyrroles. Although salts
of such metals as vanadium, niobium, cerium, and manganese were found
to facilitate the formation of 1,4-dihydro-pyrrolo[3,2-
b
]pyrroles from amines, aldehydes, and diacetyl, we confirmed that
iron salts are the most efficient catalysts. The conditions identified
(first step: toluene/AcOH = 1:1, 1 h, 50 °C; second step: toluene/AcOH
= 1:1, Fe(ClO
4
)
3
·H
2
O, 16 h,
50 °C) resulted in the formation of tetraarylpyrrolo[3,2-
b
]pyrroles in a 6–69% yield. For the first time,
very electron-rich substituents (4-Me
2
NC
6
H
4
, 3-(OH)C
6
H
4
, pyrrol-2-yl) originating
from aldehydes and sterically hindered substituents (2-ClC
6
H
4
, 2-BrC
6
H
4
, 2-CNC
6
H
4
, 2-(CO
2
Me)C
6
H
4
, 2-(TMS-C≡C)C
6
H
4
) present on anilines can be appended to the
pyrrolo[3,2-
b
]pyrrole core. It is now also possible
to prepare 1,4-dihydropyrrolo[3,2-
b
]pyrroles bearing
an ordered arrangement of
N
-substituents and
C
-substituents ranging from coumarin, quinoline, phthalimide
to truxene. These advances in scope enable independent regulations
of many desired photophysical properties, including the Stokes shift
value and emission color ranging from violet-blue through deep blue,
green, yellow to red. Simultaneously, the optimized conditions have
finally allowed the synthesis of these extremely promising heterocycles
in amounts of more than 10 g per run without a concomitant decrease
in yield or product contamination. Empowered with better functional
group compatibility, novel derivatization strategies were developed.
