Nevertheless, to the best of our knowledge, no examples
of a general method for the synthesis of stereodefined
(E,E)-dienes8 and (E)-enynes9 containing N-substituted
carbazolyl groups has been given so far.
synthesis of (E)-β-aryl vinyl halides from styrenes,12 we
have envisaged that the ruthenium-catalyzed (E)-selective
silylative coupling of N-vinylcarbazole with trimethylvi-
nylsilane followed by N-iodosuccinimide-mediated iodo-
desilylation could be a valuable synthetic method for the
one-pot conversion of N-vinylcarbazole into (E)-9-(2-
iodovinyl)-9H-carbazole. The resulting iodide could be
used as a coupling partner for the stereoselective synthesis
of carbazole-functionalized enynes and dienes.
The silylative coupling reaction of N-vinylcarbazole and
trimethylvinylsilane (1.2 equiv) was conducted follow-
ing the original procedure (RuHCl(CO)(PCy3)2 cata-
lyst (2 mol %), toluene, 24 h, 100 °C, sealed ampule
under Ar atmosphere)7a to give exclusively (E)-9-(2-
trimethylsilylvinyl)-9H-carbazole (GC yield 95%). Treat-
ment of (E)-9-(2-trimethylsilylvinyl)-9H-carbazole with
1.2 equiv of N-iodosuccinimide (NIS) in acetonitrile at
room temperature allowed isolation of stereochemically
pure (E)-9-(2-iodovinyl)-9H-carbazole 1 in 80% isolated
yield (GC yield 98%). To the best of our knowledge, this is
the first iododesilylation process disclosed for β-silylena-
mines.13 Thus, by sequencing the highly (E)-selective sily-
lative coupling of N-vinylcarbazole with a stereospecific
iododesilylation, the stereochemical fidelity of the product
is preserved.
The design of differently functionalized building blocks
and development of reaction sequences that would allow
other key structural units are of intense ongoing synthetic
interest. In this regard, stereodefined functionalized alke-
nyl halides suitable for cross-coupling reactions have been
among the most successfully exploited for olefin motif
elaboration. (E)-9-(2-Iodovinyl)-9H-carbazole would of-
fer a synthetically versatile variant of N-vinylcarbazole
which could be used as a coupling partner in palladium-
catalyzed cross-coupling reactions allowing regio- and
stereoselective construction of complex (E)-9-(but-1-en-
3-yn-1-yl)-9H-carbazoles and (E,E)-9-(buta-1,3-dien-1-yl)-
9H-carbazoles.
In this communication we report a one-pot synthesis of
(E)-9-(2-iodovinyl)-9H-carbazole 1 from N-vinyl-carba-
zole by silylative coupling/iododesilylation sequence and
its application as a new reagent in the palladium-catalyzed
Sonogashira and Suzuki-Miyaura cross-coupling reac-
tions (Scheme 1).
During the course of our experiments, we have found that
molecular iodine (1 equiv) in CH2Cl2 could also be employed
for the iododesilylation of (E)-9-(2-trimethylsilylvinyl)-9H-
carbazole at room temperature. However, it seemed to be less
effective than NIS and gave the product with moderate yield
and selectivity (GC yield 60%, E/Z= 8/2). In contrast, when
iodine monochloride ICl (1 equiv) in CH2Cl2 was applied, no
iodovinylcarbazole product was detected and decomposition
of (E)-9-(2-trimethylsilylvinyl)-9H-carbazole to N-vinylcar-
bazole and vinyl iodide was observed.
Scheme 1. New Synthetic Strategy to Highly π-Conjugated
N-Substituted Carbazoles
After several attempts we found that iododesilylation of
(E)-9-(2-trimethylsilylvinyl)-9H-carbazole in the presence
of NIS occurred efficiently also when the 3:1 mixture of
acetonitrile and toluene was employed as the solvent with-
out affecting either the reaction yield and stereoselectivity.
This result prompted us to attempt the iododesilyla-
tion step in one pot with silylative coupling without
further purification of the (E)-9-(2-trimethylsilylvinyl)-
9H-carbazole intermediate. In a typical procedure,
N-vinylcarbazole, trimethylvinylsilane (1:1.2 ratio), and
RuHCl(CO)(PCy3)2 catalyst (2 mol %) were dissolved in
dry toluene (0.5 M concentration) and heated under an Ar
atmosphere in a Schlenk bomb flask fitted with a plug
valve at 110 °C for 24 h. Next, after cooling the reaction
mixture to room temperature, a 3-fold excess of acetoni-
trile and 1.2 equiv of solid N-iodosuccinimide were added.
Treatment of the silylative coupling product with NIS
caused iododesilylation in a stereospecific manner, giving
In the past two decades, we have developed the silylative
coupling of olefins with vinyl-substituted organosilicon
compounds occurring in the presence of complexes con-
taining initially or generating in situ M-H and M-Si
bonds.10 The silylative coupling, in combination with
subsequent desilylation reactions such as Hiyama cross-
coupling and halodesilylation, appears to be a valuable
step to provide highly conjugated π-electron compounds
such as stilbenes, aryl-substituted polyenes, or styryl
halides.11
As we have previously reported, the silylative coupling
of N-vinylcarbazole with vinylsilanes catalyzed by a ruthe-
nium-hydride complex occurred stereoselectively to give
(E)-9-(2-silylvinyl)-9H-carbazoles in high yields.7 On the
basis of our recent results on the highly stereoselective
(9) To the best of our knowledge, only a single example of carbazole-
containing (E)-but-1-en-3-yne: (E)-9-(but-1-en-3-ynyl)-9H-carbazole
has been reported: Wang, B. Chin. J. Org. Chem. 2005, 1, 81.
(10) For recent reviews, see: (a) Marciniec, B. Acc. Chem. Res. 2007,
40, 943. (b) Marciniec, B. Coord. Chem. Rev. 2005, 249, 2374.
ꢀ
(12) Pawluc, P.; Hreczycho, G.; Szudkowska, J.; Kubicki, M.; Mar-
ciniec, B. Org. Lett. 2009, 11, 3390.
(13) No report in the literature has been found on the halodesilyla-
tion of β-nitrogen-substituted vinylsilanes except the iododesilylation of
β-silylvinyltoluenesulfonamides: Timbart, L.; Cintrat, J. Ch. Chem.;
Eur. J. 2002, 8, 1637.
ꢀ
(11) Pawluc, P.; Prukala, W.; Marciniec, B. Eur. J. Org. Chem. 2010,
2, 219.
Org. Lett., Vol. 13, No. 8, 2011
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