enylborane moiety, it did not couple with 4-bromoanisole
in the absence of a base. After completion of the initial
reaction with alkynylborate 1a, 1.0 equiv of 4-bromoiodo-
benzene and 3 equiv of NaOH were directly added to the
reaction mixture. The Suzuki-Miyaura coupling reaction
took place chemoselectively at the iodo site to give tetra-
substituted olefin 3 in 87% yield with excellent stereoselec-
tivity (E/Z ) <1/99). The bromoaryl moiety was retained in
the coupling product 3 due to the reactivity difference
between the iodo and bromo groups.6 Thus, the Pd/XANT-
PHOS catalyst was proven to be active enough to promote
the two different kinds of carbon-carbon bond-forming
reactions in one-pot without the need for any additional
catalyst or ligand. Through this sequential one-pot procedure,
the initial aryl bromide (i.e., 4-bromoanisole) grew into the
second-generation aryl bromide 3, which is expected to be
directly used in the reaction with the borate 1a again without
intervention of any activation or deprotection step.
the chain in a stepwise manner with high efficiency and
stereoselectivity by repetition of the sequential one-pot
procedure.
Monodispersed oligomers can be divergently synthesized
by repeating an iterative procedure which generally consists
of an extension step and an activation step.1,7 An extending
unit possessing a dormant coupling site is initially added to
a main chain (an extension step). The dormant site is then
activated to be subsequently coupled with another extending
unit (an activation step). On the contrary, the present method
to synthesize OPVs dispenses with the need for activation.8
One cycle that adds a vinylenephenylene unit consists of two
different extension steps, one extending a tetrasubstituted
vinylene unit and the other extending a phenylene unit. Both
extension steps can be executed by the same catalyst system
in one-pot. Thus, this simple method makes it practical to
synthesize a structurally well-defined oligomer of single
molecular weight in an efficient way.
A phenylenevinylene chain could be extended into two
directions by using 1,4-dibromobenzene as the starting aryl
bromide (Scheme 3). 1,4-Dibromobenzene was reacted with
Next, aryl bromide 3 was subjected to the second-round
sequential one-pot procedure. Treatment of 3 with alkynylborate
1a (1.1 equiv) followed by direct addition of 4-bromoiodo-
benzene (1.1 equiv) and NaOH to the reaction mixture
afforded the third-generation aryl bromide 4 in 92% yield
(Scheme 2). Further application of the third-round sequential
Scheme 3. Extension into Two Directions
Scheme 2. Repetition of the Sequential Procedure
the alkynylborates 1a (2.1 equiv) under the standard condi-
tions, and the subsequent double cross-coupling reaction with
4-bromoiodobenzene (2.2 equiv) furnished the dibromide 8
in 86% yield. The dibromide 8 was then reacted with the
alkynylborate 1a (3.0 equiv) to afford diborylated phenyle-
nevinylene intermediate, which was treated with 4-bro-
moiodobenzene (4.0 equiv) and NaOH, resulting in the
formation of OPV 9 in 92% yield. Interestingly, OPV 9
procedure to 4 furnished the fourth-generation aryl bromide
5 in 87% yield. The sequential procedure was repeated on 5
once to give the fifth-generation 6 in 82% yield and twice
to give the sixth-generation 7 in 85% yield. These oligomers
were readily soluble in common organic solvents like toluene,
THF, AcOEt, and chloroform and, therefore, were isolated
with high purity by column chromatography on silica gel
(5) Ishida, N.; Shimamoto, Y.; Murakami, M. Org. Lett. 2009, 11, 5434
(6) Unrau,; C, M.; Campbell, M. G.; Snieckus, V. Tetrahedron Lett.
1992, 33, 2773
.
.
(7) For iterative synthesis of OPVs by an extension/activation sequence,
see: (a) Schenk, R.; Gregorius, H.; Meerholz, K.; Heinze, J.; Mu¨llen, K.
J. Am. Chem. Soc. 1991, 113, 2634. (b) Xue, C.; Luo, F.-T. J. Org. Chem.
1
and identified by H and 13C NMR and mass spectroscopy.
Importantly, the yield of each round did not decrease as the
molecular size increased, suggesting that further elongation
by the iterative method would be possible. Thus, one
(tetrasubstituted vinylene)phenylene unit could be added to
2003, 68, 4417. (c) Iwadate, N.; Suginome, M. Org. Lett. 2009, 11, 1899
.
(8) For activation-free iterative synthesis of OPVs by two different
extension steps, see: (a) Maddux, T.; Li, W.; Yu, L. J. Am. Chem. Soc.
1997, 119, 844. (b) Itami, K.; Tonogaki, K.; Nokami, T.; Ohashi, Y.;
Yoshida, J. Angew. Chem., Int. Ed. 2006, 45, 2404.
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Org. Lett., Vol. 12, No. 14, 2010
Ishida, Naoki
