Phase tag-assisted synthesis of benzo[ b ]carbazole end-capped oligothiophenes

Article abstract of DOI:10.1021/ol302748h

The introduction and removal of a phase tag have been used to trigger cyclization events in a new synthesis of benzo[b]carbazoles. The approach has been exploited in a tag-assisted approach to new benzo[b]carbazole end-capped oligothiophenes for prelimina

Full text of DOI:10.1021/ol302748h

ORGANIC
LETTERS
2012
Vol. 14, No. 22
5744–5747
Phase Tag-Assisted Synthesis
of Benzo[b]carbazole End-Capped
Oligothiophenes
Matthew T. Levick,† Susannah C. Coote,† Iain Grace,‡ Colin Lambert,‡
Michael L. Turner,*,† and David J. Procter*,†
School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL,
U.K., and Department of Physics, Lancaster University, Lancaster, LA1 4YB, U.K.
david.j.procter@manchester.ac.uk; Michael.Turner@manchester.ac.uk
Received October 5, 2012
ABSTRACT
The introduction and removal of a phase tag have been used to trigger cyclization events in a new synthesis of benzo[b]carbazoles. The approach
has been exploited in a tag-assisted approach to new benzo[b]carbazole end-capped oligothiophenes for preliminary evaluation as
semiconductors.
The development of tagging strategies1 continues to play
a key role in the quest for more efficient synthesis. The use
of a phase tag usually introduces two synthetically redun-
dant steps to a sequence: tag introduction and tag removal.
In recent years, we have explored new strategies that allow
a highersyntheticreturntobegainedfrom the unavoidable
steps during which the tag is introduced and removed.2 We
have focused on the development of processes in which tag
introduction and removal trigger key cyclization events,
with the ultimate goal of developing efficient tag-assisted
syntheses in which tag introductionꢀcyclization and tag
removalꢀcyclization operate alongside established tag-
assisted purification technologies. We recently described
the first applications of such an approach in the synthesis
of the natural product neocryptolepine2e and in the synthe-
sis of a library of spirooxindole natural product analo-
gues for biological evaluation.2f Herein, we report a phase
tag-assisted approach in which tag introduction and tag
removal trigger cyclization events en route to benzo[b]-
carbazole end caps, which have been used to prepare novel
oligothiophenes for evaluation as organic semiconductors
(Scheme 1).
Extensive effort has been invested in the discovery of
new organic semiconducting materials, and the optimiza-
tion and understanding of their performance.3 Much of
this interest has been driven by the unique advantages
offered by organic semiconductors relative to their inor-
ganic counterparts. In particular, organic semiconductors
have the potential for low-cost, large-area device fabrica-
tion on flexible substrates. Organic semiconductors can be
† University of Manchester.
‡ Lancaster University.
(1) (a) Curran, D. P. Angew. Chem., Int. Ed. 1998, 37, 1174. (b)
Yoshida, J.; Itami., K. Chem. Rev. 2002, 102, 3693.
(2) (a) McAllister, L. A.; McCormick, R. A.; Brand, S.; Procter, D. J.
Angew. Chem., Int. Ed. 2005, 44, 452. (b) McAllister, L. A.; McCormick,
R. A.; James, K. M.; Brand, S.; Willetts, N.; Procter, D. J. Chem.;Eur.
J. 2007, 13, 1032. (c) James, K. M.; Willetts, N.; Procter, D. J. Org. Lett.
2008, 10, 1203. (d) Ovens, C.; Vogel, J. C.; Martin, N. G.; Procter, D. J.
Chem. Commun. 2009, 3101. (e) Miller, M.; Vogel, J. C.; Tsang, W.;
Merrit, A.; Procter, D. J. Org. Biomol. Chem. 2009, 7, 589. (f) Coote,
S. C.; Quenum, S.; Procter, D. J. Org. Biomol. Chem. 2011, 9, 5104.
(3) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012,
112, 2208.
r
10.1021/ol302748h
Published on Web 10/30/2012
2012 American Chemical Society

FSPE involves filtration through commercial fluorous
silica gel, washing first with a fluorophobic solvent mix,
to remove untagged impurities, and then a fluorophilic
solvent mix, to isolate the tagged product. Tagged oxi-
ndoles 2a,b were prepared using a modification of our
Pummerer9 approach and were purified conveniently using
FSPE. With the nonaflate group on the aromatic ring in
amides 1a,b, good regiocontrol was observed in the Pum-
merer cyclization and 2a,b were obtained as a separable 4:1
mixture of isomers in favor of the desired isomer shown.
Subsequent oxidation and alkylation with a range of 1,2-
bromomethylbenzenes gave sulfones 3aaꢀ3bd after pur-
ification using FSPE after each stage (Scheme 2). FSPE
proved particularly valuable for the isolation of inter-
mediates 3 as they were unstable to conventional flash
chromatography.
Scheme 1. Phase Tag-Assisted Approach to End-Capped
Oligothiophenesa
a Circle represents a thiophene-based linker.
either molecular or polymeric in nature. Oligothiophenes
are the most well studied molecular materials.4 In parti-
cular, acene end-capped oligothiophenes have been shown
tobe promising organic semiconductors for applications in
organic field effect transistors (OFETs).5 Field effect
mobilities and Ion/off values up to 0.40 cm2 Vꢀ1 sꢀ1 and
1 ꢁ 105 have been reported for devices fabricated using
vacuum evaporation.5a Devices made by solution proces-
sing show slightly lower values at 3.1 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1
and 4.5 ꢁ 104, respectively.5b In this Letter we describe the
construction of novel benzo[b]carbazole end-capped oli-
gothiophenes using tag introductionꢀcyclization and tag
removalꢀcyclization strategies alongside tag-assisted pur-
ification technologies and detail their preliminary evalua-
tion as organic semiconductors.
Scheme 2. Phase Tag IntroductionꢀCyclization and Phase
Tag RemovalꢀCyclization in a Fluorous Synthesis of
Benzo[b]carbazoles
We have previously reported a tag introductionꢀ
cyclization process2a,b based on a connective Pummerer-
type reaction6 that allows a fluorous tag7 to be introduced
and a heterocycle to be constructed in a one-pot reaction.
With the fluorous tag in place, fluorous solid-phase ex-
traction (FSPE)8 can be used to purify intermediates
without recourse to conventional purification methods.
ꢀ
(4) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066.
(5) (a) Tian, H. K.; Shi, J. W.; Yan, D. H.; Wang, L. X.; Geng, Y. H.;
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Org. Lett. 2012, 14, 2316.
Pleasingly, treatment of sulfones 3aaꢀ3bd with the
electron transfer reductant samarium(II) iodide (SmI2)10
(9) For recent reviews of Pummerer chemistry, see: (a) Smith,
L. H. S.; Coote, S. C.; Sneddon, H. F.; Procter, D. J. Angew. Chem.,
Int. Ed. 2010, 49, 5832. (b) Akai, S.; Kita, Y. Top. Curr. Chem. 2007, 274,
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A. Chem. Rev. 2004, 104, 2401.
(10) For recent reviews, see: (a) Szostak, M.; Procter, D. J. Angew.
Chem., Int. Ed. 2012, 51, 9238. (b) Szostak, M.; Procter, D. J. Angew.
Chem., Int. Ed. 2011, 50, 7737. (c) Procter, D. J.; Flowers, R. A., II;
Skrydstrup, T. Organic Synthesis Using Samarium Diiodide: A Practical
Guide; RSC Publishing: Cambridge, 2010. (d) Nicolaou, K. C.; Ellery, S. P.;
Chen, J. S. Angew. Chem., Int. Ed. 2009, 48, 7140. (e) Flowers, R. A., II.
Synlett 2008, 1427. (f) Gopalaiah, K.; Kagan, H. B. New J. Chem. 2008,
32, 607. (g) Edmonds, D. J.; Johnston, D.; Procter, D. J. Chem. Rev.
(6) (a) Miller, M.; Tsang, W.; Merritt, A.; Procter, D. J. Chem.
Commun. 2007, 498. (b) Miller, M.; Vogel, J. C.; Tsang, W.; Merrit,
A.; Procter, D. J. Org. Biomol. Chem. 2009, 7, 589. (c) Ovens, C.; Martin,
N. G.; Procter, D. J. Org. Lett. 2008, 10, 1441.
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S.; Ferritto, R.; Kim, S.-Y.; Jeger, P.; Wipf, P.; Curran, D. P. Science
1997, 275, 823. For recent reviews, see: (b) Curran, D. P. In The
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Handbook of Fluorous Chemistry; Gladysz, J. A., Curran, D. P., Horvath,
I. T., Eds.; Wiley-VCH: Weinheim, 2004. (c) Zhang, W. Tetrahedron 2003,
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Org. Lett., Vol. 14, No. 22, 2012
5745

triggered tag removal11ꢀcyclization to give carbazoles
4aaꢀ4bd in good overall yield after oxidation of the crude
product mixture using benzoquinone. The tag removalꢀ
cyclization step constitutes a new approach to benzo-
[b]carbazoles and proceeds by the generation and proto-
nation of samarium(III) enolate 5,12 followed by the
formation and Barbier cyclization of benzylic organo-
samarium(III) intermediate 6.13 Oxidation of the crude
cycloadducts 7 then gave benzo[b]carbazole products
(Scheme 3).
Scheme 3. Mechanism of Phase Tag RemovalꢀCyclization: A
New Approach to Benzo[b]carbazoles
The benzo[b]carbazoles prepared by our tag-assisted
approach can be used to generate solution-processable
end-capped oligothiophenes 9aꢀj by efficient Stille cross-
coupling14 with thienyl tributyltin coupling partners 8aꢀe
(see Supporting Information for the structures of 8aꢀe and
details of their preparation) (Figure 1). The optical and
electrochemical properties of the end-capped oligothio-
phenes were examined by UVꢀvis absorption spectro-
scopy, fluorimetry, and cyclic voltammetry. Cyclic voltam-
metry was undertaken using spin coated thin films on
platinum foil as the working electrode and Ag/AgNO3 as
a reference electrode in MeCN solution containing 0.1 M
TBAPF6 as a supporting electrolyte.
The redox behavior was referenced against ferrocene
(Fc) as an internal standard, and HOMO energy levels
were then calculated using the equation EHOMO (eV) =
(ꢀ4.8 ꢀ Eonset þ E1/2[Fc]). Representative CV curves (for
9a, 9e, and 9i) are shown in Figure 2. The estimated
HOMO levels all lie in the range ꢀ5.0 to ꢀ5.4 eV, which
(11) (a) McAllister, L. A.; Brand, S.; de Gentile, R.; Procter, D. J.
Chem. Commun. 2003, 2380. (b) Turner, K. L.; Baker, T. M.; Islam, S.;
Procter, D. J.; Stefaniak, M. Org. Lett. 2006, 8, 329. (c) McAllister, L. A.;
Turner, K. L.; Brand, S.; Stefaniak, M.; Procter, D. J. J. Org. Chem.
2006, 71, 6497. (d) McKerlie, F.; Procter, D. J.; Wynne, G. Chem.
Commun. 2002, 584. (e) McKerlie, F.; Rudkin, I. M.; Wynne, G.;
Procter, D. J. Org. Biomol. Chem. 2005, 3, 2805. (f) For a review
on sulfur and selenium linkers in phase tag-assisted synthesis, see:
McAllister, L. A.; McCormick, R. A.; Procter, D. J. Tetrahedron
2005, 61, 11527.
(12) For a review of the chemistry of Sm(III) enolates, see: Rudkin,
I. M.; Miller, L. C.; Procter, D. J. Organomet. Chem. 2008, 34, 19.
(13) For a review of SmI2-mediated Barbier reactions, see: Krief, A.;
Laval, A.-M. Chem. Rev. 1999, 99, 745.
(14) Carsten, B.; He, F.; Son, H. J.; Xu, T.; Yu, L. Chem. Rev. 2011,
111, 1493.
Figure 1. Synthesis of a library of end-capped oligothiophenes.
Org. Lett., Vol. 14, No. 22, 2012
5746

Table 1. Optical and Electrochemical Properties of 9aꢀ9j
HOMO/
eVa
LUMO/
eVb
HOMO
calcdc
LUMO
calcdd
λonset
/
Eg/
eVf
nme
9a
9b
9c
9d
9e
9f
ꢀ5.27
ꢀ5.16
ꢀ5.15
ꢀ5.22
ꢀ5.05
ꢀ5.30
ꢀ5.31
ꢀ5.24
ꢀ5.04
ꢀ5.09
ꢀ2.46
ꢀ2.40
ꢀ2.39
ꢀ2.82
ꢀ2.66
ꢀ2.76
ꢀ2.64
ꢀ2.75
ꢀ2.56
ꢀ2.61
ꢀ5.60
ꢀ5.46
ꢀ5.52
ꢀ5.37
ꢀ5.26
ꢀ5.48
ꢀ5.54
ꢀ5.42
ꢀ5.31
ꢀ5.34
ꢀ1.29
ꢀ1.18
ꢀ1.25
ꢀ1.53
ꢀ1.91
ꢀ1.26
ꢀ1.14
ꢀ1.43
ꢀ1.43
ꢀ1.47
442
450
450
517
518
489
465
498
499
500
2.81
2.76
2.76
2.40
2.39
2.54
2.67
2.49
2.48
2.48
Figure 2. UVꢀvis spectra of 9a, 9e, and 9i in CH2Cl2 solution
(left) and cyclic voltammograms (right) as thin films on Pt
electrodes.
9g
9h
9i
9j
a Calculated using the equation EHOMO (eV) =(ꢀ4.8 ꢀ Eonset +E1/2[Fc]).
b Calculated using the equation ELUMO = EHOMO þ Eg. c See the
Supporting Information for calculation of the negative of the ionization
energies. d See the Supporting Information for calculation of the negative of
the electron affinities. e Measured as dilute solutions in CH2Cl2. f Calculated
from the absorption onset measured by UVꢀvis spectroscopy.
fabricated by spin coating a 0.5% w/w solution in CHCl3
onto a bare Si/SiO2 (200 nm) wafer, followed by thermal
annealing at 100 °C and evaporation of gold electrodes
through a shadow mask. Transfer and output curves for 9e
are shown in Figure 3.
Figure 3. Transfer (right) and output (left) curves for 9e.
In summary, we have developed a new synthetic ap-
proach to benzo[b]carbazoles that utilizes key cyclization
events triggered by the introduction and removal of a
phase tag, in combination with tag-assisted purification
methods. The approach has been used to generate a library
of novel benzo[b]carbazole end-capped oligothiophene
semiconductors for application in OFET devices. Work
is currently underway to further investigate and optimize
the properties and performance of these new materials.
suggests stability under ambient conditions and good
charge injection from gold (Φ = 5.1 eV). The addition of
alkyl substituents on the outermost rings has the effect of
raising the HOMO energy level, as does the incorporation
of fused units in the central linkers.
Solution UVꢀvis spectra of compounds 9a, 9e, and 9i
are also shown in Figure 2. As expected, a red shift is
observed upon extension of the central oligothiophene
linker, while the addition of alkyl substituents has neglible
impacton the appearance of thespectra. Optical band gaps
were calculated from the absorption onset and LUMO
energy levels and then were estimated using the equation
Acknowledgment. We thank the EPSRC (S.C.C. and
CASE award to M.T.L.) and SAFC Hitech (CASE award
to M.T.L.) for financial support.
Supporting Information Available. Experimental de-
1
E
LUMO = EHOMO þ Eg. The optical and electrochemical
tails for the preparation of compounds and devices, H
and 13C NMR spectra for new compounds. This material
is available free of charge via the Internet at http://pubs.
acs.org.
properties of 9aꢀj are collated in Table 1.
A preliminary investigation into the performance of
compound 9e in OFET devices has been carried out.
Charge mobilities of 0.026 ((0.003) cm2/(V s) and Ion/Ioff
3
ratios of 2 ꢁ 104 were obtained from top-contact devices
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 22, 2012
5747