Synthesis and Electronic Properties of Donor-Acceptor π-Conjugated Siloles

Article abstract of DOI:10.1021/ja049758z

An efficient method is presented for the synthesis of novel donor?acceptor silole chromophores through selective monohalogenation of 2,5-dimetallosiloles followed by Negishi alkyne cross-coupling reactions. The electronic properties and crystal packing of

Full text of DOI:10.1021/ja049758z

Published on Web 03/09/2004
Synthesis and Electronic Properties of Donor-Acceptor π-Conjugated Siloles
A. J. Boydston, Youshi Yin, and Brian L. Pagenkopf*
Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin, Austin, Texas 78712
Received January 14, 2004; E-mail: pagenkopf@mail.utexas.edu
Siloles are being increasingly recognized as promising and
desirable π-conjugated materials for potential device applications,
and important contributions have been made in the areas of electro-
Scheme 1
1
2
and photoluminescence and nitroaromatic sensors. They are also
strong candidates for NLO applications based on the theoretical
3
4
works of Matsuzaki and Bakhshi and the successes with related
5
Scheme 2
arylene materials. The ability to control the electronic nature of
the silole chromophore by varying the functional groups at the silole
termini will be critical to fine-tuning and optimizing their properties.
Access to silole systems bearing electron donating and accepting
groups peripherally at the C(2) and C(5) positions has been impeded
by the challenges of synthesizing desymmetrized siloles.
The synthesis of functionalized 2,5-silole extended chromophores
is notoriously complicated by the incompatibility of the silole ring
with traditional alkyne protecting groups. For example, the nu-
cleophilic nature of the reaction conditions present during removal
Table 1. Summary of Data for Donor-Acceptor Siloles
of conventional orthogonal alkyne protecting groups (e.g., Me
3
Si
i
vs Pr
3
Si vs Me
2
COH) successfully employed in the synthesis of
absorption
λmax (log
emission
λ
max
space
group
6
silole
donor
acceptor
ꢀ
)
(
F
Φ × 10
-2
)
a
yield
b
other acetylenic systems results in rapid desilylation of the silole
moiety.1 In pioneering investigations, Tamao prepared asymmetric
f,7
82c
65
54
62
55
58
55
56
58
54
-d
6
7
8
9
H
H
H
429 (4.74)
435 (4.33)
443 (4.33)
445 (4.51)
456 (4.44)
457 (4.44)
465 (4.25)
471 (4.11)
476 (4.53)
496 (4.37)
520 (8.97)
528 (4.06)
559 (0.70)
558 (2.57)
649 (0.44)
594 (1.80)
598 (2.12)
613 (0.11)
611 (2.94)
siloles suitable for further functionalization by monolithiation of a
symmetrical 2,5-dibromosilole, followed by electrophilic trapping.8
We sought a more direct route to obtain asymmetric siloles through
OCH3
OCH3
NH2
OCH3
NH2
NMe2
NH2
NMe2
NMe2
Pbnc
P21/c
CN
H
NO2
CN
H
NO2
CN
NO2
d
-
10
P1h
site-specific cross-coupling methodology. Utilizing ZnCl
2
as an
11
12
Pna2
1
d
oxidizing agent for residual LiNaph, an approach that has proven
advantageous in Negishi silole cross-couplings involving aryl
halides,1 provided a satisfying 84% isolated yield of 2,5-dibro-
mosilole 3 (Scheme 1).9
-
13
14
15
Pna21
P21/c
P21/c
f
e
nd
a
Determined with reference to fluorescein. b Isolated yields based on
However, our efforts to desymmetrize the dibromosilole 3 by
selective single cross-coupling reactions failed, and formation of
the undesired biscoupled product along with unreacted starting
material was the usual result. Attempts to desymmetrize the dizinc
intermediate 2 by low temperature monobromination were stymied
by formation of only the dibromosilole. We suspected that
chlorination might be superior to bromination given the greater
deactivating effect expected from a monochlorosilole on the
nucleophilicity of the second vinyl zinc. However, chlorination of
diethynylsilane 1. c Isolated yield based on dibromosilole 3. Suitable
d
crystals not yet obtained. e Photoluminescence not detected.
transmetalation, chlorination, iodination, and two sequential cross-
couplings produced asymmetric siloles 7-15 in 54-65% isolated
overall yield based on diyne 1.
These materials are the first silole-containing extended chro-
mophores bearing electronically dissimilar functional groups at C(2)
and C(5) prepared by direct cross-coupling methodology. In this
regard, a disincentive to large-scale silole synthesis by cross-
coupling methodology was its previous reliance on stoichiometric
tin reagents11 for Stille-type chemistry. Alkyne cross-couplings
2
using N-chlorosuccinimide gave fickle results with poor ratios
of mono- to dichlorosilole (1.1:1, 84% combined yield). In contrast,
successful monochlorination (16.2:1 ratio) was achieved using
N-chlorophthalimide (NCP), and the intermediate 2-chloro-5-
metallosilole 4 could be further functionalized by iodination with
12
1
3
with 2,5-dihalo siloles under related Sonogashira conditions
proved fickle in our hands, as observed by others.7 To the best
,14
2
I to afford the new chloroiodosilole 5 in 81% yield (Scheme 2).
of our knowledge, this communication constitutes the first suc-
cessful alkyne cross-couplings under Negishi conditions.
1
5
The high light sensitivity of 5 complicated its isolation and storage,
but through extensive optimization we have found in situ cross-
coupling reactions to be extraordinarily useful for asymmetric silole
synthesis. The best results were obtained by engaging the electron-
poor acceptor alkyne in the first cross-coupling event at 40 °C,
and when complete (ca. 14 h), the donor moieties were allowed to
react at 70 °C. In this way, donor-acceptor (DA) systems 7-15
have each been prepared by an operationally simple process
requiring only a single purification step (Scheme 2 and Table 1).10
Specifically, the entire sequence consisting of cycloreduction,
The electronic absorption spectra of DA siloles 7-15, along with
the parent compound 6,10 are shown in Figure 1. All of the
compounds absorb in the visible region, and absorption maxima
strongly depend on the nature of the push-pull substituents of the
chromophore. Sequentially increasing the degree of electron delo-
calization from the parent silole 6 (A ) D ) H) to the most polar
silole (15) results in a stepwise bathochromic shift in the electronic
absorption spectra from 429 to 496 nm. These results demonstrate
experimentally that silole chromophores can be electronically fine-
3724
9
J. AM. CHEM. SOC. 2004, 126, 3724-3725
10.1021/ja049758z CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
trosymmetric space groups. This is particularly evident in comparing
the amino siloles (11 and 13) with the dimethylamino counterparts
(14 and 15) in which a hydrogen bond donor is absent. Further
exploitation of hydrogen bonding directed crystal packing with other
DA systems is currently underway in our laboratory.
In summary, we have developed a new and efficient method for
the synthesis of novel donor-acceptor silole chromophores by direct
cross-coupling reactions. The ability to manipulate the electronic
and physical properties of siloles through judicious combinations
of peripheral functional groups was also demonstrated. The entire
synthetic sequence from silane 1 to DA systems is achieved
efficiently in two steps with only a single purification process. Silole
alkene cross-couplings and utilization of 5 to prepare second-
generation siloles will be described elsewhere.
Figure 1. Electronic absorption spectra of donor-acceptor siloles in
CH2Cl2.
Acknowledgment. We thank the Robert A. Welch Foundation
and donors of the American Chemical Society Petroleum Research
Fund for partial financial support of this research. We thank Dr.
Vincent Lynch for determination of crystal structures and Professors
Allen Bard, David Vanden Bout, and Grant Willson for helpful
discussions. Y.Y. thanks Pfizer for an undergraduate research
fellowship.
Supporting Information Available: Detailed experimental pro-
cedures (PDF) and characterization of all new compounds (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Photoluminescence spectra of donor-acceptor siloles in CH2Cl2.
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(
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(
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