Rhodium-Catalyzed Synthesis and Optical Properties of Silicon-Bridged Arylpyridines

Article abstract of DOI:10.1002/chem.201605000

A convergent and regioselective synthesis of silicon-bridged 4-arylpyridines has been developed through a rhodium-catalyzed [2+2+2] cycloaddition of silicon-containing diynes with nitriles. The absorption and emission properties of these compounds have be

Full text of DOI:10.1002/chem.201605000

A Journal of
Accepted Article
Title: Rhodium-Catalyzed Synthesis and Optical Properties of Silicon-
Bridged Arylpyridines
Authors: Ryo Shintani, Nana Misawa, Ryo Takano, and Kyoko Nozaki
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Rhodium-Catalyzed Synthesis and Optical Properties of Silicon-
Bridged Arylpyridines
Ryo Shintani,* Nana Misawa, Ryo Takano, and Kyoko Nozaki*[a]
Dedication ((optional))
Abstract: A convergent and regioselective synthesis of silicon-
bridged 4-arylpyridines has been developed through a rhodium-
catalyzed [2 + 2 + 2] cycloaddition of silicon-containing diynes with
nitriles. The absorption and emission properties of these compounds
have been examined and these optical properties could be tuned by
the substituent on the benzene ring as well as by the protonation or
alkylation of nitrogen of the pyridine ring. A catalytic asymmetric
synthesis of silicon-centered axially chiral spirocyclic derivatives has
also been achieved with high enantioselectivity by using a newly
modified MeO-mop derivative as the chiral ligand. These spirocyclic
compounds were found to be CPL active, representing the first CPL
active compounds based on the chirality at silicon.
Results and Discussion
Initially,
propynyl)phenyl)silane (1a) as a model substrate for the cationic
rhodium-catalyzed [2 2] cycloaddition with ethyl
we
employed
dimethyl(phenylethynyl)(2-(1-
+
2 +
cyanoformate (2a) in the presence of (±)-binap as the ligand in
CH2Cl2 at 40 °C (Table 1, entry 1). Although potentially two
regioisomers 3aa and 3aa’ could be obtained depending on the
orientation of 2a, the reaction selectively provided desired 3aa
exclusively in 96% yield.[9,10] The change of methyl groups on the
silicon atom of 1a to isopropyl groups (1b) also gave 3ba as the
sole product (96% yield; entry 2). Other nitriles such as benzoyl
cyanide (2b) could be employed as well (72% yield; entry 3), but
less reactive nitriles such as benzonitrile resulted in almost no
reaction. The perfect regioselectivity was maintained with
substrate 1c having 1-pentynyl group on silicon instead of
phenylethynyl group (90% yield; entry 4). Introduction of an
electron-donating group such as alkoxy and amino groups at 7-
position of compounds 3 for longer p-conjugation was also
successfully achieved as demonstrated in entries 5–7 (92–97%
Introduction
Dibenzosiloles (silicon-bridged biaryls) and related compounds
have been extensively investigated due to their unique
optoelectronic properties.[1] Substituting one or both of the
benzene rings of dibenzosiloles by heteroaromatic rings is one of
the promising ways of tuning the properties of this useful class of
compounds.[1a,2] In particular, substitution by thiophenes (giving
benzothienosiloles[3] or dithienosiloles[4]) has been most widely
explored directed toward the application to solar cells. In contrast,
introduction of other heteroaromatic rings such as pyridines
(giving benzopyridosiloles[5] or dipyridosiloles[6]) has been much
less investigated, although the electron-deficient nature of
pyridines and their nucleophilic and basic nitrogen atoms could
significantly influence the overall properties. Among them, silicon-
bridged 4-arylpyridines are particularly worth investigating due to
the effective conjugation between the silicon-bridged benzene
and pyridine rings, but only a single synthetic example has been
reported to date with no description on the physical properties.[5b]
In this context, herein we describe the development of a
convergent and regioselective synthesis of silicon-bridged 4-
Table 1. Rhodium-catalyzed asymmetric [2 + 2 + 2] cycloaddition of 1 with
2.
[RhCl(C2H4)2]2
(5 mol% Rh)
)-binap (5 mol%)
NaBArF4 (10 mol%)
R R
R
R
R1
N
(
±
R2
Si
Si
N
R2
+
R1
Me
CH2Cl2, 40 °C, 17 h
EWG
EWG
(ArF = 3,5-(CF3)2C6H3)
Me
1 (1.0 equiv)
2 (1.2 equiv)
3
Yield
[%][a]
Entry
1
2 (EWG)
Product
1
2
1a (R = Me, R1 = Ph, R2 = H)
1b (R = iPr, R1 = Ph, R2 = H)
2a (CO2Et) 3aa
2a 3ba
96
96
3
1b
2b (COPh) 3bb
72
90
97
94
92
4
1c (R = iPr, R1 = nPr, R2 = H)
1d (R = iPr, R1 = Ph, R2 = OCH2OMe)
1e (R = iPr, R1 = Ph, R2 = NMe2)
1f (R = iPr, R1 = 2-Py, R2 = NMe2)
2a
2a
2a
2a
3ca
3da
3ea
3fa
arylpyridines (9H-benzo[4,5]silolo[2,3-c]pyridines) through
a
rhodium-catalyzed [2 + 2 + 2] cycloaddition of silicon-containing
diynes with nitriles,[7] including the catalytic asymmetric synthesis
of silicon-centered axially chiral spirocyclic compounds,[8] and
investigation of their optical properties.
5
6
7[b]
[a] Isolated yield. [b] The reaction was conducted using 10 mol% of Rh/(±)-
binap/2NaBArF
.
[a]
Prof. Dr. R. Shintani, N. Misawa, R. Takano, Prof. Dr. K. Nozaki
Department of Chemistry and Biotechnology, Graduate School of
Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656 (Japan)
4
Me Me
Si
Ph
CO2Et
E-mail: shintani@chembio.t.u-tokyo.ac.jp; nozaki@chembio.t.u-
tokyo.ac.jp
N
Me
3aa'
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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4
3
2
1
0
yield). It is worth mentioning that the nucleophilicity of pyridine
nitrogen atom of 3da was exploited in the reaction with methyl
triflate to give N-methylated compound 4 in 92% yield [Eq. (1)].
compound 3ba
compound 3da
compound 3ea
compound 4
iPr iPr
iPr iPr
Ph
N
Ph
OTf
Me
Si
Si
MeOTf
+
N
O
O
(1)
MeO
MeO
CH2Cl2, rt
ε
CO2Et
CO2Et
Me
Me
4: 92% yield
3da
Having established an efficient synthesis of a series of
electronically different silicon-bridged arylpyridines 3, we
examined the optical properties of these compounds. As
summarized in Table 2, we focused on the effect of a substituent
at 7-position (3ba: R2 = H, 3da: R2 = OCH2OMe, 3ea: R2 = NMe2).
In both UV-vis absorption (Figure 1) and fluorescence spectra
(Figure 2), only a small change was observed by introduction of
methoxymethoxy group (3ba vs. 3da), although the quantum yield
of 3da (0.03) became somewhat higher than that of 3ba (<0.01).
In contrast, introduction of dimethylamino group (3ea) was quite
influential in both the absorption and emission properties. Thus,
UV-vis absorption and emission band maxima were significantly
red-shifted, and the quantum yield became much higher (0.69).
Red-shift of UV-vis absorption and emission band maxima was
also observed for N-methylated compound 4, and its quantum
yield reached as high as 0.93.[11] We also examined the effect of
acid on the fluorescence behavior of compound 3da. As shown in
Figure 3, relatively weak emission was observed at 409 nm for
compound 3da (FF = 0.03), which became weaker by adding (±)-
10-camphorsulfonic acid and a new emission peak started to grow
at 512 nm with much stronger intensity (FF = 0.46 with 100 equiv
of acid).[12] These results demonstrate that the nucleophilic and
basic nitrogen atoms of compounds 3 can actually influence the
overall optical properties.
230
280
330
380
wavelength (nm)
430
480
Figure 1. UV-vis spectra of compounds 3ba (green line; at 5.4 x 10–5 M), 3da
(yellow line; at 2.1 x 10–5 M), 3ea (orange line; at 5.2 x 10–5 M), and 4 (red line;
at 1.2 x 10–5 M) in CH2Cl2 at 25 °C.
compound 3ba
compound 3da
compound 3ea
compound 4
300
400
500
600
700
wavelength (nm)
Figure 2. Fluorescence spectra of compounds 3ba (green line; at 5.4 x 10–5 M,
l
ex = 290 nm), 3da (yellow line; at 2.1 x 10–5 M, lex = 310 nm), 3ea (orange line;
at 5.2 x 10–5 M, lex = 340 nm), and 4 (red line; at 1.2 x 10–5 M, lex = 360 nm) in
CH2Cl2 at 25 °C.
7000"
6000"
5000"
4000"
3000"
2000"
1000"
0"
Taking advantage of the present rhodium-catalyzed [2 + 2 + 2]
cycloaddition for the regioselective synthesis of silicon-bridged
arylpyridines, we decided to develop its asymmetric variant
toward the enantioselective synthesis of axially chiral 9,9’-
spirobi[benzo[4,5]silolo[2,3-c]pyridine]s, a new class of chiral
spirocyclic p-conjugated compounds. As a starting point, we
employed silicon-containing tetrayne 5a as a substrate and
conducted the reaction with nitrile 2a in the presence of 10 mol%
of cationic Rh/(R)-binap[13] catalyst in CH2Cl2 at 40 °C for 17 h
(Table 3, entry 1). Under these conditions, a mixture of partially
Table 2. Optical properties of compounds 3ba, 3da, 3ea, and 4 in CH2Cl2 at
25 °C.
UV-vis absorption
max/nm (e/104 M–1cm–1
Fluorescence
max/nm (lex/nm)
Cmpd
FF (lex/nm)
l
l
)
3ba
3da
236 (3.4), 282 (1.5)[a]
239 (3.1), 296 (1.7)[b]
410 (290)[a]
409 (310)[b]
<0.01 (290)[a]
0.03 (310)[b]
300"
350"
400"
450"
500"
550"
600"
650"
700"
750"
800"
3ea
4
339 (2.0), 363 (1.8)[c]
508 (340)[d]
481 (360)[f]
0.69 (340)[e]
0.93 (380)[g]
wavelength (nm)
270 (0.9), 308 (0.8), 378 (2.5)[f]
Figure 3. Fluorescence spectra of compound 3da (excited at 310 nm) in the
presence of (±)-10-camphorsulfonic acid (black line: 0 equiv of acid; dark gray
line: 5 equiv of acid; gray line: 10 equiv of acid; light gray line: 100 equiv of acid)
at 2.1 x 10–5 M in CH2Cl2 at 25 °C.
[a] At 5.4 x 10–5 M. [b] At 2.1 x 10–5 M. [c] At 5.2 x 10–5 M. [d] At 5.2 x 10–6
M. [e] At 1.1 x 10–5 M. [f] At 1.2 x 10–5 M. [g] At 2.0 x 10–5 M.
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10.1002/chem.201605000
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cyclized silicon-bridged arylpyridine 6aa and fully cyclized desired
7aa was obtained in 25% yield and 65% yield, respectively. The
enaniomeric excesses of 6aa and 7aa were determined to be 49%
ee (S) and 16% ee (R), indicating that kinetic resolution took place
during the second cyclization step from 6aa to 7aa, and the overall
enantioselectivity was calculated to be only 2% ee (S). By
changing the ligand to (R)-MeO-mop,[14] the reactivity was
improved to give 3% yield of 6aa and 95% yield of 7aa with 23%
overall ee (S) (entry 2). The exclusive formation of 7aa in 92%
yield was achieved with somewhat higher but still unsatisfactory
enantioselectivity (40% ee (S)) by the use of ligand (R)-L1[15]
having methyl group at the 3’-position (entry 3), which we
previously employed as an effective chiral ligand for the
in high yields with 86% ee. This asymmetric catalysis was
applicable as well to the enantioselective synthesis of axially
chiral
9,9’-spirobi[benzo[4,5]silolo[2,3-c]pyridine]-3,3’(2H,2’H)-
diones 9 by the reaction of 5c with aryl isocyanates 8.[18] As shown
in Eq. (2), compounds 9ca and 9cb were obtained with 93% ee
under similar reaction conditions. The absolute configuration of
9cb was assigned to be S by X-ray crystallographic analysis.[10]
In addition, in analogy to N-methylation of achiral 3da [Eq. (1)],
spirocyclic compound 7ba could also be dimethylated by methyl
triflate to give dicationic compound 10 in 90% yield [Eq. (3)].[19]
Although some limited examples are known for the synthesis of
enantioenriched silicon-centered spirocyclic compounds,[8,20] no
(chir)optical properties have been ever reported in the literature.
In this regard, we decided to examine the optical properties of the
newly obtained enantioenriched spirocyclic compounds.
asymmetric
synthesis
of
related
silicon-stereogenic
dibenzosiloles[16] and silicon-bridged arylpyridinones.[17] To further
improve the enantioselectivity, we prepared a new chiral ligand
having isopropyl group at the 3’-position ((R)-L2) and found that
high overall enantioselectivity of 89% ee (S) could be achieved at
the cost of reactivity (39% yield of 6aa and 41% yield of 7aa; entry
4).
Me
CO2Et
Me
Ar
N
R
1. Condition A
2. Condition B
R
R
Ar
Si
Si
Ar
R
Based on the result in Table 3, entry 4, we were able to obtain
compound 7aa from 5a and 2a in 95% yield with 90% ee by the
sequential reaction with 5 mol% of Rh/(R)-L2 catalyst at 25 °C
and then with 5 mol% of Rh/(±)-MeO-mop catalyst at 40 °C as
shown in Scheme 1. Similarly, compounds 7ba and 7ca having
methoxymethoxy and dimethylamino groups, respectively, could
also be synthesized from the corresponding compounds 5 and 2a
N
Ar
Me
CO2Et
Me
7
5 (1.0 equiv)
Condition A: [RhCl(C2H4)2]2 (5 mol% Rh), (R )-L2 (5 mol%), NaBArF
(10 mol%), 2a (2.4 equiv), CH2Cl2, 25 °C
4
Condition B: [RhCl(C2H4)2]2 (5 mol% Rh), (±)-MeO-mop (5 mol%),
NaBArF4 (10 mol%), CH2Cl2, 40 °C
Me
Si
Me
Si
CO2Et
CO2Et
Table 3. Rhodium-catalyzed asymmetric [2 + 2 + 2] cycloaddition of 5a with
2a: Ligand effect.
N
N
R
Ar
Ph
N
Me
Ar
Ph
CO2Et
Me
Ar
Me
[RhCl(C2H4)2]2
(10 mol% Rh)
ligand (10 mol%)
R
N
Ar
N
NaBArF4 (20 mol%)
CO2Et
CO2Et
Ar
N
Me
Me
Ar
+
Si
Si
Si
Ar
7aa (Ar = 4-MeOC6H4): 7ba (R = OCH2OMe): 91% yield, 86% ee
95% yield, 90% ee 7ca (R = NMe2): 90% yield, 86% ee
2a (2.4 equiv)
CH2Cl2
40 °C, 17 h
N
Ar
Me
CO2Et
CO2Et
Me
6aa
Scheme 1. Rhodium-catalyzed asymmetric synthesis of spirocyclic compounds
7.
Me
7aa
(Ar = 4-MeOC6H4)
5a (1.0 equiv)
Me
O
6aa
7aa
Yield [%][a] ee [%][b]
Overall
Entry Ligand
ee [%][c]
5c (1.0 equiv)
N
Me2N
Ar
Ph
Yield [%][a] ee [%][b]
1. Condition C
2. Condition D
(2)
+
Si
Ph
Me2N
Ar
N
C O
1
2
3
(R)-binap
(R)-MeO-mop
(R)-L1
25
3
49 (S)
98 (S)
—
65
95
92
16 (R)
2 (S)
N
Ar
8 (4.0 equiv)
21 (S) 23 (S)
40 (S) 40 (S)
79 (S) 89 (S)
O
Me
9ca (Ar = Ph): 69% yield, 93% ee
9cb (Ar = 4-BrC6H4): 49% yield, 93% ee
0
4
(R)-L2
39
>99.5 (S) 41
Condition C: [RhCl(C2H4)2]2 (7–10 mol% Rh), (R )-L2 (7–10 mol%),
NaBArF4 (14–20 mol%), CH2Cl2, 30 °C
Condition D: [RhCl(C2H4)2]2 (5–10 mol% Rh), (±)-MeO-mop (5–10 mol%),
[a] Isolated yield. [b] Determined by chiral HPLC. [c] Calculated from yields
and ee values of 6aa and 7aa.
NaBArF4 (10–20 mol%), CH2Cl2, 40 °C
R
Me
Si
Me
Si
CO2Et
N+
CO2Et
PPh2
PPh2
OMe
PPh2
N
R
R
Me
MeOTf
2
Ph
N
Ph
OTf
(3)
Ph
Ph
CH2Cl2, rt
+
(R )-binap
(R )-MeO-mop: R = H
(R )-L1: R = Me
R
R
Me
N
(R )-L2: R = iPr
CO2Et
7ba (R = OCH2OMe, 86% ee)
CO2Et
10: 90% yield
Me
Me
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Table 4. Optical properties of compounds 7aa–7ca and 10 in CH2Cl2 at 25 °C.
UV-vis absorption
max/nm (e/104 M–1cm–1
Fluorescence
max/nm (lex/nm)
Cmpd
F
F (lex/nm)
|glum| (lex/nm)
l
l
)
7aa
7ba
7ca
10
280 (4.3)[a]
244 (6.6)[b]
342 (3.6), 386 (1.9)[c]
248 (3.9), 274 (2.8), 381 (3.2)[d]
424 (280)[a]
418 (300)[b]
528 (340)[b]
498 (380)[d]
0.02 (280)[a]
0.09 (300)[b]
0.51 (340)[b]
0.75 (380)[d]
2.9 x 10–3 (280)
<0.1 x 10–3 (300)
0.6 x 10–3 (340)
0.7 x 10–3 (380)
[a] At 1.0 x 10–5 M. [b] At 1.3 x 10–5 M. [c] At 3.4 x 10–5 M. [d] At 1.2 x 10–5 M.
10
As compiled in Table 4 and Figures 4 and 5, the absorption and
compound 7aa
compound 7ba
compound 7ca
emission profiles of compounds 7aa, 7ba, 7ca, and 10 were quite
similar to those of their achiral counterparts 3ba, 3da, 3ea, and 4,
respectively, except that the absorption coefficients (e) were
almost doubled due to the existence of two silicon-bridged
compound 10
5
arylpyridine moieties in each molecule. Thus, introduction of
dimethylamino groups led to red-shift in both UV-vis absorption
ε
and emission band maxima with a higher quantum yield (7aa vs.
7ca), and a similar effect was observed by N-methylation as well
(7ba vs. 10). We also measured the circularly polarized
luminescence (CPL) of these compounds and found that 7aa
0
240
290
340 390
wavelength (nm)
440
490
showed the highest activity among them, although the glum value
Figure 4. UV-vis spectra of compounds 7aa (green line; at 1.0 x 10–5 M), 7ba
(yellow line; at 1.3 x 10–5 M), 7ca (orange line; at 3.4 x 10–5 M), and 10 (red line;
at 1.2 x 10–5 M) in CH2Cl2 at 25 °C.
(luminescence dissymmetric factor) is within a comparable range
to the ones for common CPL active small organic molecules
(Table 4 and Figure 6).[21] It is also worth noting that the N-
methylation of 7ba led to improvement of the CPL activity to some
extent (7ba vs. 10).[22] These results represent the first example
of CPL active compounds based on the chirality at silicon and
could open the door for a new molecular design to induce CPL
activity.
compound 7aa
compound 7ba
compound 7ca
compound 10
Conclusions
In summary, we have developed a convergent and regioselective
synthesis of silicon-bridged 4-arylpyridines through a rhodium-
catalyzed [2 + 2 + 2] cycloaddition of silicon-containing diynes with
nitriles and these compounds were found to be emissive. The
optical properties of these compounds can be changed by the
substituent on the benzene ring as well as by the protonation or
alkylation of nitrogen of the pyridine ring. We have also developed
a catalytic asymmetric synthesis of silicon-centered axially chiral
nitrogen-containing spirocyclic compounds, a new class of chiral
spirocyclic p-conjugated compounds, with high enantioselectivity
by using a newly modified MeO-mop derivative (R)-L2 as the
chiral ligand. These spirocyclic compounds were found to be CPL
active, representing the first CPL active compounds based on the
chirality at silicon.
350
400
450
500
550
wavelength (nm)
600
650
700
750
Figure 5. Fluorescence spectra of compounds 7aa (green line; at 1.0 x 10–5 M,
l
ex = 280 nm), 7ba (yellow line; at 1.3 x 10–5 M, lex = 300 nm), 7ca (orange line;
at 3.4 x 10–5 M, lex = 340 nm), and 10 (red line; at 1.2 x 10–5 M, lex = 380 nm)
in CH2Cl2 at 25 °C.
0
ꢀꢁꢂ
ꢃꢂꢂ
ꢃꢁꢂ
ꢁꢂꢂ ꢁꢁꢂ
wavelength (nm)
ꢄꢂꢂ
ꢄꢁꢂ
ꢅꢂꢂ
Figure 6. CPL spectrum of compound 7aa (at 1.4 x 10–4 M, lex = 280 nm) in
CH2Cl2 at 25 °C.
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10.1002/chem.201605000
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Chem. Int. Ed. 2013, 52, 1520; b) M. Murai, Y. Takeuchi, K. Yamauchi,
Y. Kuninobu, K. Takai, Chem. Eur. J. 2016, 22, 6048.
Acknowledgements
[9]
For selected recent examples of regioselective pyridine synthesis by
rhodium-catalyzed intermolecular [2 + 2 + 2] cycloaddition reactions: a)
Y. Komine, A. Kamisawa, K. Tanaka, Org. Lett. 2009, 11, 2361; b) Y.
Komine, K. Tanaka, Org. Lett. 2010, 12, 1312; c) K. Kashima, K. Teraoka,
H. Uekusa, Y. Shibata, K. Tanaka, Org. Lett. 2016, 18, 2170.
Support has been provided in part by a Grant-in-Aid for Scientific
Research (B), the Ministry of Education, Culture, Sports, Science
and Technology, Japan. We thank Dr. Jiawei Xu at JASCO
Corporation for the measurement of CPL spectra and thank Prof.
Takuzo Aida at The University of Tokyo for the measurement of
CD spectra. We thank Prof. Shigehiro Yamaguchi at Nagoya
University for helpful discussions.
[10] CCDC-1509849 and CCDC-1509850 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_ request/cif.
[11] a) H. Wang, R. Helgeson, B. Ma, F. Wudl, J. Org. Chem. 2000, 65, 5862;
b) I. Richter, M. R. Warren, J. Minari, S. A. Elfeky, W. Chen, M. F. Mahon,
P. R. Raithby, T. D. James, K. Sakurai, S. J. Teat, S. D. Bull, J. S. Fossey,
Chem. Asian J. 2009, 4, 194; c) M. T. Gabr, F. C. Pigge, RSC Adv. 2015,
5, 90226.
Keywords: rhodium • asymmetric catalysis • p-conjugated
compound • fluorescence • circularly polarized luminescence
[1]
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[22] See Supporting Information for details including their CD spectra.
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Ryo Shintani,* Nana Misawa, Ryo
Takano, Kyoko Nozaki*
Page No. – Page No.
Rhodium-Catalyzed Synthesis and
Optical Properties of Silicon-Bridged
Arylpyridines
A convergent and regioselective synthesis of silicon-bridged 4-arylpyridines has
been developed through a rhodium-catalyzed [2 + 2 + 2] cycloaddition of silicon-
containing diynes with nitriles, including the catalytic asymmetric synthesis of
silicon-centered axially chiral spirocyclic compounds, and their optical properties
have also been investigated.
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