Photosubstitution of dicyanoarenes by hypervalent allylsilicon reagents via photoinduced electron transfer

Article abstract of DOI:10.1246/cl.131132

Photosubstitution of dicyanoarenes with hypervalent allylsilicon reagents in the presence of anthracene efficiently proceeded to afford the allyl-substituted product in moderate to good yields. The efficiency for the present photoreaction was much higher

Full text of DOI:10.1246/cl.131132

Received: December 4, 2013 | Accepted: December 27, 2013 | Web Released: January 9, 2014
CL-131132
Photosubstitution of Dicyanoarenes by Hypervalent Allylsilicon Reagents
via Photoinduced Electron Transfer
Daisuke Matsuoka and Yutaka Nishigaichi*
Department of Material Science, Graduate School of Science and Engineering, Shimane University,
1060 Nishikawatsu-cho, Matsue, Shimane 690-8504
(E-mail: nishigai@riko.shimane-u.ac.jp)
Photosubstitution of dicyanoarenes with hypervalent allyl-
silicon reagents in the presence of anthracene efficiently
proceeded to afford the allyl-substituted product in moderate
to good yields. The efficiency for the present photoreaction was
much higher than that for the tetracoordinate allylsilanes,
because hypervalent allylsilicon reagents possess lower oxida-
tion potential than the tetracoordinate ones, which resulted in
more efficient photoinduced electron-transfer process.
allylbenzonitrile (3a) in a low yield (Entry 1). When polar
solvents with moderate donisity such as acetonitrile and meth-
anol were employed, the product yield was still low (Entries 2
and 3). Then, polar and highly donating solvents such as DMF
and DMSO were investigated (Entries 4 and 5). In these cases,
moderate to good yields were obtained. Among the solvents
examined, DMF was found to provide the best result, affording
3a in 69% yield (Entry 4).
Next, the effect of a photosensitizer was examined in DMF
(Table 2). When naphthalene was employed, the product 3a was
obtained in only 14% yield (Entry 1). A higher yield was
observed when pyrene or anthracene was employed, which
afforded the product in 80% or 82% yield, respectively (Entries
3 and 4). These results indicated that a photosensitizer possess-
ing comparable oxidation potential to that of 2b (+1.12 V) was
more effective in the present reaction. In contrast, photoreaction
without a photosensitizer afforded the product in very low yields
(Entry 5). Therefore, a photosensitizer plays an important role
in the present reaction. Thus, we selected anthracene as an
optimized photosensitizer.
Photoreaction is one of the important reactions employing
environmentally friendly energy source for organic synthesis.
In particular, photoinduced electron-transfer reactions involving
group 14 organometallic compounds have received considerable
attention.1-7 Allylsilicon reagents such as allyltrimethylsilane
(2a) possess relatively low oxidation potential and are known to
behave as electron donors in the photoallylation of iminium
salts,3 aromatic imides,4 unsaturated nitriles,5 and aromatic
nitriles.6 Mizuno and co-workers have reported the photo-
allylation of dicyanobenzenes with allylsilanes such as 2a,
which afforded allylbenzonitriles 3, products substituted by an
allyl group for a cyano group, in good yield. However, it
required long reaction time and large excess of the reagent
(Scheme 1).6a-6e
Recently, we reported that the photoallylation reaction of
carbonyl compounds was effectively achieved by employing a
hypervalent allylsilicon reagent,7 which was hardly achieved
using allylsilane. We anticipated that the hypervalent allylsilicon
reagents could be employed for more efficient photoallylation
owing to their lower oxidation potentials than allyltrimethyl-
silane. In this paper, we will report the photoallylation reaction
of dicyanoarenes with a hypervalent allylsilicon reagent 2b
(Scheme 1).
To demonstrate the efficiency of this photoreaction using
hypervalent allylsilicon reagent 2b, the photoreaction of 1a with
Table 1. Screening of reaction solventa
DNb
/kcal mol
ET
Yield
/%c
b
Entry Solvent
¹1
¹1
/kcal mol
1
2
3
4
5
C6H6-MeCN (4:1)
0.1d
14.1
19.0
26.6
29.8
34.3d
45.6
55.4
43.8
45.1
16
20
24
69
30
MeCN
MeOH
DMF
DMSO
aReaction conditions: a mixture of 1a (0.2 mmol), 2b (0.3
mmol), and phenanthrene (0.1 mmol) in a solvent (10 mL) was
irradiated (- > 280 nm) for 7 h under N2. bSee ref 10. cIsolated
Initially, we investigated the photoreaction of p-dicyano-
benzene (1a) with 2b in various solvents in the presence of
phenanthrene as a photosensitizer.8 The results are shown in
Table 1 along with the donor numbers and ET values of the
solvents.10 A nonpolar solvent such as benzene11 afforded 4-
d
yield. Values are for benzene alone.
Table 2. Screening of photosensitizer (sens.)a
Eox of
sens./Vb
E*ox of
Entry Photosensitizer
Yield/%c
sens./Vb
CN
1
2
3
4
5
naphthalene
phenanthrene
pyrene
anthracene
none
+1.54
+1.50
+1.16
+1.09
®
¹2.45
¹2.10
¹2.18
¹2.22
®
14
69
80
82
8
light
Si
+
NC
NC
> 280 nm
1
2
3
2a :Si = SiMe3
2b :Si =
O
O
aReaction conditions: a mixture of 1a (0.2 mmol), 2b (0.3
mmol), and a photosensitizer (0.1 mmol) in DMF (10 mL) was
irradiated (- > 280 nm) for 7 h under N2. vs. SCE in MeCN.
Si
Me4N
O
O
b
c
Scheme 1. Photosubstitution reaction of dicyanoarenes.
See ref 12. Isolated yield.
Chem. Lett. 2014, 43, 559–561 | doi:10.1246/cl.131132
© 2014 The Chemical Society of Japan | 559

*
CN
h
NC
E*ox : -2.22 V
1
Ered : -1.53 V
CN
PET
NC
Eox : +1.09 V
SET
3
NC
Si
B
– CN
CN
2
Eox : 2a = +1.64 V
2b = +1.12 V
A
Si
radical
coupling
NC
Nu
D
E
C
Scheme 2. Plausible reaction mechanism.
Table 3. Photosubstitution reaction of dicyanoarenes 1 with 2ba
Entry
1
Dicyanoarene
Products (Yield/%)b
Light, λ/nm
CN
CN
CN
CN
>280
(36)
(21)
CN
3b
1b
1c
4b
4c
NC
CN
NC
CN
NC
2
3
>280
>330
(trace)c
(71)
(40)c
3c
CN
NC
(7)
CN
CN
5d
CN
1d
1e
3d
3e
CN
4d
>400
(38)
CN
CN
aReaction conditions: a mixture of 1 (0.2 mmol), 2b (0.3 mmol), and anthracene (0.1 mmol) in DMF (10 mL) was
b
c
1
d
irradiated for 7 h under N2. Isolated yield. Yield was determined by H NMR. Reaction conditions: a mixture of 1e
(0.1 mmol) and 2b (0.15 mmol) in DMF (10 mL) was irradiated without anthracene.
allyltrimethylsilane (2a) was examined under the same reaction
conditions as shown in Table 2. In the presence of anthracene, p-
dicyanobenzene (1a) was not reacted with 2a, and most of 1a
was recovered. When phenanthrene was employed as a photo-
sensitizer in place of anthracene, allylated product 3a was
obtained in 14% yield, which was much lower than that with
2b.13 In addition, only a small amount of 1a was recovered after
the photoreaction, which indicated the decomposition of 1a
in DMF under the present conditions.14 These results showed
that the reaction efficiency of photoreaction between 1a and
allylsilane 2a in DMF was lower than that of hypervalent
allylsilicon reagent 2b.
thracene is shown in Scheme 2. This photoreaction should be
initiated by the photoinduced electron transfer (PET) from
excited anthracene to 1 to afford radical ions A and B. This
process is rationalized by the oxidation potential of excited
anthracene (¹2.22 V)12 and the reduction potential of 1
(¹1.53 V).15 Then, secondary single electron transfer (SET)
occurs from 2 to radical cation A to regenerate neutral
anthracene and radical cation C. This process is also operative
because of the comparative oxidation potentials of anthracene
(i.e., reduction potential of A) and 2b. When allylsilane 2a is
employed, this step hardly proceeds because oxidation potential
of anthracene (ground state) is much lower than that of 2a.
The radical cation C is attacked on the silicon atom by a
nucleophile7b,16,17 such as DMF to promote the C-Si bond
The plausible mechanism for the photosubstitution reaction
with hypervalent allylsilicon reagents in the presence of an-
560 | Chem. Lett. 2014, 43, 559–561 | doi:10.1246/cl.131132
© 2014 The Chemical Society of Japan

cleavage and affords allyl radical D. Then, allyl radical D and
radical anion B form the C-C bond by radical coupling to afford
anionic intermediate E. Finally, elimination of cyanide ion
proceeds to afford the allyl-substituted product 3.
Acc. Chem. Res. 2011, 44, 204.
3
4
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K. Mizuno, M. Ikeda, Y. Otsuji, Chem. Lett. 1988, 1507.
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26, 461. b) K. Mizuno, K. Terasaka, M. Ikeda, Y. Otsuji,
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The reaction procedure is described in Supporting Informa-
tion.9
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
With the optimized reaction conditions in hand, we
examined the photochemical allyl-substitution reaction between
other dicyanoarenes and allylsilicon reagent 2b (Table 3). o-
Dicyanobenzene (1b) reacted with 2b to afford 2-allylbenzo-
nitrile (3b) in 36% yield along with 4-allyl-1,2-dicyanobenzene
(4b) in 21% yield as a by-product (Entry 1). This result can be
explained by the steric hindrance of the neighboring cyano
group at the ipso attack. m-Dicyanobenzene (1c) also reacted
with 2b (Entry 2); however, the substitution product, 3-allyl-
benzonitrile (3c), was hardly obtained. Instead, 4-allyl-1,3-
dicyanobenzene (4c) that was substituted by the allyl group for
the 4-position hydrogen was obtained in 40% yield as the main
product. Polycyclic dicyanoarenes such as 1,4-dicyanonaphtha-
lene (1d) and 9,10-dicyanoanthracene (1e) also reacted with 2b
to afford 1-allyl-4-cyanonaphthalene (3d) and 9-allyl-10-cya-
noanthracene (3e) in 71% and 38% yield, respectively (Entries 3
and 4).
In summary, the photoallylation reaction (i.e., photocou-
pling reaction) of dicyanoarenes with hypervalent allylsilicon
reagent 2b was found to proceed efficiently to afford allyl-
substituted product in good yield in DMF. We consider that
single electron transfer from 2b is apt to take place because of its
lower oxidation potential than that of 2a. In addition, polycyclic
dicyanoarenes was also applicable to obtain the corresponding
allylated product in moderate to good yield. Further photo-
chemical applications of hypervalent organosilicon reagents are
now under way.
5
6
7
8
9
10 Donor numbers and ET values are quoted from the following
literatures. a) Y. Marcus, J. Solution Chem. 1984, 13, 599. b)
C. Reichardt, T. Welton, Solvents and Solvent Effects in
Organic Chemistry, 4th ed., Wiley-VCH, 2010. Also see the
references cited therein. doi:10.1002/9783527632220.
11 The allylsilicon reagent 2b was not soluble enough in
benzene alone, thus 20% of acetonitrile was used as a
cosolvent.
12 M. Montalti, A. Credi, L. Prodi, M. T. Gandolfi, Handbook
of Photochemistry, 3rd ed., Taylor & Francis, Boca Raton,
2006. Also see the references cited therein. The oxidation
potentials (V) at the excited state were calculated from the
oxidation potentials at the ground state (V) and the excitation
energies (kJ).
The authors thank Dr. Takehiro Ohta and Prof. Yoshinori
Naruta, Institute for Materials Chemistry and Engineering,
Kyushu University, for their measurement of the ESI-MS
spectrum.
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E*OX = EOX ¹ 0.0104ES
13 In the previous report by Mizuno, allylated product 3a was
obtained in good yield by phenanthrene-sensitized photo-
reaction of 1a (0.15 M) with large excess 2a in acetonitrile
for 25 h. See ref 6e.
14 p-Cyanobenzamide derivative was observed in the reaction
mixture.
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Chem. Lett. 2014, 43, 559–561 | doi:10.1246/cl.131132
© 2014 The Chemical Society of Japan | 561