Thermolysis and photolysis of stable Se-nitrososelenols

Article abstract of DOI:10.1246/cl.2005.654

The thermal reactivity and photoreactivity of Se-nitrososelenols have been elucidated by using stable compounds protected by bowl-type substituents. Thermolysis of Se-nitrososelenol 1 afforded diselenide 3, the structure of which was determined by X-ray c

Full text of DOI:10.1246/cl.2005.654

654
Chemistry Letters Vol.34, No.5 (2005)
Thermolysis and Photolysis of Stable Se-Nitrososelenols
Keiichi Shimada, Kei Goto,ꢀ and Takayuki Kawashimaꢀ
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received February 7, 2005; CL-050180)
The thermal reactivity and photoreactivity of Se-nitrososele-
intact even after 60 h, and the formation of the corresponding di-
sulfide, BpqSSBpq, was not observed.7 Obviously, the reactivity
of Se-nitrososelenols toward bimolecular decomposition is much
higher than that of S-nitrosothiols. It is notable that diselenide 3
has not been obtained by other methods typical for preparation of
diselenides. For example, in the oxidation of BpqSeH (5) with
hydrogen peroxide or iodine, no formation of 3 was observed
because of the steric protection effect of the Bpq group.8
These results are also indicative of the high reactivity of Se-
nitrososelenols.
nols have been elucidated by using stable compounds protected
by bowl-type substituents. Thermolysis of Se-nitrososelenol 1
afforded diselenide 3, the structure of which was determined
by X-ray crystallography, while tetraselenide 13 was obtained
as the major product in photolysis of 1.
There has been a growing interest in the chemistry of reac-
tive nitrogen species relevant to cellular signal transduction.
While S-nitrosothiols (RSNO) have been recognized to play an
important role in storing, transporting, and releasing nitric oxide
(NO),1 several recent reports suggest that the Se-nitrosated spe-
cies of selenoproteins are also involved in NO-mediated cellular
functions.2 However, Se-nitrososelenols (RSeNO) are more la-
bile even than S-nitrosothiols because of their high tendency to
undergo bimolecular decomposition, and the chemistry of this
species has been almost unexplored owing to their instability.
Recently, we succeeded in the synthesis of the first stable Se-ni-
trososelenol 1 by using a novel bowl-type steric protection group
(denoted as Bpq), and elucidated its crystal structure and some
reactivities.3 In this communication, we report the thermolysis
and photolysis of Se-nitrososelenol 1 providing the correspond-
ing diselenide and tetraselenide, respectively. The synthesis and
thermal stability of Se-nitrososelenol 2 carrying another bowl-
type substituent (denoted as Bmt) are also delineated.
rt
no reaction
CDCl , 1 week
3
BpqSeNO
80 °C
BpqSeSeBpq
1
C D , 13 h
6
6
3 (quant.)
Scheme 1.
The crystal structure of 3 is shown in Figure 1.9 The Se–Se
ꢀ
bond length is 2.3546(5) A, which is slightly longer than those of
other crowded diaryl diselenides such as Mesꢀ2Se2 (6) (2.345(2)
10
A) and (2,6-Mes2C6H4)2Se2 (7) (2.339(1) A)11 (Mesꢀ = 2,4,6-
ꢀ
ꢀ
tri-tert-butylphenyl, Mes = 2,4,6-trimethylphenyl) although it
12
ꢀ
is shorter than that of [(Me3Si)3C]2Se2 (8) (2.388(1) A). The
C–Se–Se–C torsion angle of 3 (111.76(10)ꢂ) is larger than that
of 6 (93.3(3)ꢂ)10 but smaller than those of 7 (128.3ꢂ)11 and 8
(180ꢂ).12
We previously reported the synthesis of the stable selenenic
acid, BmtSeOH (9), protected by a Bmt group.13 The stable S-
nitrosothiol carrying the Bmt group, BmtSNO (10), was recently
synthesized by Okazaki and co-workers.14 The preparation and
X
X
≡ BpqX
1 (X = SeNO)
≡ BmtX
2 (X = SeNO)
It is well-known that, unless protected by a bulky substitu-
ent, S-nitrosothiols readily decompose to disulfides and NO, es-
pecially in the case of aromatic derivatives.1,4 Se-nitrososelenols
are likely to undergo such reaction more easily because the sele-
nium–nitrogen bond of the Se–NO group is considered to be
weaker than the sulfur–nitrogen bond of the S–NO group. du
Mont and co-workers reported that even a Se-nitrososelenol car-
rying the extremely bulky (Me3Si)3C group can be observed on-
ly at ꢁ78 ꢂC, above which temperature the compound decom-
poses forming NO.5 We recently reported that Se-nitrososelenol
1 bearing a Bpq group is stable at room temperature both in the
crystalline state and in solution; no decomposition was observed
in CDCl3 after 1 week at room temperature.3 On the other hand,
1 was converted to diselenide 3 quantitatively after heated in
C6D6 at 80 ꢂC for 13 h (Scheme 1).6 While such thermal stability
of 1 is remarkable for a Se-nitrososelenol, it is much lower than
that of the corresponding S-nitrosothiol, BpqSNO (4).7 In the
thermolysis of 4 under the same conditions, 38% of 4 remained
Figure 1. ORTEP drawing of 3 (50% probability). Selected
ꢀ
bond lengths (A), bond angles (deg) and torsion angle (deg):
Se(1)–Se(2), 2.3546(5); C(1)–Se(1), 1.933(3); C(2)–Se(2),
1.942(2); C(1)–Se(1)–Se(2), 98.60(7); Se(1)–Se(2)–C(2),
101.07(7); C(1)–Se(1)–Se(2)–C(2), 111.76(10).
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.5 (2005)
655
stability of Se-nitrososelenol 2 with the same substituent were
then examined. When selenol 11 was treated with an excess
amount of tert-butyl nitrite, quantitative formation of the corre-
sponding Se-nitrososelenol 2 was observed (Scheme 2).15 In the
77Se NMR spectrum (CDCl3), 2 showed a signal at ꢀ 2125, an
extremely low-field for organoselenium compounds, similarly
to 1 (ꢀ 2229). Although Se-nitrososelenol 2 was relatively stable
in solution, 2 was gradually converted to diselenide 12 at room
temperature. Removal of the solvent from the solution contain-
ing 2 afforded a mixture of 2 and 12, and it was difficult to isolate
2 as pure specimen. These results again indicate the readiness of
the bimolecular decomposition of a Se-nitrososelenol. Similarly
to 3, diselenide 12 is difficult to prepare by the usual method; 12
was obtained only in 4% yield by oxidation of selenol 11 with
hydrogen peroxide.13 Facile formation of diselenides 3 and 12
from the corresponding Se-nitrososelenols implies that nitrosa-
tion of a selenol followed by thermolysis of the generated Se-ni-
trososelenol serves as a good method to obtain sterically hin-
dered diselenides otherwise difficult of access.
h
ν
BpqSeNO
[BpqSe ]
[BpqSeSe ]
BpqSe Bpq
4
1
16
[Se]
[Bpq ]
13
NO
BpqH
14
17
Scheme 4.
reactions including the direct observation of selanyl radical 16
are currently in progress.
This work was partly supported by Grants-in-Aid for the
Scientific Research (12042220, 14703066 (KG), and 15105001
(TK)) and for the 21st Century COE Program for Frontiers in
Fundamental Chemistry (TK) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan. We also
thank Tosoh Finechem Corporation for the generous gifts of
alkyllithiums.
References and Notes
1
For recent reviews, see: a) N. Hogg, Annu. Rev. Pharmacol.
Toxicol., 42, 585 (2002). b) M. W. Foster, T. J. McMahon,
and J. S. Stamler, Trends Mol. Med., 9, 160 (2003). c)
D. L. H. Williams, Org. Biomol. Chem., 1, 441 (2003).
a) J. E. Freedman, B. Frei, G. N. Welch, and J. Loscalzo, J. Clin.
Invest., 96, 394 (1995). b) M. Asahi, J. Fujii, K. Suzuki, H. G.
Seo, T. Kuzuya, M. Hori, M. Tada, S. Fujii, and N. Taniguchi,
J. Biol. Chem., 270, 21035 (1995). c) Y. Miyamoto, Y. H.
Koh, Y. S. Park, N. Fujiwara, H. Sakiyama, Y. Misonou, T.
Ookawara, K. Suzuki, K. Honke, and N. Taniguchi, Biol. Chem.,
384, 567 (2003), and references therein.
1 week
CDCl , rt
t-BuONO
CDCl , rt
BmtSeH
11
BmtSeNO
BmtSeSeBmt
12
3
3
2
2
(quant. in solution)
(80%)
Scheme 2.
Photolysis of a Se-nitrososelenol was investigated with the
isolable Compound 1. While 1 was insensitive to ambient room
light, irradiation of 1 in C6D6 with a 400-W high pressure mer-
cury lamp led to complete decomposition of 1 after 2.5 h at room
temperature (Scheme 3). Tetraselenide 13 and BpqH (14) were
obtained as the main products along with a trace of selenol 5
and the dibenzoselenophene derivative 15.6
3
4
5
K. Shimada, K. Goto, T. Kawashima, N. Takagi, Y.-K. Choe,
and S. Nagase, J. Am. Chem. Soc., 126, 13238 (2004).
S. Oae and K. Shinhama, Org. Prep. Proced. Int., 15, 165
(1983).
C. Wismach, W.-W. du Mont, P. G. Jones, L. Ernst, U. Papke, G.
Mugesh, W. Kaim, M. Wanner, and K. D. Becker, Angew.
Chem., Int. Ed., 43, 3970 (2004).
The new compounds 3, 13, and 15 gave satisfactory spectral
data and elemental analyses. For details, see Supporting Infor-
mation.
K. Goto, Y. Hino, Y. Takahashi, T. Kawashima, G. Yamamoto,
N. Takagi, and S. Nagase, Chem. Lett., 2001, 1204.
The corresponding selenenic acid and selenenyl iodide were ob-
tained, respectively. The details of the reaction will be described
elsewhere.
h
ν
BpqSeNO
C D , rt, 2.5 h
6
6
6
1
Se
BpqSe4Bpq
BpqH(D) BpqSeH(D)
7
8
13
(18%)*
14
5
(36%)
(trace)
15 (trace)
* 36% based on the Bpq unit
Scheme 3.
.
.
9
Crystallographic data for 3 2C6H14 C6H6: Mr ¼ 2148:92, tri-
ꢁ
clinic, space group P1, a ¼ 15:666ð4Þ, b ¼ 18:165ð4Þ, c ¼
In contrast with thermolysis of 1, formation of diselenide 3
1
ꢀ
22:937ð5Þ A,
ꢁ ¼ 91:938ð5Þꢂ,
ꢂ ¼ 91:524ð4Þꢂ,
ꢃ ¼
was not observed by H NMR monitoring during the course of
ꢀ 3
92:373ð5Þ , V ¼ 6515ð3Þ A , Z ¼ 2, Dcalcd ¼ 1:095 g cmꢁ3
,
ꢂ
the reaction. These results can be explained by the possible
mechanism shown in Scheme 4, which is based on the following
hypotheses: (i) a part of selanyl radical 16 generated by homol-
T ¼ 120 K, R1 ¼ 0:0523 (I > 2ꢄðIÞ), wR2 ¼ 0:1271 (all data).
10 P. M. Dickson, M. A. D. McGowan, B. Yearwood, M. J. Heeg,
and J. P. Oliver, J. Organomet. Chem., 588, 42 (1999).
11 J. J. Ellison, K. Ruhlandt-Senge, H. H. Hope, and P. P. Power,
Inorg. Chem., 34, 49 (1995).
12 I. Wagner, W.-W. du Mont, S. Pohl, and W. Saak, Chem. Ber.,
123, 2325 (1990).
13 K. Goto, M. Nagahama, T. Mizushima, K. Shimada, T.
Kawashima, and R. Okazaki, Org. Lett., 3, 3569 (2001).
14 M. Itoh, K. Takenaka, R. Okazaki, N. Takeda, and N. Tokitoh,
Chem. Lett., 2001, 1206. It was reported that complete
conversion of S-nitrosothiol 10 to BmtSSBmt requires heating
in refluxing benzene for 75 h.
15 2: 77Se NMR (CDCl3, 95 MHz) ꢀ 2125. UV–vis (CHCl3) ꢅmax
467 (" ca. 160) nm. IR (CHCl3) 1582 cmꢁ1 (ꢆ(N=O)). For other
spectral data, see Supporting Information.
.
ysis of the Se–N bond of 1 further decomposes to Bpq (17) with
loss of atomic selenium, which is incorporated into 16; (ii) in dis-
elenide 3 and the corresponding triselenide, BpqSe3Bpq (18), the
steric repulsion between the two Bpq groups is very strong while
it is relieved in tetraselenide 13, and if a small amount of 3 and
18 are formed during the reaction, they are readily converted to
13 under the conditions of this photoreaction. The relative yields
of 13 and 14 are consistent with this mechanism. A trace amount
of selenol 5 and dibenzoselenophene 15 are considered to be
formed by hydrogen abstraction or intramolecular cyclization
of selanyl radical 16, respectively.
Further investigations on the detailed mechanism of these
Published on the web (Advance View) April 2, 2005; DOI 10.1246/cl.2005.654