The First Chemical Trapping of Stibinidene, a Monovalent Antimony Compound

Article abstract of DOI:10.1246/cl.2001.42

Deselenation of 1,3,5-triselena-2,4,6-tristibane bearing 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt) groups in the presence of 1,3-butadienes resulted in the formation of the corresponding [1+4]-cycloadducts of a sterically hindered stibi

Full text of DOI:10.1246/cl.2001.42

42
Chemistry Letters 2001
The First Chemical Trapping of Stibinidene, a Monovalent Antimony Compound
Takahiro Sasamori,† Yoshimitsu Arai,†† Nobuhiro Takeda, Renji Okazaki,††,# and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
†Department of Chemistry and Physics of Condensed Matter, Graduate School of Science, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
††Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received October 24, 2000; CL-000960)
Deselenation of a 1,3,5-triselena-2,4,6-tristibane bearing
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt)
groups in the presence of 1,3-butadienes resulted in the forma-
such a [1 + 4]-cycloadduct of stibinidene 3 but resulted in almost
quantitative recovery of 1.9 These results suggest that distibene 1
is not thermally dissociated into stibinidene 3 which should be
trapped as stibolene derivatives 4 if generated. Taking account of
the above-mentioned results, the formation of 4a,b as a main
product in the deselenation reaction of 2 can be reasonably
explained by the initial formation of a intermediary stibinidene 3
and the subsequent [1 + 4]-cycloaddition reaction of 3 with buta-
dienes. It should be noted that the formation of stibolenes 4a,b
here described is the first example of the intermolecular chemical
trapping of a stibinidene derivative.
tion of the corresponding [1 + 4]-cycloadducts of a sterically
· ·
hindered stibinidene (Tbt–Sb:), a monovalent species of antimo-
ny. Thermal generation of the stibinidene from the stibolene
derivatives via retro [1 + 4]-cycloaddition reaction was suggest-
ed by the diene-exchange reaction in thermolysis of the adducts
in the presence of another butadiene derivative.
In recent decades, there has been much interest in the
chemistry of low-coordinated species of heavier group 15 ele-
ments, namely doubly bonded systems and monovalent species.
As for the double-bond compounds, several examples of stable
diphosphenes (RP=PR), diarsenes (RAs=AsR), and related
compounds have been synthesized and fully characterized1–3 by
utilizing kinetic stabilization since the first isolation of a stable
diphosphene (Mes*P=PMes*; Mes*=2,4,6-tri-t-butylphenyl).4
··
By contrast, phosphinidenes (R–P:),5 phosphorus analogues of
nitrenes, have long been postulated as reactive intermediates.
· ·
Stibinidenes (R–Sb:) are also interesting monovalent species of
group 15 elements, but no stibinidene has been detected or
trapped so far probably due to its extremely high reactivity.
On the other hand, we have recently succeeded in the synthe-
sis and characterization of the first stable distibene (TbtSb=SbTbt,
1)6 and dibismuthene (TbtBi=BiTbt),7 novel doubly bonded sys-
tems between heavier group 15 elements, by taking advantage of
an efficient steric protection group, Tbt. In the final step for the
synthesis of distibene 1, that is, deselenation reaction of
(TbtSbSe)3 (2) with hexamethylphosphorous triamide (HMPT), it
Since the stibinidene adducts 4a,b thus obtained are expected
to be good precursors of stibinidene 3, the isolated cycloadduct 4a
(22.3 mg, 0.03 mmol) was heated in the presence of 2,3-dimethyl-
1,3-butadiene (34 µL, 10 equiv) in toluene-d8 (120 °C for 20 h, in
a sealed tube) to give the diene-exchanged adduct 4b in 74%
yield. Thermolysis of 4a in the absence of such a trapping reagent
resulted in the formation of distibene 1 in 55% yield, suggesting
the dimerization of the initially formed stibinidene intermediate 3.
Also, thermolysis of 4b in toluene-d8 in the presence of an excess
amount of isoprene (120 °C for 23 h and then 130 °C for 12 h, in a
sealed tube) afforded the corresponding diene-exchanged product
4a in 78% yield. These results indicate that the stibinidene 3 was
re-generated by the thermal retrocycloaddition of stibolenes 4a,b.
The formation of distibene 1 by the thermolysis of 4a in the ab-
sence of excess dienes is reasonably interpreted in terms of the
irreversible dimerization of the stibinidene intermediate 3 leading
to the formation of extremely stable Sb=Sb double-bond species
from the equilibrated mixture of stibolene 4a and 3.
is rational to postulate the initial formation of a stibinidene
· ·
(Tbt–Sb:, 3) as an intermediate. In this paper, we wish to present
the first successful trapping reaction of the intermediary stib-
inidene 3 with isoprene or 2,3-dimethyl-1,3-butadiene and also the
re-generation of 3 by thermal retro [1 + 4]-cycloaddition of the
diene adducts.
When 2 (217 mg, 0.10 mmol) was treated with HMPT (0.18
ml, 10 equiv) in the presence of isoprene (1.0 mL, 100 equiv) in
toluene (4.5 mL) at 110 °C in a sealed tube for 22 h, a small
amount of green crystals of distibene 1 (16 mg, 26%) were precip-
itated. Filtration of the reaction mixture followed by purification
with gel permeation liquid chromatography afforded stibolene
derivative 4a8 (133 mg, 62%), i.e., the [1 + 4]-cycloaddition prod-
uct of the intermediary stibinidene 3 as colorless crystals.
Stibolene 4b8 was also obtained (65%) in a similar treatment of 2
using 2,3-dimethyl-1,3-butadiene instead of isoprene. On the
other hand, thermolysis of the isolated distibene 1 in the presence
of 2,3-dimethyl-1,3-butadiene (in toluene, at 150 °C) did not give
Previous studies on phosphinidenes and arsinidenes, i.e., the
lighter congeners of stibinidenes, have shown that they readily
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
43
undergo coordination towards some transition metals to give sta-
ble terminal pnictogenidene complexes.10 These reports prompted
us to examine the thermodynamic stabilization of stibinidene 3 by
the complexation with a transition metal. However, thermal reac-
tions of the stibinidene precursors 4a (at 100 °C for 20 h) and 4b
(at 120 °C for 18 h) with W(CO)5·thf in THF in a sealed tube
afforded no expected terminal stibinidene complex 6 but the cor-
responding η1-tungsten complexes 5a11 and 5b8 in 75 and 65%
yields, respectively.
This work was partially supported by the Grant-in-Aids for
COE Research on Elements Science (No. 12CE2005) from the
Ministry of Education, Science, Sports and Culture of Japan. T.
S. thanks Research Fellowships of the Japan Society for the
Promotion of Science for Young Scientists. We are grateful to
Shin-Etsu Chemical, and Tosoh Akzo Co., Ltds., for the gener-
ous gifts of chlorosilanes and alkyllithiums, respectively.
References and Notes
#
Present address: Department of Chemical and Biological Science,
Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai,
Bunkyo-ku, Tokyo 112-8681, Japan.
1
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(1997).
All the new products here obtained showed satisfactory spectral and
analytical data. The spectral data for 4a are shown as the representative
as follows. 4a: colorless powder, mp 123–125 °C (dec.); 1H NMR (500
MHz, CDCl3) δ 0.31 (s, 54H), 1.27 (s, 1H), 1.81 (s, 3H), 1.97 (s, 1H),
2.03 (s, 1H), 2.34–1.47 (m, 2H), 3.05–3.10 (m, 2H), 5.89 (br s, 1H),
6.26 (s, 1H), 6.40 (s, 1H); 13C NMR (125 MHz, CDCl3) δ 0.74 (q), 1.02
(q), 21.4 (q), 23.6 (t), 28.6 (t), 29.9( q), 30.8 (d), 31.0 (d), 121.8 (d),
126.7 (d), 127.4 (d), 133.3 (s), 142.0 (s), 142.7 (s), 150.5 (s), 150.7 (s).
Anal. Calcd for C32H67SbSi6·H2O: C, 50.56; H, 9.14%. Found: C,
50.61; H, 8.72%.
Distibene 1 is not photochemically dissociated to give stibinidene 3 (a
medium pressure 400-W mercury lamp, Pyrex filter, in C6D6, at 70 ºC).
By contrast, diphosphene (Mes*P=PMes*) is known to be photo-
chemically labile to generate the corresponding phosphinidene; M.
Yoshifuji, T. Sato, and N. Inamoto, Chem. Lett., 1988, 1735.
2
3
The molecular structure of complex 5a was determined
unambiguously by X-ray crystallographic analysis (Figure 1),12
which is the first structural analysis of the compound having sti-
bolene framework. Of particular note among the structural fea-
tures of 5a is the quite small C(28)–Sb–C(31) angle [83.3(5)°],
which might promote the elimination of isoprene via thermal
retrocycloaddition to give terminal stibinidene complex 6. The
other bond lengths and angles are not significantly different from
those of reported stibine–tungsten complexes,13 except for the
large C(1)–Sb–W angle [130.2(2)°] which might be caused by
steric repulsion between the extremely bulky Tbt group and
W(CO)5 moiety.
4
5
6
7
8
9
10 a) A. H. Cowley, Acc. Chem. Res., 30, 445 (1997) and references cited
therein. b) P. B. Hitchcock, M. F. Lappert, and W.-P. Leung, J. Chem.
Soc., Chem. Commun., 1987, 1282. c) A. H. Cowley, B. Pellerin, J. L.
Atwood, and S. G. Bott, J. Am. Chem. Soc., 112, 6734 (1990). d) J. B.
Bonanno, P. T. Wolczanski, and E. B. Lobkovsky, J. Am. Chem. Soc.,
116, 11159 (1994).
1
11 Spectral data for 5a: pale yellow crystals; mp 200–205 °C (dec.); H
NMR (500 MHz, CDCl3) δ 0.05 (s, 18H), 0.11 (s, 18H), 0.12 (s, 18H),
1.22 (s, 1H), 1.28 (s, 1H), 1.34 (s, 1H), 1.79 (s, 3H), 2.57–2.76 (m, 2H),
3.51–3.56 (m, 2H), 5.77 (s, 1H), 6.35 (s, 1H), 6.48 (s, 1H); 13C NMR
(126 MHz, CDCl3) δ 0.79 (q), 1.14 (q), 1.45 (q), 20.9 (d), 29.5 (t), 30.4
(q), 33.3 (d), 33.8 (d), 34.1 (t), 122.4 (d), 124.3 (d), 126.6 (s), 127.6 (d),
In expectation of trapping the terminal stibinidene complex
6, thermolysis of complex 5a was carried out in the presence of
an excess amount of 2,3-dimethyl-1,3-butadiene (130 °C for 5 h,
toluene-d8, in a sealed tube) to give a tungsten complex 5b (58%)
together with the demetallated stibolene 4b (42%). Formation of
the diene-exchanged product 5b implies the generation of an
expected complex 6, but the formation of 4b suggests another
complicated pathway, that is, the initial elimination of W(CO)5
moiety from 5a prior to the diene-exchange reaction via interme-
diary stibinidene 3 as mentioned above. Then, a part of 4b thus
generated may undergo recombination with W(CO)5 moiety to
give 5b.
1
139.5 (s), 145.2 (s), 150.0 (s), 150.1 (s), 198.0 (s, JCW=126.2 Hz),
1
199.3 (s, JCW=161.4 Hz). IR (CHCl3) 2066, 1936 cm–1 (νCO). HRMS
(FAB) m/z Obsd 1065.1753 ([M+H]+), Calcd for C37H68O5SbSi6W:
1065.2230.
12 The intensity data for 5a were collected on a Rigaku/MSC Mercury
CCD diffractometer. Crystallographic data for 5a: C37H67O5SbSi6W,
fw = 1066.05, T = 173(2) K, triclinic, P1, a = 9.6414(6) Å, b =
14.8719(14) Å, c = 17.376(3) Å, α = 90.199(4)°, β = 93.007(4)°, γ =
97.7124(19)°, V = 2465.4(5) Å3, Z = 2, Dcalc = 1.436 g cm–3, R1 =
0.065, wR2 = 0.171
13 a) M. J. Aroney, I. E. Buys, M. S. Davies, and T. W. Hambley, J.
Chem. Soc., Dalton Trans., 1994, 2827. b) A. M. Hill, N. J. Holmes, A.
R. J. Genge, W. Levason, and M. Webster, J. Chem. Soc., Dalton
Trans., 1998, 825.
Further investigation on the chemistry of low-coordinated
species of heavier group 15 elements is currently in progress.