1,3,2,4-Diselenastannaboretane, a novel selenium-containing four-membered boracycle: Synthesis, structure and thermal cycloreversion into a selenoxoborane

Article abstract of DOI:10.1039/a806478d

Treatment of an overcrowded aryl trihydroborate bearing 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group with Cp₂TiSe₅ followed by the addition of Ar₂SnCl₂ (Ar = Ph or Mes) and Ph₃P resulted in

Full text of DOI:10.1039/a806478d

1,3,2,4-Diselenastannaboretane, a novel selenium-containing four-membered
boracycle: synthesis, structure and thermal cycloreversion into a
selenoxoborane
Mitsuhiro Ito,a Norihiro Tokitoh*b and Renji Okazakic
a Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-0033, Japan
b Institute for Fundamental Research of Organic Chemistry, Kyushu University, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan. E-mail: tokitoh@ms.ifoc.kyushu-u.ac.jp
c
Department of Chemical and Biological Sciences, Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai,
Bunkyo-ku, Tokyo 112-8681, Japan
Received (in Cambridge, UK) 17th August 1998, Accepted 6th October 1998
Treatment of an overcrowded aryl trihydroborate bearing
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group with
Cp2TiSe5 followed by the addition of Ar2SnCl2 (Ar = Ph or
Mes) and Ph3P resulted in the isolation of novel selenium-
containing four-membered boracycles, 1,3,2,4-diselenastan-
naboretanes, the thermolysis of which in the presence of
2,3-dimethyl-1,3-butadiene or 2,4,6-tri-tert-butylbenzoni-
trile oxide indicated the formation of a novel class of boron-
containing doubly bonded compound, an arylselenoxobor-
ane (Tbt)BNSe.
gave a novel five-membered boracycle, 1,2,4,3,5-triselena-
stannaborolane 3, as yellow crystals in 20% yield from 1
(Scheme 1). Although we have attempted the thermolysis of 3 in
the hope of cycloreversion into the selenoxoborane (Tbt)BNSe,
no change was observed even at 150 °C in the presence of
2,3-dimethylbuta-1,3-diene as a trapping reagent for a selenox-
oborane. We next examined ring contraction of 3 to the
1,3,2,4-diselenastannaboretane by deselenation with Ph3P. The
reaction proceeded smoothly and quantitatively at ambient
temperature to give the desired 1,3,2,4-diselenastannaboretane
4, a novel selenium-containing four-membered boracycle
(Scheme 1). Compound 4 showed satisfactory spectral and
analytical data, but single crystals suitable for X-ray crystallo-
graphic analysis were not obtained owing not only to the low
crystallinity but also to its high instability in solution. On the
other hand, 2,2-dimesityl-1,3,2,4-diselenastannaboretane 5,
which was prepared from 1 and dichlorodimesitylstannane in
24% yield by a synthetic method similar to that of 4,10 gave
single crystals suitable for X-ray crystallographic analysis upon
recrystallization from 1,2-dimethoxyethane. The crystal struc-
ture of 5 is shown in Fig. 1.†
The chemistry of boracycles containing heavier group 16
elements has been much less investigated than that of the
oxygen-containing counterpart such as boroxines [(RBO)3] and
other cyclic boronic esters owing to their high sensitivity toward
air and moisture.1 Since a boron atom is known to prefer planar-
tricoordinate geometry strongly, small ring compounds contain-
ing a boron atom generally have a large ring strain and are quite
reactive. Recently, we have reported the synthesis and struc-
tures of several stable 1,3,2,4-dithiametallaboretanes, novel
sulfur-containing four-membered boracycles, bearing a very
bulky and effective steric protecting group, 2,4,6-tris[bis-
(trimethylsilyl)methyl]phenyl (Tbt) group.2 We have also
described that one of those four-membered ring systems, i.e. a
1,3,2,4-dithiastannaboretane derivative, provides us with a new
and facile route to the boron–oxygen and boron–sulfur double-
bond compounds (oxoborane and thioxoborane, respectively),
the synthetic methods to which have, so far, not been well
established.3,4 By contrast, until now there have been very few
reports on selenium-containing organoboranes.5
The four-membered ring of 5 is not completely planar with
the dihedral angle between planes B(1)–Se(1)–Se(2) and Sn(1)–
Se(1)–Se(2) being 13.8° due to the steric repulsion between the
Tbt group and one of the mesityl group on the tin atom. The
geometry around the central boron atom is found to be perfectly
trigonal planar (S•B = 360°). The angle between the plane
Se
Ar
Tbt
B
(Tbt)B(OMe)2
Here, we report the synthesis and crystal structure of a
kinetically stabilized 1,3,2,4-diselenastannaboretane bearing
the Tbt group, a novel selenium-containing four-membered
boracycle, together with its thermolysis leading to the formation
of an arylselenoxoborane (Tbt)BNSe. Oxoboranes and thioxo-
boranes are known to be important intermediates in the
oxidation or sulfurization of elemental boron or other boron
compounds and are widely studied from the viewpoints of
theoretical and/or gas-phase chemistry.6,7 On the other hand,
selenoxoboranes, selenium analogues of oxoboranes and
thioxoboranes, are thought to be much less stable than
oxoboranes and thioxoboranes, and there has been only one
report on the gas-phase detection of a chloroselenoxoborane in
the reaction of elemental boron with Se2Cl2 at 1000 °C.8
Although the synthesis and structure of 1,3,2,4-diselenadibor-
etane, a dimer of a selenoxoborane, has already been reported
by Nöth and coworkers, no description was given for the
intermediary selenoxoborane.9
Sn
1
Se
Ar
4 (Ar = Ph, quant. from 3)
5 (Ar = Mes, 24% from 1)
LiAlH4
THF
Ph3P – Ph3P=Se
Ar
Ar
Se
1. Cp2TiSe5
2. Ar2SnCl2
Tbt
B
Sn
(Tbt)BH3Li(THF)3
Se Se
2
3 (Ar = Ph, 20% from 1)
H
H
Me3Si
Me3Si
SiMe3
SiMe3
Tbt
SiMe3
SiMe3
The reaction of an overcrowded trihydroborate 2, synthesized
by the reaction of (Tbt)B(OMe)2 1 with LiAlH4,2 with
titanocene pentaselenide followed by treatment with Ph2SnCl2
H
Scheme 1
Chem. Commun., 1998, 2495–2496
2495

selenoxoborane has a double-bond character like oxoborane and
thioxoborane previously reported.3,4
In summary, we have succeeded in the isolation and
crystallographic analysis of the first 1,3,2,4-diselenastannabor-
etane 4. Compound 4 undergoes thermal cycloreversion into the
overcrowded selenoxoborane (Tbt)BNSe, the formation of
which was confirmed by the intermolecular [4+2]cycloaddition
reaction with a diene.
This work was partly supported by Grants-in-Aid for
Scientific Research (No. 09239208 and 09440216) from the
Ministry of Education, Science, Sports, and Culture, Japan.
M. I. thanks Research Fellowships of the Japan Society for the
Promotion of Science for Young Scientists. We also thank Shin-
etsu Chemical and Tosoh Akzo Co., Ltds. for the generous gift
of chlorosilanes and alkyllithiums, respectively.
Notes and references
Fig. 1 ORTEP drawing of 2,2-dimesityl-1,3,2,4-diselenastannaboretane 5
with thermal ellipsoid plots (40% probability). Selected bond lengths (Å)
and angles (°). B(1)–C(1) 1.557(13), B(1)–Se(1) 1.962(11), B(1)–Se(2)
1.960(10), Se(1)–Sn(1) 2.557(1), Se(2)–Sn(1) 2.566(1), Sn(1)-C(28)
2.149(10), Sn(1)-C(37) 2.127(10), Se(1)–B(1)–Se(2) 114.1(5), Se(1)–B(1)–
C(1) 123.2(7), Se(2)–B(1)–C(1) 122.7(8), B(1)–Se(1)–Sn(1) 82.4(3), B(1)–
Se(2)–Sn(1) 82.2(4), Se(1)–Sn(1)–Se(2) 79.95(4), C(28)-Sn(1)–C(37)
117.3(3).
† Crystallographic data for 5: C45H81BSe2Si6Sn, M = 1087.07, mono-
clinic, space group P21/n, a = 21.053(5), b = 13.324(4), c = 21.341(5) Å,
b = 91.22(2)°, V = 5634(2) Å3, Z = 4, Dc = 1.271 g cm23, T = 173 K,
F(000) = 2232.00, yellow prism with dimensions 0.65 3 0.40 3 0.10 mm,
m(Mo-Ka) = 19.00 cm21, R(Rw) = 0.069(0.075). The intensity data (2q <
55°) for 1 were collected on a Rigaku AFC7R diffractometer with graphite
monochromated Mo-Ka radiation (l = 0.71069 Å), and 13870 reflections
(13517 unique) were measured. The structure of 1 was solved by direct
methods with SHELXS-86,11 expanded using Fourier techniques,12 and
refined by the full-matrix least-squares methods using the TEXSAN
crystallographic software package.13 All the non-hydrogen atoms were
refined anisotropically, while the hydrogen atoms were located in the
calculated positions. The final cycles of the least square refinement were
based on 6066 observed reflections [I > 3s|I|] and 497 variable parameters.
The maximum and minimum peaks on the final difference Fourier map
correspond to 3.48 and 21.04 e2/Å3, respectively. CCDC 182/1052.
defined by B(1)–Se(1)–Se(2) and the aromatic ring plane of the
Tbt group is 86.6°, suggesting no conjugative interaction of p-
electrons on the Tbt group with the boron atom. These structural
features of 5 are similar to those of the 2,2-dimesityl-
1,3,2,4-dithiagermaboretane derivative reported by us.2b
Since we have already shown that the 2,2-diphenyl-
1,3,2,4-dithiastannaboretane derivative, a sulfur-analogue of 4,
acts as a good precursor of a thioxoborane, (Tbt)BNS, on
thermolysis,4 the thermolysis of 4 was also carried out in the
expectation that it would dissociate into a boron–selenium
double-bonded species (selenoxoborane) (Tbt)BNSe and diph-
enylstannaneselone.
1 C. E. Housecroft, in Comprehensive Organometallic Chemistry II, ed.
E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon Press, Oxford,
UK, 1995, vol. 1, ch. 4, p. 129.
2 (a) N. Tokitoh, M. Ito and R. Okazaki, Organometallics, 1995, 14,
4460; (b) M. Ito, N. Tokitoh and R. Okazaki, Organometallics, 1997,
16, 4314.
When a toluene-d8 solution of 4 and 2,3-dimethylbuta-
1,3-diene in a sealed NMR tube was heated at 100 °C for 12 h,
the starting material completely disappeared to afford 4,5-di-
methyl-1,2-selenaboracyclohex-4-ene 6 (91%) along with the
trimer 7 of diphenylstannaneselone 9 (75%) (Scheme 2). The
formation of 6 and 7 clearly indicates the initial retro
[2+2]cycloaddition of the diselenastannaboretane ring of 4 into
the two units, i.e. selenoxoborane 8 and diphenylstannaneselone
9, followed by the [4 + 2]cycloaddition reaction of 8 with co-
existing 2,3-dimethylbuta-1,3-diene and the self-trimerization
of 9. To the best of our knowledge, this is the first example of
the trapping reaction of a selenoxoborane. Similarly, thermol-
ysis of 4 at 50 °C in the presence of 2,4,6-tri-tert-butylbenzoni-
trile oxide afforded the [3 + 2]cycloadduct of selenoxoborane
10 (59%)† together with 7 (45%). These results show that the
3 M. Ito, N. Tokitoh and R. Okazaki, Tetrahedron Lett., 1997, 38,
4451.
4 N. Tokitoh, M. Ito and R. Okazaki, Tetrahedron Lett., 1996, 37,
5145.
5 For example: W. Siebert and F. Riegel, Chem. Ber., 1973, 106, 1012; R.
Köster, G. Seidel, R. Boese and B. Wrackmeyer, Chem. Ber., 1988, 121,
1955; C. D. Habben, Chem. Ber., 1988, 121, 1967; R. Köster, G. Seidel
and M. Yalpani, Chem. Ber., 1989, 122, 1815; M. Yalpani, R. Boese and
R. Köster, Chem. Ber., 1990, 123, 707; see also ref. 9.
6 (a) Gas phase detection of the FBO molecule, see: Y. Kawashima, K.
Kawaguchi, Y. Endo and E. Hirota, J. Chem. Phys., 1987, 87, 2006 and
references cited therein; (b) for a theoretical study, see: M. T. Nguyen,
L. G. Vanquickenborne, M. Sana and G. Leroy, J. Chem. Phys., 1993,
97, 5224; (c) for a review, see: S. H. Bauer, Chem. Rev., 1996, 96,
1907.
7 R. W. Kirk and P. L. Timms, Chem. Commun., 1967, 18; C. Kirby,
H. W. Kroto and M. J. Taylor, J. Chem. Soc., Chem. Commun., 1978, 19;
C. Kirby and H. W. Kroto, J. Mol. Spectrosc., 1980, 83, 1, 130; for a
theoretical study, see ref. 6(b); for a review, see: H. W. Kroto, Chem.
Soc. Rev., 1982, 11, 435.
8 T. A. Cooper, M. A. King, H. W. Kroto and R. J. Suffolk, J. Chem. Soc.,
Chem. Commun., 1981, 353.
9 D. Männig, C. K. Narula, H. Nöth and U. Wietelmann, Chem. Ber.,
1985, 118, 3748; E. Hanecker, H. Nöth and U. Wietelmann, Chem. Ber.,
1986, 119, 1904.
10 In this reaction an inseparable mixture of 1,2,4,3,5-triselenastannabor-
etane and 1,3,2,4-diselenastannaboretane 5 was obtained, to which was
added triphenylphosphine without separation.
11 G. M. Sheldrick, SHELXS-86, in Crystallographic Computing 3, ed.
G. M. Sheldrick, C. Kruger and R. Goddard, Oxford University Press,
pp. 175–189.
Se
Ph
Ph
Tbt
B
Sn
Se
Se
Tbt
B
4
heat
6 (91%)
Tbt
B
Se
8
+
Mes*
Se
C
Mes*CNO
Tbt
B
Ph2Sn Se
N
O
9
12 P. T. Beuskens, DIRDIF94. Direct methods for difference structures–an
automatic procedure for phase extension and refinement of difference
structure factors, Technical Report of the Crystallography Laboratory,
University of Nijmegen, The Netherlands.
13 TEXSAN: TEXRAY Structure Analysis Package, Molecular Structure
Corporation, 1985.
10 (59%)
Mes* = 2,4,6-But3C6H2
(Ph2SnSe)3
7
Scheme 2
Communication 8/06478D
2496
Chem Commun., 1998, 2495–2496