Generation of 9-stannaphenanthrene and its reactivities

Article abstract of DOI:10.1246/cl.2005.1088

A neutral stannaaromatic compound, 9-stannaphenanthrene 1a bearing an efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), was successfully generated by the reaction of the corresponding chlorostannane 2 with lithium 2,2,6,6-tetramethylpiperidide in THF at -78°C. The generaion of 1a was indicated by the trapping experiments using MeOD, Mes*CNO, and 2,3-dimethyl-1,3-butadiene at the same temperature. However, 1a was found to undergo ready dimerization at room temperature to give the cis-[2 + 2] dimer 3 stereoselectiveiy. Copyright

Full text of DOI:10.1246/cl.2005.1088

1088
Chemistry Letters Vol.34, No.8 (2005)
Generation of 9-Stannaphenanthrene and Its Reactivities
Yoshiyuki Mizuhata, Nobuhiro Takeda, Takahiro Sasamori, and Norihiro Tokitohꢀ
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received April 15, 2005; CL-050520)
A neutral stannaaromatic compound, 9-stannaphenanthrene
1) n-BuLi
2) TbtSnCl3
3) LiAlH4
4) CCl4
Tbt
1a bearing an efficient steric protection group, 2,4,6-tris[bis(tri-
methylsilyl)methyl]phenyl (Tbt), was successfully generated by
the reaction of the corresponding chlorostannane 2 with lithium
2,2,6,6-tetramethylpiperidide in THF at ꢁ78 ꢂC. The generaion
of 1a was indicated by the trapping experiments using MeOD,
MesꢀCNO, and 2,3-dimethyl-1,3-butadiene at the same temper-
ature. However, 1a was found to undergo ready dimerization at
room temperature to give the cis-[2 þ 2] dimer 3 stereoselective-
ly.
Br
SnCl2
73%
Me
Tbt
Me
Cl
1) Mg
SnCl2
2) LiAlH4
Tbt
Sn
NBS
56%
3) CCl4
85%
Br
In recent years, much attention has been focused on stan-
naaromatic compounds, i.e., Sn-containing ½4n þ 2ꢃꢀ electron
ring systems. In view of the recent progress in the chemistry
of sila- and germaaromatic compounds, the synthesis of stan-
naaromatic compounds is of great interest from the standpoint
of systematic elucidation of heavier congeners of aromatic hy-
drocarbons,1 which play very important roles in organic chemis-
try. Although stannole dianions2 have been successfully synthe-
sized as stable compounds and fully characterized as ionic
stannaaromatic compounds, neutral stannaaromatic compounds
are still elusive and their properties have not been disclosed
yet so far. The main reason for the lack of neutral stannaaromatic
compounds is due to the difficulty in their synthesis and isolation
responsible for the extremely high reactivity of tin–carbon
double bonds.3 On the other hand, we have recently succeeded
in the synthesis and isolation of kinetically stabilized, neutral
sila- and germaaromatic compounds4,5 by taking advantage of
an efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)-
methyl]phenyl (Tbt). With these stable systems in hand, we have
revealed their molecular structures and reactivities and have dis-
cussed the aromaticity of sila- and germaaromatic compounds.
The successful results in the sila- and germaaromatic systems
naturally prompted us to extend this chemistry to the heavier
metallaaromatic systems of group 14 elements. Here, we report
the generation of 9-stannaphenanthrene 1a kinetically stabilized
by the Tbt group, the first neutral stannaaromatic compound
(Figure 1).
2
Scheme 1.
to the ready halogen-exchange reactions with LiBr and MgBrCl,
respectively, the initially generated halostannanes were subject-
ed to the LiAlH4 reduction followed by the rechlorination with
CCl4 in both steps with the intension of transforming into the
corresponding pure chlorostannanes.
Synthesis of 1a was first attempted by the dehydrochlorina-
tion of 2 with various kinds of bases [lithium diisopropyl amide
(LDA), lithium hexamethyldisilazide (LHMDS), 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU), and tert-butyl lithium].6 Although
no evidence for the generation of 1a was obtained in these cases,
the reaction of 2 with lithium 2,2,6,6-tetramethylpiperidide
(LTMP) at room temperature resulted in the stereoselective for-
mation of cis-[2 þ 2] dimer 36 (46%) of 9-stannaphenanthrene
1a (Scheme 2). The molecular structure of 3 was unambiguously
determined by X-ray crystallographic analysis (Figure 2).7
Tbt
LTMP (1.0 equiv.)
Sn
2
3
Sn
THF, rt, 1 h
46%
Tbt
Scheme 2.
Since the formation of 3 suggested the generation of 1a as a
transient species, the trapping experiments at low temperature
were examined. To the reaction mixture of 2 and LTMP were
added MeOD, MesꢀCNO [Mesꢀ = 2,4,6-tri(tert-butyl)phenyl],
and 2,3-dimethyl-1,3-butadiene as trapping reagents at ꢁ78 ꢂC
to give the corresponding adducts 4a (almost quantitative as
estimated by 1H NMR), 5 (43% isolated yield), and 6 (67%
isolated yield), respectively (Scheme 3). Compound 4a was
moisture-sensitive and could not be isolated as a pure compound
because of the difficulty in the separation from 2,2,6,6-tetra-
methylpiperidine, which is the inevitable by-product of this
reaction. The molecular structure of 4a was determined by the
comparison with pure 4b6 which was prepared by the reaction
Ar
Sn
SiMe3
Me3Si
SiMe3
SiMe3
Tbt =
Me3Si
1a: Ar = Tbt
1b: Ar = Ph
SiMe3
Figure 1.
9-Chloro-9,10-dihydro-9-stannaphenanthrene 2,6 the suita-
ble precursor of 1a, was prepared according to Scheme 1. Since
a considerable amount of bromostannanes were formed in the
first (stannylation) and third (reductive cyclization) steps due
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.8 (2005)
1089
naphenanthrene ring of 1b consists of a localized Sn=C unit and
a biphenyl unit in contrast to the 9-silaphenanthrene ring of 7,
the X-ray crystallographic analysis of which has revealed its
planar and ꢀ-electron-delocalized structure.10 The thermal insta-
bility of 1a and the theoretical calculations suggest the small
contribution of aromatic stabilization in the 9-stannaphenan-
threne system of 1a.
In summary, we have succeeded in the generation of 9-stan-
naphenanthrene 1a for the first time and revealed its high reac-
tivities. With the hope of isolating 1a as a stable compound, fur-
ther investigation on the introduction of additional substituent(s)
to the stannaphenanthrene skeleton and the cooperative stabili-
zation method (the contribution of kinetic and thermodynamic
stabilization) are currently in progress.
Figure 2. ORTEP drawing (50% probability) of 3.
This work was supported by Grants-in-Aid for COE Re-
search on Elements Science (No. 12CE2005), Scientific Re-
search on Priority Area (No. 14078213), Scientific Research
(A) (No. 14204064) and the 21st Century COE Program on
Kyoto University Alliance for Chemistry from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
of 2 with LiOMe in THF. The molecular structures of 5 and 6
were confirmed with the 1H, 13C, and 119Sn NMR and mass spec-
tral data,6 and were finally established by X-ray crystallographic
analysis (Figure 3).8 Since 2,3-dimethyl-1,3-butadiene is inert to
anionic species such as 9-chloro-10-lithio-9-Tbt-9,10-dihydro-
9-stannaphenanthrene, which is an alternative intermediate in
the reactions of 2 with LTMP giving 3, 4a, and 5, the formation
of a [2 þ 4] cycloadduct 6 from 2 indicates that the reaction of 2
with LTMP affords not an anionic intermediate but a neutral
stannaphenanthrene 1a. These results strongly indicate that 1a
exists as a monomer in a THF solution at ꢁ78 ꢂC.9 The thermal
instability of 1a is in sharp contrast to the high stability of Tbt-
substituted 9-silaphenanthrene (7),10 which is stable at 100 ꢂC in
C6D6. In theoretical calculations using a Ph-substituted model
1b,11 it was found that the planar conformation of the 9-stanna-
phenanthrene skeleton is not a local minimum, that is, the 9-stan-
References and Notes
1
a) V. J. Minkin, M. N. Glukhovtsev, and Y. B. Simkin, in ‘‘Aromaticity
and Antiaromaticity; Electronic and Structural Aspects,’’ Wiley,
New York (1994). b) N. Tokitoh, Acc. Chem. Res., 37, 86 (2004). c) N.
Tokitoh, Bull. Chem. Soc. Jpn., 77, 429 (2004).
2
3
a) M. Saito, R. Haga, and M. Yoshioka, Chem. Commun., 2002, 1002.
b) M. Saito, R. Haga, and M. Yoshioka, Chem. Lett., 32, 912 (2003).
For reviews, see: a) K. M. Baines and W. G. Stibbs, Adv. Organomet.
´
Chem., 39, 275 (1996). b) J. Escudie, C. Couret, and H. Ranaivonjatovo,
Coord. Chem. Rev., 178–180, 565 (1998). c) N. Tokitoh and R. Okazaki,
in ‘‘The Chemistry of Organic Germanium, Tin and Lead Compounds,’’
ed. by Z. Rappoport, Wiley, Chichester, U.K. (2002), Vol. 2, Part 1,
Chap. 13. d) V. Y. Lee and A. Sekiguchi, Organometallics, 23, 2822
(2004).
Tbt
4
5
Recent applications of the Tbt group to the stabilization of neutral silaar-
omatic compounds, see: a) A. Shinohara, N. Takeda, and N. Tokitoh,
J. Am. Chem. Soc., 125, 10804 (2003). b) N. Takeda, A. Shinohara,
and N. Tokitoh, Organometallics, 21, 4024 (2002).
Recent applications of the Tbt group to the stabilization of neutral
germaaromatic compounds, see: a) N. Nakata, N. Takeda, and N.
Tokitoh, Angew. Chem. Int. Ed., 42, 115 (2003). b) N. Nakata, N.
Takeda, and N. Tokitoh, Organometallics, 22, 481 (2003).
trapping
Sn
reagents
LTMP
2
products
THF
−78 °C
1 h
−78 °C
1a
N
Mes*
6
7
Experimental procedures and physical properties for the precursor 2 and
the products 3, 4b, 5, and 6 are described in the Supporting Information.
MeO
Tbt Sn
O
X
Tbt Sn
H
.
Tbt Sn
H
Crystal data for 3: C80H136Si12Sn2 C6H6 MW ¼ 1750:46; triclinic; space
ꢀ
ꢁ
ꢁ
ꢁ
group P1; a ¼ 12:8898ð2Þ A, b ¼ 17:9414ð3Þ A, c ¼ 22:3902ð5Þ A; ꢁ ¼
ꢂ
ꢂ
ꢂ
ꢁ 3
81:1212ð8Þ , ꢂ ¼ 82:9007ð9Þ , ꢃ ¼ 72:8807ð16Þ ; V ¼ 4872:49ð16Þ A ;
Z ¼ 2; Dcalcd ¼ 1:193 g/cm3; ꢄ ¼ 0:699 mmꢁ1; 2ꢅmax ¼ 50ꢂ; T ¼ 103
K; R1ðI > 2ꢆðIÞÞ ¼ 0:0451; wR2 (all data) = 0.1015; GOF ¼ 1:020 for
16862 reflections and 937 parameters, (CCDC 268569).
4a: X = D
4b: X = H
5
6
8
9
Crystal data for 5: C59H97NOSi6Sn MW ¼ 1123:61; triclinic; space
ꢀ
ꢁ
ꢁ
ꢁ
ꢁ 3
group P1; a ¼ 9:8361ð9Þ A, b ¼ 15:5831ð11Þ A, c ¼ 21:8919ð17Þ A;
Scheme 3.
ꢂ
ꢂ
ꢂ
ꢁ ¼ 107:957ð3Þ , ꢂ ¼ 96:281ð3Þ , ꢃ ¼ 88:936ð7Þ ; V ¼ 3172:5ð4Þ A ;
Z ¼ 2; Dcalcd ¼ 1:176 g/cm3; ꢄ ¼ 0:552 mmꢁ1; 2ꢅmax ¼ 50ꢂ; T ¼ 103
K; R1ðI > 2ꢆðIÞÞ ¼ 0:0309; wR2 (all data) = 0.0907; GOF ¼ 1:014 for
10703 reflections and 716 parameters, (CCDC 268570). Crystal data
for 6 is described in the Supporting Information.
Additional information: 1) When these trapping experiments were
performed after stirring for 1 h at ꢁ40 ꢂC, dimer 3 was obtained in ca.
30% yield. 2) Dimer 3 showed no change in C7D8 at 100 ꢂC and did
not react with MesꢀCNO in C6D6 even at 80 ꢂC, indicating no dissocia-
tion of 3 to the monomer 1a in solution.
10 A. Shinohara, N. Takeda, T. Sasamori, and N. Tokitoh, The 7th IUPAC
International Conference on Heteroatom Chemistry, Shanghai, China,
August 2004, Abstr., No. P43.
11 Theoretical calculations were carried out using the Gaussian 98 program
with density functional theory at the B3LYP level. In calculations,
LANL2DZ (for Sn) and 6-31G(d) (for C and H) basis sets were used.
The optimized structure of 1b is shown in the Supporting Information.
Figure 3. ORTEP drawing (50% probability) of 5.
Published on the web (Advance View) July 2, 2005; DOI 10.1246/cl.2005.1088