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Silaethene.

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Silaethenel"]
By Giinther Maier, Gerhard Mihm, and
Hans Peter Reisenauer"]
Encouraged by the successful matrix isolation of silabenzene"], we ventured to approach unsubstituted silaethenel". While derivative^'^] are known, unequivocal evidencer4]for the parent compound, which is especially important in order to experimentally test theoretical predict i o n ~ ' ~was
] , still awaited. We report the preparation of silaethene (5a) by retrodiene cleavage and its detection by
IR and UV spectroscopy.
1,l-Dichloro-1-sila-2,4-cyclohexadiene( 1 ~ ) and
''~~
dimethy1 acetylenedicarboxylate react to produce the adduct
(2c) (Table I), whose flash pyrolysis at 400°C in the presence of benzene with nitrogen as the carrier gas, led to dimethyl phthalate and phthalic anhydride.
Attempts to trap the presumed intermediate product,
1,l-dichloro-1-silaethene(Sc), by carefully freezing the
fragments formed by vacuum flash pyrolysis (650°C, l o u 4
torr)l6]in argon at 10 K, resulted in the detection of I R absorption bands at G= 1008,732 and 593 cm-', in addition to
those of the previously mentioned arenes. The same bands
are also recognizable when the bicyclooctadiene ( 3 ~ ) ' (Ta~'
ble I), readily accessible from (lc) and perfluoro-2-butyne,
is similarly cleaved. This observation and the fact that the
normal flash pyrolysis (650"C, 0.2 torr, product trapped at
77 K) of (3c) only yields o-bis(trifluoromethy1)benzene (4)
and tetrachlorodisilacyclobutane (6c)['], suggest that the
absorptions measured in the matrix are assignable to (Sc).
Table 1. Synthetic conditions together with physical and spectroscopic properties of compounds ( 2 ~ 1(,3 4 , (3b)and (3c). 1R (film) [cm-']; NMR: &-values
relative to TMS (in CDCl,, unless otherwise specified).
(2c): 6 h reflux in toluene; 57%; B.p.=130°C/10-2 torr.-IR: 1740, 1730,
1635, 1600 (C-C), 562 (Si-CI).-'H-NMR
(CCI,): 1.00 (2H,
1718 (W),
d), 3.40-4.30 (2H, m, +2CH3), 6.13-6.54 (ZH, m).-l3C-NMR: 15.00,
37.12,38.31,51.81, 51.93, 128.72, 131.78, 136.46, 139.32, 164.31, 165.26.-Correct elemental analysis.
( 3 ~ ) :autoclave; 5 h, 90°C; 52%; B.p.=9OoC/1O0 torr-IR: 2160 (Si-H),
1655, 1605 (C=-C).-'H-NMR: 0.82 (2H, q), 3.53 (2H, d), 3.94(1 H, d), 4.20
( l H , m), 5.90-6.33 (2H, m).-")C-NMR [a]: 6.94, 29.23, 35.19, 116.37,
127.28, 129.85, 130.38.-MS: m/e=258.0299 (calc.), 258.0291 (found).
(3b):as (3a), 50%; B.p.=9O0C/100 tom.-IR: 1655, 1605 (C=-C), 1585, 1565
(Si-D).-'H-NMR:
0.82 (2H, d), 3.92 (1 H, d), 4.20 (1 H, m), 5.88-6.28
(2H, m).-'"C-NMR[a]: 6.85, 29.26, 35.18, 116.46, 127.33, 129.85, 130.42.MS: m/e= 260.0425 (calc.), 260.0425 (found).
(3c): as ( 3 ~ ) .71%; B.p.=75"C/S tom-IR:
1660, 1610 (C=C), 570
(Si-CI).-'H-NMR:
1.01 (2H, 4). 3.94 ( I H , d), 4.14 (IH, d), 6.01-6.56
(2H,m).-"C-NMR[a]: 14.26, 36.03, 37.56, 114.78, 128.98[b], 132.04.-MS:
m/e= 325.9520 (calc.), 325.9515 (found).
[a] The C-atoms of the CF, groups are not observable under the recording
conditions. [b] Two olefinic C-atoms coincidently show the same chemical
shift.
By means of a combination of vacuum flash pyrolysis
(65OoC, lop4 torr) and matrix isolation techniques, apart
from the bands originating from (4). the IR absorptions
shown in Table 2 can also be measured. The latter disappear upon irradiation ( 2 ~ 2 5 4nm) of the ond dens ate''^.
This also occurs upon thawing of the argon matrix at 35 K.
Hereby, the corresponding 1,3-disilacyclobutanes (6a) and
(6b) are formed.
Silaethenes (5)
IR [cm-'I
UV [nm]
(SQ)
(Sb)
(54
The thermal behavior of the unchlorinated silaethene
precursors (3a) and (3b). which are readily obtained from
(la)""' and the silacyclohexadiene (1b)-prepared from
( l c ) and LiA1D4-is even more assertive.
When (3a) or (36) are heated to 420°C with Nzas the
flow gas, in the presence of cyclohexane['', (4) is smoothly
formed together with the hydrogenated or deuterated 1,3disilacyclobutanes (6a) or (6b), respectively['].
[*I Prof. Dr. G. Maier, DipLChem. G. Mihm, Dr. H. P. Reisenauer
["I
lnstitut fur Organische Chemie der Universitst
Heinrich-Buff-Ring 58, D-6300 Giessen 1 (Germany)
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. Engl. 20 (1981) No. 6/7
2239 (m), 2219 (m),
985 (w), 927 (w),
817 (s), 741 (s)
1635 (m), 1600 (m),
952 (w), 759 (s),
719 (s), 396 (w)
1008 (m), 732 (s),
593 (m)
1,3-Disilacyclobutanes (6)
IR [cm - '1
258 [a]
(60)
2160-2140 (m),
961 (s), 899 (s)
259 [a]
(66)
1677 (m),1654 (m),
786 (s), 689 (s)
246 [a]
(64
[b]
Of the bands observed, the antisymmetric and symmetric Si-H and Si-D vibrational frequencies are of considerable diagnostic use. These frequencies are found above
2200 cm-' in silabenzene (2217 ern-')''"]. The still higher
wavenumbers (2239 and 2219 cm-') suggest that the product obtained from (3a) is in fact a compound with a hydrogen atom on an spz-hybridized silicon atomPq. These
experimentally determined frequencies lie between the values obtained by calculation (2480'5c' and 216OL4Icm- I). The
correctness of the assignment follows from the shift of the
bands mentioned above to much lower wavenumbers (1635
cm-' and 1600 cm-') in the product prepared from the
deuterated precursor (36).
Comparison of the matrix UV spectrum of separately
matrix isolated arene (4) with the matrix spectra obtained
by pyrolysis of (3a). (3b) and (34, indicates that the silaethenes (Sa),(56) and (5c) are characterized by distinct UV
0 Verlag Chemie GmbH, 6940 Weinheim. 1981
0570-0833/81/0707-0597 $ 0Z.S0/0
597
maxima (Table 2)[’01.These disappear, as do the corresponding IR bands, upon irradiation or thawing of the matrices.
It should be noted that the most commonly used route to
silaethenes, i. e. the pyrolysis of the corresponding silacyc l o b ~ t a n e s ~cannot
~ . ~ ~ , be transferred to the parent compound. Instead of (5a) only propene, ethene and acetylene
can be detected as cleavage products after vacuum flash
pyrolysis“ ‘I.
The following conclusions can be drawn: 1) Unsubstituted silaethene ( 5 4 and its derivatives (Sb) and (5c) are
species capable of existence and can be identified by IR
and UV spectroscopy. 2) They are only stable under conditions of matrix isolation at 10 K. 3) On thawing of the matrix they dimerize to 1,3-disilacyclobutanes.
Received: January 14, 1981 [ Z 790a IE]
German version: Angew. Chem. 93, 615 (1981)
[I] a) G. Maier, G.Mihm, H. P. Reisenauer, Angew. Chem. 92, 58 (1980);
Angew. Chem. Int. Ed. Engl. 19, 52 (1980); b) PE spectroscopic studies:
B. Solouki, P. Rosmus, H . Bock, G. Maier, ibid. 92. 56 (1980) and 79, 51
(1980).
(21 For a summary of the silaethene problem see: a) P. Jutzi, Angew. Chem.
87,269 (1975); Angew. Chem. Int. Ed. Engl. 14, 232 (1975); b) L. E. Gusel’nikov, N. S.Numetkin, V. M . Vdouin, Acc. Chem. Res. 8,18 (1975); c)
L. E. Guselitikoo, N. S . Nametkin, Chem. Rev. 79, 529 (1979).
[3] For the latest results on the characterization of 1.1-dimethyl-1-silaethene
see: a) 0.M . Nefedou, A . K . Mal’tsev, V. N . Khabashesku. V. A . Koroleu.
J. Organomet. Chem. 201, 123 (1980); b) L. E. Guselnikou, V. V. Volkova, V. G.Aoakyan, N. S.Numetkin, ibid. 201, 137 (1980); c) P. G. Mahafly. R . Gutowsky. L. K . Montgomery, J. Am. Chem. SOC. 102, 2854
(1980); d) 1,1,2-Trimethyl-l-silaethene: 0.L. Chapman, C.-C. Chanq. J.
Kolc. M . E. Jung, J. A . Lowe. T. J. Barton, M . L. Tumey. ibid. 98, 7844
(1976); e) M . R. Chedekel, M . Skoghnd. R . L. Kreeger, H . Shechter, ibid.
98, 7846 (1976); f) I-Methyl-I-silaethene: T. J. Dmhnak, J. Michl, R .
West, ibid. 701, 5427 (1979); 703, 1845 (1981); g) for silaethenes kinetically stabilized by steric hindrance see: A . G. Brook, J. W. Harris, J.
Lemon. M . El Sheikh, ibid. 101, 83 (1979); A . G. Brook, F.Abdesaken. B.
Gurekunst. G. Gutekunst, R . K . Kallury, J. Chem. SOC.Chem. Commun.
1981, 191.
141 Our results contradict a recent publication indicating that silaethene
should be stable in the condensed phase at 77 K: N . Auner. J. Grobe. Z.
Anorg. Allg. Chem. 459, 15 (1979).
[5] Theory: a) Complete literature survey in [Zc]; b) most recent theoretical
work: J. D . Goddard, Y. Yoshioka, H . F. Schaefer I I I , J. Am. Chem. SOC.
102, 7644 (1980); see also M . Hanamura. S.Naqase. K . Morokuma, Tetrahedron Lett. 1981, 1813; c) calculated IR spectrum of silaethene: H .
B. Schlegel. S. Wove, K . Mislow. Chem. Commun. 1975,246; a recent
calculation gives the values 2346 and 2338 c m - ’ for the asymm. and
symm. Si-H vibrational frequencies respectively. ( H . B. Schleqel, personal communication.)
[6] Pyrolysis oven with directly coupled low temperature cell (Displex
Closed Refrigeration System CSA 202, Air Products), quartz pyrolysis
tube (8 x 50 mm). Oven outlet-window distance 50 mm.
[7] Diels-Alder adducts of this type have previously been suggested as potential sources of silaethene: T. J Barton. E. Kline. J. Organomet.
Chem. 42,C21 (1972); see also T. J . Barton, Pure Appl. Chem. 52,615
(1980).
[8] Identified by comparison with the published spectral data of compounds (6): a) G. Fritz, E. Matern, Z . Anorg. Allg. Chem. 426,28 (1976);
b) R . M . Irwin, 1. M . Cooke, J. Laane, J. Am. Chem. SOC. 99, 3273
(1977); c) N. Auner. J. Grobe, J. Organomet. Chem. 188, 151 (1980).
) 2004 and 1935
[9] Thereby, new IR absorptions [stemming from ( 5 ~ at
cm-’, from (5b)at 1454 and 1405 cm-’1 are observed, which could originate from silylenes (silanediylenes) or silyl radicals: D . E. Milligan, M .
E. Jacox, J. Chern. Phys. 52, 2594 (1970).
[lo] I,]-Dimethyl-1-silaethene,prepared by pyrolysis of I,l-dimethyl-1-silacyclobutane, shows an absorption maximum at 244 nm in argon at 10
K.
[ I l l a) Own attempts; b) A . K . Mal’tseu, V. N. Khabashesku, 0.M . Nefedou.
Dokl. Akad. Nauk SSSR 247,383 (1979).
598
0 Verlaq Chemie GmbH, 6940 Weinheim, 1981
Silaethene: Highly Correlated Wave Functions and
Photoelectron Spectroscopic Evidence[**]
By Pave1 Rosmus, Hans Bock, Bahman Solouki,
Giinther Maier, and Gerhard MihmI’]
Dedicated to Professor Edgar Heilbronner on the occasion
of his 60th birthday
Ab initio quantum mechanical calculations, which account for large portions of the electron correlation1’,’],
yield reliable theoretical predictions of experimentally unknown molecular propertiesL3’.Thus, ionization energies
can be calculated to an accuracy of ca. 0.1 to 0.3 eV, i. e. to
a degree of accuracy sufficient to identify unknown molecules by photoelectron spectroscopy. We have used such a
combination of theory and experiment in the search for
thermal dissociation reactions through which silaethene
could be f ~ r m e d [ ~ . ~ I .
The quality of the wave function plays an important role
in calculations of energy differences: the total energies for
silaethene calculated by us, are, to date, the most accurate
for this moleculef6]. For all electronic states treated, we
have obtained 70 to 80% of the valence electron correlation
energies. The vertical energy differences are shown in Table 1.
Method
of calculation
Xj’B,)
&’B2)
Bj’A,)
@B2)
D(*A,) E(’A,)
12.6
12.5
12.3
12.7
14.7
14.2
18.0
16.9
AEPNU-CEPA
[eVJ
[ew
7.40
8.59
“IE,”
[eV]
8.95f.1 12.5f.1 12.9f.l 14.1f.l 16.5f.2 20.4t.3
*ES‘F
23.0
21.1
The ionization energies “ZE,” are estimated from the energy differences AEscF and AEPNo-CEPA,using the assumption that approximately 75% of the correlation contributions are covered.
It should be possible to detect silaethene by photoelectron spectroscopy, in particular from the predicted first
ionization energy at 8.95 f 0.1 eV. However, in numerous
pyrolysis experiments on promising precursors, such as
1,3-disilacyclobutane, no reproducible bands in the expected region were ob~ervedl~,~].
Finally, in the thermal retrodiene cleavage of the bis(trifluoromethy1)-substituted
bicycle[51,a novel band with vibrational fine structure (Fig.
1) appears, whose intensity increases upon addition of argon to the flowing gas. Higher ionization bands are
masked by the superimposed PE spectrum of the cleaved
off component 1,2-bis(trifluoromethy1)benzene (Fig. 1).
The following additional arguments corroborate the assignment of silaethene as a retrodiene dissociation productl5]:The first ionization energy is observed in the pre-cal-
[*I
[**I
Prof. Dr. H. Bock, Dr. P. Rosmus, Dr. B. Solouki
Institut fur Anorganische Chemie der Universitat
Niederurseler Hang, D-6000 Frankfurt am Main 50 (Germany)
Prof. Dr. G. Maier, DipLChem. G. Mihm
Institut fur Organische Chemie der Universitat
Heinrich-Buff-Ring 58, D-6300 Giessen I (Germany)
For previous work on highly correlated wave functions see: H. 1. Werner. P. Rosmus. J. Chem. Phys. 73,2319 (1980). Also Part 25 on Analysis
and Optimization of Gas Phase Reactions.-Part 24: H. Bock, T. Hirabayashi. S. Mohmand, Chem. Ber., in press.
0570-0833/81/0707-0598 S 02.50/0
Anqew. Chem. In!. Ed. Enql. 20 (1981) No. 6/7
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