The first cyclic π-conjugated silylium ion: The silatropylium ion annelated with rigid σ-frameworks [9]

Full text of DOI:10.1021/ja0014685

9312
J. Am. Chem. Soc. 2000, 122, 9312-9313
Scheme 1a
The First Cyclic π-Conjugated Silylium Ion: The
Silatropylium Ion Annelated with Rigid
σ-Frameworks
Tohru Nishinaga, Yoshiteru Izukawa, and Koichi Komatsu*
Institute for Chemical Research
Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
ReceiVed April 27, 2000
Studies on silylium ion in condensed phases have attracted
much attention,1 particularly following the X-ray structural
analyses made by Lambert’s2 and Reed’s3 groups. Nevertheless,
only a few types of compounds having a silylium-ion character
have been reported so far,2-7 because in many cases the silylium
ion forms a strong coordinate bond with the solvent or
counteranion.1e-j As one of the representative π-conjugated
silylium ions, the silatropylium ion (1) is of great interest from
the viewpoints of its aromaticity, structure, and reactivity.
However, its presence in solution has been highly questioned.8
Even in the gas phase, the recent suggestion of 1 as one of the
C6H7Si+ isomers observed in a FT ICR mass spectrum9 was
disproved by a subsequent experimental study showing that it
was rearranged isomer C6H6‚SiH+.10 Theoretical studies have
indicated that 1 is less stable than “silabenzyl cation” by 9 kcal
a (a) Mesityllithium, THF, rt; (b) LiAlH4, THF, 40 °C; (c) trityl TPFPB.
mol-1 11
. Thus, the synthesis of the silatropylium ion remains as
a major challenge in organosilicon chemistry. For its realization,
we designed a modification of the seven-membered ring with rigid
σ-frameworks which involves annelation with bicyclo[2.2.2]octene
(BCO) units. In our previous study, such structural modification
was found to be the most effective in stabilizing the carbon
analogue, tropylium ion, in terms of the thermodynamic crite-
rion.12 Here we report the first example of the generation and
Figure 1. 1H NMR spectrum (400 MHz) for 2 at -50 °C in CD2Cl2.
The signals marked with +, *, and # corresponds to those for 5,
mesitylene, and triphenylmethane, respectively. A signal for the two
mesityl ring protons of 2 is overlapped with signals for triphenylmethane.
NMR characterization of the silatropylium ion 2 stabilized by
such a structural modification together with the placement of a
bulky substituent on the silylium center.
(1) (a) Pauling, L. Science 1994, 263, 983. (b) Olah, G. A.; Rasul, G.; Li,
X.; Buchholz, H. A.; Sandford, G.; Prakash, G. K. S. Science 1994, 263, 983-
984. (c) Lambert, J. B.; Zang, S. Science 1994, 263, 984-985. (d) Reed, C.
A.; Xie, Z. Science 1994, 263, 985-986. (e) Lambert, J. B.; Kania, L.; Zhang,
S. Chem. ReV. 1995, 95, 1191-1201. (f) Belzner, J. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1277-1280. (g) Schleyer, P. v. R. Science 1997, 275, 39-
40. (h) Reed, C. A. Acc. Chem. Res. 1998, 31, 325-332. (i) Maerker, C.;
Schleyer, P. v. R. In The Chemistry of Organic Silicon Compounds; Rappoport,
Z., Apeloig, Y., Eds.; John Wiley: Chichester, 1998; Vol. 2, pp 513-555. (j)
Lickiss, P. D. In The Chemistry of Organic Silicon Compounds; Rappoport,
Z., Apeloig, Y., Eds.; John Wiley: Chichester, 1998; Vol. 2, pp 557-594.
(2) Lambert, J. B.; Zang, S.; Stern, C. L.; Huffman, J. C. Science 1993,
260, 1917-1918.
The precursor silepin 4 having a mesityl (Mes) group on silicon
was prepared from dichlorosilepin 313 in 40% yield as shown in
Scheme 1. When the first attempt was made to generate 2 by
hydride abstraction from 4 with an equivalent of trityl tetrakis-
(pentafluorophenyl)borate (TPFPB) in toluene-d8 (C7D8) under
(3) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993, 262, 402-
404.
(4) (a) Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. Engl. 1997, 36,
400-401. (b) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J.
Am. Chem. Soc. 1999, 121, 5001-5008.
1
vacuum at -50 °C, a ring contraction occurred to show the H
(5) (a) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. Commun. 1993,
383-384. (b) Lambert, J. B.; Zang, S.; Ciro, S. M. Organometallics 1994,
13, 2430-2443.
NMR signals for benzene derivative 5 and mesitylene, possibly
via a nonclassical ion 5‚Mes-Si+ which corresponds to C6H6‚
SiH+ observed in the gas phase.10 On the other hand, when the
reaction was conducted in dichloromethane-d2 (CD2Cl2) at -50
(6) (a) Xie, Z.; Liston, D. J.; Jelink, T.; Mitro, V.; Bau, R.; Reed, C. A. J.
Chem. Soc., Chem. Commun. 1993, 384-386.(b) Xie, Z.; Bau, R.; Benesi,
A.; Reed, C. A. Organometallics 1995, 14, 3933-3941. (c) Xie, Z.; Manning,
J.; Reed, R. W.; Mathur, R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am.
Chem. Soc. 1996, 118, 2922-2928.
1
°C, the H and 13C NMR signals corresponding to silatropylium
ion 2 were observed14 as shown in Figure 1 and Table 1.
Apparently, C7D8 with its lower polarity and a larger molecular
size could not stabilize the cationic species by coordination,1i-j,15
while CD2Cl2 with its medium polarity is small in size for
(7) Steinberger, H.-U.; Mu¨ller, T.; Auner, N.; Maerker, C.; Schleyer, P. v.
R. Angew. Chem., Int. Ed. Engl. 1997, 36, 626-628.
(8) Olah, G. A.; Rasul, G.; Heiliger, L.; Bausch, J.; Prakash, G. K. S. J.
Am. Chem. Soc. 1992, 114, 7737-7742.
(9) (a) Murthy, S.; Nagano, Y.; Beauchamp, J. L. J. Am. Chem. Soc. 1992,
114, 3573-3574. (b) Nagano, Y.; Murthy, S.; Beauchamp, J. L. J. Am. Chem.
Soc. 1993, 115, 10805-10811.
(13) Nishinaga, T.; Izukawa, Y.; Komatsu, K. Chem. Lett. 1998, 269-
270.
(14) As to the hydride abstraction reaction, ∼0.5 equiv trityl cation was
not consumed and about 6% of 5 and mesitylene were also formed. The fate
of the possible species such as Mes-Si+ generated on the ring contraction of
2 is not known, but such a species and its degradation products would also
work to abstract hydride from 4.
(15) Schleyer, P. v. R.; Buzek, P.; Mu¨ller, T.; Apeloig, Y.; Siehl, H. U.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1471-1473. see also Olsson, L.;
Cremer, D. Chem. Phys. Lett. 1993, 215, 433-443.
(10) Jarek, R. L.; Shin, S. K. J. Am. Chem. Soc. 1997, 119, 6376-6383.
(11) Nicolaides, A.; Radom, L. J. Am. Chem. Soc. 1994, 116, 9769-9770;
1996, 118, 10561-10570; 1997, 119, 11933-11937.
(12) (a) Komatsu, K.; Akamatsu, H.; Jinbu, Y.; Okamoto, K. J. Am. Chem.
Soc. 1988, 110, 633-634. (b) Komatsu, K.; Akamatsu, H.; Aonuma, S.; Jinbu,
Y.; Maekawa, N.; Takeuchi, K. Tetrahedron 1991, 47, 6951-6996. (c)
Komatsu, K.; Nishinaga, T.; Maekawa, N.; Kagayama, A.; Takeuchi, K. J.
Org. Chem. 1994, 59, 7316-7321.
10.1021/ja0014685 CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/06/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 38, 2000 9313
Table 1. 13C NMR Chemical Shifts for 2 at -50 °C in CD2Cl2
phenyltropylium ion 6 upon going from the neutral precursor (δbh
2.88 (2H), 2.67 (4H)) to the ion (δbh 4.09, 4.06, 3.17).12c Also
the nucleus independent chemical shift23 at 1 Å above the ring
(NICS(1))24,26 and magnetic susceptibility exaltation (Λ)25,26
calculated for the mother compound 1 (NICS(1) ) -7.3, Λ )
-15.6) showed that the aromaticity of 1 approaches that of the
tropylium ion (NICS(1) ) -10.7, Λ ) -20.1).
C(sp2)
C(sp3)
tropyl
aryl
CH
CH2
CH3
175.9
153.2
149.7
144.8
143.8
128.0
118.7
36.8
35.1
34.9
25.1
24.2
23.9
25.9
21.4
approaching the silylium-ion center to coordinate to it. Cation 2
is stable at temperatures below -50 °C, but it slowly abstracts
the chloride ion from CD2Cl2 to furnish dichlorosilepin 3 at -50
°C (t1/2 ≈ 6 h).16 Such an abstraction of the chloride ion from
CH2Cl2 by the silylium ion has been previously reported.3,17 When
the temperature was raised, 2 decomposed to give 5 as was
observed in C7D8 at low temperature.
The 13C NMR signals (Table 1) for the sp2 carbons of 2 in
CD2Cl2 (δ 175.9, 153.2, 149.7 ppm) showed downfield shifts
compared with 4 (δ 152.0, 145.5, 141.8), indicating considerable
positive-charge delocalization in the seven-membered ring. The
most downfield-shifted signal (δ 175.9) is assigned to the â-carbon
judging from the calculated chemical shifts for 1 (R, 136.5; â,
167.6; γ, 141.0).18 These results imply that a contribution of the
sila-allylic resonance structure is important for the positive-charge
delocalization.
To estimate the degree of interaction between the silatropylium
ion and the solvent molecule, the optimized structure and its
energy for 1‚CH2Cl2 were calculated at the B3LYP/6-31G* level.
The Si-Cl distance (2.735 Å) for 1‚CH2Cl2 was found to be
considerably longer than that for Me3Si-Cl (2.11 Å) as a model
compound for the covalent molecule. This Si-Cl distance for
1‚CH2Cl2 was still 0.33 Å longer than that for Me3Si+‚CH2Cl2
(2.41 Å), indicating that the cyclic conjugation of the silatropylium
ion reduces the interaction between the silylium ion and CH2Cl2.
Furthermore, although some stabilization energy is present for
1‚CH2Cl2 (7.7 kcal mol-1) as estimated from the energy difference
between 1‚CH2Cl2 and the sum of 1 and CH2Cl2, it is smaller
due to the cyclic π-delocalization than the case for Me3Si+‚CH2-
Cl2 (18.2 kcal mol-1). In addition, the bulky sustituent on silicon
of 2 should have prevented the solvent molecule to approach the
silylium-ion center.
The 29Si NMR signal of 2 was observed at δ 142.9 ppm in
CD2Cl2, which is 192.2 ppm downfield-shifted compared with
the precursor silepin 4 (δ -49.3). This is taken as the clear
evidence for the silylium-ion character of 2. The observed 29Si
chemical shift is also in fair agreement with the value calculated
for the mother compound 1 (δ 158.1).18,19 This method of
calculation is known to reproduce the experimental value observed
for the trimesitylsilylium ion.20 This 29Si chemical shift of 2 is
28 ppm downfield-shifted compared with the solid-state NMR
chemical shift (δ 115) of i-Pr3Si+ (Cl6-CB11H6)- which was
shown to have some interaction between the silylium-ion center
and the chlorine atom of the counterion.6c Thus, the interaction
between the silylium-ion center of 2 and CD2Cl2 should be smaller
than the case for i-Pr3Si+ (Cl6-CB11H6)-: the ion 2 may be
considered as a nearly free silylium ion.
The chemical shift of the BCO’s bridgehead (bh) protons is
useful for judging the presence of diamagnetic ring current
because they are rigidly fixed on the same plane as the
silatropylium-ion ring.21 As shown in Figure 1, the signals for
these protons were observed at δ 3.70, 3.64, and 3.2122 ppm,
which were about 1 ppm downfield-shifted compared with those
for the neutral precursor 4 (δbh 2.81 (4H), 2.61 (2H)). This is
taken as the experimental proof for the aromatic ring current in
2. Similar downfield shifts were observed in the corresponding
In summary, we have succeeded in the first NMR observation
of the silatropylium ion with the aid of annelation with rigid BCO
frameworks and a bulky substituent on silicon. This study revealed
not only that the silatropylium ion is aromatic but that the aromatic
stabilization is effective for reducing the interaction between a
solvent and silylium ion. Efforts aiming at the synthesis of a
solvent-free isolable silatropylium ion are now under way.
(16) The possible mechanism is considered as follows: the produced
chloromethyl cation exerts an electrophilic aromatic substitution on the mesityl
ring of the generated mesitylchlorosilepin to cleave the Si-Mes bond, and
the second chloride abstraction by the chlorosilatropylium ion from the
(chloromethyl)mesitylene furnishes dichlorosilepin 3. The formation of
tetramethylbenzene-d2 (C10H12D2) was confirmed by GC-MS analysis.
(17) Kira, M.; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992, 114, 6697-
6700.
Acknowledgment. We thank Professor Robert West, University of
Wisconsin, for continued encouragement for the present study. This work
was supported by a Grant-in-Aid for Scientific Research on Priority Areas
(A) (No. 10146102) and a Grant-in-Aid for Scientific Research (A) (No.
09304060) from the Ministry of Education, Science, Sports and Culture,
Japan.
(18) Calculated at HF/GIAO/6-311+G(2df,p) (Si), 6-31G* (C, H)//B3LYP/
6-31G*.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.;
Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.;
Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski,
J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J.
V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.5; Gaussian,
Inc.: Pittsburgh, PA, 1998.
Note Added in Proof: A homoconjugated silylium ion, the
homocyclotrisilenylium ion, was recently synthesized by Sekigu-
chi’s group.27
Supporting Information Available: Experimental details describing
the synthesis and characterization of compound 4 and the generation of
2, and 1H, 13C, and 29Si NMR spectra for 2 in CD2Cl2 (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(20) Mu¨ller, T.; Zhao, Y.; Lambert, J. B. Organometallics 1998, 17, 278-
280.
JA0014685
(21) (a) Nishinaga, T.; Komatsu, K.; Sugita, N. J. Chem. Soc., Chem.
Commun. 1994, 2319-2320. (b) Nishinaga, T.; Kawamura, T.; Komatsu, K.
J. Org. Chem. 1997, 62, 5354-5362. (c) Nishinaga, T.; Wakamiya, A.
Komatsu, K. Chem. Commun. 1999, 777-778.
(23) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N.
J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317-6318.
(24) Calculated at HF/GIAO/6-31+G*//B3LYP/6-31G*.
(22) This signal resonates at a relatively high field because it would
correspond to the proton (Ha) that sticks out in the shielding zone of mesityl
ring as in the case of one of the bh protons (Ha) in the phenyltropylium ion
annelated with BCO units: 12c.
(25) Calculated at HF/CSGT/6-31+G*//B3LYP/6-31G*.
(26) For the method of calculations see Subramanian, G.; Schleyer, P. v.
R.; Jiao, H. Organometallics 1997, 16, 2362-2369.
(27) Sekiguchi, A.; Matsuno, T.; Ichinohe, M. J. Am. Chem. Soc., in press.