Synthesis, structure, and reactions of a novel triarylsilanol with a bowl-type framework: A silanol extremely resistant to self-condensation

Article abstract of DOI:10.1246/cl.2001.1258

A novel nano-scale triarylsilanol bearing a bowl-shaped framework was synthesized, the structure of which was established by X-ray crystallographic analysis. The silanol was found to be extremely resistant to self-condensation whereas it reacted easily with appropriate molecules to give the corresponding derivatives.

Full text of DOI:10.1246/cl.2001.1258

1258
Chemistry Letters 2001
Synthesis, Structure, and Reactions of a Novel Triarylsilanol with a Bowl-Type Framework:
A Silanol Extremely Resistant to Self-Condensation
Kei Goto,* Tomoko Okumura, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received August 30, 2001; CL-010850)
A novel nano-scale triarylsilanol bearing a bowl-shaped
framework was synthesized, the structure of which was established
by X-ray crystallographic analysis. The silanol was found to be
extremely resistant to self-condensation whereas it reacted easily
with appropriate molecules to give the corresponding derivatives.
almost the same as that of 4 and seems less heavier than that of 3,
because there is no ortho-substituent on the aromatic rings con-
nected to the silicon atom.
TRMS–OH (5) was prepared from bromide 6 and isolated as
colorless crystals (Scheme 1).6 In the 1H and 13C NMR spectra, all
the methyl signals appeared equivalent, indicating that the Si–C
bonds rotate rapidly on the NMR time-scale. The IR spectra of 5
in a CH2Cl2 solution showed a band at 3648 cm–1, which is assign-
able to a free SiOH. This band was found to be concentration-
independent, indicating that silanol 5 has a monomeric structure in
solution.
A variety of bulky silyl groups have been developed so far
and widely applied to the kinetic stabilization of various highly
reactive species as well as to the protection of functional groups in
organic synthesis.1 In the course of our study on the development
and application of the bowl-shaped steric protection groups,2 we
have recently designed a novel triarylmethyl group 1 (denoted as
Trm), where the radially extended m-terphenyl units forms a large
bowl-type cavity.3 The reactive species bearing the Trm group can
be prevented from dimerization or self-condensation effectively
whereas its reactivity towards appropriate molecules is not so
much degenerated because there is a relatively large space around
it. By taking advantage of the Trm group, a stable S-nitrosothiol
(TrmSNO) was successfully synthesized.3 It is expected that the
silyl group 2 (tris(2,2",6,6"-tetramethyl-m-terphenyl-5'-yl)silyl;
denoted as TRMS hereafter), where the central carbon atom of the
Trm group is replaced by a silicon atom, will be useful as a nano-
scale silyl group possessing the above-mentioned characteristics of
the Trm group. We report here the synthesis of a silanol extremely
resistant to the self-condensation by taking advantage the TRMS
group along with its crystal structure and reactivity.4,5
A single crystal of 5 grown from a hexane–benzene solution
was studied by X-ray diffraction, and its structure is shown in
Figure 2 with selected bond lengths and angles.7 The figure shows
that the OH group is surrounded from all sides and from a distance
as expected by the molecular model. It was found that 5 is solvat-
ed by one molecule of hexane, which does not show any particular
interaction with the SiOH group. Usually, silanols form strong
intermolecular OH···O hydrogen bonding to give oligomeric struc-
tures in the crystalline state. For example, Ph3SiOH (4) was
reported to have a tetrameric structure with the intermolecular
O···O distances of 2.63–2.68 Å.8 On the other hand, the shortest
intermolecular O···O distance of 5 is 7.80 Å, indicating the absence
of OH···O hydrogen bonding. Recently, Eaborn et al. reported that,
in the crystal structures of some sterically hindered silanols, the
intermolecular OH···π interaction was observed.9 Examination of
the crystal structure of 5 revealed that the OH hydrogen atom lies
above one of the 2,6-dimethylphenyl rings (ring A in Figure 2) of
the neighboring molecule at a distance of 2.64 Å from its center,
suggesting the possible presence of the intermolecular OH···π
interaction in the crystals. The IR spectrum of 5 in the solid state
(KBr) showed its ν(OH) band at 3598 cm–1, slightly lower
wavenumber than that in solution, also indicating that there is weak
Figure 1 shows the molecular models of (t-Bu)3SiOH (3),
Ph3SiOH (4), and TRMS–OH (5). Inspection of the model of 5
indicates that the central silicon atom is embedded in the shallow
bowl-shaped cavity with a diameter of about 16 Å and a depth of
about 1.5 Å. Despite the bulkiness of the whole molecule, the
steric congestion in the close vicinity of the OH group of 5 is
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1259
interaction in the solid state. These results suggest that the bowl-
shaped framework of 5 sterically prevents the formation of the
OH···O hydrogen bonding, thus enabling the weak intermolecular
OH···π interaction to be observed.
was further converted to fluorosilane 10 and hydrosilane 11 in
good yields. The reaction of 11 with elemental sulfur in decalin13
afforded the corresponding silanethiol 12. Silanethiol 12 can be
stored for more than several months in an open atmosphere.
In summary, a novel nano-scale silyl group with a bowl-
shaped cavity, the TRMS group, was designed, and a silanol and
its derivatives bearing this framework were synthesized. Further
investigations on the application of the TRMS group to the stabi-
lization of highly reactive species as well as to the synthetic use
are currently in progress.
This work was partly supported by Grants-in-Aid for
Scientific Research (Nos. 12304037 (T.K.) and 12874075
(K.G.)) from the Ministry of Education, Culture, Sports, Science
and Technology, Japan. We also thank Shin-etsu Chemical Co.,
Ltd. and Tosoh Finechem Corporation for the generous gifts of
chlorosilanes and alkyllithiums, respectively.
It is notable that the sum of the C–Si–C bond angles of 5 is
326.3°, somewhat smaller than that of Ph3SiOH (4) (331°, the
mean of those of eight fragments).8 This is in contrast with other
bulky silyl derivatives such as trimesitylsilyl azide10 and tri(9-
anthryl)silyl fluoride,11 for which the sums of the C–Si–C angles
increase to 342.1° and 345.7°, respectively. These results indi-
cate that, in the dendrimer-type silanol 5, the molecular size is
enlarged in comparison with 4 without increasing steric repulsion
among three aryl groups.
It is known that Ph3SiOH (4) is fairly resistant to self-conden-
sation and stable under acidic conditions. It was reported, howev-
er, that 4 undergoes condensation to disiloxane 7 upon heating
under basic conditions.12 Actually, when 4 was heated at 100 °C
in the presence of NaOH in water/dioxane (3:7), 4 was converted
to 7 in 60% yield (Scheme 2). By contrast, no condensation was
observed when TRMS–OH (5) was heated under the same condi-
tions (Scheme 2). Silanol 5 was also stable in the presence of
hydrochloric acid similarly to 4. In spite of such high stability
towards self-condensation, 5 readily reacted with Me3SiOH gener-
ated in situ by hydrolysis of Me3SiCl to give the unsymmetrical
disiloxane 8, indicating that there is a space around the SiOH func-
tionality of 5 large enough for its reaction with an appropriate mol-
ecule. Silanol 5 can be readily converted to various derivatives by
the usual methods (Scheme 3). Treatment of 5 with acetyl chloride
afforded the corresponding chlorosilane 9 quantitatively, which
References and Notes
1
a) R. J.-R. Hwu, S.-C. Tsay, and B.-L. Cheng, in “The Chemistry of
Organic Silicon Compounds,” ed. by S. Patai and Z. Rappoport, John
Wiley & Sons, New York (1998), Vol. 2, Part 1, p 431. b) T. W. Greene
and P. G. M. Wuts, “Protective Groups in Organic Synthesis,” 3rd ed.,
John Wiley & Sons, New York (1999).
For examples, see: a) K. Goto, N. Tokitoh, and R. Okazaki, Angew.
Chem., Int. Ed. Engl., 34, 1124 (1995). b) T. Saiki, K. Goto, N. Tokitoh,
and R. Okazaki, J. Org. Chem., 61, 2924 (1996). c) K. Goto, M. Holler,
and R. Okazaki, J. Am. Chem. Soc., 119, 1460 (1997). d) T. Saiki, K.
Goto, and R. Okazaki, Angew. Chem., Int. Ed. Engl., 36, 2223 (1997).
e) K. Goto and R. Okazaki, Liebigs Ann./Recueil, 1997, 2393
(Microreview). f) K. Goto, M. Holler, and R. Okazaki, Chem.
Commun., 1998, 1915. g) K. Goto, G. Yamamoto, B. Tan, and R.
Okazaki, Tetrahedron Lett., 42, 4875 (2001).
2
3
4
5
6
K. Goto, Y. Hino, T. Kawashima, M. Kaminaga, E. Yano, G.
Yamamoto, N. Takagi, and S. Nagase, Tetrahedron Lett., 41, 8479
(2000).
A part of this work has been presented: T. Okumura, K. Goto, and T.
Kawashima, 77th Spring Annual Meeting of the Chemical Society of
Japan, Funabashi, March 2000, Abstr., No. 3E427.
Recently, Maruoka et al. reported a tris(2,6-diphenylbenzyl)silyl group
as a highly crowded bowl-shaped protective group: A. Iwasaki, Y.
Kondo, and K. Maruoka, J. Am. Chem. Soc., 122, 10238 (2000).
5: colorless crystals; mp 262.5–263.0 °C; 1H NMR(CDCl3, 500 MHz) δ
1.92 (s, 36H), 2.51 (s, 1H), 7.00 (t, J = 1.5 Hz, 3H), 7.03–7.12 (m,
18H), 7.42 (d, J = 1.5 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 20.8 (q),
127.0 (d), 127.2 (d), 131.4 (d), 133.8 (d), 135.4 (s) 135.8 (s), 140.8 (s),
141.5 (s). IR (CH2Cl2) 3648 cm–1 (νOH). Found: C, 87.74; H, 7.16%.
Calcd for C66H64OSi: C, 87.95; H, 7.16%.
Crystallographic data for 5: C66H64OSi·C6H14, Mr = 987.43, monoclinic,
space group C2/c, a = 31.121(1), b = 15.460(1), c = 26.376(1) Å, β =
110.283(1)°, U = 11903.4(10) Å3, Z = 8, Dc = 1.102 g cm–3, T = 148 K,
µ(Mo Kα) = 0.082 mm–1, R1 = 0.0554 (I > 2σ(I)), wR2 = 0.1577 (all
data).
7
8
9
H. Puff, K. Braun, and H. Reuter, J. Organomet. Chem., 409, 119
(1991).
S. S. Al-Juaid, A. K. A. Al-Nasr, C. Eaborn, and P. B. Hitchcock, J.
Organomet. Chem., 429, C9 (1992).
10 S. S. Zigler, K. J. Haller, R. West, and M. S. Gordon, Organometallics,
8, 1656 (1989).
11 S. Yamaguchi, S. Akiyama, and K. Tamao, Organometallics, 17, 4347
(1998).
12 H. H. Szmant and G. A. Brost, J. Am. Chem. Soc., 72, 5763 (1950).
13 L. Birkofer, A. Ritter, and H. Goller, Chem. Ber., 96, 3289 (1963).