A new highly sterically demanding silyl (TEDAMS) group. Synthesis by multiple substitution of tris(diphenylmethyl)silane with diarylcarbenium ions

Article abstract of DOI:10.1016/j.tetlet.2010.05.140

A highly efficient electrochemical method for synthesis of halosilanes bearing bulky substituents was developed. Electrophilic multiple substitution on tris(diphenylmethyl)silane with diarylcarbenium ions afforded a highly sterically demanding hydrosilane

Full text of DOI:10.1016/j.tetlet.2010.05.140

Tetrahedron Letters 51 (2010) 4107–4109
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new highly sterically demanding silyl (TEDAMS) group. Synthesis by
multiple substitution of tris(diphenylmethyl)silane with diarylcarbenium ions
*
Kimitada Terao, Takashi Watanabe, Takafumi Suehiro, Toshiki Nokami, Jun-ichi Yoshida
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly efficient electrochemical method for synthesis of halosilanes bearing bulky substituents was
developed. Electrophilic multiple substitution on tris(diphenylmethyl)silane with diarylcarbenium ions
afforded a highly sterically demanding hydrosilane, tris(extended diarylmethyl)silane (TEDAMS-H),
which was converted to the corresponding bromosilane (TEDAMS-Br).
Received 1 May 2010
Revised 22 May 2010
Accepted 28 May 2010
Available online 1 June 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Organosilanes
Electrophilic substitution
Carbocation
Dendritic molecule
Silyl groups bearing extremely bulky substituents have received
significant research interest from view point of sterically
Ph
Ph
Ph Si
a
demanding substituents.1–4 For example, extensive studies have
been made on the chemistry of trityldimethylsilyl (TrDMS)
group,5–7 tris(trimethylsilyl)-silyl (TTMSS or supersilyl) group,
which contains UV labile Si–Si bond,8–12 bowl-shaped tris(2,6-
diphenylbenzyl)silyl (TDS) group13,14 and tris(2,200,6,600-tetra-
methyl-m-terphenyl-50-yl)silyl (TRMS) group15 (Fig. 1). We report
herein the synthesis of a new highly sterically demanding silyl (tri-
s(extended diarylmethyl)silyl, TEDAMS) group based on multiple
substitution of tris(diarylmethyl)silane with diarylcarbenium ions.
The present works stem from our earlier work16 on the
construction of dendritic molecules using the cation pool meth-
od.17–26 During the course of our study, we realized that a CH2SiR3
group is an excellent activating group of a phenyl group27 toward
the electrophilic substitution with diarylcarbenium ions.28,29 Thus,
we envisioned that organosilanes bearing multiple diphenylmethyl
groups serve as effective cores for construction of sterically
demanding silyl groups by electrophilic multiple substitution with
diarylcarbenium ions (Fig. 2).30,31
Me
Me
Me
Si
Me Si Si Si Me
Me Me
Me
Me
Me
Me
Ph
Ph Ph
Si
TTMSS (Supersilyl)
Ar
TDS
Ar
trityldimethylsilyl
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Si
Si
(present work)
TRMS
Ar = 2,6-dimethylphenyl
TEDAMS
Ar
ArAr
Ar Ar = p-FC6H4
Figure 1. Sterically demanding silyl groups.
Conventionally, bulky silyl groups are synthesized by nucleo-
philic substitution on silicon using the corresponding organomag-
nesium13 or organolithium15 reagents. This method requires
multisteps such as preparation of bulky organometal reagents
and manipulation of a latent halogen substituent on a silicon atom.
The present method serves as an alternative approach, which may
* Corresponding author. Tel.: +81 75 383 2726; fax: +81 75 383 2725.
E-mail address: yoshida@sbchem.kyoto-u.ac.jp (J. Yoshida).
Figure 2. Strategies for the synthesis of TEDAMS group.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.140

4108
K. Terao et al. / Tetrahedron Letters 51 (2010) 4107–4109
n-BuLi
HSiMenCl3-n
anodic
oxidation
F
F
F
F
Ph2CHLi
Ph2CH2
HSiMen(CHPh2)3-n
Bu4NBF4
CH2Cl2, -78 o
C
4
3
(10 equiv)
Me
Si
H
Si
H
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Si
H
Si
H
1 68%
2 67%
Figure 3. Preparation of hydrosilanes equipped with several diphenylmethyl
groups.
2
Ar
Ar
Ar
Ar
expand the scope of the chemistry of sterically demanding silyl
groups.
6
Ar = p-FC6H4
(67%)
Figure 5. Electrophilic multisubstitution of hydrosilane 2 with diarylcarbenium ion
4.
We initiated our work by preparing organosilicon compounds
which have multiple diphenylmethyl groups (Fig. 3). These
organosilicon compounds were prepared by the reaction of diph-
enylmethyllithium with correspondingchlorosilanes. Bis (diphenyl-
methyl)methylsilane 1 and tris(diphenylmethyl)silane 2 were
obtained in 68% and 67% yields, respectively.
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Next, we examined electrophilic multisubstitution of
bis(diphenylmethyl)methylsilane 1 with diarylcarbenium ion 4,
which was prepared from di(p-fluorodiphenyl)methane 3 (8 equiv)
by the low-temperature electrochemical oxidation based on the
‘cation pool’ method (Fig. 4).32,33 The tetrasubstituted product 5
was obtained in 41% yield, in which hydride was converted to fluo-
ride. In this case, both the substitution on the phenyl groups and
the fluorination occurred. Presumably, the hydride was abstracted
by the diarylcarbenium ion followed by the fluorination with tetra-
fluoroborate counter anion (BF4ꢀ).34 In fact, it is known that trip-
henylcarbenium perchlorate can abstract the hydride from
hydosilanes.35–37
Ar
Ar
Ar
Ar
Si
Br
Si
H
Br2
CH2Cl2, rt
Ar
Ar
Ar
Ar
Ar
Ar Ar
Ar
7
6
(87%)
Ar = p-FC6H4
Ar = p-FC6H4
However, no fluorination was observed for the reaction of
tris(diphenylmethyl)silane 2 with diarylcarbenium ion 4 (Fig. 5).
The corresponding hexasubstituted hydrosilane 6 was obtained
in 67% yield. This is probably because of higher steric hindrance
around the silicon atom in 2 than that in 1.
Finally, we investigated the conversion of hydrosilane 6 to the
corresponding bromosilane 7, which may be more useful for the
introduction of TEDAMS group (Fig. 6). It was reported that
tris(2,6-diphenylbenzyl)silane (TDS-H) was converted to the corre-
sponding bowl-shaped bromosilane quantitatively by the reaction
with Br2 in 1 h at ꢀ78 °C.13 Hydrosilane 6 was recovered when it
was treated with Br2 at ꢀ78 °C, indicating that hydride in 6 is more
sterically hindered than TDS-H. However, the corresponding
Figure 6. Conversion of hydrosilane 6 to bromosilane 7. H (white), C (gray), Si
(purple), F (light green), Br (red) are shown in each colors.38,39
bromosilane 7 was obtained in 87% when the reaction was carried
out at room temperature. Bromosilane 7 has reasonable stability
and can be handled in open air, indicating that the bromosilane
moiety is kinetically stabilized by bulky dendritic substituents.
As observed in the computationally optimized structure of bro-
mosilane 7, steric hindrance of substituents is significant (see Sup-
plementary data for details).
anodic
oxidation
F
F
F
F
Bu4NBF4
4
CH2Cl2, -78 o
C
3
(8 equiv)
In summary, we have developed an efficient method for the
construction of a highly sterically demanding silyl (TEDAMS) group
using multiple substitution on the phenyl groups of tris(diphenyl-
methyl)silane with diarylcarbenium ions generated by the ‘cation
pool’ method. Further optimization of the present method and
applications of this highly sterically demanding silyl group are un-
der investigation in our laboratory.
Ar
Ar
Ar
Ar
Ar
Me
Si
Me
Si
F
H
1
Ar
Ar
Acknowledgments
Ar
5 (41%)
Ar = p-FC6H4
The authors gratefully acknowledge partial financial supports
from a Grant-in-Aid for Scientific Research and a Grant-in-Aid for
Figure 4. Electrophilic multisubstitution of hydrosilane 1 with diarylcarbenium ion 4.

K. Terao et al. / Tetrahedron Letters 51 (2010) 4107–4109
4109
17. Yoshida, J.; Suga, S.; Suzuki, S.; Kinomura, N.; Yamamoto, A.; Fujiwara, K. J. Am.
Chem. Soc. 1999, 121, 9546–9549.
18. Suga, S.; Suzuki, S.; Yamamoto, A.; Yoshida, J. J. Am. Chem. Soc. 2000, 122,
10244–10245.
the Global COE program from the MEXT, Japan. T.N. thanks the Asa-
hi Glass Foundation for financial supports. The authors thank Dr.
Keiko Kuwata of Kyoto University for MS analyses.
19. Yoshida, J.; Suga, S. Chem. Eur. J. 2002, 8, 2651–2658.
20. Suga, S.; Watanabe, M.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 14824–
14825.
Supplementary data
21. Suga, S.; Nishida, T.; Yamada, D.; Nagaki, A.; Yoshida, J. J. Am. Chem. Soc. 2004,
126, 14338–14339.
22. Okajima, M.; Suga, S.; Itami, K.; Yoshida, J. J. Am. Chem. Soc. 2005, 127, 6930–
6931.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.05.140.
23. Maruyama, T.; Suga, S.; Yoshida, J. J. Am. Chem. Soc. 2005, 127, 7324–7325.
24. Suga, S.; Matsumoto, K.; Ueoka, K.; Yoshida, J. J. Am. Chem. Soc. 2006, 128,
7710–7711.
References and notes
25. Maruyama, T.; Mizuno, Y.; Shimizu, S.; Suga, S.; Yoshida, J. J. Am. Chem. Soc.
2007, 129, 1902–1903.
26. Nokami, T.; Shibuya, A.; Tsuyama, H.; Suga, S.; Bowers, A. A.; Crich, D.; Yoshida,
J. J. Am. Chem. Soc. 2007, 129, 10922–10928.
27. Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165–195.
28. Okajima, M.; Soga, K.; Nokami, T.; Suga, S.; Yoshida, J. Org. Lett. 2006, 8, 5005–
5007.
29. Okajima, M.; Soga, K.; Watanabe, T.; Terao, K.; Nokami, T.; Suga, S.; Yoshida, J.
Bull. Chem. Soc. Jpn. 2009, 82, 594–599.
1. Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry; Wiley-
Interscience: New York, 2000.
2. Organosilicon Chemistry V; Auner, N., Weis, J., Eds.; Wiley-VCH: Weinheim,
2003.
3. Protective Groups in Organic Synthesis; Greene, T. W., Wuts, P. G. M., Eds., 3rd
ed.; Wiley: New York, 1999.
4. Kocienski, P. J. Protecting Groups, 3rd ed.; Thieme: Stuttgart, 2005.
5. Ager, D. J.; Fleming, I. J. Chem. Res. Synop. 1977, 6–7.
6. Groaning, M. D.; Brengel, G. P.; Meyers, A. I. J. Org. Chem. 1998, 63, 5517–5522.
7. Dilley, A. S.; Romo, D. Org. Lett. 2001, 3, 1535–1538.
8. Kanabus-Kaminska, J. M.; Hawari, J. A.; Griller, D.; Chatgilialoglu, C. J. Am. Chem.
Soc. 1987, 109, 5267–5268.
9. Elsner, F. H.; Woo, H.-G.; Tilley, T. D. J. Am. Chem. Soc. 1988, 110, 313–314.
10. Bock, H.; Meuret, J.; Ruppert, K. Angew. Chem., Int. Ed. Engl. 1993, 32, 414–416.
11. Brook, M. A.; Balduzzi, S.; Mohamed, M.; Gottardo, C. Tetrahedron 1999, 55,
10027–10040.
12. Boxer, M. B.; Akakura, M.; Yamamoto, H. J. Am. Chem. Soc. 2008, 130, 1580–
1582. and references cited therein.
13. Iwasaki, A.; Kondo, Y.; Maruoka, K. J. Am. Chem. Soc. 2000, 122, 10238–10239.
14. Shirakawa, S.; Maruoka, K. Tetrahedron Lett. 2003, 44, 281–284. and references
cited therein.
30. Diphenylmethyldimethyl (benzhydryldimethyl) group has been used as
a
protecting group: Roberson, Z.-H.; Woerpel, K. A. Org. Lett. 2000, 2, 621–623.
31. Peng, Z.-H.; Woerpel, K. A. Org. Lett. 2000, 2, 1379–1381.
32. The electrochemical method in organic synthesis: for example, Yoshida, J.;
Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev. 2008, 108, 2265–2299.
33. Moeller, K. D. Tetrahedron 2000, 59, 9527–9554.
34. When hydrosilane 1 was treated with 1.0 equiv of diarylcarbenium ion 2, the
corresponding fluorosilane was obtained as
determined by NMR).
a major product (67% yield
35. Corey, J. Y. J. Am. Chem. Soc. 1975, 97, 3237–3238.
36. Corey, J. Y.; Gust, D.; Mislow, K. J. Organomet. Chem. 1975, 101, C7–C8.
37. Lambert, J. B.; Sun, H.-n. J. Am. Chem. Soc. 1976, 98, 5611–5615.
38. CONFLEX6: Goto, H.; Osawa, E. J. Chem. Soc., Perkin Trans. 2 1993, 187–198. and
references cited therein.
39. MMFF94s: Halgren, T. A. J. Comput. Chem. 1999, 20, 720–729. and references
cited therein.
15. Goto, K.; Okumura, T.; Kawashima, T. Chem. Lett. 2001, 1258–1259.
16. Nokami, T.; Ohata, K.; Inoue, M.; Tsuyama, H.; Shibuya, A.; Okajima, M.; Soga,
K.; Suga, S.; Yoshida, J. J. Am. Chem. Soc. 2008, 130, 10864–10865.