Novel meta-selective Friedel-Crafts acylation of phenylsilsesquioxane

Article abstract of DOI:10.1246/cl.2010.1082

The Friedel-Crafts acylation of octaphenylsilsesquioxane (T ₈Ph₈) (1) with Cl₂CHOCH₃ or CH ₃COCl at low temperature followed by subsequent hydrolysis provided the octakis(macylphenyl)octasilsesquioxanes (T₈(C₆ H ₄R-m)₈); R = CHO (2a) and R = COCH₃ (2b) in high yields. The remarkable metaselectivity of 2a and 2b in more than 99:1 and 93:7 meta/para ratios was confirmed by ¹HNMR and X-ray analyses, and further elucidated via theoretical calculations.

Full text of DOI:10.1246/cl.2010.1082

doi:10.1246/cl.2010.1082
1082
Published on the web September 4, 2010
Novel meta-Selective Friedel-Crafts Acylation of Phenylsilsesquioxane
Yoshiteru Kawakami and Yoshio Kabe*
Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-1293
(Received July 14, 2010; CL-100632; E-mail: kabe@kanagawa-u.ac.jp)
The Friedel-Crafts acylation of octaphenylsilsesquioxane
(T8Ph8) (1) with Cl2CHOCH3 or CH3COCl at low temperature
followed by subsequent hydrolysis provided the octakis(m-
acylphenyl)octasilsesquioxanes (T8(C6H4R-m)8); R = CHO (2a)
and R = COCH3 (2b) in high yields. The remarkable meta-
selectivity of 2a and 2b in more than 99:1 and 93:7 meta/para
ratios was confirmed by 1H NMR and X-ray analyses, and
further elucidated via theoretical calculations.
Scheme 1. meta-Selective acylation of T8Ph8.
Polyhedral oligomeric silsesquioxanes (POSSs)1a represent-
ed by the formula (RSiO3/2)n (n = 6, 8, 10, 12, +) have been
intriguing the scientific community for over half a century. The
octahedral octasilsesquioxanes (T8), having a cubic Si8O12 cage
with a variety of functional groups attached to each silicon
vertex, are of especially significant importance due to their
highly symmetric rigid framework and multifunctionality. These
functionalized octasilsesquioxanes (T8) have shown great
potential for many applications such as supports for transition-
metal catalysts,1b liquid crystals,1c dendrimers,1d-1f and network
solids.1g Because of this wide variety of potential applications,
the need for a variety of functionalized silsesquioxanes has
increased.
Functionalization of octasilsesquioxanes (T8) has generally
been achieved via either hydrosilylation of octahydridosilses-
quioxane (T8H8),2 the Heck coupling and cross-metathesis of
octavinylsilsesquioxane (T8(CH=CH2)8),3 or substitution or
cross coupling of octabenzyl- and octaarylsilsesquioxanes
(T8(C6H4X)8, X = CH2I and I).4 An alternative pathway to
functionalized octasilsesquioxanes (T8) was demonstrated by
the electrophilic substitution of octaphenylsilsesquioxane
(T8Ph8) to give T8(C6H4R)8 (R = NO2, Br, and SO3H).5,6
However, electrophilic substitution often gives limited sub-
stitutional selectivity. For example, nitration of T8Ph8 in
fuming nitric acid gives a 20:65:15 ortho:meta:para mixture
of T8(C6H4NO2)8. Bromination of T8Ph8 gives a 20:15:65
ortho:meta:para mixture of T8(C6H4Br)8,5b although iodination
of T8Ph8 with ICl at ¹40 °C gives T8(C6H4I-p)8 with >93%
para-substitution.4b Here we have found a novel meta-selective
Friedel-Crafts formylation, as well as the acylation of T8Ph8 (1),
toward a goal of the functionalization of silsesquioxane by acyl
groups, as shown in Scheme 1.
Figure 1. X-ray structures of (a) 2a¢CHCl3 and (b) 2b. All
ellipsoids are shown at the 30% probability level. Hydrogen
atoms are omitted for clarity except for the formyl proton in 2a.
increased to more than 99:1 (Run 2 in Table 1). However, both
yields were still low (21% and 41% yields determined by
1H NMR) as evidenced by recovered SiPh groups observed in
the 1H NMR spectra of the crude mixtures.6 In order to improve
the yields, larger amounts of Cl2CHOCH3 and AlCl3 were used
at the same temperature (¹40 °C), as shown in Table 1 (Runs 3
and 4). The use of 2.0 equivalents of Cl2CHOCH3 and 4.0
equivalents of AlCl3 resulted in 86% yield, and octakis(m-
formylphenyl)octasilsesquioxane (2a) was obtained in 70%
isolated yield (>99:1 meta/para) by column chromatography.6
1
In the H NMR spectrum of 2a, the spin-coupling pattern of the
aromatic protons at 8.34 (s), 8.03 (d), 8.01 (d), and 7.61 (t) ppm
confirmed assignment of the meta-substituted structure.6 Crys-
tallization of 2a from mixed solvent of chloroform and hexane
gave crystals suitable for X-ray analysis, which disclosed eight
meta-formyl groups attached to each vertex of the T8 cage, as
shown in Figure 1a.6
In the first experiment, Cl2CHOCH3 (6.0 mmol) in ca.
10-mL CH2Cl2 was added dropwise into a ca. 30-mL CH2Cl2
solution of octaphenylsilsesquioxane (1) (T8Ph8, 4.0 mmol based
on Ph groups) and AlCl3 (4.0 mmol). After 2 h of stirring at
0 °C, subsequent hydrolysis provided a mixture of formylated
products T8(C6H4CHO)n(Ph)8¹n (n = 1-8), as shown in Table 1
(Run 1 and Scheme 1). The 96:4 meta/para ratio of the
The acetylation of T8Ph8 (1) was also carried out using
CH3COCl, giving
a
mixture of acetylated products
T8(C6H4COCH3)n(Ph)8¹n (n = 1-8). The results are summarized
in Table 1 (Runs 5-8 and Scheme 1). The meta/para ratio was
determined by 1H NMR integration of the acetyl methyl protons
at around 2.5 ppm.6 Although the use of the optimized
formylation reaction conditions gives a high meta/para ratio
(>99:1), the yield is unacceptably low, as shown in Table 1
(Run 5). When the reaction temperature was raised from ¹40 °C
1
formylated products was determined by H NMR integration of
the formyl proton at around 10.0 ppm.6 When the temperature
was reduced from 0 to ¹40 °C, this high meta/para ratio further
Chem. Lett. 2010, 39, 1082-1083
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1083
Table 1. Acylation condition and yields of 2a and 2b
a
Run
Reagent
(equiv)a
AlCl3
Temp/°C
Time/h
meta/para
Conversionb
1
2
3
4
5
6
7
8
Cl2CHOMe (2a)
1.5
1.5
2.0
2.0
2.0
2.0
4.0
4.0
1.0
1.0
2.0
4.0
4.0
4.0
4.0
4.0
0
¹40
¹40
¹40
¹40
¹20
0
2
4
4
4
4
96/4
>99/1
>99/1
>99/1
>99/1
98/2
21.0
41.0
76.5
86.8 (70.3)c
5.7
50.9
90.4 (72.9)c
99.5 (90.2)c
CH3COCl (2b)
4
4
92/8
93/7
0
16
c
aBased on Ph group. bYields of acylated phenyl groups determined by 1H NMR. Isolated yields of T8(C6H4R)8 (R = CHO and
COCH3).
2081. For POSS-supported transition-metal catalysts: b) F. J.
Feher, T. A. Budzichowski, Polyhedron 1995, 14, 3239. For
liquid crystals: c) J. W. Goodby, G. H. Mehl, I. M. Saez, R. P.
Tuffin, G. Mackenzie, R. Auzély-Velty, T. Benvegnu, D.
Plusquellec, Chem. Commun. 1998, 2057. For the dendrimers:
d) X. Zhang, K. J. Haxton, L. Ropartz, D. J. Cole-Hamilton,
R. E. Morris, J. Chem. Soc., Dalton Trans. 2001, 3261. e) F. J.
Feher, K. D. Wyndham, D. Soulivong, F. Nguyen, J. Chem. Soc.,
Dalton Trans. 1999, 1491. f) K. Tanaka, K. Inafuku, K. Naka, Y.
Chujo, Org. Biomol. Chem. 2008, 6, 3899. For the network
solids: g) C. Zhang, F. Babonneau, C. Bonhomme, R. M. Laine,
C. L. Soles, H. A. Hristov, A. F. Yee, J. Am. Chem. Soc. 1998,
120, 8380.
For the hydrosilylation of T8H8: a) V. W. Day, W. G. Klemperer,
V. V. Mainz, D. M. Millar, J. Am. Chem. Soc. 1985, 107, 8262. b)
P. A. Agaskar, J. Chem. Soc., Chem. Commun. 1992, 1024. c)
P. A. Agaskar, J. Am. Chem. Soc. 1989, 111, 6858. d) M. A. Said,
H. W. Roesky, C. Rennekamp, M. Andruh, H.-G. Schmidt, M.
Noltemeyer, Angew. Chem., Int. Ed. 1999, 38, 661.
For Heck coupling and cross-metathesis of T8(CH=CH2)8:
a) P.-A. Jaffrès, R. E. Morris, J. Chem. Soc., Dalton Trans.
1998, 2767. b) G. Cheng, N. R. Vautravers, R. E. Morris, D. J.
Cole-Hamilton, Org. Biomol. Chem. 2008, 6, 4662. c) Y. Itami,
B. Marciniec, M. Kubicki, Chem.®Eur. J. 2004, 10, 1239. d)
M. Y. Lo, C. Zhen, M. Lauters, G. E. Jabbour, A. Sellinger,
J. Am. Chem. Soc. 2007, 129, 5808.
Figure 2. ·-Complexes for the acetylation reaction of T8Ph8.
to ¹20 to 0 °C, the yield increased from 5% to 50% to 90%
(Runs 6 and 7). Finally, elongation of reaction time from 4 and
16 h led the acetylated product in 99% yield (Run 8). Even
at 0 °C, the meta/para ratio (93:7) was slightly decreased.
Purification of the acetylated products by column chromatog-
raphy gave pure octakis(m-acetylphenyl)octasilsesquioxane (2b)
in 90% isolated yield. Similar to 2a, the 1H NMR showed
aromatic protons at 8.38 (s), 8.07 (d), 8.00 (d), and 7.53 (t) ppm,6
and X-ray analysis revealed that substitution of each of the eight
phenyl groups of the T8 cage occurred at the meta-position, as
shown in Figure 1b.6
Although a few alternative studies of meta-functionalization
have been reported recently,7 no meta-substitution via traditional
Friedel-Crafts reaction has been discussed. In order to under-
stand the meta-selectivity, four different methods were used to
calculate the charge density for the aromatic rings in the T8Ph8
and related phenylsilanes such as PhSiMe3, PhSiCl3, and
PhSi(OMe)3.6 All calculations show the highest negative charge
at the ipso-position, and the second highest at the meta-position
from the silyl-substituent.6 When the acyl cation cannot attack
the ipso-position sterically and electronically to afford the ·-
complex 3, the reaction occurs at the meta-position to give 4
(Figure 2), a trend which has precedents.6 Thus, the ·-complex
4 derived from meta attack of the acyl cation is stabilized by
·-³ conjugation compared to other ·-complexes 5 and 6
derived from ortho- and para attack leading exclusively to meta-
substitution (Figure 2).8
2
3
4
5
For substitution and cross coupling of T8(C6H4X)8, X = CH2I,
SiMe3, and I: a) F. J. Feher, T. A. Budzichowski, J. Organomet.
Chem. 1989, 379, 33. b) M. F. Roll, M. Z. Asuncion, J. Kampf,
R. M. Laine, ACS Nano 2008, 2, 320.
For electrophilic substitution of T8Ph8: a) R. Tamaki, Y. Tanaka,
M. Z. Asuncion, J. Choi, R. M. Laine, J. Am. Chem. Soc. 2001,
123, 12416. b) C. M. Brick, Y. Ouchi, Y. Chujo, R. M. Laine,
Macromolecules 2005, 38, 4661. c) C. Hartmann-Thompson, A.
Merrington, P. I. Carver, D. L. Keeley, J. L. Rousseau, D. Hucul,
K. J. Bruza, L. S. Thomas, S. E. Keinath, R. M. Nowak, D. M.
Katona, P. R. Santurri, J. Appl. Polym. Sci. 2008, 110, 958.
See the Supporting Information for details of the experimental
procedure, spectroscopic and X-ray data, and theoretical calcu-
lations, which is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
a) R. J. Phipps, M. J. Gaunt, Science 2009, 323, 1593. b) C. J.
Rohbogner, G. C. Clososki, P. Knochel, Angew. Chem., Int. Ed.
2008, 47, 1503. c) J. P. Flemming, M. B. Berry, J. M. Brown,
Org. Biomol. Chem. 2008, 6, 1215. d) H. W. G. van Herwijnen,
U. H. Brinker, J. Org. Chem. 2001, 66, 2874.
6
7
In conclusion, a unique meta-selective Friedel-Crafts for-
mylation and acetylation of octaphenylsilsesquioxane (T8Ph8)
has been accomplished. The reasons for the meta-selectivity are
understood in terms of the charge density of the aromatic ring as
well as stabilization of the ·-complex intermediate.
8
In contrast to the charge-controlled reaction of a hard elecrophile
such as ClCHOCH3+, CH3CO+, and NO2+, soft electrophile
such as Br+ and I+ probably undergo a frontier orbital controlled
reaction to affored ortho, para-selectivity. Summary of regio
selectivity of electrophilic aromatic substitution to phenylsilanes
is also involving Supporting Information.6
References and Notes
1
For recent reviews of polyhedral oligomeric silsesquioxanes: a)
D. B. Cordes, P. D. Lickiss, F. Rataboul, Chem. Rev. 2010, 110,
Chem. Lett. 2010, 39, 1082-1083
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/