Synthesis and characterization of (perfluoroaryl)borane-functionalized carbosilane dendrimers and their use as lewis acid catalysts for the hydrosilation of acetophenone

Article abstract of DOI:10.1021/om0205187

The synthesis and characterization of (perfluoroaryl)borane-functionalized carbosilane dendrimers and their use as Lewis acid catalysts for the hydrosilation of acetophenone is discussed. Use of a phenyl spacer and positioning the -OMe group in the meta p

Full text of DOI:10.1021/om0205187

4300
Organometallics 2002, 21, 4300-4302
Syn th esis a n d Ch a r a cter iza tion of
(P er flu or oa r yl)bor a n e-F u n ction a lized Ca r bosila n e
Den d r im er s a n d Th eir Use a s Lew is Acid Ca ta lysts for
th e Hyd r osila tion of Acetop h en on e
Roland Roesler, Bryan J . N. Har, and Warren E. Piers*,1
University of Calgary, Department of Chemistry, 2500 University Drive NW,
Calgary, Alberta, Canada T2N 1N4
Received J uly 2, 2002
Summary: Carbosilane dendrimers capped with 4, 8,
and 12 perfluoroarylborane Lewis acids are prepared via
a self-catalyzed silation of the aryl ether 1 with the
appropriate Si-H terminated dendrimer scaffold. The
dendrimers were fully characterized by spectroscopic
methods and MALDI-TOF mass spectrometry and tested
as catalysts for the hydrosilation of acetophenone using
triethylsilane, exhibiting only slightly inferior activities
in comparison to B(C6F5)3.
catalysts are comparable to (or even better than6) those
of the parent mononuclear systems, but “dendritic
effects” which lower activity have been observed in some
systems. These effects include leaching of the active
metal away from the dendrimeric support,7 dimerization
of active sites on the periphery of the dendrimer,8 and
other steric effects resulting from a high local density
of catalyst sites.
While transition-metal-functionalized dendrimers have
received much attention, few if any examples of den-
drimers adorned with main-group Lewis acids have
appeared.9 Our interest in the chemistry of the strong
Dendrimers are aesthetically pleasing macromol-
ecules formed in iterative synthetic protocols which
build branches from a geometrically defined core such
that the final structures are essentially monodisperse.2
In recent years, research on dendrimers has evolved
from form to function in that well-defined dendrimeric
scaffolds can be adorned at the periphery with groups
that serve some chemical purpose: e.g. catalysis,3 light
harvesting,4 or (as in carborane-containing dendrimers)
neutron capture therapy.5 In the catalysis area, the
focus has mainly been on incorporation of transition-
metal-based catalysts, and several examples of the
preparation and behavior of dendritic catalysts with
well-defined mononuclear organometallic analogues has
been achieved. In general, activities of the dendrimeric
10
organometallic Lewis acid B(C6F5)3 as a catalyst for
organic reactions such as hydrosilation11 and allylstan-
nation12 of carbonyl functions, as well as its important
role as a cocatalyst in olefin polymerization by single-
site catalysts,13 led us to prepare dendrimeric versions
of this catalyst to explore their behavior in these areas.
Since B(C6F5)3 is a relatively expensive Lewis acid, the
possibility of implementing removable dendritic versions
of this catalyst is a primary motivation of this work.
In addition to the above-mentioned reactions, B(C6F5)3
is also an efficient catalyst for the silation of aryl methyl
ethers ArOMe to form ArOSiR3 and CH4 in high yields
and with trivial workup procedures.14 This is thus an
ideal reaction for capping the well-known carbosilane
dendrimers with (perfluoroaryl)borane groups, provided
a suitable borane-functionalized ArOMe reagent can be
prepared. The mechanism of this ether silation reaction
involves the activation of the silane by B(C6F5)3 through
partial abstraction of the silane hydride, followed by
nucleophilic attack of the developing silylium ion by the
(1) Phone: 403-220-5746.Fax: 403-289-9488.E-mail: wpiers@ucalgary.ca.
S. Robert Blair Professor of Chemistry 2000-2005. E. W. R. Steacie
Memorial Fellow, 2001-2003.
(2) (a) Buhleier, E. W.; Wehner, W.; Vo¨gtle, F. Synthesis 1978, 155.
(b) Denkewalter, R. G.; Kolc, J .; Lukasavage, W. J .; U.S. Patent
4,289,872, Sept 15, 1981; U.S. Patent 4,360,646, Nov 23, 1982; U.S.
Patent 4,410,688, Oct 18, 1983. (c) Tomalia, D. A.; Baker, H.; Dewald,
J . R.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J .; Ryder, J .; Smith, P.
Polym. J . (Tokyo) 1985, 17, 117. (d) Tomalia, D. A.; Baker, H.; Dewald,
J . R.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J .; Ryder, J .; Smith, P.
Macromolecules 1986, 19, 2466. (e) Newkome, G. R.; Yao, Z.-Q.; Baker,
G. R.; Gupta, K. J . Organomet. Chem. 1985, 50, 2003.
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Organometallics 2000, 19, 4025. (b) Reetz, M. T.; Lohmer, G.; Schwick-
ardi, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1526.
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J . C.; Kooijman, H.; van Haaren, R. J .; van der Made, A. W.; Spek, A.
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(8) Miedaner, A.; Curtis, C. J .; Barkley, R. M.; DuBois, D. L. Inorg.
Chem. 1994, 33, 5482.
(9) A scandium-containing dendrimer has been employed as a Lewis
acid catalyst: Reetz, M. T.; Giebel, D. Angew. Chem., Int. Ed. 2000,
39, 2498.
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Chardac, F. Chem. Rev. 2001, 101, 2999. (c) Newkome, G. R.; He, E.;
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L. J .; King, A. S. H.; Martin, I. K. Chem. Soc. Rev. 2002, 69. (f) Keijsper,
J . J .; van Leeuwen, P. W. N. M.; van der Made, A. W. Eur. Patent EP
0456317, 1991; Int. Research, U.S. Patent 5,243,079, 1993; Chem.
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L. A.; van Koten, G. Acc. Chem. Res. 1998, 31, 423.
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(11) Parks, D. J .; Blackwell, J . M.; Piers, W. E. J . Org. Chem. 2000,
65, 3090.
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74. (b) Adronov, A.; Frechet, J . M. J . Chem. Commun. 2000, 1701. (c)
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10.1021/om0205187 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 09/17/2002

Communications
Organometallics, Vol. 21, No. 21, 2002 4301
Sch em e 1
Sch em e 2
ether substrate.15 Loss of methane from [Ar(Me)Of
SiR3]+[HB(C6F5)3]- gives the ArOSiR3 product and
regenerates the borane. It follows from this mechanistic
picture that, in addition to a strongly Lewis acidic
borane catalyst, this reaction also requires a sufficiently
nucleophilic ArOMe substrate to proceed efficiently.
With these considerations in mind, we prepared low-
generation dendrimers incorporating 4, 8, and 12 borane
centers using the appropriate carbosilane dendrimer
framework and the borane 1, synthesized as shown in
Scheme 1. The installation of a phenyl linker between
the -OMe and (perfluoroaryl)borane functions was
necessitated by the fact that the ArOMe function in the
borane 2 was not basic enough to engage in the silation
reaction. Use of a phenyl spacer and positioning the
-OMe group in the meta position minimized the pos-
sibility of conjugation between the aryl ether group and
the boron center.16 The aryl ether was synthesized using
standard copper coupling methodology,17 while the B-C
bond-forming step utilized transmetalation with the
Ar2SnMe2 reagent18 and ClB(C6F5)2.19 Although other
reagents were explored (Ar2Zn, for example), this tin-
based route produces 1 cleanly with no aryl group
redistribution and is therefore the method of choice thus
far, although forcing conditions are required to drive
this step to completion. Multinuclear NMR data for 1
(e.g. 11B NMR, 59.7 ppm) are consistent with its form-
ulation as a three-coordinate borane and do not point
to any intermolecular aggregation involving -OMe
coordination to the boron center. Furthermore, by the
Childs method,20 1 has a Lewis acid strength essentially
identical with that of the parent borane B(C6F5)3.
Coupling of borane 1 with carbosilane dendrimers21
terminated by Si-H functions proceeds smoothly with
evolution of CH4, as shown in Scheme 2, to give the
polyfunctional boranes 3a -c, containing 4, 8, and 12
borane functions, respectively. In these reactions, the
borane serves as its own catalyst, and since it is present
in essentially 100% loading, the reactions smoothly
proceed to completion (8 h at room temperature), giving
essentially quantitative yields of high-purity (>95%)
dendrimers. The dendrimeric products are readily soluble
in both saturated hydrocarbon and arene solvents and
have been fully characterized using 1H, 11B, 13C,19F, and
29Si NMR spectroscopy and provide samples for which
elemental analyses are consistent with the expected
empirical formulas. In the case of 3c, the 500 MHz
HMQC spectrum accumulated with a cryoprobe reveals
a highly monodisperse structure, with only trace signals
attributable to defect structures detectable. Negative ion
MALDI-TOF mass spectrometry confirms that mol-
ecules of mass m/z 2769 (3a ), 5850 (3b), and 8532 (3c)
are the products of these reactions (Figure 1). The
parent ion peaks are detectable but much less intense
than peaks which arise from the dendrimers plus a
fluoride ion, or an equivalent of LiF. This pattern is
apparent in all three spectra; in addition, peaks arising
from the loss of a branch (m/z 569) via C-O bond
cleavage are apparent, particularly in the spectrum for
3a . All three compounds are moderately stable in air
(15) Blackwell, J . M.; Foster, K. L.; Beck, V. H.; Piers, W. E. J . Org.
Chem. 1999, 64, 4887.
(16) Synthesis of a borane with a para-substituted aryl ether was
also carried out, but the Lewis acidity of the borane was slightly lower
than that of the meta isomer, and so the latter borane was used for
the synthesis of dendrimers.
(17) Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Tokito, S.;
Taga, Y. J . Am. Chem. Soc. 2000, 122, 1832.
(18) Chambers, R. D.; Chivers, T. J . Chem. Soc. 1964, 4782.
(19) (a) Chambers, R. D.; Chivers, T. J . Chem. Soc. 1965, 3933. (b)
Parks, D. J .; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17,
5492.
(20) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J . Chem. 1982,
60, 801.
(21) (a) For a review of carbosilane dendrimers see: Lang, H.;
Lu¨hmann, B. Adv. Mater. 2001, 13, 1523. (b) For synthetic procedures
to the carbosilane scaffolds used here, see: Seyferth, D.; Son, D. Y.;
Rheingold, A. L.; Ostrander, R. L. Organometallics 1994, 13, 2682.

4302 Organometallics, Vol. 21, No. 21, 2002
Communications
F igu r e 2. Hydrosilation of acetophenone as followed by
the appearance of product over time using B(C6F5)3 (dia-
mond), 3a (circle), 3b (crosshatch), and 3c (square) at 5%
catalyst loading ([B] normalized) and -35 °C.
the linking point are two possible degradation reactions.
The former process may be prevented by incorporating
perfluoroborole groups23 instead of -B(C6F5)2 termini,
while to address the latter scenario, routes to (perfluo-
roaryl)borane dendrimers without these Si-O bonds are
currently being developed.
In conclusion, we have prepared the first examples
of low-generation dendrimeric molecules functionalized
by an important class of (perfluoroaryl)borane. While
perfluoroarylborate dendrimers have appeared,24 the
dendrimers reported herein are more versatile by virtue
of the many applications that B(C6F5)3 has in both
organic25 and organometallic13 chemistry. In addition
to further exploring the organic chemistry preliminarily
discussed here, we are evaluating these dendrimers as
olefin polymerization cocatalysts and developing proce-
dures aimed at recycling the catalyst.
F igu r e 1. MALDI-TOF spectra for dendrimers 3a -c ac-
cumulated using a benzo[a]pyrene matrix. The most in-
tense peaks shown are actually doublets corresponding to
the dendrimer plus a fluoride ion and the dendrimer
plus LiF.
but are seriously hygroscopic and aquo adducts are
formed readily.22 Thus, the samples for MALDI-TOF
were prepared under air-free and anhydrous conditions.
To test the efficacy of these (perfluoroaryl)borane-
functionalized dendrimers in comparison to the parent
borane B(C6F5)3, we examined the hydrosilation of
acetophenone by HSiEt3 as a test reaction.11 This
hydrosilation proceeds cleanly to essentially 100% con-
version over the course of several minutes at 5% catalyst
loadings at room temperature. To conveniently monitor
Ack n ow led gm en t. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada in the form of a Postdoctoral Fellowship to R.R.,
a Discovery Grant to W.E.P., and an E. W. R. Steacie
Fellowship to W.E.P. (2001-2003). We gratefully ac-
knowledge Prof. Susan Lees-Miller and Mr. Mohamed
Bader (Biological Sciences, University of Calgary) for
the use of their MALDI-TOF mass spectrometer, Dr.
Randy Whittal (University of Alberta) for MALDI-TOF
analysis of dendrimers, and Prof. Hans Vogel and Dr.
Deane Mcintyre (Biological Sciences, University of Cal-
gary) for the use of their 500 MHz NMR spectrometer.
1
the reaction by H NMR spectroscopy and compare the
various catalysts, the reaction was followed at -35 °C
over the course of a few hours. As Figure 2 shows, the
dendrimers perform well in comparison to the parent
borane at these loadings, with 3a and 3b retaining
∼80% of the activity. The more crowded 3c operates at
a somewhat slower rate but is still an effective catalyst
under these conditions. These results suggest that each
boron center in the dendrimers is functioning as an
independent catalyst for this hydrosilation reaction and
that, under the conditions of the reaction, deleterious
dendrimer effects are minimal. It should be noted,
however, that in the absence of ketone substrate the
dendrimers do react with triethylsilane over the course
of several hours in ways which are as yet not fully
understood. Boron-carbon bond cleavage to form B-H
moieties19b or silation of the Si-O bonds15 present at
Su p p or tin g In for m a tion Ava ila ble: Text giving full
experimental details for the synthesis of all new compounds,
plus spectroscopic and other characterization data. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
OM0205187
(23) Chase, P. A.; Piers, W. E.; Patrick, B. O. J . Am. Chem. Soc.
2000, 122, 12911.
(24) (a) Becke, S.; Denninger, U.; Mager, M.; Becke, S.; Windisch,
H. (Bayer AG). Eur. Patent PCT/EP99/01558, March 11, 1999. (b)
Mager, M.; Becke, S.; Windisch, H.; Denninger, U. Angew. Chem., Int.
Ed. 2001, 40, 1898.
(22) Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometallics
2001, 20, 4927 and references therein.
(25) (a) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 345. (b)
Ishihara, K.; Yamamoto, H. Eur. J . Org. Chem. 1999, 527, 7.