Well-defined boron-containing polymeric Lewis acids

Article abstract of DOI:10.1021/ja020773i

A general new route to well-defined polymeric Lewis acids via borylation of silylated polymers is reported. Trimethylsilylated polystyrene (PS-Si) of controlled molecular weight and low polydispersity (PDI <1.15) was obtained via atom transfer radical polymerization (ATRP) of 4-(trimethylsilyl)styrene. The functional polymer PS-Si was quantitatively borylated using BBr₃ to give poly(4-dibromoborylstyrene) (PS-B), a novel soluble boron-containing polymeric Lewis acid. PS-B readily reacted with nucleophiles serving as a precursor to a family of new polymers with boron centers of variable Lewis acidity. Reaction of PS-B with Cu(C₆F₅) gave the highly Lewis acidic polymer poly[4-bis(pentafluorophenyl)borylstyrene], the first polymeric analogue of tris(pentafluorophenyl)borane. Copyright

Full text of DOI:10.1021/ja020773i

Published on Web 10/05/2002
Well-Defined Boron-Containing Polymeric Lewis Acids
Yang Qin, Guanglou Cheng, Anand Sundararaman, and Frieder Ja¨kle*
Department of Chemistry, Rutgers UniVersity-Newark, 73 Warren Street, Newark, New Jersey 07102
Received June 3, 2002
Scheme 1. Synthesis of Dibromoborylated Polystyrene (PS-B)
Organoboron compounds play a major role as catalysts and
cocatalysts in organic transformation including polymerization
reactions.1 In a quest for superior catalysts, bifunctional organobo-
ranes have been developed and, in several cases, have been shown
to display enhanced reactivity and selectivity over their monofunc-
tional counterparts.2 However, the synthesis of organoboron
polymers has been a challenging task that only recently was met
with success leading to the discovery of new supported reagents
and immobilized catalysts.3 Boron-containing polymers also serve
as intermediates in the synthesis of functionalized polymers with
polar side groups4-6 and are used as preceramic7 and photolumi-
nescent8 materials.
similarity of the hydrodynamic volume of polystyrene and PS-Si.
A kinetic study of the polymerization reaction confirmed the
“living” nature of the polymerization, thereby demonstrating the
applicability of this methodology to the preparation of a wide variety
of polymers of different molecular weight and architecture, includ-
ing copolymers.
Side-chain organoboron polymers have been prepared from
organoboron monomers using a variety of polymerization tech-
niques including standard free radical polymerization,9 metathesis
polymerization,10 and Ziegler-Natta polymerization.4 Alternatively,
organic polymers or resins can be modified in a postpolymerization
modification step. In this case, the boron centers are typically
attached via multistep polymer modification reactions with low
degrees of functionalization (<40%).11 The direct borylation of
polystyrene with X2BH under forcing conditions has been described,
but occurs with low selectivity.12 In an exciting new development,
transition-metal-catalyzed borylation of polyolefins has been re-
ported by Hartwig and Hillmyer to yield boronate-functionalized
polymers in one step.6
However, a method for the synthesis of soluble boron polymers
of controlled architecture, molecular weight, and degree of func-
tionalization, in which the substituents on boron can be readily
exchanged and consequently the strength of the Lewis acid centers
can be fine-tuned, is currently not known. Such a process would
provide an opportunity to investigate the binding behavior of
nucleophiles to polymeric Lewis acids in depth. Moreover, studies
of this type are likely to result in the discovery of new polymer-
supported Lewis acid catalysts as well as cocatalysts for Ziegler-
Natta polymerization. We report here a new route to well-defined
polymeric Lewis acids via controlled radical polymerization of a
silicon-functionalized monomer and subsequent borylation of the
silylated polymer. The resulting reactive boron polymers serve as
precursors to a family of well-defined polymers bearing Lewis acid
centers.
The introduction of Lewis acidic boron centers into the polymer
side chains was achieved by silicon boron exchange using BBr3, a
strong Lewis acid which is known to cleave Si-C(sp2) bonds under
mild conditions (ambient temperature) with nearly quantitative
yields and high selectivity.15 PS-Si was treated with a slight excess
of BBr3 (Caution! BBr3 is toxic and highly corrosive) in CH2Cl2
at ambient temperature over a period of 12 h (Scheme 1).
Importantly, this procedure did not lead to precipitation of the
polymer. The soluble borylated polymer PS-B was isolated in 81%
yield after precipitation into hexanes. Alternatively, the polymer
may most conveniently be used in situ for further exchange of the
bromide substituents with nucleophilic reagents.
Selective and quantitative cleavage of the Si-C(sp2) bonds in
PS-Si to form PS-B was confirmed by multinuclear NMR
spectroscopy. Importantly, the signal due to the trimethylsilyl
1
substituents of PS-Si in the 29Si (δ ) -4.4), H (δ ) 0.24), and
13C (δ ) -0.5) NMR spectra completely disappeared. A new set
of sharp signals developed as a result of formation of Me3SiBr
(δ(29Si) ) 30.3, δ(1H) ) 0.59, δ(13C) ) 4.5). Attachment of
dibromoboryl groups to the side chains is evident from a broad
signal in the 11B NMR spectrum at δ ) 54 (h1/2 ) 2800 Hz), in a
region typical of arylboron dibromides. The chemical shift favorably
compares with that for the model compound 4-(dibromoboryl)-
cumene of δ ) 55.8. The attachment of a strongly electron-
withdrawing substituent on the phenyl rings is further reflected in
the 1H and 13C NMR spectra. Upon borylation, the protons in ortho-
position to the functional group experience a pronounced downfield
shift from δ ) 7.4-7.0 in PS-Si to δ ) 8.2-7.6 in PS-B (for
4-(dibromoboryl)cumene: δ ) 8.07). A similar trend is observed
for the carbon atoms in ortho-position to the boryl groups which
resonate at δ ) 138.5 (PS-Si: δ ) 133.8/133.5) (Figure 1). A broad
signal at δ ) 136.4 can be attributed to the carbon atom bearing
the boryl substituent, and two closely spaced resonances (δ ) 153.3,
152.9) are observed for the ipso-carbon atom closest to the polymer
backbone. Importantly, our data do not show any sign of isomer-
ization reactions at the phenyl rings or of polymer degradation,
but rather indicate selective and quantitative borylation of PS-Si
by BBr3.
4-Trimethylsilylstyrene was polymerized in anisole (50%) ac-
cording to a typical protocol for atom transfer radical polymerization
(ATRP)13,14 initiated with 2 mol % 1-phenylethyl bromide (PEBr)
and catalyzed by CuBr/pentamethyldiethylenetriamine (PMDETA)
(68.5% conversion within 5 h at 110 °C). The molecular weight
and dispersity of the resulting polymer PS-Si were determined by
GPC analysis relative to polystyrene standards to be Mw ) 6500
and PDI ) 1.13. Molecular weight analysis by static light-scattering
analysis gave similar results (Mw ) 6810), indicating a close
* To whom correspondence should be addressed. E-mail: fjaekle@
andromeda.rutgers.edu.
9
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J. AM. CHEM. SOC. 2002, 124, 12672-12673
10.1021/ja020773i CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
difference between the meta- and para-fluorine atoms of ∆δm,p
)
13.0 for the free acid that significantly decreases to ∆δm,p ) 6.4 in
the polymeric Lewis acid-crotonaldehyde adduct. PS-BARF may
be viewed as a polymeric analogue of the important Lewis acid
tris(pentafluorophenyl)borane.17
In summary, we have developed an efficient new method for
the introduction of Lewis acidic boron centers into the side chains
of organic polymers. Facile substituent exchange reactions on boron
lead to a new family of well-defined polymeric Lewis acids which
lend themselves to further studies in catalysis and materials science.
Figure 1. Phenyl/methyl regions of the 13C spectra of PS-Si and PS-B.
Acknowledgment. Acknowledgment is made to the donors of
the Petroleum Research Fund, administered by the ACS, and to
the Rutgers University Research Council for financial support of
this research. GPC and LS instrumentation was purchased with
support from NSF (MRI 0116066).
Supporting Information Available: Experimental procedures and
data for all polymers described and for model compounds thereof.
Kinetic plots for the polymerization of 4-trimethylsilylstyrene, Zimm
plot for PS-BPin, and 19F NMR spectrum of PS-BARF (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. GPC traces for PS-Si and PS-BOBu.
PS-B readily reacts with nucleophiles, thereby serving as a
precursor to a number of other polymers with boron centers of
variable Lewis acidity. When PS-B was treated with Me3SiOEt or
THF, the weakly Lewis acidic polymeric boronates, PS-BOEt and
PS-BOBu, were obtained in 90 and 83% isolated yield, respectively.
Interestingly, when PS-BOEt was subsequently reacted with pinacol
in toluene at ambient temperature, the ethoxy substituents were
readily replaced to yield the air-stable polymer PS-BPin containing
a cyclic boronate moiety. PS-BOEt and PS-BPin were freeze-dried
from benzene and readily isolated to give white solid materials.
The isolated yield for the glassy polymer PS-BOBu is slightly lower
due to losses during precipitation into hexanes.
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An upfield-shifted 11B NMR signal for polymers PS-BOR (δ )
25) confirms the introduction of alkoxy substituents on boron. The
molecular weight was determined by GPC analysis to be in a similar
range as that of the precursor PS-Si, whereas the polydispersity
remained virtually identical (see Figure 2). This suggests that both
the borylation and the subsequent substituent exchange occur
without cleavage of the backbone or significant cross-linking.
Static light-scattering measurements were performed to further
confirm the high selectivity and absence of cross-linking upon
borylation of PS-Si. Comparative data as derived from Zimm plots
of PS-Si and PS-BPin (see Table S2 of the Supporting Information)
clearly show that the average number of polymer repeat units and
the polydispersity remain unchanged.
The highly Lewis acidic polymer PS-BARF was obtained by
reaction of PS-B with Cu(C6F5) in 74% yield after precipitation
into hexanes. The relative Lewis acidity of the boron centers was
estimated by treatment with crotonaldehyde according to Childs’
method16 to 0.60 (BBr3 ) 1.0). A pronounced Lewis acidity of
PS-BARF was further confirmed by a large chemical shift
JA020773I
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