ROMP-Boranes as Moisture-Tolerant and Recyclable Lewis Acid Organocatalysts

Article abstract of DOI:10.1021/jacs.0c05454

Although widely used in catalysis, the multistep syntheses and high loadings typically employed are limiting broader implementation of highly active tailor-made arylborane Lewis acids and Lewis pairs. Attempts at developing recyclable systems have thus far met with limited success, as general and versatile platforms are yet to be developed. We demonstrate a novel approach that is based on the excellent control and functional group tolerance of ring-opening metathesis polymerization (ROMP). The ROMP of highly Lewis acidic borane-functionalized phenylnorbornenes afforded both a soluble linear copolymer and a cross-linked organogel. The polymers proved highly efficient as recyclable catalysts in the reductive N-alkylation of arylamines under mild conditions and at exceptionally low catalyst loadings. The modular design presented herein can be readily adapted to other finely tuned triarylboranes, enabling wide applications of ROMP-borane polymers as well-defined supported organocatalysts.

Full text of DOI:10.1021/jacs.0c05454

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Communication
ROMP-Boranes as Moisture Tolerant and
Recyclable Lewis Acid Organocatalysts
Fernando Vidal, James McQuade, Roger Lalancette, and Frieder Jaekle
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05454 • Publication Date (Web): 05 Aug 2020
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Fernando Vidal, James McQuade, Roger Lalancette, and Frieder Jäkle*
Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102, United States.
KEYWORDS “Frustrated” Lewis pairs, triarylboranes, recyclable catalyst, ROMP, reductive amination.
ABSTRACT: Although widely used in catalysis, the multi-step syntheses and high loadings typically employed are limiting
broader implementation of highly active tailor-made arylborane Lewis acids and Lewis pairs. Attempts at developing recy-
clable systems have thus far met with limited success, as general and versatile platforms are yet to be developed. We demon-
strate a novel approach that is based on the excellent control and functional group tolerance of ring-opening metathesis
polymerization (ROMP). The ROMP of highly Lewis acidic borane-functionalized phenylnorbornenes afforded both a sol-
uble linear copolymer and a crosslinked organogel. The polymers proved highly efficient as recyclable catalysts in the re-
ductive N-alkylation of arylamines under mild conditions and at exceptionally low catalyst loadings. The modular design
presented herein can be readily adapted to other finely tuned triarylboranes, enabling wide applications of ROMP-borane
polymers as well-defined supported organocatalysts.
The remarkable success of main-group Lewis acids (LA) in
general, and electron-deficient triarylboranes (BAr3) such
as B(C6F5)3 in particular, is clearly evident in the wide vari-
ety of chemical transformations they have been applied to,1
many of them associated with the concept of “frustrated”
Lewis pairs (FLPs).2 Striking examples include hydrogena-
tion,3 hydrosilylation,4 CO2 reduction,5 defunctionaliza-
tion,6 C-H bond activation,7 and Lewis pair (LP) polymeri-
zation.8 Although these reactions no longer require the use
of costly, scarce, and oftentimes toxic transition-metal cat-
alysts, as homogeneous catalysts the boranes cannot be
easily recycled and require separation from the products.
This is exacerbated by the need for multi-step syntheses to
prepare tailored halogenated BAr3 catalysts and the high
loadings typically employed in catalytic processes.9
challenging and has remained largely limited to vinyl-ad-
dition methods (Figure 1, top).13, 17
Immobilization of organoboranes is essential to overcome
these problems,10 but so far very few solid supports are
available, among them modified silica,11 metal-organic
frameworks (MOFs),12 transient micelles,13 and covalent
polymer networks.14 Limitations arise due to difficult ma-
terial syntheses and manipulations, complex or inadequate
characterizations, underperforming stability, catalyst
leaching, and/or insufficient understanding of the true
molecular environment around the catalytic site. Contra-
rily, polymeric materials obtained by controlled conver-
sion of functional monomers offer access to well-defined
and tunable (“homogeneous-like”) catalytic sites, conven-
ient characterization, and facile manipulations.15 In the
rich field of boron-containing polymers, both (intrinsically
less perfect) polymer modification and direct polymeriza-
tion routes are commonly employed.16 However, the direct
polymerization of highly Lewis acidic BAr3 monomers is
Figure 1. Selected examples of poly(triarylborane)s and
poly(borate)s obtained by direct polymerization methods.
Despite the great success of ROMP for immobilizing other
homogeneous organocatalysts,18 and unlike the case of
tetracoordinated borates (Figure 1, bottom right),19 the
ROMP of triarylboranes remains unexplored to the best of
our knowledge. Herein, we propose a versatile new ap-
proach to polymeric Lewis acids and Lewis pairs by ROMP
of functional norbornenes that offers the following benefits
(Figure 1, bottom left): (a) a modular monomer platform,
allowing facile tuning of the steric/electronic environment
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at boron, (b) perfect compatibility between the growing
Page 2 of 7
Encouraged by the high Lewis acidity and FLP activity of
B1, as well as the premise of recyclability based on the fa-
vorable stability, we pursued the ROMP with Grubbs’ 3rd
generation catalyst (G3). Using a [G3]/[M] ratio of 1:100 in
anhydrous THF, a linear random copolymer PB1 was ob-
tained containing 90 mol% of (exo)-5-phenyl-2-nor-
bornene (NBEPh) and 10 mol% of B1 (Scheme 1 and SI). 1H
NMR analyses of reaction aliquots indicated full monomer
conversion within less than 3 min (Figure S22), thus
polymerizations were quenched after 5 min with excess vi-
nylene carbonate (VC) as terminating agent.22 The fast
polymerization kinetics are attributed to both the favora-
ble exo-geometry of the norbornene moiety (endo-isomers
polymerize more slowly) and the high Lewis acidity of the
monomer itself. As such, B1 acts as a scavenger for 3-bro-
mopyridine, Br-py (the dissociating L-type ligand on G3),
and thus accelerates the ROMP propagation rate. Indeed,
a stoichiometric mixture of B1 and Br-py in C6D6 confirmed
the instantaneous formation of the LP B1·Br-py (Scheme 1,
and Figures S34-S36). Subsequent addition of excess
BF3·OEt2 regenerated the free borane B1 almost quantita-
tively. Based on these findings, BF3·OEt2 was injected into
the polymerization mixture prior to isolation to remove Br-
py. Then, precipitation in wet MeCN on the benchtop un-
der air yielded the polymers in the form of white colloidal
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site of the polymer main-chain and the LA, (c) retention of
structural integrity and catalytic activity after polymeriza-
tion, and (d) facile manipulation of the polymer architec-
ture to impart recyclability.
The synthesis of the tricoordinate organoborane monomer
B1, inspired by other 2:1 hetero-tri(aryl)boranes with en-
hanced H2O tolerance,20 was accomplished on gram scale
and in 48% combined yield as illustrated in Scheme S1 (SI).
The Lewis acidity of B1 (acceptor number, AN = 73, as de-
termined by the Gutmann-Beckett method in CDCl3) is
close to that of B(C6F5)3 (AN = 81.8)21. As expected, B1 forms
classical LPs with coordinating solvents, such as MeCN or
THF, and the corresponding hydroxo complexes
[H·Solv]+[B1·OH]− in the presence of air and moisture (Fig-
ures S30-S33). Importantly, these species remain stable at
room temperature for >10 days in MeCN, enabling bench-
top manipulations, and even at 80°C show less than ~25%
degradation over 72 hours (Figures S12-S19). We also veri-
fied that, in combination with the appropriate LB, B1 is
competent to engage in an FLP-mediated intermolecular
activation of H2 (Scheme 1 and Figures S41-S43). First, a
stoichiometric mixture of B1 and 1,4-diazabicyclo[2.2.2]oc-
tane (DABCO) forms a non-interacting pair in C6D6 as in-
dicated by 1H, 19F, and 11B NMR spectra that are identical to
those of the isolated species. Second, a solution of
B1/DABCO rapidly and irreversibly activates H2O to form
the [H·DABCO]+[B1·OH]− ion pair via deprotonation of the
Brønsted acidic B1-aqua complex (Figures S37-S40). Fi-
nally, replacing an inert atmosphere of N2 with H2 fur-
nishes the ammonium hydridoborate [H·DABCO]+[B1·H]−
in 85% yield after 3 days at room temperature. A doublet at
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suspensions. Spectroscopic analyses of PB1 by H, 19F, and
1
11B NMR confirmed the presence of stabilized triarylborane
moieties as a mixture of solvent (MeCN/THF) adducts and
hydroxylated species (Figures S23-S25).
Next, we set out to explore the catalytic performance of the
molecular and ROMP-boranes. We selected the reductive
amination of carbonyls as a convenient model reaction to
probe their utility as recyclable catalysts in the presence of
H2O.23 At the outset, using 5 mol% of B1 and 1.2 eq of
PhMe2SiH as reducing agent, the reductive N-alkylation of
aniline with benzaldehyde under inert conditions pro-
duced N-benzylaniline in quantitative yield within 24 h at
20 °C (Table S3, entries 1-4).24 This is remarkable as neither
BPh3 nor B(C6F5)3 display any catalytic activity at room
temperature and are only effective at 100 °C.23a, 23c Increas-
ing the temperature to 60 °C (80 °C) in THF (MeCN) low-
ered the reaction time to a matter of minutes (Table S3,
entries 5-6). Importantly, these reactions can be performed
by simple manipulation on the benchtop using undried
solvents and reagents (Table S3, entries 7-8), and regard-
less of the MeCN/THF vol:vol content (Table S4). To inves-
tigate whether B1 remains active, competence checks were
performed by replenishing the testing NMR tubes with a
second load of all the reagents 24 h after the first reaction
was completed (Table S3). Under identical reaction condi-
tions, B1 showed no loss of activity regardless of the reac-
tion temperature and exposure to moisture and air.25 Fur-
thermore, up to 6 consecutive refilling cycles in wet CD3CN
furnished quantitative yields with only 1 mol% of B1 (Fig-
ures S109-S110).26 Gratifyingly, the polymeric ROMP-
borane catalyst PB1 performed similarly well under homo-
geneous conditions in a wet MeCN/THF solvent mixture,
rapidly yielding 100% of N-benzylaniline with only 1.2 eq of
hydrosilane (see SI).
11
−19.8 ppm in the B NMR, an upfield shift of the 19F NMR
resonances, and a characteristic B-H broad signal at 3.64
ppm in the H NMR all corroborate this archetypal FLP-
mediated transformation.
1
Scheme 1. Reactivity of norbornene monomer B1 and
its polymerization to generate ROMP-boranes.
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Having confirmed the catalytic proficiency in the N-alkyl-
Table 1. Substrate scope for reductive amination. a
1
ation of aniline, we further explored the substrate scope of
the molecular and polymer-bound catalysts. Anilines con-
taining electron-withdrawing and donating groups, as well
as secondary arylamines, underwent reductive amination
in quantitative yields with loadings of B1 and PB1 as low as
0.5 mol% (Table 1, entries 1-6). Conversions could be ex-
traordinarily fast, as exemplified by the reaction between
benzocaine and benzaldehyde (turn-over-frequency, TOF
= 2400 h−1, entry 3). An exception is the reductive amina-
tion of the more basic benzylamine which did not proceed
even with a large hydrosilane excess, likely due to irrevers-
ibly deactivation of the [BenzNH3]+[LA·OH]− species (entry
7).23a Variation of the keto component also gave excellent
results as acetophenone, 5-methylfurfural and substituted
benzaldehydes were converted to product in high yield by
only 0.5 mol% of B1 or PB1 as the catalyst (Table 1, entries
8-13). The lower reactivity of benzophenone is attributed
to steric effects.
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Entry
#
Time
(min)
PhMe2SiH
(eq.)
Yield (%) b Yield (%) b
Product
B1
100
99
PB1
100
99
1
2
2a
2b
2c
2d
2e
2f
60
30
1.2
1.2
1.2
3.5
1.2
1.2
3.5
3.5
3.5
3.5
1.2
1.2
1.2
The remarkable similarities between the molecular and
polymer-bound catalysts reflected in Table 1 suggests that
the structure/reactivity dependence of the LA was retained
when embedded in the polymer framework. Direct step-
wise monitoring of the model catalytic reaction by NMR
spectroscopy in wet THF-d8 provided further evidence
3
5
100
100
100
100
0
100
100
100
100
0
4
60
5
1440
60
6
11
(Figure 2). The resonances in the 19F and B NMR spectra
of PB1 matched almost perfectly with those of B1, except
for the expected broadening of the polymer signals. These
signals indicated that the triarylborane moieties engaged
in Lewis and Brønsted acid/base equilibria prior to the ad-
dition of the reducing agent. However, a dominant boro-
hydride complex, ([P/B1–H]−), characterized by a doublet
7
2g
2h
2i
1440
1440
1440
60
8
67.1 c
2.8 c
100
100
100
100
62.2 c
1.1 c
100
100
73.3
100
9
10
11
12
13
2j
11
2k
2l
30
at −20.9 ppm in the B NMR, was systematically found at
the end of the catalytic process regardless of the solvent
and moisture content (Figures S60-S77). This is in good
agreement with the expected hydrosilylation mechanism
(Scheme S2). The catalyst resting state, although not pre-
viously reported for other BAr3,23a, 23c was remarkably stable
in wet MeCN over seven days at room temperature, which
bodes well for developing recyclable catalyst systems.27
1440
1440
2m
aAmine = 2.88 mmol; ketone = 2.40 mmol; catalyst = 0.012 mmol; PhMe2SiH
= 2.88 or 8.40 mmol; solv. = 2 mL (MeCN for B1; MeCN/THF 75:25 for PB1);
[mesitylene] = 0.6 M (IS). bDetermined by 1H NMR. cDetermined by GC/MS.
δ = −20.9 ppm
J 1H-B = 87.5 Hz
▼
▼
▼
♠
(d)
−20
−21
−22
+ 1.2 eq PhMe2SiH
60 °C, 10 min
(e)
(c)
(b)
+ 1.0 eq PhCHO
Cl
Cl
B(C6F5)2
Cl
Cl
B(C6F5)2
PhH2N
solv
+ 1.2 eq PhNH2
Cl
H
Cl
B(C6F5)2
Cl
Cl
B(C6F5)2
(a)
HO
−130
−136
−142
−148
−154
−160
−166
60
40
20
0
−20
19F NMR (ppm)
11B NMR (ppm)
Figure 2. Sequential 19F NMR and 11B NMR spectra (wet THF-d8, 25 °C) during the in-situ model reductive N-alkylation: (a) 5 mol%
of catalysts B1 (black line) and PB1 (dark cyan line); (b) 1.2 eq. of aniline; (c) 1.0 eq. of benzaldehyde; (d) 1.2 eq. of dimethylphen-
ylsilane, 60 °C, 10 min. (e) Schematic structures of borane species detected in solution (▼) C6F5H; (♠) Ar2BOH and ArB(OH)2.
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Page 4 of 7
In an effort at developing a more general recycling ap-
thetic strategy lends itself to constructing other heterogen-
ized BAr3 whose FLP activity and scope can be tailored by
design. Further development of ROMP-boranes is also ex-
pected to offer access to boron-containing materials with
desirable stimuli-responsive, self-healing, and optoelec-
tronic properties.16b, 17e, 28
proach, polymer resin PB2 containing 3.33 × 10−4 mol/g of
borane units was synthesized by addition of a crosslinking
agent (1,4-di(norbornen-2-yl)benzene, NBE2Ph) during a
second block copolymer chain extension by ROMP
(Scheme 1 and SI). The crosslinker was added in the second
block to ensure full conversion of B1 before gelation takes
place and because the expected architecture with grafted
triarylborane chains should provide favorable access for
substrates and reagents. The ROMP-resin successfully pro-
moted the reductive N-alkylation of aniline with benzalde-
hyde in quantitative yields either as swollen organogel
(80% THF) or as heterogeneous particles (100% MeCN) in
bench-top reactions (Figures 3 and S111-S116). Excitingly,
high yield of N-benzylaniline (94.1%) was obtained with
just 0.5 mol% catalyst and in the absence of solvent, hint-
ing at a path to greener and easier alternatives that avoid
wasteful product purifications (Figures S117-S119). How-
ever, a more rapid decrease in activity with neat reagents
in consecutive cycles compared with THF/MeCN solutions
points to a beneficial stabilizing effect of coordinating sol-
vents under open-air conditions. Thus, further improve-
ments are expected from optimization of the recycling pro-
cedure and implementation of continuous-flow methods.
In comparison, a control polynorbornene resin prepared
without B1 monomer was unable to catalyze this reaction
(Figure S120).
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8
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The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Full details on monomer and polymer synthesis and charac-
terization, NMR data, and catalytic reaction protocols, (PDF).
Crystallographic data for [H·DABCO]+[B1·OH]−, (CIF).
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*fjaekle@newark.rutgers.edu.
NSF CHE-1609043, CHE-1904791, CRIF-0443538, MRI-
1229030, MRI-1039828; Rutgers University SEED grant
(202607), New Jersey Higher Education Equipment Leasing
Fund (ELF III 047-04)
The authors declare no competing financial interests.
(a)
(b)
We thank Dr. Lazaros Kakalis for assistance with solid-state
11B NMR measurements. This material is based upon work sup-
ported by the National Science Foundation (NSF) under
Grants CHE-1609043 and CHE-1904791. The Bruker 500 MHz
NMR spectrometer (MRI-1229030), the SEM (MRI-1039828)
and the X-ray diffractometer (CRIF 0443538) used in this
study were acquired with support by the NSF and Rutgers
University. Equipment in the Polymer and Nanomaterials Fa-
cility at Rutgers University Newark (PolyRUN) was acquired
with support by the New Jersey Higher Education Equipment
Leasing Fund (ELF III 047-04) and maintained with support
from Rutgers University through SEED grant 202607.
(c)
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THF (80%)
MeCN (100%)
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cycle
Figure 3. (a) SEM of swollen resin PB2. (b) Photograph of
ROMP-borane gel PB2 in 80:20 (v:v) THF/MeCN. (c) Consec-
utive reaction yields in the reductive amination of ani-
line/benzaldehyde catalyzed by recycled 5 mol% PB2 (for de-
tails of reaction conditions see SI).
In summary, the attachment of a chemically and sterically
tuned triarylborane to a solid support was realized by
ROMP of a functionalized phenylnorbornene. Outstand-
ing catalytic performance was achieved as exemplified in
the bench-top reductive N-alkylation of arylamines, re-
quiring far lower temperatures and catalyst loadings than
previously reported. The immobilization and recycling of
the organocatalyst was accomplished by integration into a
covalently crosslinked polymer network. The modular syn-
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M. J. Expanding Water/Base Tolerant Frustrated Lewis Pair
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(24)
The
direct catalytic hydrosilylation of
N-
benzylideneaniline (the intermediate imine) in anhydrous CD3CN
proceeded even faster at 20 °C (less than 30 min).
(25) Contrarily, only 17% conversion to reduced amine was
observed during the second run in toluene at 20 °C, suggesting
significant deactivation of B1 in non-coordinating solvents even
under moisture-free manipulations.
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(26) Longer reaction times in CD3CN were required to
overcome dilution effects after each new substrate addition.
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