Hydrosilylation and Mukaiyama aldol-type reaction of quinolines and hydrosilylation of imines catalyzed by a mesoionic carbene-stabilized borenium ion

Article abstract of DOI:10.1039/d1ob01056e

Aldimines and ketimines containing electron-donating and electron-withdrawing groups can be hydrosilylated with borenium catalysts at as low as 1 mol% catalyst loading at room temperature, providing the corresponding secondary amines in excellent yields. Reactions with 2-phenylquinoline gave the 1,4-hydrosilylquinoline product selectively which can be further functionalized in a one-pot synthesis to give unique γ-amino alcohol derivatives. Control experiments suggest that the borenium ion catalyzes both the hydrosilylation and subsequent addition to the aldehyde.

Full text of DOI:10.1039/d1ob01056e

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Biomolecular Chemistry
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Hydrosilylation and Mukaiyama aldol-type
reaction of quinolines and hydrosilylation of
imines catalyzed by a mesoionic
carbene-stabilized borenium ion†
Cite this: Org. Biomol. Chem., 2021,
19, 6786
Received 31st May 2021,
Accepted 2nd July 2021
Joshua J. Clarke,a Karthik Devaraj,a Brian P. Bestvater,a Ryoto Kojima,a,b
DOI: 10.1039/d1ob01056e
a,c
rsc.li/obc
Patrick Eisenberger,a Joseph F. DeJesusa and Cathleen M. Crudden
*
Aldimines and ketimines containing electron-donating and elec- has been expanded to encompass the use of phosphonium,16,17
tron-withdrawing groups can be hydrosilylated with borenium cat- silylium,18,19 carbenium20,21 and borenium ions as catalysts
alysts at as low as 1 mol% catalyst loading at room temperature, for the FLP-type activation of small molecules.22–24
providing the corresponding secondary amines in excellent yields.
Carbene-supported borenium ions have been shown by the
Reactions with 2-phenylquinoline gave the 1,4-hydrosilylquinoline Stephan25,26 and Crudden groups27 to be effective catalysts for
product selectively which can be further functionalized in a one- the activation of H2 and subsequent reduction of CvN func-
pot synthesis to give unique γ-amino alcohol derivatives. Control tionalities. The introduction of a formal positive charge onto
experiments suggest that the borenium ion catalyzes both the an already electron deficient boron centre results in increased
hydrosilylation and subsequent addition to the aldehyde.
Lewis acidity without the need for exhaustive fluorination of
substituents on boron, providing considerable flexibility in
terms of catalyst design and synthesis. In particular, 1,2,3-tria-
zolylidene (or mesoionic carbene [MIC])-stabilized borenium
ions such as 1+ have been shown to reduce imines and
N-heterocycles at low H2 pressure and ambient temperatures.27
Based on their strong reducing power under mild reaction con-
ditions, we chose to explore the activity of borenium ions
in the catalytic hydrosilylation of more complex targets.
Interestingly, there are only a few reports describing the use of
borenium ions as catalysts for hydrosilylation, including those
by Denmark,28 Jäkle29 and more recently Ashley.30 In addition
to providing new examples to these key studies, we also
demonstrate a secondary role for the Lewis acidic borenium
ion, namely activation of carbonyl groups in a two-step
approach to the synthesis of more complex organic molecules,
namely γ-aminoalcohols via Mukaiyama aldol-like reactions
(Scheme 1).
Introduction
Amines are important functional groups found in high-value
chemicals such as polymers,1 agrochemicals,2 and pharmaceu-
ticals.3 The hydrosilylation of imines provides a practical and
direct synthetic approach for the synthesis of amines.
Transition metal catalysts have been workhorses in this area4,5
but due to their challenging removal from amine-containing
products,6,7 alternatives have been explored.
Piers and co-workers reported that B(C6F5)3 could reduce
carbonyl8 and iminyl9 functionalities. This reaction was a
seminal example of the now ubiquitous frustrated Lewis pair
(FLP) reductions, characterized by bulky Lewis acids and Lewis
bases working in concert to activate small molecules, includ-
ing silanes10 and dihydrogen.11,12
FLPs traditionally depend on neutral, perfluorinated-aryl
boranes to achieve the high Lewis acidity and significant steric
effects required for their unique reactivity.13–15 This concept
Results and discussion
We began our work with borenium ion 1+ (Scheme 2), which
was previously shown by our group to have high catalytic
activity for the hydrogenation of imines.27 In particular, the
triazolylidene framework enables the introduction of a hydro-
gen atom on one of the wingtip groups, to reduce steric con-
straints around the reactive boron centre. Because it is a stron-
ger sigma donor, the mesoionic carbene results in a more
hydridic borohydride intermediate, without sacrificing any of
aDepartment of Chemistry, Queen’s University, Chernoff Hall, Kingston, Ontario, K7L
3N6, Canada. E-mail: cruddenc@chem.queensu.ca
bDivision of Applied Chemistry, Graduate School of Engineering, Hokkaido
University, Sapporo, Hokkaido 060-8628, Japan
cInstitute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo,
Chikusa, Nagoya 464-8602, Japan
†Electronic supplementary information (ESI) available: Synthetic procedures,
spectral data, crystallographic information. CCDC 2086430. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/d1ob01056e
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Table 1 Silane screening for reduction of ketimine 3aa
Temp.
(°C)
Catalyst loading
(mol%)
Time
(h)
Yieldb
(%)
Entry
Silane
1c
2
3
4
5
6
7
Et3SiH
60
10
1
1
1
1
1
0.1
48
2
3.5
1
<0.25
<0.25
1
88
64
98
99
99
99
99
PhMe2SiH
PhMe2SiH
Et2SiH2
Ph2SiH2
PhSiH3
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
PhSiH3
Scheme 1 a) Borenium-ion catalyzed protocol for the hydrosilylation
of imines (b) sequential hydrosilylation/aldehyde addition leading to
γ-aminoalcohols.
a Reactions performed with 0.25 mmol of 3a in a J-young tube in
0.5 mL CD2Cl2. r.t. = room temperature. b Determined by 1H NMR rela-
tive to remaining starting material. c Conducted on a 0.1 mmol scale.
loadings as low as 0.1 mol% were sufficient, leading to com-
plete conversion in one hour at room temperature (entry 7).
Using optimized conditions, the reaction scope was exam-
ined (Scheme 3). Aldimines with electronically neutral- (3b) or
electron-donating groups (3c and 3l) reacted well, giving the
resulting secondary amines in good to excellent yields. The tri-
fluoromethyl group was tolerated under our reaction con-
ditions, although a higher catalyst loading was required (3e,
Scheme 2 Borenium ion synthesis.
the Lewis acidity of the starting borenium ion.27 Moreover, the 5 mol%). This indicates that small but not substantial
catalyst can be prepared in a straightforward protocol from amounts of hydrodefluorination may be occurring, which is a
readily available starting materials.27
known reaction pathway for strongly Lewis acidic silylium and
Triazolium salt 2 is prepared via the well-established alumenium cations.35
copper-catalyzed [3 + 2] Huisgen cycloaddition31 between an
Imines containing reducible groups such as an ester (3g),
azide and alkyne followed by aryl quaternization at nitrogen nitro (3f) and an alkenyl (3h) substituent were resistant to
by diphenyliodonium tetrafluoroborate mediated by copper hydrosilylation, although longer reaction times, increased cata-
(Scheme 2).32 From triazolium salt 2, a one-pot procedure con- lyst loading or higher reaction temperatures were required.
sisting of deprotonation with strong base (typically NaHMDS) N-Tosyl imine 3j was competently reduced to the corres-
in the presence of 9-BBN dimer produces borenium ion ponding amine 4j, a product that can be transformed into a
precursor 1-H directly. Facile hydride abstraction by primary amine via sulfone deprotection. Lack of steric hin-
[Ph3C][B(C6F5)4] at room temperature in CH2Cl2 then gives bor- drance about nitrogen in 3m resulted in decreased reactivity,
enium ion 1+. It should be noted that the nature of the coun- necessitating 5 mol% catalyst loading, possibly due to Lewis
terion is important to the stability of the borenium ion, since acid–Lewis base adduct formation between the substrate and
−
counterions with direct F–M bonds such as BF4 can result in catalyst. Finally, diimines 3n and 3o could be reduced to the
fluoride transfer to boron and quenching of Lewis acidity.33,34
corresponding diamines, even with sterically cumbersome
With the aim of developing a general protocol for the hydro- mesityl or diisopropylphenyl substituents. Imines bearing het-
silylation of imines, we selected ketimine 3a as a model sub- eroaromatic substituents were examined, and did lead to
strate for investigation of reaction conditions (Table 1). Since product, but these reactions were not clean, leading to chal-
compound 4a proved difficult to isolate, preliminary reactions lenges with obtaining the product in sufficient purity.
were performed in a J-Young tube in CD2Cl2 to facilitate moni-
To further test the catalytic ability of borenium ion 1+, we
toring of reaction progress. Initial test reactions using triethyl- explored the possibility of reducing quinolines, which are
silane as the terminal reductant proceeded sluggishly even at more challenging due to the need to disrupt aromaticity. With
elevated temperatures and relatively high catalyst loadings of 2-phenylquinoline 5 as the test substrate, we demonstrated
10 mol% (entry 1). However, decreasing the bulk of the silane that, as seen with imine hydrosilylations, bulkier alkyl silanes
t
resulted in an appreciable increase in rate, such that full con- such as Et3SiH and Bu2SiH2 were ineffective at room tempera-
version was achieved even at loadings as low as 1 mol%, at ture with 5 mol% catalyst loading (Table 2). However, switch-
room temperature (entries 2–6). With phenylsilane, catalyst ing to PhMe2SiH gave the singly hydrosilylated product 6 in
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almost quantitative yield after 24 h (Table 2, entry 3). As in the
case of imines, PhSiH3 proved to be the most effective, giving
quantitative reduction in less than 20 minutes (Table 2,
entry 6).
Surprisingly, we obtained only the 1,4-hydrosilylation
product 6 even with two equivalents of phenylsilane (6 equiva-
lents of hydride), with the regiochemistry indicative of an
outer sphere mechanism.36 The prototypical Lewis acid for
FLP-reductions, B(C6F5)3,37 has been shown to doubly hydrosi-
lylate quinolines generally under more forcing conditions
(elevated temperature) with larger excesses of hydrosilane and
smaller 2-substitutents on the N-heterocyclic ring (H or Me).
Upon exposure to air, 6 readily disproportionates into 2-phe-
nylquinoline and 2-phenyltetrahydroquinoline, unlike acyclic
N-silyl enamine varieties which are reportedly air stable,38
making isolation of these molecules difficult.
However, the observation of selective mono-reduction pro-
vided the opportunity to employ 6 as an N-silyl enamine
nucleophile. N-silyl enamines are rare and underutilized com-
pared with the more common dialkyl enamines.39–42 A compe-
tition experiment between bis-N-silyl enamines and enol
ethers placed N-silyl enamines at half the reactivity of enol
ethers and 7 times less reactive than their silyl enol ether
counterparts.43 Silyl enamines have very few known uses to
date in organic synthesis, apart from the synthesis of
densely functionalized pyridines,44 dichlorocyclopropanes and
oxazines.43
Taking advantage of the ability to generate silylenamines
through our method, we examined the reactivity of 6-SiPhH2 in
the Mukaiyama aldol reaction. Reactions between 6-SiPhH2
and 2-naphthaldehyde gave a complex mixture by 1H NMR
spectroscopy, potentially due to secondary reactivity of the
remaining Si–H bonds. When using 6-SiPhMe2, the reaction
was much cleaner, giving full conversion to 7 (Ar = 2-naphthal-
dehyde) within one hour of aldehyde addition (Scheme 4).
Quinolines containing aromatic substituents in the 2-posi-
tion were readily hydrosilylated under our optimized con-
ditions to give the corresponding dihydroquinolines 6 in >90%
yield. Because of the sensitivity of these compounds to dispro-
portionation, they were not isolated, but were treated with
aldehyde directly. Subsequent NaBH4 reduction followed by
deprotection of the silyl group furnished tetrahydroquinoline
derivatives 8 in modest to good yields. Of the four possible dia-
stereomers, this process yields only two diastereomers of the
γ-amino alcohols, which could be separated by column chrom-
atography (Scheme 4). The diastereoselectivity determined
after chromatography was within error of that assessed on the
crude mixture.
Scheme 3 Borenium ion-catalyzed reduction of aldimines and keti-
mines. Values below the respective compounds are isolated yields
obtained after purification. Catalyst loadings vary from 1–5%. Substrate-
specific information is given in the ESI† along with specific reaction
times.
Table 2 Hydrosilylation of 2-phenylquinolinea
Entry
Silane
Time
Yield (%)b
1
2
3
4
5
6
Et3SiH
72 h
24 h
24 h
<2 h
<20 min
<20 min
0
0
To assist in elucidating the stereochemistry of the two dia-
stereomers, X-ray quality crystals of 8b′ were obtained by layer-
ing pentane into a toluene solution of 8b′. The crystallized dia-
stereomer has a syn relationship between protons on C2 and
C3 of the tetrahydroquinoline ring (Scheme 4), and an anti
relationship to the exocyclic benzylic proton. This relationship
was corroborated in bulk by 1D NOE NMR spectroscopic
experiments (Table S2†). The other diastereomer (8b) pos-
tBu2SiH2
PhMe2SiH
Ph2SiH2
Et2SiH2
PhSiH3
Quantitative
Quantitative
Quantitative
Quantitative
a Reactions performed with 0.20 mmol of 5 at room temperature in
J-young tubes. b Conversion was determined by 1H NMR relative to
remaining starting material since the instability of compounds 6 pre-
cluded their isolation.
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Scheme 5 Proposed catalytic cycle for the Mukaiyama aldol-like reac-
tivity of N-silyl enamine 6 with aldehyde 9 catalyzed by 1+.
consistent with quenching of the borenium ion by Cl−. These
results suggest that 1+ catalyzes both the hydrosilylation and
the subsequent reaction with aldehyde, acting as both a FLP
catalyst and a traditional Lewis acid. This finding is consistent
with the low activity of silyl enamines, which typically require
activation by an external fluoride nucleophile and provides
interesting possibilities for the use of FLP-type catalysts in
multi-step transformations.38,44
A proposed catalytic cycle for the involvement of the bore-
nium ion in the Mukaiyama aldol is depicted in Scheme 5.
The first step involves the coordination of borenium ion 1+ to
the incoming aldehyde 9,45 activating it towards nucleophilic
attack by the weak N-silyl enamine nucleophile 6. After attack,
a 1,5-silyl shift from nitrogen to oxygen permits catalyst release
and turnover. Our catalyst is very sterically demanding
(implied by its propensity to activate dihydrogen in a FLP
manner46) while also being less electrophilic than silylium
ions according to the Gutmann–Beckett test,46,47 thus its
release from 12 should be favoured over desilylation.
Scheme 4 Borenium ion dual catalysis in the preparation of quinoline-
derived γ-aminoalcohols. Substituents on the tetrahydroquinoline ring
are syn for both diasteroemers (only one enantiomer is shown for
clarity), and the diastereomeric mixture refers to stereochemistry at the
alcohol-bearing carbon. The yields as written are the combined yield of
the two diastereomers isolated by column chromatography with their
relative ratios denoted below the yield, as determined by examination of
the crude 1H NMR spectrum. For 8d, where the yield refers to the minor
syn diastereomer. a Crude d.e. for this reaction is calculated to be
60 : 40.
sesses a syn relationship between all three protons, based on
1D NOE NMR experiments and analysis of relative coupling
constants, Table S1.†
Conclusions
To determine the role of the borenium ion in the
Mukaiyama aldol reaction, a quenching experiment was con-
ducted under standard conditions (5 mol% catalyst loading),
in which 6 mol% of [NBu4][Cl] was added to the reaction
mixture to quench the borenium ion prior to the addition
of aldehyde. Interestingly, under these conditions, the
Mukaiyama aldol-like reaction with benzaldehyde did not
proceed. Analysis of crude 1H NMR spectra showed that 6a was
unreacted after treatment with chloride, thus leaving the N–Si
bond unbroken. Additionally, 11B NMR spectroscopy revealed
In summary, a MIC-stabilized borenium ion has been shown
to be a highly efficient catalyst in the hydrosilylation of both
imines and quinolines. The borenium ion then catalyzes the
further derivatization of N-silyl enamines through reaction
with a variety of aldehydes to produce γ-aminoalcohols via
Mukaiyama aldol-like reactivity.
Author contributions
a new resonance at −1.0 ppm after chloride addition, consist- Reaction discovery and optimization was carried out by J.J.C.,
ent with a four-coordinate boron species in which the chloride B.P.B., R.K. and P.E., while diastereomer separation, purifi-
is bound to the boron atom. This change is also associated cation and characterization were carried out by J.J.C. and K.D.
with disappearance of the borenium ion signal at 82.4 ppm, Crystallization of 8d′ was carried out by J.F.D. The manuscript
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was written by J.J.C. and C.M.C. with assistance from J.F.D. 17 V. Fasano, J. H. W. Lafortune, J. M. Bayne, M. J. Ingleson
and K.D.
and D. W. Stephan, Chem. Commun., 2018, 54, 662.
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Conflicts of interest
The authors have no conflicts to declare.
20 V. Fasano, J. E. Radcliffe, L. D. Curless and M. J. Ingleson,
Chem. – Eur. J., 2017, 23, 187–193.
21 E. R. Clark and M. J. Ingleson, Angew. Chem., Int. Ed., 2014,
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