Silver-Catalyzed Hydroboration of C-X (X = C, O, N) Multiple Bonds

Article abstract of DOI:10.1021/acs.orglett.1c00106

AgSbF6 was developed as an effective catalyst for the hydroboration of various unsaturated functionalities (nitriles, alkenes, and aldehydes). This atom-economic chemoselective protocol works effectively under low catalyst loading, base- A nd solvent-free moderate conditions. Importantly, this process shows excellent functional group tolerance and compatibility with structurally and electronically diverse substrates (>50 examples). Mechanistic investigations revealed that the reaction proceeds via a radical pathway. Further, the obtained N,N-diborylamines were showcased to be useful precursors for amide synthesis.

Full text of DOI:10.1021/acs.orglett.1c00106

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Silver-Catalyzed Hydroboration of C−X (X = C, O, N) Multiple Bonds
Vipin K. Pandey, Chandra Shekhar Tiwari, and Arnab Rit*
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ABSTRACT: AgSbF6 was developed as an effective catalyst for the
hydroboration of various unsaturated functionalities (nitriles, alkenes,
and aldehydes). This atom-economic chemoselective protocol works
effectively under low catalyst loading, base- and solvent-free moderate
conditions. Importantly, this process shows excellent functional group
tolerance and compatibility with structurally and electronically diverse
substrates (>50 examples). Mechanistic investigations revealed that the
reaction proceeds via a radical pathway. Further, the obtained N,N-
diborylamines were showcased to be useful precursors for amide
synthesis.
loading) specially designed ligand systems, additional bases,
and additives are often necessary to achieve highest activity.
Among the coinage metals, even though substantial
advancements in the Cu- and Au-catalyzed hydroboration of
unsaturated functionalities have been made,8 the use of Ag in
these reactions are still underdeveloped (Scheme 1).9 Environ-
ment friendly silver ion is advantageous over several other
transition metals as it is less toxic and can function as a Lewis
acid, weak base, or general oxidant as per the needs of the
reaction.10 Further, silver ion is reported to bind many
unsaturated functionalities via π-coordination which stabilizes
atalytic hydroelementations are fundamental transforma-
Ctions with high scientific and commercial relevance that
pioneer numerous carbon−carbon and carbon−heteroatom
bond forming reactions and are thus extensively employed in
the production of cermet, natural products, agrochemicals,
polyesters, dyes, pharmaceuticals, etc.1 Among various hydro-
elementation reactions, hydroboration, the addition of a B−H
moiety across various unsaturated functionalities such as
nitriles, alkenes, alkynes, carbonyls, etc., has drawn the growing
interest of researchers worldwide.1f,2 Amidst the common
unsaturated moieties, the CN bond in organonitrile com-
pounds is exceptionally robust because of the high bond
dissociation energy (750.0 kJ/mol), which makes its
functionalization very challenging.3 Traditionally, nitrile is
reduced utilizing pyrophoric reagents such as LiAlH4 or
NaBH4, but these protocols remained less attractive due to
poor yields, product selectivity, and generation of large
quantities of inorganic waste.4 On the other hand, direct
hydrogenation suffers from harsh conditions (higher pressure/
reaction temperature), needs of Lewis acid additives, and
mostly precious metal catalysts along with poor product
selectivity.5 Thus, catalytic CN bond hydroboration has
evolved to be a preferred route for the selective transformation
of nitriles due to milder reaction conditions and better
chemoselectivity.6 Further, the generated amines after
hydrolysis find applications as sythons in various fields such
as the productions of dyes, polyesters, pharmaceuticals, as well
as agrochemical compounds.7,2d Hence, developing efficient
catalytic methods for nitrile hydroboration continued to be of
great value. Most of the reported nitrile dihydroboration
methods rely on precious metals, which include transition as
well as f-block elements, although a few reports utilizing base
metals and main-group elements have also been documented.6
However, along with the metal precatalysts (mostly high
Scheme 1. Overview of Prior AgI-Catalyzed Hydroboration
Reaction and This Work
Received: January 12, 2021
Published: February 15, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00106
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1681−1686
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a
the intermediate during various reaction pathways making it a
suitable catalyst for the functionalization of unsaturated
bonds.11
Scheme 2. Scope for the Dihydroboration of Nitrile
Motivated from our interest in Ag-catalyzed organic
transformations and previous experience with the hydro-
boration of terminal alkynes (Scheme 1b),9b we focused on
developing Ag-mediated hydrofunctionalizations of unsatu-
rated functionalities. Herein, we describe a ligand- and base-
free, simple Ag-salt (AgSbF6) catalyzed maiden nitrile
dihydroboration under solvent-free modest reaction condition.
Further, we investigated the suitability of our protocol in
alkene hydroboration which selectively produces the linear
alkyl boronic acid derivatives via exclusive anti-Markovnikov
addition. Incredibly, our protocol works smoothly and
efficiently for diverse terminal alkenes under milder solvent-
free conditions and afforded superior activity compared to the
protocol reported by Bi et al.9c Additionally, we employed the
present protocol toward aldehyde hydroboration which
afforded diversified alcohols after hydrolysis in excellent yields.
To explore the viability of nitrile dihydroboration, initially
benzonitrile (0.5 mmol) was treated with H-Bpin (1.1 mmol)
in the presence of AgSbF6 (2 mol %) under neat condition.
After stirring for 24 h at room temperature, the desired
dihydroborated product 3a was obtained only in 50% yield
(Table 1; entry 1) and no monoborylation was observed.
a
Reaction condition: Nitrile (0.5 mmol), AgSbF6 (0.005 mmol), H-
Bpin (1.2 mmol), 60 °C, 12 h. Yields are based on 1H NMR analysis
b
using hexamethylbenzene as internal standard. AgSbF6 (0.01 mmol).
c
d
e
48 h. 2.4 mmol H-Bpin. 24 h.
or -withdrawing substituents (-F, -CF3, -Br) at the para-
position reacted rapidly (12 h) to deliver essentially the
quantitative yields of the products 3b−f demonstrating
negligible electronic influence of the substituents. Substituents
at other position of the phenyl ring (3g,h) and polycyclic
aromatic ring are also well-tolerated (3i). Remarkably, the
presence of potentially reactive ester functional groups was also
permitted and resulted in exclusive hydroboration of the nitrile
group (3j). We noticed slightly reduced activity (82% yield)
for the heteroaromatic thiophene based nitrile (1k) in
comparison with other aromatic nitriles, while no detectable
conversions for 2-pyridinecarbonitrile, which might be due to
catalyst deactivation via coordination by the pyridine nitro-
gen.12a−c Gratifyingly, the tetrahydroboration of terephthaloni-
trile (1l) was also smoothly attained to furnish 3l in excellent
yield (96%). In addition to the aromatic nitriles, our catalytic
system also performs well for the aliphatic compounds. At first,
(substituted)benzyl cyanides (1m,n) were tested which
delivered the desired products in moderate yield of 60−71%,
although longer reaction duration was needed. The scope of
the protocol was further expanded to various challenging
nitriles such as isobutyronitrile, (2-chloro)acetonitrile, cyclo-
pentanecarbonitrile, and mesitylacetonitrile (1o−s).12a,13 Sat-
isfyingly, all of these aliphatic nitriles are active and afforded
the corresponding N,N-diborylamines (3o−s) as well in good
yields (66−78%).
Moreover, we have isolated the corresponding aryl
ammonium chlorides in selected cases by treating the obtained
N,N-diborylamines (3a−f,i) with ethereal HCl (0.05 M),
which led to the isolation of desired products as fine white
powders in excellent yields (90−96%) after workup (Scheme
3). To showcase further applicability of the obtained N,N-
diborylamines, which has remained underdeveloped,14 we
studied their reactivity toward carboxylic acid. Accordingly,
3a−e were chosen and reacted with benzoic acid at 120 °C,
which under the elimination of (Bpin)2O (see Supporting
Information) produced the corresponding amides (4a′−e′),
finding diverse applications in pharmaceutical industry and
biochemistry, in high yields (81−98%) (Scheme 3). This
a
Table 1. Optimization of Reaction Condition
b
entry
AgSbF6 (mol %)
temperature ( °C)
time (h)
yield (%)
1
2
3
4
5
2
2
2
1
1
1
RT
60
60
60
60
60
24
24
24
12
12
12
50
80
79
94
>99
trace
c
d
e
6
a
Reaction condition: Benzonitrile (0.5 mmol), H-Bpin (1.1 mmol).
b
c
d
1
Yields are based on H NMR analysis. 0.5 mL toluene. 1.2 mmol
e
of H-Bpin. B2pin2 (1.2 mmol) instead of H-Bpin and toluene (0.5
mL) were used.
Elevating the reaction temperature to 60 °C, enhanced the
yield (80%) of the product 3a, while the use of organic solvent
did not have any significant impact (entry 2 and 3). Further,
lowering the catalyst loading (1 mol %) and reaction duration
(12 h) under same conditions leads to the formation of 3a in
94% yield (entry 4), which was further improved to
quantitative reaction when H-Bpin was marginally increased
to 2.4 equiv (entry 5). Silver salts such as AgBF4, Ag2CO3 were
also found to provide similar results. On the basis of these
observations, 1 mol % of AgSbF6 under neat condition at 60
°C was set as the optimal reaction condition. To the extent of
our knowledge, a simple silver salt catalyzed nitrile hydro-
boration has not been developed until now.
With the optimized reaction conditions, we explored the
scope of our present protocol and we were pleased to find that
electronically, sterically, and structurally diverse nitriles
(aromatic, heterocyclic, aliphatic as well as polyaromatic
nitriles) 1a−s were smoothly dihydroborated to provide 3a−
s in good to excellent yields (up to >99%; Scheme 2).
Aromatic nitriles with either electron-donating (-Me, -OMe)
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products (6a−f) in excellent yields. These observations
demonstrate that the present catalytic system is negligibly
affected by sterics and electronics of the substrates (5b−f) and
displays better activity under milder reaction condition than
the previous reports by Bi et al. (10 mol % AgOAc in toluene
at 120 °C)9c and Thomas et al. (10 mol % LiAlH4 at 110 °C;
provides regioisomeric mixture).15b Further, 2-vinylnaphtha-
lene (5g) also furnished the β-borylated product 6g in high
yield (90%). Importantly, the hydroboration of styrene with
para-acetoxy substituent (5h) proceeded with the selective
hydroboration of alkene though with reduced activity (50%
yield). Further, the applicability of our protocol was not only
bound to aromatic alkenes, it was also well suited to aliphatic
alkenes. Both long chain primary (5k−m) and cyclic secondary
alkyl substituted terminal alkenes (5n) were effectively
hydroborated with this catalytic system leading to fantastic
yields of 6k−n (88−93%). Moreover, our simple catalyst
system works more efficiently and regioselectively for
allylbenzenes 5i,j than the previous Rh-phospine based
expensive systems by Crudden et al.16 However, our present
system failed to catalyze the hydroboration of internal alkenes
(e.g., trans-stilbene) and α-substituted styrene derivatives (e.g.,
α-methyl styrene).
Scheme 3. Access to Primary Amines and Amides from the
Obtained N,N-Diborylamines
method of using N,N-diborylated amines instead of primary
amines for amide synthesis provides synthetic advantage as
direct reaction between primary amines and carboxylic acid in
the absence of any catalyst requires very high temperature of
250 °C and 40 bar of pressure.15a
To further define the efficacy of our Ag-catalyzed hydro-
boration protocol, we then focused on the hydroboration of
alkene functionality. At the outset, we probed the hydro-
boration of styrene (5a) with H-Bpin employing 2 mol % of
AgSbF6 at ambient temperature for 24 h under neat condition,
which sluggishly provided the hydroborated product 6a (35%
conversion), however with exclusive anti-Markovnikov selec-
tivity (Table S1). When the reaction temperature was slightly
elevated to 60 °C, complete conversion to β-hydroborated
product (6a) was observed. Further, when AgSbF6 loading was
reduced to 1 mol % under same reaction condition 94%
conversion to 6a was observed. Performing the reaction with
slightly higher loading of AgSbF6 (1.5 mol %) but reducing the
reaction duration (12 h) was not useful (80% conversion),
while reaction duration of 24 h resulted in complete
conversion.
After the success in styrene and nitrile hydroboration, we
broadened the aptness of our simple Ag-salt based protocol by
applying to room temperature hydroboration of aldehydes.
Satisfyingly, a range of aldehydes could be hydroborated near
quantitatively within a few hours, which after hydrolysis with
methanol delivered the corresponding alcohols in excellent
yields (Scheme 5). Aromatic aldehydes containing both
a
Scheme 5. Scope for the Hydroboration of Aldehyde
With these optimal conditions, we probed the substrate
scope and functional group tolerance of this protocol by
applying to electronically and sterically varying alkenes, and
satisfyingly, very good to excellent yields of the linear alkyl
boronic acid derivatives were observed via exclusive anti-
Markovnikov addition (Scheme 4). Styrenes containing
electron-donating or -withdrawing (including halogen) groups
at the para- as well as ortho-position underwent facile
hydroboration to regioselectively deliver the anti-Markovnikov
a
Reaction condition: Aldehyde (1 mmol), AgSbF6 (0.005 mmol), H-
b
Scheme 4. Substrate Scope for the Hydroboration of
Styrene
Bpin (1.25 mmol), 1−6 h; isolated yields of the alcohols. Conversion
a
c
1
is based on the H NMR analysis w.r.t. aldehyde. 1 mol % AgSbF6.
d
2.5 mmol H-Bpin.
electron-donating (p-Me and p-OCH3) and -withdrawing
halogen (p-Cl/F, m-Cl/Br and o-Br) substituents are well-
permitted to readily yield the final alcohols. 1-Naphthaldehyde
is also active, which delivers the alcohol 9i in 94% yield.
Further, this apparently simple catalyst system also works
flawlessly for aldehydes decorated with heterocyclic rings (7j−
l) with no evidence of dearomatization. The double hydro-
boration of terephthaldehyde (7m) is possible as well. Lastly,
the aliphatic aldehydes such as cyclohexanecarboxaldehyde
(7n) and butyraldehyde (7r) are also near quantitatively
hydroborated. Pleasingly, sterically encumbered pivalaldehyde
also got quantitatively hydroborated (8s). Further, our present
a
Reaction condition: Styrene (0.5 mmol), AgSbF6 (0.0075 mmol), H-
b
c
Bpin (0.65 mmol), 60 °C, 24 h. 0.01 mmol of AgSbF6. AgSbF6
(0.005 mmol) and 0.55 mmol of H-Bpin.
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protocol also works, although relatively less effectively, for
ketones as exemplified by the 83% hydroboration of
acetophenone at 65 °C for 12 h (see Supporting Information).
To assess the suitability in chemoselective hydroboration, at
the onset we studied the hydroboration of a 1:1 mixture of
benzaldehyde and benzonitrile and sole hydroboration of
benzaldehyde (>95%, Scheme 6a) was detected. Similarly,
On the basis of our experimental findings along with
previous report,9b,c a most probable reaction mechanism is
proposed (Scheme 8). An electron transfer from H-Bpin (via
Scheme 8. Plausible Mechanism for the Ag(I)-Catalyzed
Hydroboration of Multiple Bonds
Scheme 6. Chemoselective Hydroboration
homolytic cleavage of B−H bond) to AgSbF6 might reduce AgI
to Ag0 under the formation of [BpinSbF6] and a hydrogen
radical which via reaction with another molecule of H-Bpin
releases H2 gas and a Bpin radical. Combination of the
generated Bpin radical with styrene/aldehyde produce a
benzylic radical (A), which then reacts with dihydrogen to
yield the final hydroborated product via exclusion of a
hydrogen radical. Parallelly, an electron shift from the
generated Ag0 to [BpinSbF6] possibly regenerate AgSbF6 and
a Bpin radical, which may then couple to a hydrogen radical to
close the cycle. We believe that a similar mechanism might also
be operating for the nitrile hydroboration.
In summary, we have demonstrated a simple silver salt
catalyzed efficient protocol for the hydroboration of various
unsaturated functionalities (nitriles, alkenes, and aldehydes)
under ligand- and base-free milder conditions, which provided
an easy access to a library of valuable hydroborated products as
well as the corresponding end products. This simple catalytic
system provides excellent regio- (exclusive anti-Markovnikov
addition to terminal alkene) and chemoselectivities (prefer-
ence for nitrile over ester and aldehyde over alkene, nitrile, and
nitro functionalities). Primary mechanistic studies show that
the present protocol follows a radical pathway. Development of
the present catalytic system expands the applicability of simple
silver salts in synthetic chemistry and provides a new domain
for further exploration.
a
b
1 mol % AgSbF6. Isolated yields.
exclusive hydroboration of benzaldehyde (>99%) was recorded
in the case of benzaldehyde and styrene mixture (Scheme 6a).
Further, examinations of substrates incorporated with two
different functionalities (1j and 7o−q; Scheme 6b) revealed
that the present method is exclusively selective for the nitrile
over an ester moiety (3j), whereas aldehyde is preferred for
hydroboration over the alkene, nitrile, and nitro functionalities
(8o−q), which make our present protocol a chemoselective
one.
In order to comprehend the reaction mechanism, various
controlled experiments were executed. Preliminary observation
includes that mixing of H-Bpin to AgSbF6 was accompanied by
1
a brisk release of H2 gas as evidenced by in situ H NMR
analysis9b along with the formation of a black precipitate.
However, the mercury poisoning test assures that the process is
homogeneous.17a Additionally, we observed no product
formation when performed in the presence of radical
scavengers like TEMPO, BHT, and CuCl2 (Scheme 7) and
the formation of a Bpin radical during the reaction is suggested
by the 11B NMR and mass analysis (see Supporting
Information). These results demonstrate that the catalytic
cycle is promoted via a radical route17b and complete
shutdown of the reaction in the presence of CuCl2 implies
the involvement of a single electron transfer (SET) process.17b
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00106.
Experimental details, characterization data, and NMR
spectra of the synthesized compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Scheme 7. Radical Scavenger Experiment
Arnab Rit − Department of Chemistry, Indian Institute of
Technology Madras, Chennai 600036, India; orcid.org/
0000-0003-0224-8387; Email: arnabrit@iitm.ac.in
Authors
Vipin K. Pandey − Department of Chemistry, Indian Institute
of Technology Madras, Chennai 600036, India
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Letter
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00106
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge DST and SERB, India (Project No.
DST/INSPIRE/04/2015/002219 and ECR/2016/001272)
for the financial support. V.K.P. and C.S.T. thank the IIT
Madras for research fellowship. We also thank the Department
of Chemistry, IIT Madras for the instrumental facility.
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