Zinc-Catalysed Hydroboration of Terminal and Internal Alkynes

Article abstract of DOI:10.1002/asia.201900839

A regioselective hydroboration of alkynes has been developed by using commercially available zinc triflate as a catalyst, in the presence of catalytic amount of NaBHEt₃. The reaction tolerates a wide range of terminal alkynes having several synthetically useful functional groups and proceeds regioselectively to furnish hydroborated products in moderate to excellent yields. This system shows moderate chemoselectivity towards terminal C≡C bond over terminal and internal C=C bond and internal C≡C bond.

Full text of DOI:10.1002/asia.201900839

CHEMISTRY
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Accepted Article
Title: Zinc Catalysed Hydroboration of Terminal and Internal Alkynes
Authors: Souvik Mandal, Sayantan Mandal, and Geetharani K
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Zinc Catalysed Hydroboration of Terminal and Internal Alkynes
Souvik Mandal, Sayantan Mandal, K. Geetharani*[a]
Abstract: A regioselective hydroboration of alkynes has been
developed by using commercially available zinc triflate as a catalyst,
in the presence of catalytic amount of NaBHEt3. The reaction
alkynes with HBdan12 (dan= 1,8-diaminonapthalene) and
the borylation of aryl diazonium salt and aryltriazene with
B2pin213 was achieved using Zn(OTf)2 and Zn(ClO4)2 as a
catalysts, respectively. Also, Aldridge and co-workers
developed a palladium catalysed borylation of aryl halides
and acyl chlorides using air stable zincboryl reagent.14
tolerates
a wide range of terminal alkynes having several
synthetically useful functional groups and proceeds regioselectively
to furnish hydroborated products in moderate to excellent yields.
This system shows moderate chemoselectivity towards terminal
C≡C bond over terminal and internal C=C bond and internal C≡C
bond.
NacnacZn-H
(Nacnac
=
N,N’-bis-2,6-
diisopropylphenyldiketiminate) catalysed hydrosilylation and
hydroboration of N-heterocycles has been recently reported
by Nikonov et al. (scheme 1c).15 During the preparation of
this manuscript, Ingleson and co-workers reported zinc
catalysed C−H borylation of terminal alkynes and the
hydroboration of internal alkynes.16 Although there are
excellent examples related to the Zn-catalysed borylation of
organic substrates reported, the Zn-catalysed hydroboration
of alkynes is scarcely explored. Herein, we are reporting a
simple and efficient zinc catalyst for regio- and
chemoselective hydroboration of terminal alkynes. This
approach is also applicable to internal alkynes.
Organoborane compounds emerged as a versatile synthetic
intermediate in organic synthesis and it has been
extensively used in medicinal chemistry, functional
materials and pharmaceutical research.1 Among them, vinyl
boronate ester derivatives are well known for the
construction of substituted olefins and dienyl moieties in
organic synthesis via Suzuki-Miyaura cross coupling
reaction.2 Hydroboration of alkyne is one of the proficient
techniques demonstrated for the synthesis of alkenyl
boronates.3 Over the last decade, various transition metal
catalysts as well as metal-free systems were extensively
investigated for the stereo and regioselective hydroboration
of alkynes.4-6 Transition metal complexes of Co, Ru, Rh, Pd
and Ir, have been largely employed to achieve this
transformation. However, recently, the first-row transition
metals such as Fe, Co, Ni, Cu and Zn are becoming the
primary choices of catalyst, as they are earth abundant,
less expensive and less toxic.
Organozinc compounds were known to serve as a
transmetalating regent in Negishi coupling type reactions,7
but the catalytic borylation using zinc as a catalyst has been
hardly explored. In 2008 Nozaki and co-workers reported
the first hydroboration of α,β-unsaturated ketone using
borylzinc reagent (scheme 1a).8 Mankad and co-workers
developed a series of bimetallic base metal catalyst that
utilizes metal-metal cooperativity for photoinduced C-H
borylation,9 in which, a Cu-Fe catalyst was found to be
superior than the Zn-Fe catalyst. Along the same line,
Uchiyama and co-workers reported diethylzinc catalysed
borylation of aryl halides and borylzincation of benzynes.10
They have also demonstrated the borylzincation of terminal
alkynes leading to alkenylboranate esters upon treating with
water (scheme 1b). More recently, pioneering research
concerning the zinc catalysed borylation reactions of alkyl
halides,11a aryl halide,11b and dual C-X and C-H borylation
of aryl halides,11c have been reported by Marder and co-
workers. Further, the dehydrogenative borylation of terminal
Scheme 1: Examples of a) Borylation using borylzinc as a transfer
reagent, b) alkyne hydroboration via borylzincate intermediate, c)
hydroboration of N-heterocycles, d) This work: Zinc catalysed
hydroboration of alkynes.
We commenced our catalytic studies using phenylacetylene
(1a) as a model substrate, pinacolborane (HBpin, 1.2 equiv) as
the boron source, Zn(OTf)2 as the catalyst. Reaction between 1a
and HBpin using Zn(OTf)2 as a catalyst at room temperature did
not induce any hydroboration reaction (Table 1, entry 1). When
NaBHEt3 was used as a hydride source, the hydroboration
product 1b was obtained in 14% yield (entry 2). Subsequent
increment of temperature from room temperature to 80 °C
increases the yield of the alkenyl boronate ester from 14% to
92% (entries 2-5). NaBH4 as the hydride source yielded 80% of
the product albeit with less regioselectivity (entry 6). Control
experiments confirmed the essential role of the zinc catalyst
(entry 7). While zinc dust only provided a trace amount of
[a]
S. Mandal, S. Mandal, Dr. K. Geetharani
Department of Inorganic and Physical Chemistry
Indian Institute of Science, Bangalore-560012
India
E-mail: geetharani@iisc.ac.in
Supporting information for this article is given via a link at the end of
the document.
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hydroborylated product (entry 8), ZnCl2 and ZnI2 resulted
comparable yields (entries 9 and 10). When the reaction was
carried out at the catalyst loading of 2.5 mol% the yield
decreases to 68%, however, increasing the catalyst loading did
not significantly influence the product yield (entries 11 and 12).
Changing the solvent from toluene to other polar solvents did not
improve the yields (entries 13-15).
the terminal C-C double bond, internal C-C triple bond and
internal C-C double bond. Finally, we tested the efficiency of the
Zn-catalysed hydroboration of the more challenging
disubstituted alkynes. We were delighted to see the good yields
of hydroborated products for symmetrical alkynes with either aryl
(19a) or alkyl substituents (20a-21a). Unsymmetrical internal
alkynes (22a-24a) were subjected to hydroboration reaction, the
vinyl boronated esters were obtained in good yield with
moderate regioselectivity. Moreover, when the substrate
containing electron withdrawing groups, such as 4-
ethynylbenzonitrile, phenyl propargyl ether were used, trace
amount of borylated products were observed. Using a substrate
containing alcoholic moiety (9-ethynyl-9H-fluorenol) under the
standard conditions employed, showed the incompatibility of the
hydroxyl group.
Table 1 Optimisation of the reaction conditions for Zn catalysed
hydroboration of phenylacetylene (1a).[a]
entry
catalyst
additive
solvent
T (°C) yield
(%)[b]
1
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
-
-
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
RT
RT
40
60
80
80
80
80
80
80
80
80
80
80
80
trace
14
42
80
92
80
11
8
Table 2: Substrate scope of zinc catalysed hydroboration of
2
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBH4
terminal and internal alkynes.[a]
3
4
5
6
7
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
8
Zn dust
ZnCl2
9
90
88
68
91
60
67
75
10
11[c]
12[d]
13
14
15
ZnI2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
1,2-DME
Dioxane
[a] Standard conditions: 1a (0.2 mmol), catalyst (5 mol %), additive (5 mol %),
HBpin (1.2 equiv), solvent (1.0 mL) for 10 h unless otherwise stated. [b] Yields
were determined by 1H NMR using nitromethane as an internal standard. [c]
2.5 mol% of catalyst and 2.5 mol% of additive were used. [d] ]10 mol% of
catalyst and 10 mol% of additive were used.
Having identified the optimum conditions for hydroboration of
phenylacetylene (1a) with 5 mol% Zn(OTf)2, 5 mol% NaBHEt3,
1.2 equiv of HBpin (see supporting information for details), we
then explored the substrate scope. At first, the hydroboration
reaction of several terminal and internal alkynes were examined
(Table 2). Phenylacetylene derivatives having electron-donating
(2a and 3a) and -withdrawing groups (4a and 5a) were efficiently
hydroborylated to the corresponding alkenyl boronate ester in
excellent yields (85-97%). We observed the good yield of
hydroborated product for the sterically encumbered ortho-
substituted substrate (6a). In the case of 1,4-diethynylbenzene,
both mono- (7b, 58%) and di-hydroborated product (8b, 75%)
can be obtained selectively by modifying the reaction conditions.
The hydroboration of 3-ethynylpyridine (9a) and 3-
ethynylthiophene (10a) took place smoothly, affording the
alkenyl boronate esters in good yields. Besides aromatic alkynes,
the scope of the zinc catalyst system was extended to aliphatic
terminal alkynes. The compounds 1-decyne (11a) as well as 1-
cyclohexyl-prop-2-yne (12a) gave the borylated products in
yields up to 75%. Remarkably, alkynes having cyclic side chain
(13a-15a) could also be easily transformed to the corresponding
hydroborated products, despite their ring strain. The terminal C-
C triple bond in 16a-18a was hydroborated preferentially over
Reaction conditions: [a] alkyne (1 mmol), Zn(OTf)2 (5 mol%), NaBHEt3 (5
mol%), HBpin (1.2 equiv), toluene (2 mL) for 10 h at 80 °C unless otherwise
stated. Yields were determined by 1H NMR, using nitromethane as an
internal standard. Isolated yields were given in parentheses. [b] Isolated
yield using ZnCl2 (5 mol%) as a catalyst. [c] Reaction was performed using
10 mol% of Zn(OTf)2 and NaBHEt3, and 2.4 equiv of HBpin for 24 h. [d]
Reaction was performed using 10 mol% of Zn(OTf)2 and NaBHEt3, and 2
equiv of HBpin for 24 h. [e] Reaction was performed using 2 equiv of HBpin.
[f] Trace amount of bisborylated (alkene and alkyne hydroboration) product
was observed by GC-MS analysis. [g] Trace amount of isomers of 17b was
observed by GC-MS analysis. [h] Reaction was performed using 1.5
equivalent of HBpin. [i] Ratio of the regioisomers was determined by 1H NMR
spectroscopy.
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(hydrozincation), similar to that of the previously reported
hydroboration reactions.4s,4u,6d,6i
Selective hydroboration of alkyne in the presence of other
reducible functional groups is challenging and elusive. To probe
On the other hand, Tsuchimoto et al. reported in the
dehydrogenative borylation reaction that the Zn(OTf)2 can
activate the B-H bond, which initiates the evaluation of H2 gas
upon reaction with phenylacetylene.12 Based on this preceding
study, we monitored the stoichiometric reaction of 1a with in situ
generated active species A in a sealed system by 1H NMR
spectroscopy; we did not observe any production of H2, which
suggests that the deprotonation mechanism is not operated
under our reaction conditions. Further, Marder et al. reported
that the alkyl halide borylation catalysed by ZnCl2 seems to
involve one-electron processes.11 Hence, we examined the
hydroboration reaction in the presence of radical scavenger
9,10-dihydroanthracene, the desired product was obtained in
good yield (see SI for details). This result excluded the
possibility of a radical mechanism.
the chemoselectivity of our catalyst system,
a series of
experiments were performed as shown in Scheme 2. The
competitive intermolecular hydroboration reaction in the
presence of equimolar amount of 1a, 25a and HBpin was
performed using Zn(OTf)2 as a catalyst (5 mol%). The results
demonstrated that the presence of an alkene has little impact on
hydroboration of terminal alkyne, and the desired alkenyl
boronate ester was obtained in 55% yields (Scheme 2a). The
competitive intermolecular hydroboration of internal and terminal
alkynes was also conducted (Scheme 2b), and the 1H NMR
analysis shows moderate chemoselectivity towards terminal
over the internal alkyne.
Scheme
2 Chemoselectivity experiments (a-b; using standard reaction
conditions: alkynes or alkenes (0.2 mmol), Zn(OTf)2 (5 mol %), NaBHEt3 (5
mol %), HBpin (1 equiv), toluene (1.0 mL) for 10 h at 80 °C). Yields were
determined by 1H NMR, using nitromethane as an internal standard.
Scheme 4. Stoichiometric reaction between: a) zinc triflate and sodium
triethylborohydride, and further with 1a, b) B and HBpin.
To assess the regioselectivity of this method, deuterium
labelling experiments were carried out using PhC≡CD and
HBpin. We observed a resonance at δ = 6.22 ppm in the 2H
NMR spectroscopy, indicating a cis orientation of deuterium and
phenyl group in 1b-D (Scheme 3a). Similarly, the regioselective
hydroboration product (1b-D') was obtained from the reaction of
1a with DBpin, indicating the cis arrangement of deuterium and
In summary, we have demonstrated the utility of
commercially available zinc triflate catalyst system in the
hydroboration of a variety of internal and terminal alkynes. The
wide substrate scope, functional group tolerability and
chemoselectivity are some of the salient features of our system.
Further studies on the mechanism and synthetic applications of
this method are in progress in our laboratory.
2
Bpin moiety, as the resonance observed at δ = 7.32 ppm in H
NMR spectrum (Scheme 3b).
Acknowledgements
K.G. thanks the SERB (EMR/2016/000367), DST inspire faculty
award (DST/INSPIRE/04/2015/000785) and Indian Institute of
Science (IISc) start-up research grant, for funding. S.M. & S.M.
thanks Kishore Vaigyanik Protsahan Yojana (KVPY) for
fellowship. We also thank Prof. S. Ramakrishnan, IISc
Bangalore for the GC-MS facility and the AllyChem Co. Ltd.,
China for a gift of B2pin2.
Scheme 3. Deuterium labelling experiment: a) Hydroboration of 1a-D with
HBpin. b) Hydroboration of 1a with DBpin
To shed light on the reaction mechanism, a series of
stoichiometric experiments were performed. An equimolar
amount of Zn(OTf)2 with NaBHEt3 was mixed, the formation of
BEt3 was observed by the 11B NMR spectroscopy, however,
several attempts to obtain a characterizable active species were
not successful (scheme 4a). Subsequent addition of
phenylacetylene (1a) resulted into the side on coordination of 1a
to the zinc center (species B, scheme 4a). Further addition of
stoichiometric amount of HBpin led the quantitative yield of the
desired product (scheme 4b). Based on these observations, it is
possible that the mechanism of the present hydroboration
reaction might proceed via alkenyl zinc intermediate where
insertion of alkyne to Zn-H bond is likely to be the key step
Conflict of interest
There is no conflict of interest to declare
Keywords: boronate ester • chemoselective • homogeneous
catalysis • hydroboration • zinc
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
A zinc‐catalysed regioselective
hydroboration of alkynes has
been achieved. This method
shows an excellent substrate
Souvik Mandal, Sayantan
Mandal, K. Geetharani*
Page No. – Page No.
scope
chemoselectivity
terminal alkyne over other
reducible functional groups.
and
moderate
towards
Zinc Catalysed Hydroboration
of Terminal and Internal
Alkynes
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