H2-Acceptorless Dehydrogenative Boration and Transfer Boration of Alkenes Enabled by Zirconium Catalyst

Article abstract of DOI:10.1002/anie.201908931

The first example of an efficient and direct dehydrogenative boration of alkenes for vinyl boronate ester synthesis was achieved using a zirconium catalyst. Our methodology avoids using precious transition metals, additional hydrogen acceptors, high temperatures, and long reaction times, which were required to overcome the reducing ability of borane, to give alkyl boronate esters. Detailed mechanistic studies revealed a reversible reaction pathway and further suggested applying the zirconium complex as a “shuttle catalyst” for transfer boration, which thus sidesteps the use of relatively sensitive borane.

Full text of DOI:10.1002/anie.201908931

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International Edition: DOI: 10.1002/anie.201908931
German Edition: DOI: 10.1002/ange.201908931
Boration
H2-Acceptorless Dehydrogenative Boration and Transfer Boration of
Alkenes Enabled by Zirconium Catalyst
Xiaonan Shi, Sida Li, and Lipeng Wu*
Dedicated to Lanzhou University on the occasion of its 110th anniversary
Abstract: The first example of an efficient and direct
dehydrogenative boration of alkenes for vinyl boronate ester
synthesis was achieved using a zirconium catalyst. Our
methodology avoids using precious transition metals, addi-
tional hydrogen acceptors, high temperatures, and long reac-
tion times, which were required to overcome the reducing
ability of borane, to give alkyl boronate esters. Detailed
mechanistic studies revealed a reversible reaction pathway and
further suggested applying the zirconium complex as a “shuttle
catalyst” for transfer boration, which thus sidesteps the use of
relatively sensitive borane.
produce saturated alkyl boronate esters. Nevertheless, when
a Rh catalyst[18] was chosen, the dehydrogenative boration of
alkenes was achieved in the presence of sacrificial alkenes or
at higher temperatures (50–1508C) (Scheme 1a). Recently, an
V
inyl boronate esters (VBEs) are important compounds
used as versatile intermediates in synthetic organic chemistry
[1]
to forge C C and C X (X = F, Cl, Br, I, O, N) bonds.[2] Their
utility has been demonstrated in the synthesis of natural and
biologically active compounds.[3] Compared with saturated
À
À
=
alkyl boronate esters, C C bonds offer more opportunity to
obtain versatile boron-containing compounds.[4] Recent
results also revealed the potential medicinal and pharma-
ceutical properties of VBEs.[5] Despite the existence of
valuable applications, there are only few approaches for
VBE preparation.[2c] Traditional methods, including the trans-
metalation between trialkyl borates and 1-alkenyl lithium or
magnesium reagents[6] or the hydroboration of alkynes[7] are
limited inherently by restricted substrate availability and low
functional group compatibility. Other recently developed
approaches, for instance, the metal-catalyzed boration of
alkenyl halides,[8] the boryl-Heck reaction of alkenes with
chloroborane,[9] and alkene cross-metathesis with VBEs[10]
requires prefunctionalized alkenes or boranes which are
more expensive and not as widely available than the
corresponding alkenes and boranes.
Scheme 1. Methods for transition-metal-catalyzed synthesis of vinyl
boronate esters: a) dehydrogenative boration of alkenes in the pres-
ence of hydrogen acceptors; b) “dehydrogenative” boration of alkenes
using B2pin2; c) our current methodologies.
iron-catalyzed dehydrogenative boration was reported, in
which 2 equivalents of norbornene were required.[19] Thus, in
order to avoid the hydroboration reactivity of borane,
diborane was introduced,[18b,20] but still in some cases ketone
or alcohol was required as the HBpin scavenger (generated
from B2pin2)[20f–h] (Scheme 1b). Consequently, a mild, effi-
cient, and direct catalytic H2-acceptorless dehydrogenative
boration of alkenes with borane remains underdeveloped.
Herein, we report a straightforward and efficient system
enabled by an earth-abundant zirconium catalyst that produ-
ces a series of vinyl boronate esters from alkenes and borane
with release of H2 at 258C in 10 min. Mechanistic studies
guided us to further exploit the zirconium complex as
a “shuttle catalyst”[21] for the transfer boration of alkenes,
which avoids the use of relatively sensitive HBpin in boration
reactions (Scheme 1c).
Alternatively, the direct dehydrogenative boration of
readily available alkenes is an attractive methodology to
prepare VBEs.[5,11] Yet less successful examples were reported
due to the overwhelming hydroboration activities of borane
[17]
using Rh,[12] Ir,[13] Pd,[14] Ni,[15] Co,[16] and Fe catalysts
to
[*] X. Shi, S. Li, Dr. L. Wu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Suzhou Research Institute of LICP
Lanzhou Institute of Chemical Physics (LICP)
Chinese Academy of Sciences, Lanzhou, 730000 (P. R. China)
E-mail: lipengwu@licp.cas.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201908931.
In our model reaction a solution of 0.2 mmol styrene (1a)
and 0.2 mmol of HBpin (2a) in 1 mL toluene was allowed to
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react with 5 mol% of catalyst at 258C. Initially, commercially
available zirconocene complexes were tested and we found
that both Cp2ZrCl2 and Cp2ZrMe2 showed no activity and
styrene was recovered (Table 1, entries 1 and 2). When the
Table 1: Catalyst and temperature optimization in the zirconium-cata-
lyzed dehydrogenative boration.[a]
Entry
Cat.
T [8C]
Yield [%][b]
3a
4a
5a
1
2
3
4
5
6
7
Cp2ZrCl2
Cp2ZrMe2
Cp2ZrCl2
Cp2ZrHCl
Cp2ZrH2
CpZrCl3
–
25
25
100
25
25
25
0
0
0
23
92
0
0
0
11
1
1
30
0
0
0
0
3
3
0
0
100
0
[a] Reaction conditions: 1a (0.2 mmol), HBpin (0.2 mmol, 1.0 equiv),
5 mol% cat., 1 mL toluene in 15 mL pressure tube, heated at the
temperature given, 10–720 min. [b] GC yield.
reaction mixture was heated to 1008C with Cp2ZrCl2 as the
catalyst, only the saturated hydroboration product 4a was
produced in 11% yield (Table 1, entry 3). With Cp2ZrHCl,
which is known to be active in the hydroboration of
alkynes,[22] we could get the dehydrogenative boration
product 3a in 23% yield along with 4a and 5a in less than
4% combined yield (Table 1, entry 4). Notably, when we
applied freshly made Cp2ZrH2 as the catalyst, 3a was
produced in 92% yield as a single E-stereoisomer with
excellent regioselectivity (linear product > 99%) within
10 min at 258C (Table 1, entry 5). In contrast, with CpZrCl3
as the catalyst, only the saturated product 4a was observed in
30% yield (Table 1, entry 6). It should be noted that there was
no reaction at all when the system was heated at 1008C
without any catalyst (Table 1, entry 7).
With the optimized reaction conditions established
(Table 1, entry 5: 0.2 mmol of alkene, 0.2 mmol HBpin,
5 mol% Cp2ZrH2, 258C), we investigated the generality of
our system (Scheme 2). We found that the reaction worked
well with electron-donating (2-Me) and -withdrawing groups
(2-F, 2-Cl) at the ortho position of the aryl alkenes. Thus,
linear products 3b–3d were obtained in 85–99% yield
without detection of the branched products.[12b,23] When the
position of the functional group was changed from ortho to
meta position, 98–99% selectivity for the linear product was
observed with 93–97% yield of the halogen-substituted vinyl
boronate esters (3e–3g). For the para-substituted aryl
alkenes, slightly lower regioselectivities (94–99%) were
observed with excellent functional group compatibility for
alkyl, aryl, -NMe2, halogen, and -CF3 groups (3h–3o). For the
substrate with the -COOMe group, 3p was obtained in only
9% yield along with unconverted alkene. Interestingly, the
Scheme 2. Cp2ZrH2-catalyzed dehydrogenative boration of various
alkenes; numbers in brackets refer to regioselectivities, when applica-
ble. Conditions: alkene (0.2 mmol), HBpin (0.2 mmol, 1.0 equiv),
5 mol% Cp2ZrH2, 1 mL toluene in 15 mL pressure tube; yields of
isolated products are given. [a] 258C. [b] GC yield. [c] 1008C.
[d] 3.0 equiv of HBpin..
protic -NH2 group, which was expected to react with HBpin,
was tolerated and 3q was obtained in 93% yield. Thus, our
methodology represents the first direct dehydrogenative
boration system that tolerates easily reacting protic groups.
2-Vinylnaphthalene and disubstituted styrene both gave the
corresponding product 3r and 3s in 97% and 90% yield,
respectively. Styrene with a methyl substituent at the a-
position furnished the expected product 3t in 67% yield along
with unreacted starting materials. For indene, a cyclic alkene
substrate, the dehydrogenative boration product 3u was
obtained in 74% yield. Ferrocenyl-substituted vinyl boronate
ester 3v was obtained in 87% yield and 95% linear selectivity
starting from vinylferrocene. Extending the substrate scope to
heteroaromatic alkenes was also successful. Thus N-methyl-3-
vinylindole and 2-vinylthiophene underwent efficient dehdyr-
ogenative boration in up to 85% yield without detection of
the branched product (3w–3x). With diphenylvinyl-
phosphine, which was a tough substrate for other systems,
3y was produced in 93% yield.[24] Interestingly, our method-
ology also worked for long-chain alkene such as octene
though 3z was obtained in lower yield and regioselectivity.
Thus, our methodology not only achieved the first facile and
direct dehydrogenative boration of alkenes with boranes but
2
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also yielded products that were not tolerated by other
precious metal systems.
We also demonstrated the practical synthetic application
of our methodology on a 20 mmol scale: vinyl boronate ester
3a was obtained in 88% yield indicating the scale-up
potential of our procedure. Follow-up chemistry was carried
out to demonstrate the synthetic utility of VBEs. A series of
transformations were performed under mild conditions with
easy operations (Scheme 3). First, transformations involving
Scheme 4. Control experiments and proposed reaction mechanism.
analysis of the gas phase (Figures S5 and S6). When saturated
boronate ester 4a was subjected to the standard reaction
conditions, no formation of 3a was observed, which precluded
the hydroboration–dehydrogenation pathway (Scheme 4c).
Deuterium-labeling experiments were carried out next. When
d8-styrene was used as the substrate and reacted with HBpin,
22% and 13% H-incorporation into the a and b position of
3a was detected (Scheme 4d and Figure S3). On the other
hand, when styrene reacted with DBpin, 13% and 17% d-
incorporation into the a and b position of 3a was observed
(Scheme 4e and Figure S4). We also found that in the
presence of H2 and Cp2ZrH2, ethylbenzene was produced in
56% yield from 3a (Scheme 4 f). The labeling experiments
and the production of ethylbenzene clearly indicated that the
formation of 3a was reversible. Based on those observation,
a reaction mechanism initiated by “Cp2Zr” was proposed
(Scheme 2g). First, release of H2 from Cp2ZrH2 produces
“Cp2Zr” in the reaction solution.[27] Then oxidative addition
of HBpin gives Cp2Zr(H)Bpin species.[27a,28] Insertion of
Scheme 3. Practical synthetic application of vinyl boronate esters.
a) Cu(OAc)2, Et3N, air, EtOH, 258C; b) Cu(OAc)2, DMF/EtOH, air,
258C; c) KHF2, H2O/MeOH, 0–258C; d) I2, NaOH, Et2O, 258C; e) 4-
Iodoanisole, Pd(dba)2, tBu3P, THF, 258C; f) Oxoneꢀ , acetone, 0–258C.
the boronate ester group were carried out: 3a was converted
to trans-vinyl ether 4 and (1E,3E)-1,4-diphenylbuta-1,3-diene
5 in 71% and 50% yield, respectively. Treatment of 3a with
KHF2 in CH3CN afforded trans-potassium trifluoroborate 6 in
76% yield. Iodination and coupling with aryl iodide produced
(E)-b-aryl vinyl iodide 7 and unsymmetrical stilbene 8 in
excellent yields (98% and 95%). Phenylacetaldehyde 9 was
obtained in 82% yield from 3a after oxidation. Then the
derivations of the vinyl group were conducted: with CuTC as
a catalyst, hydroboration of 3a to synthetic useful gem-
diboronate ester 10 was realized.[25] Finally, cyclopropanation
of 3a produced cyclopropyl boronate esters 11, which was
difficult to make using other strategies.[26]
À
alkene into the Zr B bond forms the zirconium boryl alkyl
intermediate, which after b-H elimination provides the final
product VBE with the regeneration of Cp2ZrH2.
In order to understand the reaction mechanism, several
control experiments were conducted. Firstly, the standard
reaction with styrene was performed in a J-Young NMR tube
in d8-toluene, and the peak for H2 at 4.55 ppm was observed
(Scheme 4a and Figure S1). When Cp2ZrH2 was dissolved in
d8-toluene, no peak for Zr-H was seen, instead the H2 peak at
4.55 ppm was detected (Scheme 4b and Figure S2), which
meant that Cp2ZrH2 in toluene solution could easily release
H2 to give “Cp2Zr”, and this was further confirmed by GC
The reversibility of the dehydrogenative boration inspired
us to study the use of Cp2ZrH2 as a “shuttle catalyst” for
transfer boration reactions. “Shuttle catalysis” is an emerging
methodology that enables the catalytic transfer of function-
alities. Advantages are that it can avoid the use of toxic and
difficult-to-handle reagents.[21,29] We found that at 1008C,
using 4-methylstyrene and 2 equivalents of 3a as the model
reaction, the transfer boration product 3h was obtained in
60% yield. Further heating the reaction to 1308C produced
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3h in 80% yield. Compound 3a was found to be very stable
under ambient conditions, and it thus represents a replace-
ment of the relatively sensitive HBpin. With these advantages
and the optimized reaction conditions in hand, we studied the
application of Cp2ZrH2 as a “shuttle catalyst” for transfer
boration reactions of other alkenes (Scheme 5). The reaction
Keywords: boration · shuttle catalysis · transfer boration ·
zirconium
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Scheme 5. Cp2ZrH2-catalyzed transfer boration of various alkenes.
Conditions: substituted alkene (0.1 mmol), 3a (0.2 mmol, 2 equiv),
5 mol% Cp2ZrH2, 1 mL toluene in 15 mL pressure tube; GC yields are
given.
was quite general with a series of mono- and disubstituted aryl
alkenes with yields up to 82%. Functional groups such as -F,
-Cl, and -CF3 were also tolerated. Currently, we propose that
the oxidative addition of H2 to “Cp2Zr” at higher temperature
regenerates Cp2ZrH2 which is responsible for the transfer
boration. Further investigations on the detailed mechanism
are underway.
In conclusion, the first facile and direct acceptorless
dehydrogenative boration of alkenes with borane was ach-
ieved using Cp2ZrH2 as the catalyst. Our methodology avoids
the use of 1) precious transition metals; 2) hydrogen accept-
ors; 3) higher temperatures and longer reaction times. Thus,
synthetically useful vinyl boronate esters were produced at
room temperature in less than 10 min. Practical applications
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transformations of a vinyl boronate ester to a series of
functional compounds. Mechanistic studies revealed a rever-
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Acknowledgements
We are grateful to the National Natural Science Foundation
of China (91845108) and Natural Science Foundation of
Jiangsu Province (BK20180246) for generous financial sup-
port and the National Program for Thousand Young Talents
of China for support to start the lab. We thank Prof. Senmiao
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Manuscript received: July 17, 2019
Revised manuscript received: August 26, 2019
Accepted manuscript online: September 4, 2019
Version of record online: && &&, &&&&
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Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Boration
X. Shi, S. Li, L. Wu*
&&&& — &&&&
H2-Acceptorless Dehydrogenative
Boration and Transfer Boration of Alkenes
Enabled by Zirconium Catalyst
The first facile and direct H2-acceptorless
dehydrogenative boration of alkenes was
achieved at room temperature in less
than 10 min. Mechanistic studies
which suggested the application of the
zirconium catalyst as a “shuttle catalyst”
for transfer boration, thus avoiding the
use of relatively sensitive borane
reagents.
revealed a reversible reaction pathway,
6
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
These are not the final page numbers!