Hydroboration of Alkynes Catalyzed by Pyrrolide-Based PNP Pincer-Iron Complexes

Article abstract of DOI:10.1021/acs.orglett.7b01995

To utilize iron complexes as catalysts, the application of a well-designed ligand is critical to control the reactivity of the iron center. Recently, our group has succeeded in the synthesis of iron complexes bearing a pyrrolide-based PNP pincer ligand an

Full text of DOI:10.1021/acs.orglett.7b01995

Letter
pubs.acs.org/OrgLett
Hydroboration of Alkynes Catalyzed by Pyrrolide-Based PNP Pincer−
Iron Complexes
Kazunari Nakajima, Takeru Kato, and Yoshiaki Nishibayashi*
Department of Systems Innovation, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
*
S Supporting Information
ABSTRACT: To utilize iron complexes as catalysts, the application of a
well-designed ligand is critical to control the reactivity of the iron center.
Recently, our group has succeeded in the synthesis of iron complexes
bearing a pyrrolide-based PNP pincer ligand and their application to the
catalytic transformation of dinitrogen into ammonia under mild reaction
conditions. As an extensive study, we report the iron-catalyzed
hydroboration of alkynes with pinacolborane, where the corresponding
E-isomers are obtained selectively.
evelopment of synthetic approaches to prepare organo-
boron compounds is quite important because these
D
compounds can be directly used for various transformations
1
including a carbon−carbon (C−C) bond formation. In
particular, boronic acids are quite useful reagents because of
their stability and ease of handling. To obtain boronic acids,
transition-metal-catalyzed hydroboration across C−C multiple
Figure 1. Iron complexes bearing a pyrrolide-based PNP pincer ligand.
2
bonds is a straightforward and efficient method. Although
precious metals have played a central role in achieving the
transformation, the use of cheap and earth-abundant metals has
attracted considerable attention in recent years.
fixation motivated us to apply these iron complexes to organic
transformations. In the course of this attempt, we have found
these iron complexes effectively catalyze hydroboration of
alkynes with 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (pinacol-
borane: HBpin) at room temperature to give the corresponding
Iron is the most earth-abundant transition metal and seems
to be nontoxic. Therefore, the use of iron complexes as catalysts
3
for organic transformations is quite attractive. In the past few
(
E)-alknenyl boranes as exclusive products. Herein, we report
years, several groups have reported iron-catalyzed hydro-
4
,5a
our investigation on the iron-catalyzed hydroboration of
alkynes with pinacolborane under ambient reaction conditions.
At first, the optimization of reaction conditions was carried
out by using phenylacetylene (5a) as a model substrate. Typical
results are shown in Table 1. In the presence of 1 mol % of iron
complexes (1−4), reactions of 5a with 1.1 equiv of HBpin in
hexane as a solvent were carried out at room temperature for 16
h (Table 1, entries 1−4). The use of iron−chloro complex (1)
and iron−methyl complex (2) gave the corresponding (E)-
alkenyl borane (6a) in low to moderate yields (Table 1, entries
boration of alkynes.
In 2013, Enthaler and co-workers
reported the first iron-catalyzed hydroboration of alkynes by
4a
using Fe (CO) as a catalyst. Since this first report, several
2
9
groups have reported iron-catalyzed reactions by using various
iron complexes as catalysts. However, these catalytic reactions
4a,c,e
usually require heating conditions
or the use of activators
such as highly reactive reductants and basic reagents for iron
4
b−d
complexes.
As a result, development of a novel reaction
system to promote the catalytic reaction under mild conditions
with broad scope of substrates is of significant importance.
To control the reactivity of iron complexes, the design of
ligands around the iron center occupies a critical role. Recently,
a pyrrolide-based PNP pincer ligand (2,5-bis-
1
and 2, respectively). The use of iron−hydride complex (3)
promoted the hydroboration effectively to give 6a in 88% yield
Table 1, entry 3). Iron−dinitrogen complex 4 also worked as
(
an effective catalyst under the same reaction conditions (Table
1, entry 4). Other solvents such as toluene and THF were also
applicable, although the yields of 6a were slightly lower than
that in hexane as a solvent (Table 1, entries 5 and 6).
Separately, we confirmed no reaction occurred at all when the
reaction was performed in the absence of the iron complex
(
phosphinomethyl)pyrrolide) has emerged as a variant of the
6
−9
anionic pincer ligand.
Although various transition metal
complexes bearing a pyrrolide-based PNP pincer ligand have
been synthesized, the application of the complexes to catalytic
7
reactions has been limited to only a few cases. Our group has
succeeded in the synthesis of a series of iron complexes bearing
(
Table 1, entry 7).
a pyrrolide-based PNP pincer ligand (1−4), as shown in Figure
8
1
. We also found some of these iron complexes work as
effective catalysts to convert dinitrogen into ammonia under
mild reaction conditions. The success in the catalytic nitrogen
Received: June 30, 2017
8
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01995
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Letter
a
Table 1. Optimization of Reaction Conditions
an alkyne with a 3-thiophene ring was available to gain the
corresponding product (6k) in 73% yield. Not only aromatic
alkynes but also aliphatic alkynes can be applied to this reaction
system to give the corresponding products (6l and 6m) in good
yields. Unfortunately, internal alkyne cannot be applied to this
reaction system. The reaction of diphenylacetylene afforded the
desired product (6n) in 21% yield, despite the use of 5 mol %
of catalyst and 2.0 equiv of HBpin.
b
entry
catalyst
solvent
yield of 6a (%)
1
2
3
4
5
6
7
1
hexane
hexane
hexane
hexane
toluene
THF
21
43
2
To obtain insight on chemoselectivity of alkynes and alkenes,
reactions with enynes were examined as shown in Scheme 2.
c
3
88 (88)
4
81
79
78
0
3
Scheme 2. Hydroboration of Alkynes and Alkenes
3
none
hexane
a
5
a (0.30 mmol) was reacted with HBpin (0.33 mmol) in the
presence of catalyst complex (0.0030 mmol) in solvent at room
b
c
temperature for 16 h. NMR yield. Isolated yield.
Next, we examined reactions of various alkynes with
pinacolborane under the optimized reaction conditions. Typical
results are shown in Scheme 1. The use of p-tolylacetylene and
a
Scheme 1. Reactions of Alkynes 5 with HBpin
When a conjugated enyne (5o) was applied in this reaction
system, the hydroboration of the alkyne moiety proceeded
selectively (Scheme 2a). Furthermore, we tested the 4-
ethynylstyrene (5p) as a substrate bearing both terminal alkyne
and alkene moieties (Scheme 2b). Interestingly, the selective
transformation of the alkyne moiety afforded 6p in 73% yield.
An analogous intermolecular reaction with a 1:1 mixture of p-
tolylacetylene (5b) and styrene was also examined, where
dominant transformation of 5b was observed with almost
quantitative recovery of styrene (Scheme 2c). In these
reactions, products via hydroboration of the alkene moiety
were not observed in the crude mixture. These results indicate
that the present reaction system provides the completely
selective hydroboration of alkynes even in the presence of the
alkenes. In several previous examples of iron catalysts,
hydroboration of both alkenes and alkynes was performed
4
b,d,e
under the same reaction conditions,
whereas selective
5b
a
hydroboration of alkynes over terminal alkenes was rare.
5
(0.30 mmol) was reacted with HBpin (0.33 mmol) in the presence
Additionally, we found the higher turnover number (TON)
of 3 can be achieved at a higher reaction temperature. The
reaction of 5a with 1.1 equiv of HBpin in the presence of 0.1
mol % of 3 at 60 °C for 16 h afforded the corresponding
product 6a in 73% yield (Scheme 3). Separately, we confirmed
that the reaction in the absence of 3 even under the heating
conditions proceeded sluggishly. Accordingly, the TON is
of 3 (0.0030 mmol) in hexane at ambient temperature for 16 h.
b
c
d
e
Isolated yield. 3 mol % of 3 was used. 5 mol % of 3 was used. 2
f
equiv of HBpin was used. NMR yield.
m-tolylacetylene gave the corresponding alkenyl boranes (6b
and 6c) in good yields, whereas the use of o-tolylacetylene did
not give the desired product (6d) in reasonable yield, probably
due to the steric hindrance. Reactions of alkynes bearing fluoro,
chloro, and trifluoromethyl groups at the benzene ring were
successful to give 6e−h, although slightly higher catalyst
loading was required to obtain 6g and 6h in good yields.
Alkynes bearing coordinating functional groups such as ester
and ether at the benzene ring were compatible with this
reaction system, and the desired products (6i and 6j) were
obtained in good yields. As well as alkynes with a benzene ring,
Scheme 3. Reaction at High Temperature
B
DOI: 10.1021/acs.orglett.7b01995
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estimated to be 710, and this is the highest TON ever reported
for the iron-catalyzed hydroboration reactions of alkynes with
hydroboranes.
To obtain mechanistic insight, treatment of 3 with an excess
amount of HBpin was carried out (Scheme 4a). After the
ASSOCIATED CONTENT
Supporting Information
■
*
S
4
,5
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01995.
Experimental details, additional discussion, spectroscopic
data (PDF)
Crystallographic data (CIF)
Scheme 4. Generation and Reactivity of Iron−Boryl
Complex 7
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: ynishiba@sys.t.u-tokyo.ac.jp.
ORCID
Kazunari Nakajima: 0000-0001-9892-5877
Yoshiaki Nishibayashi: 0000-0001-9739-9588
Notes
The authors declare no competing financial interest.
reaction, formation of iron−boryl complex 7 was observed as a
new species. We succeeded in the isolation of 7 in a separate
manner, and an ORTEP drawing is shown in Figure 2. We also
ACKNOWLEDGMENTS
This work was supported by CREST, JST (JPMJCR1541). We
thank JSPS KAKENHI Grant Numbers 17H01201, 15H05798,
■
1
6K05767, and 16KT0160 from the Japan Society for the
Promotion of Science (JSPS) and the Ministry of Education,
Culture, Sports, Science and Technology of Japan (MEXT).
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(
(
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(
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DOI: 10.1021/acs.orglett.7b01995
Org. Lett. XXXX, XXX, XXX−XXX