Practical and selective hydroboration of aldehydes and ketones in air catalysed by an iron(ii) coordination polymer

Article abstract of DOI:10.1039/c9gc00078j

The in air catalytic hydroboration of ketones and aldehydes with pinacolborane by an iron(ii) coordination polymer (CP) is carried out under mild and solvent-free conditions. The precatalyst is highly active towards a wide range of substrates including functionalized ketones and aldehydes in the presence of KO^tBu as an activator, achieving a high turnover number (TON) of up to 9500. Excellent chemoselectivity to aldehydes over ketones was also revealed, which is in sharp contrast with the results obtained under inert atmosphere using the same catalyst system. This catalyst observed here is not only highly efficient but also recyclable for reuse for at least 5 times without losing its effectiveness.

Full text of DOI:10.1039/c9gc00078j

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DOI: 10.1039/C9GC00078J
Green Chemistry
ARTICLE
Practical and selective hydroboration of aldehydes and ketones in
air catalysed by an iron(II) coordination polymer
Guoqi Zhang,* Jessica Cheng, Kezia Davis, Mary Grace Bonifacio and Cynthia Zajaczkowski
Received 00th January 20xx,
Accepted 00th January 20xx
The in air catalytic hydroboration of ketones and aldehydes with pinacolborane by an iron(II) coordination polymer (CP)
is carried out under mild and solvent-free conditions. The precatalyst is highly active towards a wide range of substrates
including functionalized ketones and aldehydes in the presene of KOtBu as an activator, achieving a high turnover number
(TON) of up to 9500. Excellent chemoselectivity to aldehydes over ketones was also revealed, which is in sharp contrast
with the results obtained under inert atmosphere using the same catalyst system. This catalyst observed here is not only
highly efficient but also recyclable for reuse for at least 5 times without losing its effectiveness.
DOI: 10.1039/x0xx00000x
www.rsc.org/
hydroboration of aldehydes only.12 More recently, Geetharani
and coworkers reported a simple iron(II) amide complex that is
also effective for the hydroboration of aldehydes and ketones,
Introduction
Metal-catalyzed hydroboration of carbonyl compounds using
pinacolborane (HBpin) has attracted considerable attention in
recent years, as it provides synthetically important borate ester
intermediates which could be readily converted into the
corresponding alcohols.1 While many stoichiometric methods
are known for the hydrogenation of carbonyls by hydride
reagents, the functional group tolerance and chemoselectivity
between aldehydes and ketones were often a challenge.2 In the
past decade, selective hydroboration of carbonyls has been
intensively explored by metal catalysts, in particular those
containing transition metals,3 rare earth4 and main group
metals,5 although an uncatalyzed method for aldehyde
hydroboration is recently reported.6 Among the most attractive
metal-catalyzed hydroboration of carbonyl compounds, iron, as
the most abundant transition metal, has only found applications
for this reaction in a few reports very recently, although iron is
well known for hydroboration and hydrosilylation of olefins.7-9
In 2017, Findlater and coworkers reported on iron-catalysed
hydroboration of carbonyl compounds under N2 atmosphere
using a simple Fe(acac)3 salt (acac = acetylacetonate, 10 mol%
of catalyst loading) in the presence of an air-sensitive activator,
NaHBEt3.10 The chemoselectivity of aldehydes over ketones was
also disclosed. Later on, Bai et al. developed a dimer iron(II)
phosphoranimide complex for the selective hydroboration of
aldehydes over ketones.11 In 2018, the Baker group also
revealed a dimeric FeII complex based on a N2S2 ligand that was
suitable for the
with chemoselectivity favoring the former.13
Despite such advances, the current state of catalyst
development for carbonyl hydroboration still suffers from non-
trivial catalyst synthesis (typical metal-ligand complexes), low
catalytic efficiency, limited scope of substrates and poor
functional group tolerance.1 It is also noticed that a typical
hydroboration (of alkenes, alkynes and carbonyls) usually
requires inert atmosphere (due to the use of air- and moisture-
sensitive borane reagents),1,3-6 and most of them involve air-
sensitive metal complex catalysts or activators, thus diminishing
its interest for synthetic utilization in industry. In addition, the
majority of metal-catalysed hydroboration reported thus far
proceeded under homogeneous conditions and catalyst
recycling could be very troublesome. From a viewpoint of green
chemistry, practical methods for the hydroboration of carbonyl
compounds are extremely limited. In 2017, Zhao’s group
developed
a
transition-metal-free method for the
hydroboration of ketones, aldehydes, alkenes and alkynes,
where sodium hydroxide was used to promote the reactions of
a range of substrates in toluene or benzene-d6.5f Later on,
Hreczycho and coworkers disclosed a catalyst-free and solvent-
free method for the hydroboration of aldehydes under mild
conditions.6 However, this method displayed poor effectiveness
in ketone hydroboration even at elevated temperature.
Therefore, a greener methodology that utilizes higher-
efficiency and less-expensive catalysts (ideally recyclable) and
enables the reaction in air without the use of toxic organic
solvents is highly desired.
Department of Sciences, John Jay College and The Graduate Center, The City
University of New York, New York, NY 10019, USA. Email: guzhang@jjay.cuny.edu
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Hydroboration of carbonyls
(where tpy = 2,2′;6′,2′′-terpyridine) further reveals the
View Article Online
importance of a polymeric structure in 1DfoOrI:e10ff.1e0c3t9iv/Ce9rGeCa0c0ti0v7i8tyJ
(entry 8). Solvent effect was also studied and we found that the
reactions proceeded more slowly in toluene, benzene or
pentane compared to that in THF, and even worse in diethyl
ether giving only 20% GC yield of 2 in 30 min (entries 9-12). In
contrast, we performed the reaction under the same conditions
(1/KOtBu in THF, rt), but under N2, only 12% of 2 was detected
in 30 min (entry 13). Finally, it was noted that the reaction was
equally efficient without a solvent (entry 14) and high yield
(95%) was still accomplished when decreasing the loading of
precatalyst 1 to 0.01 mol%, corresponding to a high TON of
9500.
O
OBpin
metal-catalyzed or uncatalyzed
O
B
+
R2
R2
H
R1
R1
O
inert atmosphere
H
(HBpin)
This work: Green, Catalytic hydroboration of carbonyls in air
air-stable, recyclable FeII catalyst
OBpin
R2
O
+
HBpin
R1
R2
in air AND solvent-free
R1
H
- In air and solvent-free
- High efficiency
- Excellent functional group tolerance
- Chemoselective for aldehydes over ketones
- Reusable and recyclable
Scheme 1. Fe-catalysed hydroboration of ketones and aldehydes.
1
(0.1 mol%)
O
OBpin
H
KOtBu (2 mol%)
We have been in particular interested in reduction catalysis
concerning unsaturated bonds by using Earth-abundant cobalt
and iron catalysts14 and have reported a two-dimensional
iron(II) coordination polymer (1) based on a divergent 4′-(4-
diphenylaminophenyl)-4,2′;6′,4′′-terpyridine ligand (L) that
catalyzed the heterogeneous hydroboration of carbonyl
compounds using HBpin with KOtBu as an activator.15
Interestingly, this catalyst system showed excellent catalytic
activity towards ketone substrates, yet very poor performance
for aldehyde hydroboration under inert atmosphere (Scheme
1). This is in sharp contrast with other known iron(II)
hydroboration catalysts that usually favor aldehydes over
ketones.15 Upon further investigation of the unique
chemoselectivity of 1, we found that 1 displayed higher catalytic
efficiency for hydroboration of a broad range of carbonyl
compounds in air and under mild, solvent-free conditions, as
well as excellent chemoselectivity towards aldehydes over
ketones.
+
HBpin
H
THF, RT, 16 h
R
N2
yield: 15%
1
(0.1 mol%)
O
OBpin
KOtBu (2 mol%)
+
HBpin
H
R
H
neat, RT, 0.5 h
in air
yield: >99%
Scheme 2. The comparison of 1-catalysed hydroboration of
benzaldehyde with pinacolborane under N2 or in air (up), and crystal
structure of 1 showing the FeIIN4Cl2-coordination environment (down,
left) and a 2-D framework (down, right).
Next, we applied the green, iron-catalysed method for the
hydroboration of benzaldehyde to more functionalized
aldehydes as well as ketones under neat conditions in air (0.1
mol% 1 and 2 mol% KOtBu). A range of electronically and
sterically differentiated aldehydes were first explored and the
results are summarized in Table 2. For these examples, the
hydroborated products (2) have been analysed by GC-MS as
intermediates, and the corresponding primary alcohols (3) were
obtained subsequently after purification by silica gel column
chromatography, due to the known silica-promoted hydrolysis
of boronate esters that has been observed previously.15
Herein, we report a green and practical approach to
heterogeneous hydroboration of carbonyl compounds in air
catalysed by an air-stable iron(II) coordination polymer. To our
knowledge, heterogeneous carbonyl hydroboration was only
rarely reported involving the use of air- and moisture-sensitive
metal reagents.16,17
Results and discussion
Initially, we conducted the reaction of benzaldehyde with HBpin
by using catalyst 1 (0.1 mol% based on FeL2Cl2 unit) in the
presence of KOtBu (2 mol%) in air and found complete
hydroboration in only 30 min under solvent-free conditions
(Scheme 2). To compare, the same reaction proceeded very
sluggishly under N2 atmosphere affording 15% boronate ester
after 16 h as reported previously.15 Further screening of
reaction conditions was carried out and the results are
summarized in Table 1. It was found that other base additives
such as KOtBu, KOH and K2CO3 were also comparable activators
for Fe-catalysed hydroboration in air (entries 2-4, Table 1). The
control experiments showed that the precatalyst 1 alone was
inactive without added KOtBu (entry 5), while the reaction
occurred in the presence of KOtBu with poor yield of 2 (19%,
entry 6). The combination of free ligand and KOtBu also
promoted hydroboration, however, the yield was very low
(entry 7). A direct comparison of the catalytic activity between
polymeric framework 1 and the discrete complex Fe(tpy)Cl2
Table 1. Condition Screening for 1-Catalysed Hydroboration of
Benzaldehyde with Pinacolborane in air.a
OBpin
O
[Fe]
H
HBpin
+
in air, 25 o
C
2
Entry Catalyst
Activator
KOtBu
NaOtBu
KOH
K2CO3
-
KOtBu
KOtBu
KOtBu
Solvent
THF
THF
THF
THF
THF
THF
THF
THF
Yield /%b
1
2
3
4
5
6
7
8
1
1
1
1
1
-
L
99
99
97
98
<5
19
25
18
Fe(tpy)Cl2
2 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
O
OBpin
H
4
9
1
1
1
1
1
1
1
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
Et2O
20
66
70
62
12
99
95
View Article Online
DOI: 10.1039/C9GC00078J
H
98 (94)
10
11
12
13c
14
15d
toluene
benzene
pentane
THF
-
-
O
O
O
O
(2d)
OBpin
O
S
88 (81)
94 (90)
5
H
H
S
(2e)
OBpin
H
6c
O
H
a Conditions: benzaldehyde (1.0 mmol), HBpin (1.1 mmol), catalyst 1 (0.1
mol%), activator (2 mol%) and solvent (1 mL), 25°C, 30 min, in air.
b
F3C
F3C
(2f)
Determined by GC-MS analysis with hexamethylbenzene as an internal
7c
95 (83)
99 (84)
90 (85)
95 (90)
O
OBpin
H
standard. c Reaction run under N2. d 0.01 mol% 1 was used.
H
O2N
O2N
(2g)
Benzyl alcohol, 3a, was isolated in 92% yield after the standard
reaction in air within 30 min, while complete conversion was
detected (entry 1, Table 2). Substituted benzaldehydes bearing halo,
electron-donating and electron-withdrawing groups are all suitable
substrates, affording the desired alcohols 3a-f in good to excellent
yields (entries 2-6). While reaction with 4-methoxybenzaldehyde
proceeded a little sluggishly giving boronate ester (2c) in 80% GC
yield after 4 h, 4-trifluoromethylbenzaldehyde containing a strongly
electron-withdrawing group was converted to the corresponding
alcohol in higher yield within the same period. It is worth mentioning
that all these substrates show poor to moderate reactivity with the
same catalyst under N2 atmosphere.15 Interestingly, benzaldehydes
containing reducible groups such as nitro or ester functionality were
selectively hydroborated at the aldehyde part under standard
conditions with the corresponding alcohols (3g and 3h, entries 7 and
8) being isolated with excellent yields. 1-Pyrenecarboxaldehyde was
also readily hydroborated to give the resulting adduct in high yield
within a period of 4 h (entry 9). In addition, benzene-1,4-
dicarbaldehyde can be fully hydroborated with the use of 2.2 equiv.
of HBpin, providing the desired diol in 90% yield (entry 10).
Aldehydes with S- or N-heterocycle also showed equal activity as
benzaldehyde, leading to isolation of the resultant alcohols (3k and
3l) in excellent yields. Trans-cinnamaldehyde containing conjugated
C=C and C=O bonds was selectively hydroborated at the C=O bond in
the presence of 2.2 equiv. of HBpin (entry 13). Finally, several
aliphatic aldehydes were examined and all found to be active
substrates for 1-catalysed hydroboration, with reactions being
furnished in 30 min (entries 14-17).
8c
O
OBpin
H
H
MeOOC
MeOOC
(2h)
9c
O
OBpin
H
OBpin
H
(2i)
10c,d
O
H
H
H
O
OBpin
(2j)
O
OBpin
H
11
12
98 (92)
99 (91)
H
S
S
(2k)
O
OBpin
H
H
N
N
(2l)
OBpin
13d
93 (85)
O
(2m)
OBpin (2n)
OBpin (2o)
14
15
16
99
99
90
O
O
O
5
5
OBpin
(2p)
OBpin (2q)
17
98
O
a
Conditions: aldehydes (1.0 mmol), HBpin (1.1 mmol), catalyst 1 (0.1 mol%)
and KOtBu (2 mol%), rt, 30 min and in air. b Yields of hydroborated products
(2) were determined by GC-MS analysis with hexamethylbenzene as an
internal standard. Isolated yields of the corresponding alcohols (3) are shown
in parenthesis. c Reaction run for 4 h. d 2.2 equiv. HBpin was used.
With success in aldehyde hydroboration, we also explored the
potential of this catalyst for ketone hydroboration in air, even though
it has been previously revealed that this catalyst was effective for
hydroboration of a variety of ketones under N2. A facile ketone
hydroboration without protection with inert atmosphere will be
more attractive with regard to green chemistry. Thus, a variety of
functionalized ketones were tested and the results are presented in
Table 3. Again, acetophenone and other aryl ketones with electron-
donating and -withdrawing groups can be converted smoothly into
the corresponding boronate esters in 1 h, with high isolated yields of
secondary alcohols 5a-f (entries 1-6, Table 3). Cyclopropyl phenyl
ketone was also converted with appreciable yield as compared to the
same reaction under N2 (entry 7).15 Tetralone, 2-nathphaldehyde and
2-acetylferrocene were excellent substrates, providing the
corresponding alcohols 5h-j with high yields (entries 8-10). Whereas
an , -unsaturated ketone underwent selective 1,2-addition to give
5i with a moderate yield after 4 h when two equiv. of HBpin was used
(entry 11), both heteroaryl ketone (3-acetylpyridine) and aliphatic
ketones (cyclohexanone, cyclopentanone, acetone and 2-
heptanone) were readily hydroborated in good to high yields with
HBpin in 1 h (entries 12-16). It was noted that heteroaryl ketones
Table 2. Substrate scope of iron(II)-catalysed aldehyde hydroboration in air.a
1
(0.1 mol%)
KOtBu (2 mol%)
O
OBpin
silica, rt
+
HBpin
R
OH
R
H
neat, rt, 30 min
R
H
in air
3
2
Entry
1
Substrate
Product 2
Yield (%)b
99 (92)
O
OBpin
H
H
(2a)
OBpin
O
2
99 (91)
80 (75)
H
H
Cl
O
Cl
(2b)
3c
O
OBpin
H
H
O
(2c)
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Journal Name
including 3- or 4-acetylpyridine were challenging substrates when conditions (0.5 mol% of 1 was used in THF. Scheme 3). It was
the reaction was conducted under N2 (entry 12).15 This represents an observed that hydroboration only occurred atVietwheArtickleeOtonlninee
DOI: 10.1039/C9GC00078J
extra advantage of ‘in air’ hydroboration catalyzed by 1.
position, displaying excellent selectivity over alkene
hydroboration and possible cyclic ether ring-opening reactions
that could be promoted by Lewis acid.19 Thus, compound 6 was
readily isolated after boronate ester hydrolysis as the only
product in good yield.
Table 3. Substrate scope of iron(II)-catalysed ketone hydroboration in air.a
1
(0.1 mol%)
OH
O
OBpin
KOtBu (2 mol%)
silica, rt
+
HBpin
R1 R2
R1 R2
neat, rt, 1 h
R1 R2
in air
5
4
[Fe] (0.5 mol%)
KOtBu (2 mol%)
1)
Entry
1
Substrate
Product 4
Yield (%)b
H
H
H
H
O
O
O
O
O
O
OBpin
O
O
98 (95)
90 (85)
95 (90)
in air
rt, 4 h,
HBpin
+
2)
SiO2, rt
y. 76%
(2.0 eq.)
OH
(4a)
(4b)
MeO
MeO
O
OBpin
2
3
OMe
OMe
6
Scheme 3. Fe-catalysed selective hydroborative reduction of rotenone
in air.
O
OBpin
To ascertain whether the polymeric framework structure of
1 is the key to high catalytic activity observed here, we prepared
two analogues of 1 featuring discrete structures of iron(II)
complexes, and compared their catalytic performance (Scheme
4). Thus, iron(II) complexes 7 and 8 containing convergent
ligands, 4′-(4-dimethylamino)phenyl-2,2′;6′,2′′-tpy and 4′-(4-
Cl
Cl
(4c)
O
OBpin
OBpin
4
95 (88)
O
O
(4d)
O
5
95 (90)
90 (82)
O2N
F3C
diphenylamino)phenyl-2,2′;6′,2′′-tpy,
were
synthesized,
O2N
F3C
(4e)
(4f)
6c
O
respectively, in good yields through a facile procedure (see ESI).
7 and 8 were then examined for the homogeneous
hydroboration of benzaldehyde in air under neat conditions. It
was observed that while 7 (0.1 mol%) catalysed hydroboration
of benzaldehyde in 85% yield within 30 min, 8 showed higher
activity that is identical to 1. This finding indicates that the
ligand structure has an impact on the catalytic reactivity and the
N,N-diphenylamino substituent on tpy might have played an
important role in promoting the formation of active catalytic
species. Furthermore, we applied discrete complex 8 as a
precatalyst for the hydroboration of several functionalized
aldehydes and ketones as illustrated in Scheme 4. The results
show that for all examples the yields were either comparable to
or slightly lower than those obtained by using 1 as a precatalyst
under identical conditions, suggesting that the polymeric
structure of 1 might not be determining for the excellent
catalytic activity in air.
OBpin
O
7
75 (70)
OBpin
(4g)
O
OBpin
8
98 (71)
95 (84)
96 (91)
(4h)
O
OBpin
9
(4i)
O
OBpin
10
Fe
Fe
(4j)
11c
12
70 (62)
O
OBpin
(4k)
O
OBpin
99 (92)
<5d
N
O
N
OBpin
(4l)
13
14
85
82
(4m)
OBpin
OBpin
O
O
(4n)
(4o)
15
16
99
96
O
OBpin
(4p)
a
Conditions: ketones or aldehydes (1.0 mmol), HBpin (1.1 mmol), catalyst 1
b
(0.1 mol%), KOtBu (2 mol%), rt, 1 h and in air. Yields of hydroborated
products (2) were determined by GC-MS analysis. Isolated yields of alcohols
(3) are shown in parenthesis. c Reaction run for 4 h. d Reaction run under N2.
To further demonstrate the applicability of this method for
reduction of more complex ketone substrates, we examined the
reaction of HBpin with rotenone,18 a naturally occurring
products used as a pesticide and insecticide, under Fe-catalysed
4 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
A.
O
View Article Online
O
[Fe] (0.1 mol%)
KOtBu (2 mol%)
1
(0.1 mol%)
N
Cl
O
O
DOI: 10.1039/C9GC00078J
OBpin
R2
KOtBu (2 mol%)
OBpin
R
R
+
HBpin
+
HBpin
N
Fe
+
+
N
+
+
neat, rt, 30 min
in air
R2
R1
R1
Cl
neat, rt, 30 min
N
in air
99%
97%
1 mmol
1 mmol
1 mmol
7
8
, R = Me
, R = Ph
1
(0.1 mol%)
KOtBu (2 mol%)
+
OBpin
HBpin
O
O
O
5
5
3
3
neat, rt, 30 min
in air
OBpin
H
OBpin
OBpin
1 mmol
1 mmol
99%
98%
1 mmol
OBpin
OBpin
1
(0.1 mol%)
H
B.
KOtBu (2 mol%)
neat, rt, 2 h
in air
OH
[Fe]
O
S
O
O
+
HBpin
1 mmol
85%
99%
7
8
SiO2, rt
92%
87%
OBpin
99%
9
1 mmol
, 86% yield
Scheme 5. Fe-catalysed intra- and inter-molecular chemoselective
OBpin
OBpin
hydroboration of aldehydes vs ketones.
O
N
Furthermore, we studied the reducible functional group
compatibility by conducting a simple robustness screen with
iron catalyst in air. This method was first introduced by the
Glorius group aiming to develop a fast and feasible method to
test functional group tolerance in a catalytic reaction, before a
complicated substrate containing multiple functional groups
was employed.20 Using the same method, we examined the Fe-
catalysed hydroboration of benzaldehyde in air under the
standard conditions in the presence of additives bearing various
reducible groups (1.0 eq.) as shown in Table 4. Products of each
reaction were analysed by GC using an internal standard after
30 min. The results revealed that the hydroboration of
benzaldehyde was not affected by the presence of reducible
functionalities such as alkene, alkyne, nitrile, nitro, imine and
pyridine (entries 1-6), while it was almost quenched by N,N-
dimethylformamide (DMF, entry 7). It was noted that in all
reactions, no products attributed to the additive hydroboration
were detected, even in the case of DMF.
64%a
8
88%
55%
96%
OBpin
OBpin
OBpin
98%
OBpin
5
8
91%
92%
88%
Scheme 4. Homogeneous Fe-catalysed hydroboration of carbonyl
compounds in air. Conditions: ketones or aldehydes (1.0 mmol), HBpin (1.1
mmol), 7 or 8 (0.1 mol%), KOtBu (2 mol%), rt, 30 min (for aldehydes) or 1 h
(for ketones) and in air. Yields of boronate esters were determined by using
GC-MS analysis. a Reaction run for 4 h.
Previously, it was found that under N2 the catalyst was
inactive in the presence of an equimolar mixture of
acetophenone and benzaldehyde, and the chemoselective
hydroboration could not be established.15 We are further
interested to explore the chemoselectivity between aldehyde
and ketone when the reaction is catalyzed by 1 as a precatalyst
in air (Scheme 5). Thus, when acetophenone (1 mmol) was
mixed with benzaldehyde (1 mmol) and one equiv. of HBpin
under standard conditions, quantitative hydroboration of
aldehyde was observed with almost full recovery of the ketone.
Likewise, the competing reaction between aliphatic aldehyde
and ketone revealed the same selectivity for aldehyde over
ketone. In addition, the intramolecular chemoselective
hydroboration using 4-acetylbenzaldehyde resulted in the
isolation of a primary alcohol (9) in good yield and no doubly
hydroborated product was isolated, indicating excellent
chemoselectivity as well. This is significant as it shows that
catalysis with precatalyst 1 in air outperforms that at inert
atmosphere, in terms of both its generality and
chemoselectivity. Although similar chemoselectivity has been
Table 4. The reducible functional group tolerance of benzaldehyde
hydroboration.a
1
(0.1 mol%)
KOtBu (2 mol%)
additive
O
OBpin
(1.0 eq.)
H
+
HBpin
neat, rt, 30 min
(1.0 eq.)
in air
2a
Entry
1
Additive
Yield of 2a (%)a
99
2
3
99
99
N
previously
reported
for
transition
metal-catalysed
NO2
4
5
97
99
hydroboration of carbonyls,3,10-13 this example that features a
heterogeneous catalysis operating in air is unique.
N
6
7
99
15
N
O
H
N
a
Determined by GC-MS analysis with hexamethylbenzene as an
internal standard.
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Comparing to homogeneous catalysis, reactions under HBpin and activator KOtBu would produce a reducing species,
View Article Online
heterogenous conditions with solid catalysts, such as metal- HB(OtBu)(Pin), which is responsible for the generation of a
DOI: 10.1039/C9GC00078J
organic frameworks (MOFs) and metalated porous organic critical Fe-H active species (Scheme 6).7 The as-formed Fe-H
polymers, are particularly attractive due to the recyclability and species allows facile carbonyl insertion to form an iron alkoxide
reusability of catalysts,21 as well as improved catalytic activity in intermediate that is reactive towards HBpin. As a result of σ-
some cases.22 Although effective hydroboration of carbonyl bond metathesis of HBpin, boronate ester product will be
compounds could be also promoted by homogeneous released and regenerate the active Fe-H species, closing the
precatalyst 7 in air, the utilization of coordination polymer 1 as catalytic cycle.
a precatalyst offers an opportunity for recycling and reusing the
L4FeIICl2
catalyst. Thus, the reusability of 1 was established using
benzaldehyde as a model substrate (Fig. 1). The reaction was set
up in air with standard conditions using 0.1 mol% of 1 and 2
mol% of KOtBu for the 1st run. The reaction was then monitored
by GC analysis (65% yield of 2a) after 15 min and the solid
precatalyst could be readily isolated from the reaction mixture
by removing the liquid followed by washing with diethyl ether
for three times in a fume hood. At this end, the loss of iron from
the precatalyst was analysed by Inductively coupled plasma
optical emission spectrometry (ICP-OES). The iron content in
the liquid sample extracted from reaction mixture of the 1st run
was determined to be 7 ppm, indicating a negligible loss of Fe
from 1. The precatalyst was dried in the air and then reused for
the 2nd run by recharging the substrates and additional activator
(2 mol%). After 15 min, 63% yield of 2a was detected by GC
analysis. Following the same procedure, it was further
confirmed that the precatalyst can be recycled and reused for
at least 5 more cycles without losing its effectiveness (Fig. 1).
[O]
KOtBu
HBpin
LnFeIII
HB(OtBu)(Pin)
Bpin
O
O
R1
R2
LnFe-H
R1
R2
H
LnFe
LnFe
H
Bpin
R1
R2
O
O
R1
R2
FeLn
O
HBpin
R1
R2
Scheme 6. Plausible mechanism of Fe-CP catalysed hydroboration of
carbonyl compounds in air.
O
OBpin
1
(0.1 mol%)
KOtBu (2 mol%)
H
+
HBpin
neat, rt, 15 min
in air
Conclusions
(1.1 eq.)
2a
In summary, in this work we report an unusual ‘in air’
hydroboration of aldehydes and ketones under solvent-free and
heterogeneous conditions by a FeII-based coordination polymer
as precatalyst. A wide range of functionalized aldehyde and
ketone substrates have been successfully transformed to
primary and secondary alcohols through a hydroboration-
hydrolysis procedure. The iron(II) coordination polymer
precatalyst is air-stable, recyclable and tolerant of other
reducible functional groups. Excellent chemoselectivity on
aldehydes over ketones was also observed. The current method
represents a practical, green and operationally simple pathway
to the hydroboration of carbonyl compounds utilizing the Earth-
abundant metal iron.
Experimental
Fig. 1. The recycling experiments for 1-catalysed hydroboration of
benzaldehyde in a reaction period of 15 min with the bar graph showing the
GC yields of 2a at different runs.
General Considerations. The catalytic reactions were carried
out under a nitrogen atmosphere using standard glove-box
technique. Anhydrous grade solvents and liquid reagents used
were obtained from Aldrich or Fisher Scientific and stored over
4 Å molecular sieves. FT-IR spectra were recorded on a
Shimadzu 8400S instrument with solid samples under N2 using
In view of the important role of the ‘in air’ conditions played
in enabling effective hydroboration of both aldehydes and
ketones, we tentatively proposed an “oxygen-participating”
catalytic cycle for the iron-catalysed hydroboration (Scheme 6).
Initially, the limited catalytic activity of the FeII precatalyst under
N2 is presumably oxidized to an FeIII precatalyst in air, as it has
been previously found by Findlater et al. that trivalent Fe(acac)3
was effective for hydroboration of a range of aldehydes and
ketones in the presence of NaHBEt3 as activator.10 According to
previous work by Thomas and coworkers, the reaction between
a Golden Gate ATR accessory. H NMR and 13C NMR spectra
1
were obtained at room temperature on a Bruker AV 400, 500 or
600 MHz NMR spectrometer, with chemical shifts (δ)
referenced to the residual solvent signal. GC-MS analysis was
obtained using a Shimadzu GCMS-QP2010S gas chromatograph
mass spectrometer. Inductively coupled plasma optical
6 | J. Name., 2012, 00, 1-3
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1
C. C. Chang, R. Kinjo, ACS Catal. 2015, 5, 3238-3259.
View Article Online
emission spectrometry (ICP-OES) analyses were conducted by
Robertson Microlit Laboratories in the US to determine the loss
of iron for the recycling experiments. Fe(tpy)Cl2 and Complex 1
were synthesized according to published procedure.15,23
2 (a) B. T. Cho, Chem. Soc. Rev. 2009, 38, 443–452; A. Togni, H.
Grützmacher, Catalytic HeterofunctionDaOlizI:a1t0io.1n0;39W/Cil9eGyC-V0C00H7:8J
Weinheim, 2001; (b) M. B. Smith, J. March, March’s
Advanced Organic Chemistry; 6th ed.; Wiley-Interscience:
Hoboken, NJ, 2007; pp 1703−1869.
General Procedure for 1-Catalysed Hydroboration in Air. In a fume
hood, precatalyst 1 (1.08 mg, 1.0 μmol, 0.1 mol% based on the
[Fe(L)2Cl2] unit) and KOtBu (2.2 mg, 20 μmol) was loaded in a 3.8 mL
glass vial equipped with a stir bar and the vial was open to air.
Aldehyde or ketone (1.0 mmol) and pinacolborane (140.8 mg, 1.1
mmol) were then added. The reaction mixture was allowed to stir in
air at room temperature for indicated times shown in Tables 2 and 3.
After completion of the reaction, the crude reaction mixture was first
analyzed by GC-MS using a dilute CH2Cl2 solution, and then the
product was isolated by flash column chromatography on SiO2 using
ethyl acetate/hexane (1 : 10) as an eluent. All isolated products were
characterized by 1H and 13C NMR spectroscopies.
3 (a) Ru: A. Kaithal, B. Chatterjee, C. Gunanathan, Org. Lett. 2015,
17, 4790-4793; Zn: (b) P. A. Lummis, M. R. Momeni, M. W. Lui,
R. McDonald, M. J. Ferguson, M. Miskolzie, A. Brown, E.
Rivard, Angew. Chem., Int. Ed. 2014, 53, 9347-9351; Co: (c) J.
Guo, J. Chen, Z. Lu, Chem. Commun. 2015, 51, 5725-5727; J.
Wu, H. S. Zeng, J. Cheng, S. P. Zheng, J. A. Golen, D. R. Manke,
G. Q. Zhang, J. Org. Chem. 2018, 83, 9442-9448; (d) Cu: S.
Bagherzadeh, N. P. Mankad, Chem. Commun. 2016, 52, 3844-
3846; (e) H. Zeng, J. Wu, S. Li, C. Hui, A. Ta, S.-Y. Cheng, S.
Zheng, G. Zhang, Org. Lett. 2019, 21, 401-406; TiII: (f) A. A.
Oluyadi, S. Ma, C. N. Muhoro, Organometallics 2013, 32, 70;
MnII: (g) G. Zhang, H. Zeng, J. Wu, Z. Yin, S. Zheng, J. C.
Fettinger, Angew. Chem. Int. Ed. 2016, 55, 14369-14372; (h)
V. Vasilenko, C. K. Blasius, H. Wadepohl, L. H. Gade, Angew.
Chem. Int. Ed. 2017, 57, 8393-8397; Ni: (i) A. E. King, S. C. E.
Stieber, N. J. Henson, S. A. Kozimor, B. L. Scott, N. C. Smythe,
A. D. Sutton, J. C. Gordon, Eur. J. Inorg. Chem. 2016, 1635-
1640.
Competing Experiment for Selective Hydroboration of Ketone vs.
Aldehyde. In a fume hood, precatalyst 1 (1.08 mg, 1.0 μmol, 0.1
mol% based on the [Fe(L)2Cl2] unit) and KOtBu (2.2 mg, 20 μmol) was
loaded in a 3.8 mL glass vial equipped with a stir bar and the vial was
open to air. Then, acetophenone (120.0 mg, 1.0 mmol),
benzaldehyde (106.0 mg, 1.0 mmol) and pinacolborane (128.0 mg,
1.0 mmol) were added sequentially. The reaction mixture was
allowed to stir at room temperature for 30 min. The products were
analyzed by GC using hexamethylbenzene as an internal reference.
4 (a) S. F. Chen, D. D. Yan, M. Q. Xue, Y. B. Hong, Y. M. Yao, Q.
Shen, Org. Lett. 2017, 19, 3382-3385; (b) W. F. Wang, X. C.
Shen, F. Y. Zhao, H. Jiang, W. W. Yao, S. A. Pullarkat, L. Xu, M.
T. Ma, J. Org. Chem. 2018, 83, 69-74; (c) Z. Y. Zhu, P. Dai, Z. J.
Wu, M. Q. Xue, Y. M. Yao, Q. Shen, X. G. Bao, Catal. Commun.
2018, 112, 26-30.
5 (a) Z. Yang, M. Zhong, X. Ma, S. De, C. Anusha, P. Parameswaran,
H. W. Roesky, Angew. Chem. Int. Ed. 2015, 54, 10225-10229;
(b) V. K. Jakhar, M. K. Barman, S. Nembenna, Org. Lett. 2016,
18, 4710-4713; (c) A. Harinath, J. Bhattacharjee, H. P. Nayek,
T. K. Panda, Dalton Trans. 2018, 47, 12613-12622; (d) M.
Arrowsmith, T. J. Hadlington, M. S. Hill, G. Kociok-Kohn, Chem.
Commun. 2012, 48, 4567-4569; (e) D. Mukherjee, A. Ellern, A.
D. Sadow, Chem. Sci. 2014, 5, 959-964; (f) Y. L. Wu, C. K. Shan,
J. X. Ying, J. Su, J. Zhu, L. L. Liu, Y. F. Zhao, Green Chem. 2017,
19, 4169-4175; (g) G. Zhang, J. Wu, H. Zeng, M. C. Neary, M.
Devany, S. Zheng, P. A. Dub, ACS Catal. 2019, 9, 874-884.
Recycling and Reusing Experiment for Hydroboration of
Acetophenone. In a fume hood, precatalyst 1 (1.08 mg, 1.0 μmol, 0.1
mol% based on the [Fe(L)2Cl2] unit) and KOtBu (2.2 mg, 20 μmol) was
loaded in a 3.8 mL glass vial equipped with a stir bar and the vial was
open to air. Benzaldehyde (106.0 mg, 1.0 mmol) and pinacolborane
(140.8 mg, 1.1 mmol) were then added. The reaction mixture was
allowed to stir at room temperature in air for 15 min. The reaction
mixture was analyzed by using GC analysis to obtain the yield of 2a.
Then the solid was centrifuged out of suspension and extracted with
diethyl ether for three times. The organic extract (1 mL) was
combined and analyzed by ICP-OES. The loss of iron from the
precatalyst was determined to be 7 ppm. The solid precatalyst that
was recovered was then dried under reduced pressure and then
placed in a small vial, to which additional KOtBu (2.1 mg, 20 μmol)
was added. Then, Benzaldehyde (106.0 mg, 1.0 mmol) and
pinacolborane (140.8 mg, 1.1 mmol) were added. The resultant
mixture was stirred at room temperature in air for 15 min and the
product was analyzed by using GC analysis again. This procedure was
repeated for another 4 times and the results of in total 6 runs are
summarized in Figure 1.
6
H. Stachowiak, J. Kazmierczak, K. Kucinski, G. Hreczycho,
Green Chem. 2018, 20, 1738-1742.
7
J. H. Docherty, J. Peng, A. P. Dominey, S. P. Thomas, Nat.
Chem. 2017, 9, 595-600.
8
9
J. V. Obligacion, P. J. Chirik, Nat. Rev. Chem. 2018, 2, 15-34.
(a) X. Chen, Z. Y. Cheng, Z. Lu, Org. Lett. 2017, 19, 969-971; (b)
A. J. MacNair, C. R. P. Millet, G. S. Nichol, A. Ironmonger, S. P.
Thomas, ACS Catal. 2016, 6, 7217-7221; (c) M. Espinal-Viguri, C.
R. Woof, R. L. Webster, Chem. Eur. J. 2016, 22, 11605-11608;
(d) L. Zhang, D. Peng, X. Leng, Z. Huang, Angew. Chem. Int. Ed.
2013, 52, 3676-3680.
10 S. R. Tamang, M. Findlater, J. Org. Chem. 2017, 82, 12857-
12862.
11. T. Bai, T. Janes, D. Song, Dalton Trans. 2017, 46, 12408-12412.
12 U. K. Das, C. S. Higman, B. Gabidullin, J. E. Hein, R. T. Baker,
ACS Catal. 2018, 8, 1076-1081.
Conflicts of interest
There are no conflicts to declare.
13 A. Baishya, S. Baruah, K. Geetharani, Dalton Trans. 2018, 47,
9231-9236.
Acknowledgements
14 (a) G. Zhang, B. L. Scott, S. K. Hanson, Angew. Chem. Int. Ed.
2012, 51, 12102-12106; (b) G. Zhang, K. V. Vasudevan, B. L.
Scott, S. K. Hanson, J. Am. Chem. Soc. 2013, 135, 8668-8681;
(c) G. Zhang, S. K. Hanson, Chem. Commun. 2013, 49, 10151-
10153; (d) G. Zhang, Z. Yin, J. Tan, RSC Adv. 2016, 6, 22419-
22423; (e) Z. Yin, H. Zeng, J. Wu, S. Zheng, G. Zhang, ACS Catal.
2016, 6, 6546-6550; (f) G. Zhang, Z. Yin, S. Zheng, Org. Lett.
2016, 18, 300-303; (g) G. Zhang, J. Wu, H. Zeng, S. Zhang, Z.
We acknowledge the support of funding from the PSC-CUNY awards
(60328-0048, 61321-0049), the PRISM program and the Seed grant
from the Office for Advancement of Research at CUNY John Jay
College.
Notes and references
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Page 8 of 8
ARTICLE
Yin, S. Zheng, Org. Lett. 2017, 19, 1080-1083; (h) G. Zhang, J.
Journal Name
View Article Online
Wu, M. Wang, H. Zeng, J. Cheng, M. C. Neary, S. Zheng, Eur. J.
Org. Chem. 2017, 5814-5818; (i) G. Zhang, J. Wu, S. Li, S. Cass,
S. Zheng, Org. Lett. 2018, 20, 7893-7897.
DOI: 10.1039/C9GC00078J
15 L. Li, E. Liu, J. Cheng, G. Q. Zhang, Dalton Trans. 2018, 47, 9579-
9584.
16 N. Eedugurala, Z. Wang, U. Chaudhary, N. Nelson, K. Kandel, T.
Kobayashi, I. I. Slowing, M. Pruski, A. D. Sadow, ACS Catal.
2015, 5, 7399-7414.
17 K. Manna, P. Ji, F. X. Greene, W. Lin, J. Am. Chem. Soc. 2016,
138, 7488-7491.
18 F. B. La Forge, H. L. Haller, L. E. Smith, Chem. Rev. 1993, 12,
181-213.
19 (a) Z. Li, R. S. Assary, A. C. Atesin, L. A. Curtiss, T. J. Marks, J.
Am. Chem. Soc. 2014, 136, 104-107; (b) V. L. Weidner, C. J.
Barger, M. Delferro, T. L. Lohr, T. J. Marks, ACS Catal.
2017, 7, 1244-1247.
20 K. D. Collins and F. Glorius, Nat. Chem., 2013, 5, 597-601.
21 (a) W. Wang, L. Cui, P. Sun, L. Shi, C. Yue, F. Li, Chem. Rev.
2018, 118, 9843-9929; (b) T. Drake, P. Ji, W. Lin, Acc. Chem.
Res. 2018, 51, 2129-2138; © Z. Lin, N. C. Thacker, T. Sawano, T.
Drake, P. Ji, G. Lan, L. Cao, S. Liu, C. Wang, W. Lin, Chem. Sci.
2018, 9, 143-151.
22 (a) Q. Sun, Z. Dai, X. Liu, N. Sheng, F. Deng, X. Meng, F.-S. Xiao,
J. Am. Chem. Soc. 2015, 137, 5204-5209; (b) W. Wang, C. Li, L.
Yan, Y. Wang, M. Jiang, Y. Ding, ACS Catal. 2016, 6, 6091-6100;
(c) L. Wang, D. W. Agnew, X. Yu, J. S. Figueroa, S. M. Cohen,
Angew. Chem., Int. Ed. 2018, 57, 511-515.
23 J. Peng, J. H. Docherty, A. P. Dominey, S. P. Thomas, Chem.
Commun. 2017, 53, 4726-4729.
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