Efficient hydroboration of carbonyls by an iron(II) amide catalyst

Article abstract of DOI:10.1039/c8dt01998c

An easily prepared iron(ii) amide precatalyst enables the selective hydroboration of carbonyls with HBpin (pinacolborane) in the absence of any additive. The reactions proceed with low catalytic loading (1-3 mol%) under mild reaction conditions and display wide functional group compatibility. Aldehydes are selectively hydroborated in the presence of other reducible functional groups, such as ketones, alkenes, nitriles, esters, amides, acids and halides.

Full text of DOI:10.1039/c8dt01998c

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Received 00th January
2
0xx,
Efficient Hydroboration of Carbonyls by Iron(II) Amide Catalyst
a
a
a
Ashim Baishya, Swavalina Baruah and K. Geetharani*
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An easily prepared iron (II) amide compound enables the selective Subsequently, a handful of first-row transition metal-based
8
hydroboration of carbonyls with HBpin in the absence of any catalysts (Ti, Mn, Co, Cu and Zn) are documented in literature
additive. The reactions proceed with low catalytic loading (1-3 for the hydroboration of carbonyl compounds.
mol%) under mild reaction conditions and display wide functional
In this context, the catalytic activity of iron-based
group compatibility. Aldehydes are selectively hydroborated in complexes has recently been received much attention due to
the presence of other reducible functional groups, such as its high abundance, low cost, and less toxicity. Numerous iron-
9
ketones, alkenes, nitriles, esters, amides, acids and halides.
catalysed selective hydrosilylation of alkenes and alkynes,
10
carbonyl compounds, and hydroboration of alkenes and
1
1
The versatility of organoboranes towards targeted
synthesis of organic molecules has directed numerous
methods to incorporate boron substituents into organic
alkynes have been reported. Surprisingly, reports that
employ iron as a catalyst for hydroboration of aldehydes and
ketones are scarce. In 2017, Bai and co-workers reported the
iron catalysed hydroboration of aldehydes and ketones using a
1
substrates. Typically, metal hydrides are widely used for the
1
2
reduction of carbonyl compounds, where they mostly require
the addition of equimolar mixture of reagents to carbonyl
compounds to form the organoboranes which can be easily
bulky Fe
2 3 4 2 3 4
(NPtBu ) complex. The Fe (NPtBu ) was prepared
from a salt metathesis reaction of iron dichloride and lithium
1
3
phosphoranimide Li(NPtBu
co-workers reported room temperature hydroboration of
aldehydes and ketones by employing Fe(acac) as the pre-
catalyst. The active catalyst species iron(II) hydride was in
situ generated from the reductive activation of Fe(acac) by
NaHBEt . Very recently, Baker and co-workers have prepared
an imine-coupled [Fe-N complex from the reaction of
Fe{N(SiMe ] with benzothioazolidine ligand, to catalyse
only the hydroboration of aldehydes. Herein, we have
3
). At the same time, Findlater and
1
b
hydrolysed to alcohols. Notably, transition metal catalyzed
hydroboration of unsaturated molecules is one of the direct
and selective methods to synthesise organoboranes, because
of their excellent atom economic and controllable chemo- and
3
1
4
3
1
regioselectivity. In the past decades, much attention has been
3
drawn towards the catalytic hydroboration of C=X (X = O, N
and heteroatoms) bonds, which provides facile and efficient
access to enantiomerically pure alcohols and amines
respectively, as they are valuable synthetic building blocks for
2 2 2
S ]
[
3 2 2
) }
1
5
16
]
revealed the catalytic activity of [Fe{N(SiMe
3
)
2
}
2
(1) for the
2
the preparation of bioactive molecules. Numerous examples
4
of transition metals (Rh, Ru, Mo, Pd, Ni, Ir, Re), main group
hydroboration of aldehydes and also ketones. In addition,
chemoselective hydroboration of aldehydes over other
functional groups including ketones, alkenes, nitriles, esters,
amides, acids, and halides were also achieved.
5
6
7
elements, alkali metals, and rare earth metals as catalysts
have been reported for the hydroboration of carbonyl
compounds. Owing to the high economic cost, low abundance
We performed the catalytic activity of
1 for the reduction
and performance limitations associated with precious metal of carbonyl compounds using HBpin (pinacolborane) as a
catalysts, there is always a quest for the development of
competent cheap, earth abundant, first row transition metal
catalysts that are readily available for these transformations.
boron source. The reaction conditions were optimised using
benzaldehyde (2a) as a model substrate, and are shown in
Table 1. Reaction of 2a with HBpin in the presence of 5 mol%
of
1 as a catalyst in THF yielded the borate ester
C H CH OBpin, 2b in excellent yield (99%, entry 1) at room
6
4
2
temperature within 30 min. In contrast, the use of commercial
sources of iron(II) salts, such as FeCl and FeBr gave the
2
2
desired products in lower yields, 42% and 56% respectively
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(
entries 2 and 3), which may be attributed to poor solubility of affording the corresponding borate esters (Table 2). Many
the iron salts in THF. Notably, when FeCl was used as a synthetically important functional groups including methoxy
catalyst, only a trace amount of the product was obtained 5a 6a), cyano (12a), hydroxy (7a, 13a) and nitro (14a) groups
entry 4). Also, performance of the reaction in the absence of were well tolerated under the reaction conditions. Even
yielded only a trace amount of the desired product (entry 5), halides were compatible with the selective hydroboration of
3
(
,
(
1
7
b,17
which is accordance with the recent literature report.
Solvents other than THF proved inferior (entries 6-9). A was observed. Importantly, 4-hydroxybenzaldehyde (13a) gave
reasonably good yield of 78% was obtained upon using a a 79% yield of the corresponding borate ester ([Fe-N
carbonyl groups (8a-11a) and no hydrodehalogenated product
2 2 2
S ]
1
5
reduced catalyst loading of 0.5 mol% (entry 10) within 30 min, catalyst yielded 39%). This wide functional group tolerability
1
2
whereas 1.0 mol% of the catalyst gave a better yield of 83% is rare and better than that of Fe
2
(NPtBu
L3, where L1 = SIMes example having -CN and -OH groups) and Fe(acac)
1,3-bis-(2,4,6-trimethylphenyl) imidazolin-2-ylidene), L2 no example having -CN, -OH and -NO groups) catalytic
3
)
4
(contains no
1
4
(
entry 11). Addition of ligands such as L1
-
3
(contains
(
=
2
IMes (1,3-bis(2,4,6-trimethylphenyl) imidazole-2-ylidene) and systems. Further, the sterically hindered ortho-tolualdehyde,
L3 = PPh , to the reaction mixture did not enhance the yield of gave hydroborated product in a high yield (4b: 99%).
3
hydroborated products (entries 12-14). Gratifyingly, increasing Additionally, the α, β-unsaturated aldehyde was found to react
the reaction time to 1 h resulted in an excellent yield of 98% exclusively with the carbonyl functionality in good yield 16b
1
8
using 1.0 mol% of catalyst (entry 15).
(71%), with no observable reduction of the alkene moiety.
Table 1. Optimisation of the reaction conditions for the Fe(II)- Table 2. Hydroboration of aldehydes catalysed by the iron(II) amide
a
a
catalysed hydroboration of benzaldehyde, 2a.
complex 1.
Entry
Catalyst (mol %)
Time
min)
Solvent
Yield
b
(
(%)
1
2
3
4
Fe[N{SiMe
3
}
2
]
2
(5)
30
30
30
30
THF
THF
THF
THF
99
FeCl
FeBr
2
(5)
(5)
(5)
42
2
56
trace
FeCl
3
5
6
7
8
9
-
30
30
30
30
30
30
30
30
THF
trace
87
Fe[N{SiMe
Fe[N{SiMe
Fe[N{SiMe
Fe[N{SiMe
Fe[N{SiMe
Fe[N{SiMe
Fe[N{SiMe
3
3
3
3
3
3
3
}
2
}
2
}
2
}
2
}
2
}
2
}
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
(5)
(5)
2
Et O
DME
64
(5)
Benzene
Toluene
THF
58
(5)
69
1
1
1
0
1
2
(0.5)
(1)
78
THF
83
c
(1)
THF
81
d
e
1
1
1
3
4
5
Fe[N{SiMe
Fe[N{SiMe
Fe[N{SiMe
3
3
3
}
2
}
2
}
2
]
2
]
2
]
2
(1)
(1)
(1)
30
30
60
THF
THF
THF
75
80
98
a
Reaction conditions: 2a (0.25 mmol, 1 equiv) and HBpin (0.30 mmol,
a
b
.2 equiv) at room temperature. Yields were determined by H NMR
1
Reaction conditions: aldehyde (0.25 mmol, 1 equiv), HBpin (0.30 mmol, 1.2
1
equiv), 1 (1 mol%) and THF (1 mL). Borate ester yields were determined by
analysis, using nitromethane as an internal standard and are averaged
c
from two experiments. The reaction was performed in the presence
1
H NMR analysis, using nitromethane as an internal standard. Isolated yields
b
d
of L1 (SIMes). The reaction was performed in the presence of L2
of alcohols are given in parentheses unless otherwise stated. Primary
e
IMes). The reaction was performed in the presence of L3 (PPh
alcohol was not isolated upon hydrolysis, probably due to the volatile nature
of the product.
(
3
).
With the optimised reaction conditions in hand, we first
investigated the scope of the aldehydes. Monitoring the
The heteroaromatic aldehyde, 17a, was also tolerated and
the reaction afforded the corresponding borate ester in a good
yield (80%). Reaction with isophthalaldehyde (18a) resulted a
diborate ester in good yield (85%). Further, the aliphatic
substrate also gave an excellent reaction yield of 97% (20b).
However, the lower alkyl chain aldehydes reacted poorly with
HBpin to give the hydroborated products (19b and 21b).
1
11
catalytic hydroboration reactions by
H
and
B NMR
spectroscopy revealed the formation of the corresponding
borate ester products in high yield. The desired primary
alcohols were obtained in good to excellent yields upon
hydrolysis. Aldehydes with both electron-donating and -
withdrawing groups underwent hydroboration efficiently
2
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Next, we sought to evaluate the functional-group tolerance of towards acidic functional group. Further, the competitive
the ketonic substrates (Table 3). The best catalytic system identified hydroboration reaction between electron-rich (5a), -neutral (2a)
for the hydroboration of ketones comprised of 3 mol% of catalyst 1 and -poor (11b) substrates exhibits the selectivity for electron-rich
and 1.2 equiv of HBpin in THF at room temperature with a reaction aldehyde (Scheme 1c).
time of 10 h (see Supporting Information (SI) for details). A variety
of useful functional groups such as –Me (23a), -OMe (24a, 25a) and
-Ph (32a) were found to have little to no effect on the reduction
process. The substrate 25a, containing a sterically encumbered
substituent, afforded the desired borate ester in good yield.
Substrates containing halogen substituents both in the aromatic
ring as well as in the alkyl chain were reduced in moderate to good
yields at room temperature (26a-29a). The presence of stronger
electron-withdrawing -NO
lower yields of borate esters, albeit at slightly higher temperature
50 °C).
2
group on acetophenone (30a, 31a) gave
(
Scheme 1. Chemoselective hydroboration reactions.
Table 3. Hydroboration of ketones catalysed by the iron(II) amide
a
complex 1.
Having established the scope and utility of the hydroboration
method, attention was devoted towards understanding the
1
2
mechanistic features of this transformation. Compared to Bai’s
1
4
and Findlater’s work, where they postulated Fe-H as a reactive
species for the hydroboration of carbonyls, all our attempts to
isolate the Fe-H species from 1 and HBpin were unsuccessful. In
order to understand the potential intermediate in the catalytic
cycle, stoichiometric reactions of 1 with HBpin and benzaldehyde
were performed seperately. The dark brown solution obtained from
the reaction between 1 and HBpin, showed
a
paramagnetic
1
resonance in the H NMR spectrum due to unidentified species,
1
1
wheras the B NMR and GC-MS analyses display the formation of
Bpin-N(TMS) . The reaction mixture of 1 and benzaldehyde exhibits
a mixture of diamagnetic and paramagnetic resonances in the
a
2
Reaction conditions: ketone (0.25 mmol, 1 equiv), 1 (3 mol %), HBpin (0.3
1
H
mmol, 1.2 equiv) and THF (1 mL). Borate esters yields were determined by
1
NMR spectrum. In both the cases, formation of Fe-H was not
observed either in NMR, or in IR studies. However, in situ
monitoring of the stoichiometric reaction of 1 with HBpin and 2a
showed the formation of Fe-H complex as observed in FTIR (ν Fe-H
H NMR analysis using nitromethane as an internal standard. Isolated
yields of secondary alcohols are given in parentheses unless otherwise
b
c
stated. Using 2.0 equiv of HBpin. Reaction was performed at 50 °C.
Selective reduction of carbonyls in the presence of other
unsaturated functional groups is a challenging task in modern
organic synthesis. To assess the chemoselectivity of our catalyst
system, a series of competitive reactions were performed (Scheme
-
1
1
658 cm ). When the reaction was performed using DBpin, the shift
-1
in the peak value to 1204 cm corresponding to Fe-D stretching
frequency was observed, which further supports the possible
19
formation of iron-hydride species. These results hinted that the
labile ligand formed in the reaction mixture would presumably
stabilize the in situ generated highly reactive iron hydride species
and thus prevent catalyst degradation, which is in agreement with
1
). The intermolecular selective hydroboration reactions were
performed using equimolar mixtures of aldehyde (2a, 5a or 11a),
ketone (22a, 24a or 27a) and HBpin in the presence of 1 (1 mol%).
The results indicated that aldehyde is preferentially hydroborated
regardless of the electronic effect of the functional groups, as
10b
Tilley’s
report on hydrosilylation of carbonyl compounds by
1
iron(II) amide complex.
evidenced from the H NMR spectroscopy (Scheme 1a). Also, using
To rule out any possible formation of nanoparticles during the
catalytic reaction, we carried out the mercury poisoning experiment
an excess of aldehyde and ketone with respect to HBpin gave
comparable selectivity for the hydroboration of the aldehyde (see
Figure S4). The intramolecular hydroboration of aldehydes in the
presence of various functional groups (ketone, ester, amide and
1
2
and the catalyst filtrate experiment. Both experiments had no
effect on the hydroboration reactions as evidenced by the product
yields (see SI for details). These results are consistent with a
1
acid) were also conducted (Scheme 1b). The H NMR analyses show
1
0b
homogeneous catalytic systems.
excellent chemoselectivity towards the reduction of aldehydes.
These selectivity results validate the significance of this catalyst
Kinetic measurements to determine the catalyst resting state
and the substrate dependence of the catalytic process were carried
out (see SI for details). The rate of the reaction was monitored using
1
2, 14
system over other iron-catalysts.
underwent selective hydroboration of the aldehydic group (Scheme
b), which demonstrates the outstanding tolerability of the catalyst
Notably, 4-formylbenzoic acid
4
-methylbenzaldehyde as the substrate in the presence of lower
1
loading of catalyst 1. The kinetics for the catlayitc activity
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dependence on both HBpin and aldehyde were analysed by H NMR
experiments, in which the initial concentration of each component
was varied keeping the others constant. Increase in the reaction
rate was observed when we increased the initial concentration of
either the aldehyde or HBpin, suggesting that the catalytic system is
dependent on both of these concentrations. Whereas, for a
reaction run with an equimolar concentration of aldehyde and
Acknowledgement
K.G. thanks SERB (EMR/2016/000367) and Indian Institute of
Science (IISc) start-up research grant for funding. A. B. is
grateful to the SERB for national postdoctoral fellowship. S. B.
thanks IISc Bangalore for research fellowship.
HBpin using 1 as
a catalyst, we observed that the rate of
consumption of HBpin was greater than the aldehyde, which may
indicate the activation of the precatalyst by HBpin. It is interesting
to note that these observations are consistent with the
Notes and references
1
(a) A. Suzuki, In Boronic Acids: Preparation and Applications
in Organic Synthesis and Medicine, D. G. Hall, Ed., Wiley-VCH:
Weinheim, Germany, 2005, pp 123; (b) H. C. Brown and S.
Krishnamurthy, Tetrahedron, 1979, 33, 567-607; (c) H.
2 2 2
hydroboration of carbonyl compounds by [Fe-N S ] complex.
Further, kinetic studies for the catalytic resting state experiments
revealed that the conversion to product is highest in the first cycle
and a drop in the conversion was observed (from 90% to 30%) in all
the subsequent additions (see SI for details) along with the
unreacted substrates. The less pronounced activation of the
recycled catalyst may be due to relatively long lived nature of the
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0
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500
1000 1500 2000 2500 3000 3500 4000 4500
Time (s)
Figure 1. Effect of concentration on rate of formation of product by
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3
4
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In summary, we have developed a simple iron(II) amide as a
highly active catalyst for the hydroboration of a range of aldehydes
and ketones without any additive. The reaction proceeds in high
yields with broad functional group compatibility and exhibits
excellent chemoselectivity for aldehydes over ketones. The
extremely low cost and low toxicity of the reagent enables the large
scale synthesis of 1° and 2° alcohols. The detailed mechanism of the
catalytic cycle is currently under scrutiny and further utilization of
this simple metal amide as a catalyst precursor is the subject of
ongoing investigation in our laboratory.
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Conflicts of interest
Guan, J. Am. Chem. Soc., 2010, 132, 8872-8873; (j) N.
Eedugurala, Z. Wang, U. Chaudhary, N. Nelson, K. Kandel, T.
Kobayashi, I. I. Slowing, M. Pruski and A. D. Sadow, ACS
There are no conflicts to declare.
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DOI: 10.1039/C8DT01998C
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A simple iron(II) amide pre-catalyzed chemo-selective hydroboration of carbonyl
compounds has been achieved in absence of any additive.