Homoleptic Zinc-Catalyzed Hydroboration of Aldehydes and Ketones in the Presence of HBpin

Article abstract of DOI:10.1002/ejic.201901276

Here, we report the reaction between N-phenyl-o-phenylenediamine and pyrrole-2-carboxaldehyde to afford the N-phenyl-o-phenyl-enediiminopyrrole ligand {L-H2} in quantitative yield. A one-pot reaction between {L-H2} and diethylzinc (ZnEt₂) in a 2:1 ratio afforded the homoleptic zinc metal complex [{L-H}₂Zn] (1). The solid-state structures of ligand {L-H2} and zinc complex 1 were confirmed using X-ray crystallography. Further, complex 1 was used for chemoselective hydroboration of aldehydes and ketones in the presence of pinacolborane (HBpin) at ambient temperature to produce the corresponding boronate esters in high yield.

Full text of DOI:10.1002/ejic.201901276

A Journal of
Accepted Article
Title: Homoleptic zinc–catalyzed hydroboration of aldehydes and
ketones in the presence of HBpin
Authors: Gobbilla Sai Kumar, Adimulam Harinath, Rajrani Kawaria, and
Tarun Kanti Panda
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10.1002/ejic.201901276
European Journal of Inorganic Chemistry
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Homoleptic zinc–catalyzed hydroboration of aldehydes and
ketones in the presence of HBpin
Gobbilla Sai Kumar, [‡, a] Adimulam Harinath,[ a] Rajrani Kawaria,[a] and Tarun K. Panda*[a]
‡,
Abstract:
Here, we report the reaction between N-phenyl-o-phenylenediamine
and pyrrole-2-carboxaldehyde to afford the N-phenyl-o-phenyl-
enediiminopyrrole ligand {L-H2} in quantitative yield. A one-pot
reaction between {L-H2} and diethylzinc (ZnEt2) in a 2:1 ratio afforded
the homoleptic zinc metal complex [{L-H}2Zn] (1). The solid-state
structures of ligand {L-H2} and zinc complex 1 were confirmed using
reduction of aldehydes over other active functional groups such
as -NH2, -OH, esters, amides, and nitriles.16,25,33a,35
While incorporating easily accessible, less toxic, highly abundant,
environment-friendly metals as catalysts to perform various
organic transformations is always a challenge to chemists, it is
very important in terms of green chemistry.26 Zinc is one of the
most abundant metals amongst other metals in the periodic table,
and also has specific biological significance.27 Economic viability,
high natural abundance, and biocompatibility have allowed Zn (II)
to play a significant role in synthetic chemistry.28 Unlike reactions
catalyzed by transition metals, zinc-catalyzed reactions are
mechanistically distinct for specific reactions such as
dehydrocoupling of alcohols with silanes.29 The first organo-zinc
compound, diethyl zinc, was prepared from zinc and ethyl iodide
by Frankland et al. It paved the way for several potential
applications of organo-zinc compounds in the evolution of
synthetic organic chemistry.30 Very recently, P.S. Baran et al.
reported the use of dialkylzinc as a precursor for the formation of
a C-C bond, using active ester and alkyl zinc reagents.31
Surprisingly, the report of zinc-mediated hydroboration of
carbonyl compounds is very rare.32
X-ray crystallography. Further, complex
1
was used for
chemoselective hydroboration of aldehydes and ketones in the
presence of pinacolborane (HBpin) at ambient temperature to
produce the corresponding boronate esters in high yield.
Introduction
Organoboranes are synthetic intermediate products obtained
during various chemical transformations. They are versatile and
serve as building blocks in organic syntheses.1 Boronate ester
compounds are preferred over other organometallic reagents
owing to their nontoxicity and stability.2 Hence, a wide range of
metal catalysts have been used in the hydroboration of various
unsaturated moieties, including alkenes, alkynes, imines, or
nitriles, and carbonyl compounds.3,4,5,6 Hydroboration of carbonyl
compounds is a significant transformation for the synthesis of
functionalized primary or secondary alcohols. Traditional methods
used in the reduction of carbonyl compounds require
stoichiometric reducing agents, such as NaBH4 or LiAlH4, and H2,
used under high pressure.7,8 In contrast to hydroboration, the
hydrosilylation of carbonyl compounds has been explored more.9
There are reports of a wide variety of metal catalysts from main
group elements, including catalysts based on alkali or alkaline
earth metals, namely, Al,10 Si,11 Ge,12 Sn,13 as well as rare earth
metal catalysts such as those based on La14 and Yb,15 which have
been used in the hydroboration of carbonyl compounds (Figure 1).
Transition metals such as Ti,16 Mn,17 Fe,18 Mo,19 Rh,20 Ru,21 Co,22
and Cu23 have also been used as the bases for developing
catalysts. Notably, Hreczycho et al. avoided the use of metal
catalysts and reported the hydroboration of aldehydes under
catalyst- and solvent-free conditions; however, the reaction they
reported required harsh conditions to be effective.24 There are
reports that a limited catalyst was used for the chemoselective
[a] G. S. Kumar, Dr. A. Harinath, R. Kawaria, Prof. T. K. Panda
Department of Chemistry, Indian Institute of Technology, Hyderabad
Kandi -502 285, Sangareddy, Telangana, India.
‡
both the authors contributed equally
Email: tpanda@iith.ac.in.
Figure 1. Selected metal catalysts for the hydroboration of carbonyl
compounds.
Homepage: https://sites.google.com/site/tkpandagroup/home
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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10.1002/ejic.201901276
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In the course of our ongoing research, we have developed a
number of strategies using earth-abundant metal catalysts for
various organic transformations, including hydroboration of
various unsaturated moieties.33 In continuation of this, we are
keen to explore the use of a zinc catalyst in the chemoselective
hydroboration of carbonyl compounds. In this context, we report
here the synthesis and solid-state structure of a homoleptic zinc
complex supported by the N-phenyl-o-phenylenediiminopyrrolyl
ligand {L-H2}. Further, we describe the catalytic efficiency of zinc
metal complex 1 as a catalyst for hydroboration of a wide variety
of aldehydes and ketones in the presence of pinacolborane
(HBpin).
Figure 2. Solid-state structure of ligand {L-H2}. Selected bond lengths (Å) and
bond angles (°) are given: N(1)–C(4) 1.372(3), C(4)–C(5) 1.428(3), N(2)–C(5)
1.279(3), N(3)–C(11) 1.394(3), C(4)–C(3) 1.382(3), C(1)–C(2) 1.362(3), N(2)–
C(5)–C(4) 125.6(2), N(3)–C(11)-C(6) 119.3(2), N(1)–C(4)–C(5) 124.68(19).
CCDC No. 1967376.
Results and Discussion
Synthesis and structure of ligand {L-H2} and homoleptic zinc
metal complex (1).
The N-phenyl-o-phenylenediiminopyrrole ligand {L-H2} was
prepared by the condensation reaction between N-phenyl-o-
phenylenediamine and pyrrole-2-carboxaldehyde in methanol at
room temperature (Scheme 1). The ligand {L-H2} was
characterized using standard spectroscopic and analytical
techniques. The molecular structure of ligand {L-H2} was
confirmed by single-crystal X-ray diffraction analysis (Figure 2).
Scheme 2. Synthesis of zinc metal complex 1.
The solid-state structure of zinc complex 1 in Figure 3 confirms
the coordination of two ligand moieties around the zinc center.
Metal complex 1 crystallizes in the monoclinic P21/c space group.
The coordination around the zinc center is four-fold. The geometry
around the zinc center is a distorted tetrahedral with two {L-H2}
(N-phenyl-o-phenylenediiminopyrrolyl)
ligands
coordinated
around it. The Zn–Nimine bond lengths are 2.055(4) Å and 2.086(4)
Å, which are slightly longer than the Zn–Npyrolyl distances, i.e.,
1.973(4) Å and 1.964(4) Å present in complex 1. The different
bond lengths of Zn–Nimine and Zn–Npyrolyl indicate the anionic
coordination of Npyrolyl and the neutral coordination of Nimine to the
Zn(II) ion respectively.
Scheme 1. Synthesis of Schiff base ligand {L-H2}.
1
In the H NMR spectrum, ligand {L-H2} exhibited a broad signal
for pyrrole-NH at  = 9.52 ppm. A sharp singlet resonance signal
in the 1H and 13C{1H} spectra of {L-H2} at  = 8.34 ppm and C =
148.6 ppm respectively indicate the corresponding presence of
imine (N=CH) proton and carbon. Single crystals of ligand {L-H2}
were grown by slowly evaporating the CH3CN solution at -35 °C.
The ligand crystallizes in the monoclinic P21/c space group.
The zinc (II) complex (1) was prepared by the reaction between
{L-H2} and ZnEt2 in a 2:1 molar ratio in toluene at room
temperature with the elimination of ethane gas (C2H6) (Scheme
2). Complex 1 is formed by the coordination of two ligand
fragments around the zinc (II) center, which was confirmed by
solid-state X-ray diffraction analysis (Figure 3). The
disappearance of pyrrole-NH in the 1H NMR of complex 1 further
confirms the formation of the metal complex. The signals in the 1H
and 13C{1H} spectra of complex 1 at  = 8.05 ppm and C = 153.6
ppm respectively indicate the presence of the imine (N=CH)
bond.34 Single crystals of complex 1 suitable for X-ray analysis
were grown from the toluene solution at -35 °C.
Figure 3. Solid-state structure of zinc metal complex 1. Selected bond lengths
(Å) and bond angles (°) are given: Zn(1)–N(1) 1.964(4), Zn(1)–N(3) 1.973(4),
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Zn(1)–N(2) 2.055(4), Zn(1)–N(4) 2.086(4), N(4)–C(22) 1.299(6), N(2)–C(5)
1.309(6), N(1)–Zn(1)–N(2) 85.52(17), N(1)–Zn(1)–N(3) 144.64(18), N(1)–
Zn(1)–N(4) 108.83(17), N(2)–Zn(1)–N(3) 115.80(17), N(2)–Zn(1)–N(4)
122.95(16), N(3)–Zn(1)–N(4) 83.85(16). CCDC No. 1967377.
with electron-donating functional groups such as methoxy, iPr,
and methyl groups attached to the aromatic center resulted in the
completion of aldehydes conversion quantitatively (Table 2,
entries 2-4). An increase in reaction time to two hours was
observed when the substrates with electron-withdrawing groups
such as fluoro- and chloro- (para and ortho) were employed,
indicating slightly slower reactivity by electron-withdrawing groups,
with a yield of 99%, 96%, and 97% respectively (Table 2, entries
6-8). Thus, the substituents in ortho, meta, and para positions of
benzaldehyde showed good reactivity with HBpin in the presence
of catalyst 1. The o-nitrobenzaldehyde was selectively reduced at
the carbonyl moiety to produce the corresponding boronate ester
2i with a yield of 96% (Table 2, entry 9) under two hours. The
substrate with bis-substituents such as 2,6-dichloro, 2,4-difluoro,
and 3,4-difluorobenzaldehyde also worked smoothly to give the
corresponding boronate esters 2j, 2k, and 2l respectively with
excellent yield (Table 2, entries 10-12). 4-Trifluoro-
methoxybenzaldehyde was also completely converted to the
boronate ester 2m with a quantitative yield (99%) in the presence
of HBpin and catalyst 1 (Table 2, entry 13). In the presence of
catalyst 1 and HBpin, the 3,4,5-trimethoxybenzaldehyde was
converted smoothly to the corresponding boronate ester 2n at
ambient temperature (Table 2, entry 14). Similarly, near
quantitative conversion of 3-phenoxybenzaldehyde to the
corresponding boronate ester 2o was achieved under optimum
reaction condition (Table 2, entry 15).
Hydroboration of carbonyl compounds.
The initial reaction was carried out by treating 4-
methoxybenzaldehyde with pinacolborane (HBpin) in the
presence of 1 mol% and 3 mol% of catalyst 1 for three hours
(Table 1, entries 1 and 2). When the catalyst loading was
increased up to 5 mol%, we observed complete conversion within
one hour of reaction (Table 1, entry 3). In addition to neat
conditions, the reaction also proceeded smoothly when organic
solvents such as toluene, THF, hexane, and CDCl3 were used
(Table 1, entries 4-7).
Scheme 3. Hydroboration of aldehydes and ketones catalyzed by zinc complex
1.
Table 1. Screening of catalyst 1 for hydroboration of 4-methoxy-
benzaldehyde.
Homoleptic zinc complex 1 plays a key role in the hydroboration
of aldehydes and ketones. The reactivity of ketones is slower than
the reactivity of aldehydes. Initially, we conducted the
hydroboration of benzophenone with HBpin under ambient
conditions (Table 2, entry 16). We observed that the reaction was
completed after four hours under neat conditions at room
temperature, yielding 98% of the product 2p. The increase in
reaction time with respect to that of aldehyde can be attributed to
the slow reactivity of ketones in comparison to aldehydes. We
further explored the hydroboration of ketones with HBpin in the
presence of catalyst 1. The reaction of 2'-nitroacetophenone with
HBpin afforded the corresponding boronate ester in excellent
yields 2q (Table 2, entry 17), leaving the NO2 moiety unaffected.
2’-bromoacetophenone was also converted to the corresponding
product 2r in quantitative yields of up to 98% (Table 2, entry 18).
The reduction of aromatic ketones with electron-donating (p-Me,
Table 2, entry 19) and electron-withdrawing groups (p-Br, Table
2, entry 20) also proceeded smoothly, yielding excellent quantities
of the corresponding boronate esters (2s, 98%, and 2t, 97%
respectively). Moreover, the reaction of HBpin with
propiophenone and cyclopentanone also occurred smoothly at
room temperature under neat conditions (Table 2, entries 21 and
22). Further, the conversion of 1-tetralone with HBpin to the
corresponding ester 2w (97%) was also accomplished smoothly
in the presence of catalyst 1 (Table 2, entry 23). Catalyst 1 was
effective in the conversion of 2'-aminoacetophenone, 4'-
aminoacetophenone, and 3-hydroxybenzaldehyde (Table 2,
entries 24-26) to the corresponding boronate esters 2x, 2y and 2z
respectively in good yields while leaving the functional amine
(NH2) and hydroxy (OH) moieties unaffected.
S. No. Substrate
Catalyst
(mol%)
Time
Solvent
Yieldb
(%)
1.
2.
3.
4.
5.
6.
7.
4-OMePhCHO
4-OMePhCHO
4-OMePhCHO
4-OMePhCHO
4-OMePhCHO
4-OMePhCHO
4-OMePhCHO
1
3
5
5
5
5
5
3
2
1
1
1
1
1
Neat
Neat
70
85
99
97
95
Neat
Toluene
THF
Hexane 96
CDCl3 90
Reaction conditions: HBpin (1 mmol), followed by aldehyde (1
mmol) in a specified solvent under the neat condition at rt. bYield
was calculated by using 1,3,5-trimethoxybenzene (20 mol%) as
an internal standard.
Using these optimized conditions, we further explored the
hydroboration of various carbonyl compounds with a wide range
of functional groups. All the reactions were analyzed by the NMR
spectrum in the presence of 1,3,5-trimethoxybenzene as an
internal standard. The reaction between HBpin and aldehydes
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10.1002/ejic.201901276
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13.
2
2
99
Table 2. Substrate scope for the hydroboration of aldehydes and
ketones using metal complex 1a.
(2m)
S.
Substrate
Product
Time Yield
No.
(h)
(%)
14.
99
98
1.
2.
1
99
(2n)
(2o)
(2a)
15b.
2
1
98
(2b)
16.
4
98
3.
4.
1
1
99
99
(2p)
(2q)
(2c)
(2d)
17.
18.
4
4
95
98
5.
6.
1
2
88
99
(2r)
(2s)
(2t)
(2e)
(2f)
19.
20.
4
4
98
97
7.
2
96
(2g)
(2h)
(2i)
21.
22.
4
4
98
97
(2u)
(2v)
8.
9.
2
2
97
96
23.
4
97
10.
11.
12.
2
2
2
99
99
99
(2w)
(2x)
24b.
25b.
4
4
97
98
(2j)
(2k)
(2y)
(2z)
26b.
4
97
(2l)
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Reaction conditions: aCatalyst 1 (5 mol%), HBpin (1 mmol),
followed by aldehydes (1 mmol) and ketones (1 mmol) in neat
condition at rt. bReaction was carried out in toluene. Yields were
determined using 1,3,5-trimethoxybenzene (20 mol%) as an
internal standard in reaction mixtures.
Further, in order to examine the chemoselectivity of the zinc-
catalyzed hydroboration reaction, we conducted various reactions
using complex 1 (Scheme 4, entries 1-5). Catalyst 1 selectively
reduces aldehydes over ketones, which shows the exclusive
reactivity of aldehydes, and we observed a 95% conversion
(Scheme 4, entries 1-2). Further, we performed the hydroboration
of 4-acetamidobenzaldehyde and 4-acetoxybenzaldehyde with
HBpin in the presence of catalyst 1 to obtain an exclusive mono-
boronate ester 3c, which showed the selective reduction of
aldehydes over amide and acetate groups (Scheme 4, entries 3
and 4). The same methodology was employed with respect to
methyl 4-formylbenzoate, and the reaction was carried out with
HBpin in the presence of catalyst 1. We achieved a 97%
conversion to the corresponding mono-boronate ester 3e
(Scheme 4, entry 5).
Scheme 5. A most plausible mechanism for the hydroboration of aldehydes and
ketones using zinc complex 1.
We have described a plausible mechanism based on previous
reports (Scheme 5). In the first step, the carbonyl group forms an
adduct (II) with zinc metal complex (1) which causes polarization
of the carbonyl bond to make the carbonyl carbon more
electrophilic. In the next step, the nucleophilic attack by HBpin
leads to the formation of a four-membered transition state (III),
which transfers hydride ion to the carbonyl carbon center. The
formation of the intermediate species (IV) and elimination of the
boronate ester regenerates the active zinc metal catalyst 1.
Further, to establish the mechanism, we performed the 1:1
reaction
of
zinc
complex
1
separately
with
4-
methoxybenzaldehyde and HBpin and observed that catalyst 1 is
quite stable in the presence of two substrates (Figure FS 98-100
in ESI). Additionally, the complete conversion of 3-
hydroxybenaldehyde (Table 2, entry 26) indicates that metal
hydrides are not present as intermediates to form a cross-
dehydrocoupling product.36
Conclusions
To summarize, we have successfully synthesized and established
the solid-state structure of zinc complex 1 supported by N-phenyl-
o-phenylenediiminopyrrole {L-H2} ligand. Zinc metal complex 1
was utilized for the hydroboration of aldehydes and ketones under
neat and ambient conditions to give corresponding boronate
esters in excellent yield. Catalyst 1 exhibited good chemo-
selectivity toward a wide range of active functional groups,
including -F, -Cl, -OMe, -NH2, and -OH. The results show the
versatile nature of catalyst 1 in the hydroboration of aldehydes
and ketones. The proposed mechanism promotes the neutral
coordination of carbonyl oxygen to the Lewis-acidic zinc metal
center.
Scheme 4. Selective hydroboration of aldehydes over ketones catalyzed by
metal complex 1.
Plausible Mechanism
In contrast to the traditional mechanism where the metal hydride
acts as an active catalyst, very recently, various research groups
have reported the use of a Lewis acid-based mechanism for the
hydroboration of carbonyl compounds.15,18c,35
Experimental Section
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Synthesis of ligand {L-H2}.
This work is supported by the JICA FRIENDSHIP Project under
the Collaboration Kick-starter Program (CKP). G. S. K. thanks the
Inspire Fellowship India (IF180108) and A. H. thanks CSIR, India
for their respective Ph.D. fellowships. We also thank Prof. Kazushi
Mashima and Prof. Hayato Tsurugi, Osaka University, Japan, for
their generous support.
A 25 mL Schlenk flask was charged with N-phenyl-o-phenylenediamine
(500 mg, 5.25 mmol) in MeOH (5 mL) and pyrrole-2-carboxaldehyde (968
mg, 5.25 mmol) in MeOH (5 mL) was added to it. A catalytic amount of
glacial acetic acid was added to the reaction mixture. The reaction mixture
was stirred for six hours at room temperature. A yellow-colored precipitate
formed at the bottom of the flask. The solvent was removed and washed
with cold MeOH (5 mL). The compound was dried in vacuo to obtain ligand
{L-H2} as a yellow-colored solid. Yellow-colored crystals were grown from
CH3CN at -35 °C.
Keywords: Hydroboration • carbonyl compounds • zinc •
chemoselectivity.
Yield: 950 mg (70 %). 1H NMR (400 MHz, CDCl3):  9.52 (s, 1H, PyNH),
8.34 (s, 1H, N=C-H), 7.32 - 7.24 (m, 3H, ArH), 7.19 - 7.17 (m, 2H, ArH),
7.11 - 7.07 (m, 2H, ArH), 6.99 - 6.95 (m, 2H, ArH), 6.86 - 6.83 (m, 1H, PyH),
6.71 - 6.70 (m, 1H, PyH), 6.33 - 6.31 (m, 1H, PyH). 13C NMR (101 MHz,
CDCl3): C = 148.58, 142.52, 138.76, 138.52, 131.26, 129.39, 126.69,
123.24, 121.85, 119.85, 119.81, 117.72, 116.76, 115.35, 114.13, 110.84.
Elemental Analysis: C17H15N3 (261.3): Calcd. C 78.13, H 5.79, N 16.08.
Found C 77.82, H 5.62, N 15.77.
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Yield: 190 mg (90 %). 1H NMR (400 MHz, CDCl3):  8.05 (s, 1H, N=C-H),
7.11 - 7.09 (m, 2H, ArH), 7.03 - 6.99 (m, 3H, ArH), 6.92 - 6.91 (m, 2H, ArH),
6.83 - 6.78 (m, 2H, ArH), 6.39 - 6.38 (m, 1H, PyH), 6.32 - 6.30 (m, 1H,
PyH), 5.33 (s, 1H, PyH). 13C NMR (101 MHz, CDCl3): C 153.56, 143.35,
139.31, 137.74, 137.69, 136.95, 128.88, 125.77, 123.48, 121.37, 121.00,
120.70, 119.08, 118.59, 114.67. Elemental Analysis: C34H28N6Zn (585.99):
Calcd. C 69.69, H 4.82, N 14.34. Found C 69.32, H 4.53, N 14.13.
General procedure for hydroboration of aldehydes and ketones.
Hydroboration of aldehydes and ketones with HBpin was performed using
the standard protocol. In the glove box, zinc complex 1 (0.05 mmol) and
the corresponding aldehyde or ketone (1 mmol) were charged into a 25
mL Schlenk tube, which was followed by the addition of HBpin (1 mmol) to
the reaction mixture. The reaction mixture was stirred at room temperature
to complete the conversion of the corresponding aldehyde or ketone. The
progress of the reaction was monitored using 1H NMR spectrum. The
observation of a new CH2 peak in the case of aldehyde and CH peak in
the case of ketone indicated the conversion of the corresponding
derivatives.
Supporting Information
The hydroboration of aldehydes and ketones using HBpin in the
presence of zinc metal complex 1 was performed under neat
conditions at room temperature. The 1H, 13C{1H}, and 11B NMR for
the final products are given in the electronic supporting
information. Crystallographic data for ligand {L-H2} and metal
complex 1 are also set out in the electronic supporting information.
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Acknowledgments
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10.1002/ejic.201901276
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Entry for the Table of Contents
FULL PAPER
The chemo-
selective
hydroboration
of aldehydes
and ketones
Hydroboration of carbonyl compounds
Gobbilla Sai Kumar,[a] Adimulam Harinath,[a] Rajrani Kawaria,[a]
and Tarun K Panda*[a]
Page No. – Page No.
with
HBpin
using earth-
abundant
zinc catalyst
1 at ambient
temperature
is reported.
Homoleptic zinc–catalyzed hydroboration of aldehydes and
ketones in the presence of HBpin
This article is protected by copyright. All rights reserved.