Efficient and selective carbonyl hydroboration catalyzed by a lithium NCN-Pincer magnesiate complex [Li(THF)4][NCN-MgBr2]

Article abstract of DOI:10.1016/j.jorganchem.2018.04.040

The well-defined lithium NCN-Pincer magnesiate complex [Li (THF)₄][NCN-MgBr₂] 2 based on a bis(imino)aryl ligand has been successfully prepared, crystallographic characterized and employed as an efficient catalyst for hydroboration o

Full text of DOI:10.1016/j.jorganchem.2018.04.040

Accepted Manuscript
Efficient and selective carbonyl hydroboration catalyzed by a lithium NCN-Pincer
magnesiate complex [Li(THF) ][NCN-MgBr ]
4
2
Man Luo, Jia Li, Qian Xiao, Shilong Yang, Fan Su, Mengtao Ma
PII:
S0022-328X(18)30294-8
10.1016/j.jorganchem.2018.04.040
JOM 20432
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 March 2018
Revised Date: 28 April 2018
Accepted Date: 30 April 2018
Please cite this article as: M. Luo, J. Li, Q. Xiao, S. Yang, F. Su, M. Ma, Efficient and selective carbonyl
hydroboration catalyzed by a lithium NCN-Pincer magnesiate complex [Li(THF) ][NCN-MgBr ], Journal
4
2
of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.04.040.
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ACCEPTED MANUSCRIPT
Efficient and selective carbonyl hydroboration catalyzed by a lithium
NCN-Pincer magnesiate complex [Li(THF)4][NCN-MgBr2]
Man Luo a, Jia Li a, Qian Xiao a, Shilong Yang b, Fan Su b, Mengtao Ma a,*
a College of Science, Nanjing Forestry University, Nanjing 210037, China
b Advanced Analysis and Testing Center, Nanjing Forestry University, Nanjing 210037,
China
*Email: mengtao@njfu.edu.cn
Abstract: The well-defined lithium NCN-Pincer magnesiate complex
[Li(THF)4][NCN-MgBr2] 2 based on a bis(imino)aryl ligand has been successfully
prepared, crystallographic characterized and employed as an efficient catalyst for
hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin)
at room temperature. Both aldehyde and ketone hydroboration proceeded efficiently
(>99% conversion in <15 min) and could be directly carried out in a CDCl3 solution.
In addition, chemoselective hydroboration of aldehydes over ketones was also
achieved under these conditions.
Keywords: Mg-Li complex; Hydroboration; Aldehyde; Ketone
1. Introduction
The transformations of organic unsaturated substrates play an important role in
organic synthesis since the resultant products are a class of versatile synthetic
intermediates [1]. Catalytic hydroboration of carbonyl compounds with pinacolborane
(HBpin) was one of these key transformations. The formed borate ester intermediates
could be further employed for the commodity and materials synthesis etc, on the other
hand, it was also easily converted into the corresponding alcohols [2-3]. Metal
borohydrides such as NaBH4 have been commonly used for hydroboration, however,
they frequently show poor functional-group tolerance. The catalytic hydroboration of
carbonyl compounds has been extensively studied by many research groups using
transition metal [4], main group compound [5-19], and even rare-earth metal catalysts
respectively [20-22]. The bimetallic complexes generally have a synergistic effect on
catalysis [23]. We have found that the catalytic activity of the sterically bulky
Schiff-base Mg-Na bimetallic ionic complex is superior to the corresponding neutral
monometallic magnesium complexes in the similar hydrosilylation of ketones with
(EtO)3SiH recently [24]. To our surprise, compared to the mono-metal catalysts, no
bimetal catalyzed hydroboration of carbonyl compounds has been reported hitherto.
Herein we report the synthesis and crystallographic characterization of a lithium
NCN-Pincer magnesiate complex [Li(THF)4][NCN-MgBr2] 2 and its application in
the hydroboration of aldehydes and ketones which displayed a good operational
convenience and high efficiency.

ACCEPTED MANUSCRIPT
2. Results and discussion
As depicted in Scheme 1, the bis(imino)aryl NCN pincer ligand 1 was treated
n
with one equiv. of BuLi in THF to generate the corresponding lithium salt, then
further reacted with equimolar amounts of MgBr2. The complex 2 was obtained as
colorless crystals in good yield. The crystal structure of complex 2 and selected bond
lengths and angles are shown in Figure 1. The geometry of the magnesium atom in 2
is a distorted trigonal bipyramid with the two imine nitrogen atoms of the ligand in
the apical position and one carbon atom of the ligand and two bromine atoms in the
equator which is almost identical to the ideal value of 360o. The Mg atom in 2 is
essentially coplanar with the NCN plane. The geometry of four-coordinated cationic
lithium atom in 2 is slightly distorted tetrahedral, coordinated by four oxygen atoms
from the coordinated THF molecules. The bond length of Mg‒Br [2.4733 (14) Å,
2.4771(15) Å] in 2 is slightly shorter than that of the β-diketiminate Mg-Li bimetallic
complex [25]. The bond angle of Br(1)‒Mg(1)‒Br(2) is 116.24(6)°.
Scheme 1. Synthesis of complex 2.
Figure 1. Molecular structure of 2 (50% thermal ellipsoids; hydrogen atoms omitted).
Selected bond lengths (Å) and angles (°): Mg(1)‒Br(1) 2.4733 (14), Mg(1)‒Br(2)
2.4771(15), Mg(1)‒N(1) 2.598(2), Mg(1)‒C(1), 2.105(4), N(1)‒C(5) 1.275(3),
C(2)‒C(5) 1.466(4), Br(1)‒Mg(1)‒Br(2) 116.24(6), C(1)‒Mg(1)‒Br(1) 121.50(11),
C(1)‒Mg(1)‒Br(2) 122.25(11), Br(1)‒Mg(1)‒N(1) 98.98(5), Br(2)‒Mg(1)‒N(1)
99.95(6), N(1)‒Mg(1)‒N(1)′ 143.70(11).

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In order to explore potential utility of the above NCN pincer ligand-based
complex 2, we investigated its application in the catalytic hydroboration of aldehydes
and ketones. It was noted that the blank reaction between benzaldehyde and HBpin
was observed to only afford less than 5% yield in the absence of the catalyst at room
temperature after 15min [12]. However, the same reaction with 1 mol% catalyst
loading of 2 gave a quantitative yield in C6D6 at room temperature within 15 min.
Compared with the reported hydroboration of benzaldehyde catalyzed by the similar
Table 1. Hydroboration of aldehydes catalyzed by 2
Time Yield
(min) (%)a
Time Yield
(min) (%)a
Entry
1
Product
Entry
9
Product
15
15
99
99
15
15
99
99
2
3
10
11
15
15
15
99
92
99
15
15
15
99
99
99
4
5
12
13
6
7
15
15
99
99
14
15
15
15
99
99
8
15
99
16
15
99
a The reaction was monitored by 1H NMR spectroscopy.

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calcium iodide complex [9], the complex 2 showed a slightly higher activity (Mg-Li:
1 mol% catalyst, 15 min, 99% yield; VS Ca: 2 mol%, 40 min, 90-99% yield). To our
surprise, the same reaction could be also carried out in a CDCl3 solution and gave the
same high yield (Table 1, entry 1). This is quite different from the previously reported
metal catalyzed aldehyde hydroboration which is strictly performed in purified C6D6
solution in general. The reason could be that complex 2 is less air- and
moisture-sensitive compared to other metal complexes, particularly for the similar
magnesium alkyl or hydride complexes. Due to the very higher price of C6D6, we
chose the cheaper CDCl3 as solvent that directly used from commercial available and
no further purified to expand the scope of aldehydes hydroboration.
Generally, the hydroboration of various aldehydes with 1 mol% catalyst loading
of 2 could be completed within 15 min at room temperature in quantitative conversion.
For example, the hydroboration of a series of benzaldehyde derivatives with
electron-donating and electron-withdrawing groups such as -Me, -Cl, -F, -NO2 and
-OMe all afforded the corresponding alkoxypinacolboronate esters in 99% yields
(Table 1, entries 2, 3, 5-12) except for the ortho-chloro substituted benzaldehyde gave
92% yield (Table 1, entry 4). Furthermore, cyclohexylaldehyde, α, β-unsaturated
cinnamaldehyde, heterocyclic aldehyde 2-thiophenecarboxaldehyde, and even
9-anthraldehyde were also hydroborated in full conversion within 15 min (Table 1,
entries 13-16).
Encouraged by the above aldehyde hydroboration results, we then examined the
hydroboration of a wide range of ketones. As expected, the hydroboration of more
sterically bulky ketonic carbonyl functions catalyzed by Mg-Li bimetallic complex 2
required slightly higher catalyst loading than that of aldehydes to achieve high yields.
The initial investigation on the hydroboration of acetophenone with 1 mol% catalyst
loading of 2 only afford 80% yield in CDCl3 within 15 min, however, increasing the
amount of catalyst to 5 mol%, 99% yield of product was obtained in the same time
(Table 2, entry 1). Compared with the similar reported calcium iodide catalyst, Mg-Li
bimetallic catalyst showed slightly higher efficiency (Mg-Li: 5 mol% catalyst, 15min,
99% yield VS Ca: 3 mol% catalyst, 5h, 95% yield) [9]. In most cases, the ketone
hydroboration reaction was completed within 15 min in quantitative yield. The
hydroboration of acetophenone substrates
with
electron-donating or
electron-withdrawing group such as -Me, -OMe, -NO2, and -F all produced the
corresponding borate ethers in 99% yield (Table 2, entries 2, 4-7). The reaction of
4-acetylbenzonitrile required a slightly longer time (30 min) to get 99% yield (Table
2, entry 8). Under the same reaction condition, for the 2,4,6-trimethylacetophenone,
only 80% conversion was observed in 15 min (Table 2, entry 3). This once again
highlighted the importance of steric factors in the hydroboration reactions. The
hydroboration of dialkyl ketones also finished with full yield in a very short time
(Table 2, entries 9, 10). Changing methyl moiety of acetophenone to isopropyl or
phenyl substituent, the quantitative yield was obtained in 15 min as well (Table 2,
entries 11, 12). The results of ketone hydroboration indicated that Mg-Li bromide
complex 2 was more reactive than the closely related calcium iodide complex
(73-95%) [9].

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Table 2. Hydroboration of ketones catalyzed by 2
Time Yield
Time Yield
(min) (%)a
Entry
1
Product
Entry
7
Product
(min) (%)a
15
99
15
99
2
3
4
5
6
15
15
15
15
15
99
80
99
99
99
8
30
15
15
15
15
99
99
99
99
99
9
10
11
12
a The reaction was monitored by 1H NMR spectroscopy.
In addition, we also examined the chemoselectivity of hydroboration of aldehyde
and ketone. The mixture solution of benzaldehyde (1 equiv.), acetophenone (1 equiv.)
and HBpin (1 equiv.) was treated with 1 mol% complex 2 at room temperature. The
benzaldehyde was completely converted into the corresponding borate ether in 15
min, however, acetophenone remained almost intact. This is similar to the previous
reports of other metal catalysts [9, 20-22].
Scheme 2. Chemoselective hydroboration of aldehyde/ketone catalyzed by 2.

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3. Conclusions
In summary, we have successfully synthesized a lithium NCN-Pincer magnesiate
complex [Li(THF)4][NCN-MgBr2] 2. The catalytic investigation revealed that it could
be employed as an efficient and chemoseletive catalyst for the hydroboration of
aldehydes and ketones with HBpin at room temperature in a relative cheaper CDCl3
solution. To the best of our knowledge, this is the first example of Mg-Li bimetallic
complex catalyzed hydroboration of aldehydes and ketones. This protocol provided
another operational convenience and efficient method for catalytic hydroboration of
carbonyl compounds.
4. Experimental Section
All air-sensitive compounds were carried out using standard Schlenk-line or
glovebox techniques under high-purity argon. Diethyl ether, toluene, THF and hexane
were dried and distilled from molten sodium. 1H, 13C{1H} and 11B NMR spectra were
recorded at 25 °C with a Bruker Avance III 600 MHz spectrometer and were
referenced to the resonances of the solvent used. Elemental analysis was performed
by the Elemental Analysis Laboratory of the Advanced Analysis and Testing Center
at Nanjing Forestry University. Melting points were determined with an INESA-WRR
apparatus and are uncorrected. Ligand 1 was prepared according to literature
procedure [26]. CDCl3 (99.8%) and HBpin (97.0%) were from CIL and TCI
respectively and no further purified. Other reagents were used as received.
Synthesis of complex 2
nBuLi (1.8 mL, 2.5 M in hexane, 4.50 mmol) was added dropwise to a THF (30
mL) solution of ligand 1 (2.30 g, 4.33 mmol) at -78°C and was stirred for 1 h. The
reaction solution was warmed to -40 °C to stir for another 2 h and then was added to a
THF suspension (20 mL) of MgBr2 (0.82 g, 4.45 mmol) at -78 °C. The reaction
mixture was allowed to warm to room temperature and stirred for 2 days. The yellow
solution was concentrated to ca. 10 mL to afford a large amount of crystals and the
supernatant solution was concentrated again to afford a second crop of complex 2
1
(Yield 2.71 g, 67%). m.p 223-225 °C, H NMR (C6D6, 600 MHz): δ 8.29 (s, 2 H,
CH=N), 7.32-7.05 (m, 9 H, Ar-H), 3.53 (m, 20 H, CH(CH3)2+CH2O of THF overlap),
13
1.33 (m, 16 H, CH2 of THF), 1.19 (d, 24 H, CH(CH3)2). C{1H} NMR (C6D6, 151
MHz): δ 171.2 (CH=N), 149.2, 144.1, 140.3, 137.7, 132.4, 127.1, 124.9, 123.2 (Ar-C),
68.3 (CH2O), 25.6 (CH2), 28.3 (CH(CH3)2), 23.6 (CH(CH3)2). Anal Calcd for
C48H71Br2LiMgN2O4: C, 61.91; H, 7.69; N, 3.01. Found C, 62.25; H, 7.94; N, 2.87.
General procedure for catalytic hydroboration of aldehydes or ketones
In a glove box, catalyst 2 (aldehydes: 1 mol%, ketones: 5 mol%) was added to a
solution of aldehydes or ketones (0.25 mmol) and HBpin (1.2 equiv.) at room
temperature in a common NMR tube which was charged with CDCl3 (0.4 mL). The
progress of the reaction was monitored by 1H NMR and 11B NMR.

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X-ray crystal structure determination
Crystallographic data for complex 2 is given in Table 3. Diffraction data was
collected on a Bruker D8 VENTURE PHOTON 100 diffractometer using a
graphite-monochromated MoKα radiation (0.71073Å) at 135 K in the ω-2θ scan mode.
In all cases, an empirical absorption correction by SADABS was applied to the
intensity data. The structure was solved by direct methods and refined on F2 by
full-matrix least-squares methods using the SHELXTL crystallographic software
package. All non-hydrogen atoms were refined anisotropically with hydrogen atoms
included in calculated positions (riding model). CCDC 1830603 contains the
supplementary crystallographic data for complex 2. The data can be obtained free of
charge
from
The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
Table 3. Crystallographic data for complex 2.
Formula
Crystal system
a (Å)
C48H71Br2LiMgN2O4 Mr
931.12
orthorhombic
21.7324(9)
21.0958(10)
10.5148(5)
4820.6(4)
1.283
Space group
α (°)
β (°)
γ (°)
Z
Pnma
90(4)
90(4)
90(4)
4
135(2)
1960
0.1041
b (Å)
c (Å)
V (A3)
Dcalc (g cm-3)
µ (mm-1)
R1 (obs data)
T (K)
1.737
0.0498
F (000)
wR2 (obs data)
Acknowledgements
We thank financial supports from the National Natural Science Foundation of
China (21772093) and Postgraduate Research & Practice Innovation Program of
Jiangsu Province (KYCX17_0844).
References
[1] (a) J. Zhang, Z. Xie, Acc. Chem. Res. 47 (2014) 1623; (b) R. J. Trovitch, Acc.
Chem. Res. 50 (2017) 2842; (c) J. W. B. Fyfe, A. J. B. Watson, Chem. 3 (2017) 31.
[2] C. C. Chong, R. Kinjo, ACS Catal. 5 (2015) 3238.
[3] S. J. Geier, C. M. Vogels, S. A. Westcott, ACS Symp. Ser. 1236 (2016) 209.
[4] (a) U. K. Das, C. S. Higman, B. Gabidullin, J. E. Hein, R. T. Baker, ACS Catal. 8
(2018) 1076 and references cited therein. (b) S. Bagherzadeh, N. P. Mankad,
Chem. Commun. 52 (2016) 3844.
[5] M. Arrowsmith, T. J. Hadlington, M. S. Hill, G. Kociok-Köhn, Chem. Commun.
48 (2012) 4567.
[6] K. Manna, P. Ji, F. X. Greene, W. Lin, J. Am. Chem. Soc. 138(2016) 7488.
[7] L. Fohlmeister, A. Stasch, Chem. Eur. J. 22 (2016) 10235.
[8] M. Ma, J. Li, X. Shen, Z. Yu, W. Yao, S. A. Pullarkat, RSC Adv. 7 (2017) 45401.
[9] S. Yadav, S. Pahar, S. S. Sen, Chem. Commun. 53 (2017) 4562.

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[10] D. Mukherjee, H. Osseili, T. P. Spaniol, J. Okuda, J. Am. Chem. Soc. 138 (2016)
10790.
[11] H. Osseili, D. Mukherjee, K. Beckerle, T. P. Spaniol, J. Okuda, Organometallics
36 (2017) 3029.
[12] Y. Wu, C. Shan, J. Ying, J. Su, J. Zhu, L. L. Liu, Y. Zhao, Green Chem. 19
(2017) 4169.
[13] T. J. Hadlington, M. Hermann, G. Frenking, C. Jones, J. Am. Chem. Soc. 136
(2014) 3028.
[14] Y. Wu, C. Shan, Y. Sun, P. Chen, J. Ying, J. Zhu, L. Liu, Y. Zhao, Chem.
Commun. 52 (2016) 13799.
[15] Z. Yang, M. Zhong, X. Ma, S. De, C. Anusha, P. Parameswaran, H. W. Roesky,
Angew. Chem., Int. Ed. 54 (2015) 10225.
[16] V. K. Jakhar, M. K. Barman, S. Nembenna, Org. Lett. 18 (2016) 4710.
[17] C. C. Chong, H. Hirao, R. Kinjo, Angew. Chem., Int. Ed. 54 (2015) 190.
[18] D. Wu, R. Wang, Y. Li, R. Ganguly, H. Hirao, R. Kinjo, Chem 3 (2017) 134.
[19] M. K. Bisai, S. Pahar, T. Das, K. Vanka, S. S. Sen, Dalton Trans. 46 (2017)
2420.
[20] V. L. Weidner, C. J. Barger, M. Delferro, T. L. Lohr, T. J. Marks, ACS Catal. 7
(2017) 1244.
[21] S. Chen, D. Yan, M. Xue, Y. Hong, Y. Yao, Q. Shen, Org. Lett. 19 (2017) 3382.
[22] W. Wang, X. Shen, F. Zhao, H. Jiang, W. Yao, S. A. Pullarkat, L. Xu, M. Ma, J.
Org. Chem. 83 (2018) 69.
[23] G. van Koten, R. A. Gossage, Top Organomet. Chem. 54 (2016) 17.
[24] M. Ma, X. Shen, W. Wang, J. Li, W. Yao, L. Zhu, Eur. J. Inorg. Chem. (2016)
5057.
[25] M. Ma, X. Shen, Z. Yu, W. Yao, L. Du, L. Xu, Chin. J. Inorg. Chem. 36 (2016)
72.
[26] W. Gao, D. Cui, J. Am. Chem. Soc. 130 (2008) 4984.

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Highlights
1.
A
new
well-defined
lithium
NCN-Pincer
magnesiate
complex
[Li(THF)4][NCN-MgBr2] has been prepared.
2. The carbonyl hydroboration has been efficiently catalyzed by the Mg-Li complex.
3. Chemoselective hydroboration of aldehydes over ketones was observed.