Magnesium-catalyzed hydroboration of organic carbonates, carbon dioxide and esters

Article abstract of DOI:10.1039/d0dt00465k

A low-valent magnesium(i) complex [(^XylNacnac)Mg]₂ was employed as a highly efficient precatalyst for the hydroboration of a variety of cyclic and linear organic carbonates, polycarbonates, CO₂ and esters with HBpin under mild conditions. The resultant boronates can be used for the preparation of the corresponding value-added diols, triols or alcohols through hydrolysis.

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DOI: 10.1039/D0DT00465K
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Magnesium-catalyzed hydroboration of organic carbonates,
carbon dioxide and esters
Xu Cao,a† Weifan Wang,a† Kai Lu,a Weiwei Yao,*b Fei Xue*a and Mengtao Ma*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The low-valent magnesium(I) complex [(XylNacnac)Mg]2 was pressurized hydrogen gas or hazardous metal-hydride reducing
employed as a highly efficient precatalyst for the hydroboraton of agents. Hence catalytic hydroboration of organic carbonates is an
a variety of cyclic and linear organic carbonates, polycarbonates, interesting and possible alternative to the corresponding
CO2 and esters with HBpin under mild conditions. The resultant hydrogenation. However, only few examples of hydroboration of
boronates can be used for the preparation of the corresponding organic carbonates have been reported hitherto.14,15 Very recently
value-added diols, triol or alcohols through hydrolysis.
Leitner et al. used transition metal manganese pincer complex
[Mn(Ph2PCH2SiMe2)2NH(CO)2Br] as a catalyst for the hydroboration
Due to their high stability under ambient conditions, organic of a variety of organic carbonates with HBpin as reducing agent.
carbonates are frequently used as green solvents in synthesis and However, there are still significant limitations such as the
catalysis1 and therefore the reduction of organic carbonates is a very requirement of additional base (NaOtBu) and high reaction
challenging task. Recently, reduction of organic carbonates has temperature (90 °C).14a During the preparation of this manuscript,
attracted considerable attention because they can be readily Rueping et al. reported dibutylmagnesium catalyzed hydroboration
prepared from CO2 or CO etc renewable C1 feedstock, and further of carbonates quite recently. Nevertheless, it was still required to be
reduced to produce methanol derivatives.2,3 On the other hand, carried out at relatively high reaction temperature (65-85 °C) and in
cyclic organic carbonates synthesized by the direct coupling between toluene solvent.15 Meanwhile, hydroboration of esters is an efficient
carbon dioxide and epoxides can be performed as protecting groups catalytic reduction process that avoids using traditional aggressive,
or synthetic precursors to obtain the corresponding diols which can functional-group intolerant reagents such as LiAlH4 or LiBH4 for the
be used in a wide range of chemicals and materials.4 These lead to a construction of complex molecules, preparation of materials and fine
practical two-step route for the conversion of greenhouse gas CO2 chemical production. To the best of our knowledge, few examples of
into methanol, value-added diols or their derivatives which is a esters hydroboration are reported because ester reduction is
promising solution to emerging global energy problem. Catalytic thermodynamically more challenging than aldehyde and ketone
hydrogenation is an excellent method to reduce cyclic organic conversion.16-20
carbonates to form a variety of diols. However, it required highly
Compared to the transition metal, the main group metal
pressurized and flammable hydrogen gas and high reaction magnesium is earth-abundant, cheap, and environmentally friendly.
temperature as well as using transition metals such as Ru, Mn, Fe Since the first room temperate stable magnesium(I) complexes has
complexes as catalysts commonly.2,3,5-10
been prepared by Jones et al.,21 considerable efforts have been made
Recently, hydroboration of unsaturated organic compounds has to study their reactivity,22,23 however, no catalytic application of
gained increasing attention due to its high efficiency.11-12 For magnesium(I) compounds have been reported except our two
example, various magnesium complexes have been employed as examples. Our group recently found that the magnesium(I) complex
efficient catalysts for the hydroboration of a series of unsaturated could be employed as an efficient precatalyst for the cyanosilylation
substrates such as aldehydes, ketones, isocyanates, imines, amides, and hydroboration of a variety of aldehydes, ketones, nitriles and
pydidines, nitriles, and alkynes reported by Hill, Harder, Sadow, alkynes under mild conditions.24 We wondered whether the low-
Okuda, Nembenna, Lin, Rueping et al., respectively.13 Moreover, valent magnesium(I) catalyst could be extend to apply in the
boranes such as pinacolborane (HBpin) are relatively stable and easy- hydroboration of carbonates and esters. To our delight, we found
to-handle thus circumventing the use of highly flammable and that the highly efficient hydroboration of a wide range of cyclic and
linear organic carbonates, carbon dioxide and esters catalyzed by
various low-valent magnesium(I) complexes have been achieved
under mild reaction condition. Herein, we reported these results.
a.
College of Science, Nanjing Forestry University, Nanjing 210037, China
b. College of Pharmacy, Nanjing University of Chinese Medicine, Nanjing 210023,
We embarked on our investigation employing ethylene
China
carbonate as a model substrate and HBpin as a reductant under neat
condition at room temperature. Initially, the hydroboration of
ethylene carbonate with 4 equiv. of HBpin was performed in the
† These authors contribute equally.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. 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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Table 1 Optimization of ethylene carbonate hydroboration
the amount of HBpin to 3.2 equivalents, full conversion was achieved
View Article Online
O
in 6 h at room temperature under neat condition too (Table 1, entry
DOI: 10.1039/D0DT00465K
1 mol% Mg(I)
rt
C
Bpin
O
17). Decreasing the catalyst loading of 1 to 0.1 mol%, 99% yield was
achieved in 8 h at 40 °C as well (Table 1, entries 18, 19). When the
precursor of 1, (XylNacnac)MgI(OEt2), was tested under identical
reaction conditions, a relatively low yield (85%) was obtained (Table
1, entry 20). Hence, the new main group Mg(I)-catalyzed protocol is
much more efficient than the transition metal Mn-catalyzed
hydroboration of carbonates (Mg: 40 °C, 0.1 mol%, 8h, 99% yield vs
Mn: 90 °C, 0.1 mol% (with 0.3 mol% NaOtBu), 8 h, 42-95% yield) and
that of the divalent Mg(II) catalyst (Mg(I): rt, 1 mol%, 6 h, neat, 95-
99% yield vs Mg(II): 65-85 °C, 1 mol%, 3-8 h, in toluene-d8, 88-95%
yield).14,15
n HBpin
+
+
O
O
H3C
OBpin
O
Bpin
2a
T
(h)
24
24
3
6
6
6
6
6
6
6
6
6
6
Yield
Entry
n
Cat
Solvent
(%)a
0
1
2
3
4
5
6
7
8
9
10
11
12
13
4
4
4
4
4
4
4
4
4
4
4
4
4
none
none
1
-
-
-
-
-
-
-
traceb
84
99
98
96
83
0
1
[(DippXylNacnac)Mg]2
[(MesNacnac)Mg]2
[(DippNacnac)Mg]2
Table 2 Hydroboration of carbonates catalyzed by 1
1
1
1
1
1
1
THF
CHCl3
Hexane
Et2O
Benzene
Toluene
O
34
79
99
99
99
C
Xyl
N
Xyl
N
Mg Mg
O
O
pinBO OBpin
or
1 mol% Mg(I) 1
or
O
3.2 HBpin
+
H3C OBpin
+
neat, rt, 6h
N
N
R
OBpin
2
R
C
R
Xyl
Xyl
O
O
(1)
2
Yield
(%)a
Entry
Carbonates
Products
1,4-
Dioxane
14
4
1
6
99
O
BpinO
BpinO
BpinO
O
1
2
2a
2b
2c
2d
99
97
96
99
15
16
17
18
19
20
3
3
3.2
3.2
3.2
3.2
1
1
1
1
1
6
12
6
6
8
-
-
-
-
-
-
82
99
OBpin
OBpin
O
O
99
O
92c
99c
85
O
O
OBpin
OBpin
3
4
5
6
7
8
O
(
XylNacnac)MgI(OEt2)
6
O
a The reaction was monitored by 1H NMR spectroscopy. b 100 °C. c 0.1
mol%, 40 °C.
BpinO
BpinO
BpinO
O
O
O
O
O
absence of any catalyst, no desired product was observed at room
temperature after 24 h. Increasing the temperature to 100 °C, only
trace yield was obtained even after 24 h which is similar to the result
reported by Leitner et al. (Table 1, entries 1, 2).14a To our delight,
when 1 mol% of magnesium(I) complex [(XylNacnac)Mg]2 (1, Xyl = 2,6-
Me2C6H3) was added into the above reaction mixture, 84% yield was
obtained after 3 h and ethylene carbonate was fully converted into
the corresponding boronate ester ethylene glycol and methyl
boronate only after 6 h at room temperature (Table 1, entries 3, 4).
Methyl boronate is the actual reduction product from the central
carbon of the ethylene carbonate group that originated from carbon
dioxide. Subsequently, the catalytic performance of different
sterically hindered magnesium(I) complexes [(ArNacnac)Mg]2 (where
Ar = DippXyl, Mes and Dipp; DippXyl =2,6-iPr2C6H3/2,6-Me2C6H3, Mes
= 2,4,6-Me3C6H2, Dipp = 2,6-iPr2C6H3) was examined. It can be seen
that all these Mg(I) complexes can be used as efficient catalysts in
the hydroboration of ethylene carbonate, however, the catalytic
activity of Mg(I) complexes gradually decreased with increasing steric
hindrance (Table 1, entries 5-7). Encouraged by these promising
results, solvent effect was briefly screened. When tetrahydrofuran
(THF) was used as a solvent, no hydroborated product was observed
in 6 h (Table 1, entry 8). The reason could be that THF acted as a
coordinated solvent to react with 1 to produce the corresponding
magnesium adduct [(XylNacnac)Mg(THF)]2 and resulted in the
deactivation of magnesium(I) complex. When chloroform or hexane
was added, the corresponding hydroboration yield was 34% and 79%
respectively (Table 1, entries 9, 10). However, to our surprise, when
solvents such as diethyl ether, benzene, toluene, and 1,4-dioxane
were employed, 99% yield was obtained in 6 h as neat conditions
(Table 1, entries 11−14). Lowering the amount of HBpin to ideal 3
equivalents, a relatively lower yield (82%) was obtained in 6 h.
Nevertheless, prolonging the reaction time to 12 h, 99% yield could
be achieved as well (Table 1, entries 15, 16). When slightly increasing
O
O
Ph
O
OBpin
2e
2f
99
99b
99
O
O
Ph
O
O
HO
OBpin
O
OBpin
OBpin
BpinO
BpinO
O
O
2g
2h
O
Ph
O
OBpin
99
O
Ph
O
O
O
BpinO
BpinO
OBpin
OBpin
9
2i
2j
99
96
99
99
98
99
98
99
95
O
O
O
10
11
12
13
14
15
16
17
O
O
O
O
2k
2l
BpinO
Ph
OBpin
O
OBpin
Ph
O
O
Ph
O
O
PhOBpin
EtOBpin
2m
2n
2o
2p
2q
Ph
Ph
O
O
O
O
O
EtOBpin + MeOBpin
MeOBpin (3 equiv.)
O
O
O
O
O
O
O
OBpin
O
O
BpinO
OBpin
O
O
18
2c
96c
n
aThe reaction was monitored by 1H NMR spectroscopy. b 4.2 HBpin.
c0.5 ml C6D6.
2 | J. Name., 2012, 00, 1-3
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With the optimized reaction conditions in hand, we explored the Table 3 Hydroboration of esters catalyzed by 1
View Article Online
O
1
substrate scope of hydroboration reaction with various cyclic and
1 mol %
DOI: 10.1039/D0DT00465K
OBpin
+ R2 OBpin
R2
+
2 HBpin
R1
linear structures carbonates (Table 2). All reactions underwent
successful hydroboration using 1 mol% catalyst loading of 1 at room
temperature under neat condition within 6 h. Five-membered ring
carbonates with methyl and ethyl group gave very high yields similar
to ethylene carbonate (Table 2, entries 2,3). Hydroboration of
glycerol derivatives with methyl and benzyl and free OH group also
produced the desired boronate products in excellent yield (Table 2,
entries 4-6). It needs to be noted that when 4-vinyl-1,3-dioxolan-2-
one was used, hydroboration occurred at the carbonate functional
group instead of terminal olefin group (Table 2, entry 7). Ethylene
carbonate bearing aromatic substituent such as phenyl was
efficiently reduced with high yield in 6h as well (Table 2, entry 8). Six-
membered ring carbonates also gave excellent yields of products
(Table 2, entries 9-11). Surprisingly, in the aforementioned report by
Leitner et al., the substrate 5,5-dimethyl-1,3-dioxan-2-one gave
significantly lower yield of product (only 39%) due to its low solubility
in pinacolborane.14 Gratifyingly, similar phenomenon did not occur
in our subvalent Mg-catalyzed system, 96% yield was observed
(Table 2, entry 10). In contrast to the lower conversion of linear
carbonates hydroboration catalyzed by Mn catalyst,14 the Mg
catalysts showed high reaction activities as their cyclic congeners. To
our delight, both dibenzyl carbonate and diphenyl carbonate
proceeded smoothly in quantitative yield (Table 2, entries 12,13). In
the Mn-catalytic system, the hydroboration of ethyl methyl and
dimethyl carbonate only offered 42% and 54% yield, respectively.12
However, hydroboration of the corresponding linear carbonates
catalyzed by magnesium 1 gave up to 99% yield (Table 2, entries 14-
17). Additionally, our catalytic system was found to catalyze the
hydroboration depolymerization of polycarbonates to the
corresponding diboronate ester and methyl boronate. For example,
polypropylene carbonate, which can be prepared from propylene
and CO2, underwent hydroboration in the presence of 1 to produce
boronate ester propylene glycol and methyl boronate in 96% yield
(Table 2, entry 18).
R1
O
rt, 1h
3
Yield
Entry
Substrate
Products
(%)a
O
OBpin
OBpin
1
2
3a
3b
99
O
O
99
96
95
96
O
O
Br
OBpin
OBpin
(CH2)5
+
Br
3
4
5
3c
3d
3e
O
(CH2)5
O
OBpin
+
OBpin
O
O
Ph
OBpin
Ph
O
Ph
O
OBpin
OBpin
S
+
6
7
3f
99
95
O
S
O
O
BpinO
O
OBpin
3g
O
O
OBpin
OBpin
8
9
3h
3i
99
96
O
OBpin
OBpin
O
C6H13
O
C6H13
a The reaction was monitored by 1H NMR spectroscopy.
Encouraged by the above organic carbonates hydroboration
results, we then examined the similar esters hydroboration that can
provide the corresponding alcohols and diols. We started our
investigation with the reduction of ethyl acetate as a model substrate
with 2 equiv. of HBpin under various reaction conditions. In the
absence of any catalyst, the hydroboration between ethyl acetate
and 2 equiv. of HBpin did not gave any alkoxyl boronate product in
C6D6 after 1 h. Even increasing temperature to 70 °C, no any
conversion could be observed after 15 h (Table S1, entries 1,2). This
is similar to that the catalyst free hydroboration of benzyl benzoate
exhibited a very negligible conversion.20 However, when 5 mol%
catalyst of 1 was added into the above mixture, the reaction
completed in less than 10 min at room temperature. Lowering the
catalyst loading of 1 to 1 mol%, a slightly longer reaction time (40
min) was required (Table S1, entries 3,4). When in solvent-free
condition, the hydroboration of ethyl acetate also completed in 1 h
(Table S1, entry 5). Decreasing the catalyst loading of 1 to 0.1 mol%,
the reaction proceed smoothly with 99% yield in 5 h (Table S1, entries
6,7).
Success in the hydroboration of ethyl acetate promoted us to
explore the substrate scope of a variety of esters. Hydroboration of
various esters containing a wide range of functional groups including
aliphatic, alkyl halides, conjugated double bonds, aromatic and
thiophene all provided high yields in short time. Result of these
catalytic esters hydroboration is summarized in Table 3. The
corresponding alkoxyl boronate ester products were obtained in very
high yield under 1 mol% of 1 in 1 h at room temperature for all esters
tested. It should be noted that this catalytic system shows selectivity
for the ester group over olefin group (Table 3, entry 4). The
hydroboration of cyclic esters including lactide catalyzed by 1 also
To demonstrate the practical applicability of the new developed
protocol, two representative substrates (2c, 2f) were selected to
prepare the corresponding diol and triol products via column
chromatography, respectively. After successive hydroboration and
hydrolysis, the corresponding desired 1,2-butanediol and glycerol
were obtained in high total isolated yields (Scheme 1).
Scheme 1. Catalytic synthesis of boronate esters and alcohols
HO
1 mol% Mg(I) 1
BpinO
SiO2
O
OH
yield 94%
OH
OBpin
3.2 HBpin
+
+
+
H3C OBpin
CH3OH
C
O
neat, rt, 6h
Air
O
conv. 99%
HO
SiO2
Air
1 mol% Mg(I) 1
BpinO
O
O
HO
OBpin
4.2 HBpin
+
+
+
CH3OH
H3C OBpin
C
O
neat, rt, 6h
OH
OBpin
conv. 99%
yield 92%
The high catalytic activity of the low-valent magnesium(I) 1 in the
formation of methyl boronate ester from various carbonates
prompted us to explore its ability to reduce CO2 directly under similar
conditions. To our delight, low-valent magnesium(I) 1 is able to
catalyze carbon dioxide to afford the desired methoxy boronate in
high yield using 5 mol% of 1 at 100 °C for 15 h (Scheme 2).
Scheme 2. Hydroboration of CO2 catalyzed by 1
5 mol% Mg(I) 1
CO2
+
HBpin
CH3OBpin
+
O(Bpin)2
100 oC, 15h
(1 atm)
96% yield
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produced the corresponding dialkoxyl boronates in quantitative yield
that could be further hydrolyzed to diols (Table 3, entries 7-9).
In order to gain further insight into the reaction mechanism of
5, 3238; (b) S. J. Geier, C. M. Vogels and S. A. Westcott, ACS
View Article Online
Symp. Ser., 2016, 1236, 209; (c) H. Yoshida, ACS Catal., 2016,
DOI: 10.1039/D0DT00465K
6, 1799.
carbonate hydroboration catalyzed by magnesium(I) complexes, 12 Catalyst-free borylation and hydroboration reactions: (a) E. A.
some stoichiometric reactions between magnesium(I) 1 and HBpin
or carbonate were attempted. The stoichiometric reaction between
1 and ethylene carbonate in 1:1 or 1:2 molar ratio has been
performed, however, no new intermediate was observed via 1H and
13C NMR spectra. The dimeric magnesium boryloxide complex
[(XylNacnac)Mg(μ-OBpin)]2 was obtained from the stoichiometric
reaction between 1 and HBpin (1:2 or excess molar ratio etc). This is
same to the report by Jones et al. that reactions of three
magnesium(I) dimers with different equivalents of HBpin (1:2, 1:5 or
1:20) lead to complex mixtures of many products including a plethora
of magnesium(II) complexes with oxygen-based fragments.25 Based
on the complex reactivity of magnesium(I) 1 towards HBpin it is
Romero, J. L. Peltier, R. Jazzar and G. Bertrand, Chem.
Commun., 2016, 52, 10563; (b) K. Kuciński and G. Hreczycho,
ChemSusChem, 2017, 10, 4695; (c) J. Kaźmierczak, K. Kuciński,
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2509; (d) H. Stachowiak, J. Kaźmierczak, K. Kuciński and G.
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Bhattacharjee and T. K. Panda, Chem. Commun., 2019, 55,
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Xue, ACS Omega, 2019, 4, 6775; (g) W. Wang, M. Luo, D. Zhu,
W. Yao, L. Xu and M. Ma, Org. Biomol. Chem., 2019, 17, 3604;
(h) W. Wang, M. Luo, W. Yao, M. Ma, S. A. Pullarkat, L. Xu and
P. H. Leung, New J. Chem., 2019, 43, 10744.
difficult to propose a mechanism. We also conducted the reaction of 13 For various magnesium complexes used in hydroboration
ethyl formate (the plausible intermediate) with HBpin, the
corresponding hydroboration products were found only in the
presence of 1 (Scheme S1).
reactions: (a) M. Arrowsmith, T. J. Hadlington, M. S. Hill and
G. Kociok-Köhn, Chem. Commun., 2012, 48, 4567; (b) M.
Arrowsmith, M. S. Hill and G. Kociok-Köhn, Chem. Eur. J., 2013,
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Hill, G. Kociok-Köhn, D. J. Liptrot, M. F. Mahon and C.
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In conclusion, we have demonstrated the excellent catalytic
performance of magnesium(I) 1 for the hydroboration of a wide
range of CO2-derived cyclic five- and six-membered carbonates,
linear carbonates, polycarbonates, and even carbon dioxide under
mild condition. The new main group low-valent magnesium(I)
catalytic system provides an indirect route for the conversion of CO2
into methanol, value-added diol and even triol in high yield. The
Mg(I)-catalyzed protocol is efficient than the corresponding
transition metal manganese and divalent magnesium(II) catalyzed
hydroboration of organic carbonates. In addition, it could act as an
efficient precatalyst in the catalytic hydroboration of various esters
under mild conditions as well. Further studies of the reaction
mechanism and other potential catalytic applications of
magnesium(I) complexes are in progress in our group.
We thank financial support from the National Natural Science
Foundation of China (21772093), the Natural Science Foundation
of Jiangsu Province, China (BK20181421) and PAPD
.
There are no conflicts to declare.
Notes and references
1
2
3
4
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COMMUNICATION
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DOI: 10.1039/D0DT00465K
Content
O
OBpin
pinBO
C
O
R
O
1-0.1 mol% Mg(I)
+
rt, neat, 1-6h
HBpin
+
CO2
H3C OBpin
28 examples
up to 99% yield
or
O
R'
R'/R
OBpin
O
The magnesium(I) complex was employed as a highly efficient precatalyst for the
hydroboration of carbonates and esters under mild conditions.