Reduction of Cyclic and Linear Organic Carbonates Using a Readily Available Magnesium Catalyst

Article abstract of DOI:10.1021/acscatal.9b04086

Efficient reduction of cyclic and linear organic carbonates catalyzed by a readily available earth alkaline catalyst has been achieved. The described homogenous reaction based on a ligand-free magnesium catalyst provides an indirect route for the conversion of CO₂ into valuable alcohols. The reaction proceeds with high yields under mild reaction conditions, with low catalyst loading and short reaction times, and shows a broad applicability toward various linear and cyclic carbonates. Additionally, it can be applied for the depolymerization of polycarbonates.

Full text of DOI:10.1021/acscatal.9b04086

Research Article
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2019, 9, 11634−11639
Reduction of Cyclic and Linear Organic Carbonates Using a Readily
Available Magnesium Catalyst
Marcin Szewczyk,† Marc Magre,† Viktoriia Zubar,† and Magnus Rueping*,†,‡
†Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, North Rhine-Westphalia, Germany
‡KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), 23955-6900 Thuwal, Makkah
Province, Saudi Arabia
S
* Supporting Information
ABSTRACT: Efficient reduction of cyclic and linear organic
carbonates catalyzed by a readily available earth alkaline
catalyst has been achieved. The described homogenous
reaction based on a ligand-free magnesium catalyst provides
an indirect route for the conversion of CO2 into valuable
alcohols. The reaction proceeds with high yields under mild
reaction conditions, with low catalyst loading and short
reaction times, and shows a broad applicability toward various linear and cyclic carbonates. Additionally, it can be applied for
the depolymerization of polycarbonates.
KEYWORDS: magnesium, carbonates, hydroboration, carbon dioxide, alkaline base earth-abundant metal
requirements of an ecologically and economically benign
process (Scheme 1).
ecent years have shown progress in the application of
carbon dioxide as a C1 building block.1 One of the
R
industrially applied approaches for the reduction of atmos-
pheric CO2 emissions is the use of this greenhouse gas in
reactions with alcohols or epoxides to form organic carbonates.
Especially, the latter method is based on an economic and
environmental rationale as it not only provides commercially
important chemicals but also utilizes readily available
epoxides.2 The following reduction of organic carbonates
results in a two-step route conversion of CO2 into methanol
and value-added diols or their derivatives.
Scheme 1. MgBu2-Catalyzed Hydroboration of Carbonates
However, reduction of organic carbonates is disfavored
because of their high stability. Therefore, organic carbonates
can be used as solvents, even in reductive transformations
involving metal hydride species.3 For this reason, only few
examples of carbonate reductions have been reported to
date.4,5 Direct hydrogenation, the most common method for
the reduction of carbonates has been studied in the past.
However, all protocols are based on transition-metal catalysts
and often require high pressures and temperatures.4
Boranes are often used as alternative reducing agents in
order to circumvent high-pressure hydrogenations with the use
of the flammable hydrogen gas. Thus, the hydroboration of
carbonates is an interesting alternative. Recently, Leitner and
co-workers reported an interesting transition metal-catalyzed
reduction of organic carbonates using pinacolborane as a
reducing agent.5 However, efficient transition metal-free
protocols based on earth alkaline metals remain an elusive
goal. Thus, the use of a readily available magnesium catalyst
which could be used in low catalyst loadings for the reduction
of CO2-derived organic carbonates to value added alcohols
would be an important advancement in achieving the
Furthermore, the successful development of such a process
may also be extended to the recycling of polycarbonates with
the simultaneous formation of valuable diols and methanol.
Based on our interest in earth alkaline metal catalysis and in
particular magnesium catalysis, we wondered whether a readily
available magnesium catalyst could be applied in the reduction
of CO2-derived carbonates.
Although magnesium is one of the most abundant metals on
earth, the application of magnesium-based catalysts for organic
reactions is still relatively unexplored compared with other
Received: September 23, 2019
Revised: October 24, 2019
© XXXX American Chemical Society
11634
DOI: 10.1021/acscatal.9b04086
ACS Catal. 2019, 9, 11634−11639

ACS Catalysis
Research Article
metals.6−8 Recently, magnesium complexes, such as β-
diketiminato−magnesium complexes,9,10 have been employed
for the hydroboration of unsaturated polarized bonds,
including aldehydes, ketones, and esters.11−13 Our group
reported a protocol for the selective hydroboration of terminal
and internal alkynes catalyzed by commercially available
MgBu2.11a Mechanistic studies revealed that in the presence
of the reductant HBpin, an active BuMgH species is formed in
situ.14 Given the growing interest in earth alkaline metal-
catalyzed organic transformations and the valorization of CO2
and its derivatives, we decided to examine Mg-catalyzed
reduction of carbonates as well as the extension to the
recycling of polycarbonates with simultaneous generation of
valuable diols and methanol (Scheme 1).
To develop a practical reduction protocol for the rather
stable and challenging cyclic carbonates, we started to explore a
magnesium-catalyzed protocol which should proceed with low
catalyst loading and under mild reaction conditions. Hence, we
chose cyclic carbonate 1a as our model substrate and run initial
screening of solvents, as well as the reaction under solvent-free
conditions, at ambient temperature (Table 1, entries 1−4).
excellent yields were obtained (Table 2). Ethylene carbonate
(1b) and its derivatives bearing aliphatic substituents, such as
Me (1c), Et (1d), n-Hex (1e), and even sterically hindered t-
Bu (1f), were efficiently reduced with excellent yields within 3
h. Hydroboration of glycerol derivatives with methyl (1h) and
benzyl (1i) as protecting groups, as well as free OH group (1g)
resulted in the formation of the desired products in excellent
yields. Chemoselective reduction of unsaturated carbonate (1j)
was also possible, and no product of reduction of C−C double
bond was observed. Cyclic cis- and trans-diol derivatives (2k
and 2l, respectively) were obtained with excellent yields
regardless of the stereochemistry of the starting material. Six-
membered ring organic carbonates also underwent efficient
hydroboration, affording the corresponding 1,3-diol derivatives
(2m−2p) bearing various alkyl and aryl substituents. The
lower activity observed in the case of 1,3-dioxan-2-one (1m)
can be attributed to the lower solubility of the starting material.
Linear carbonates, especially aliphatic ones, usually require
harsher conditions in order to achieve good conversions.15 To
our delight, by simply prolonging the reaction time to 4 h, we
were able to reduce methyl, ethyl, and benzyl carbonates (1q−
1s) with full conversions and excellent yields. Phenol-derived
carbonates, both cyclic and linear (1t, 1u), required an
elevated temperature to yield the corresponding products in
90−92% yield. Further exploration focused on the depolyme-
rization of polycarbonates. Polypropylene carbonate (1v)
which can be made from propylene and CO2, smoothly
underwent hydroboration in the presence of the magnesium
catalyst. In general, we were delighted to see that our earth
alkaline metal catalyst based on low-cost and commercially
available MgBu2 competes favorably with the existing protocols
in terms of broader substrate scope and milder reaction
conditions. To demonstrate the practical applicability of the
developed reaction, a gram-scale reduction of carbonate 1g was
carried out (Scheme 2). After hydrolysis in acidic conditions,
we obtained the deprotected alcohol 3 in 82% yield.16
Table 1. Optimization of MgBu2-Catalyzed Hydroboration
a
of Carbonate 1a
b
entry
solvent
cat. (%) temp. (°C) time (h) yield (%)
1
2
3
4
5
6
7
8
toluene-d8
THF-d8
benzene-d6
neat
toluene-d8
toluene-d8
toluene-d8
toluene-d8
toluene-d8
toluene-d8
5
5
5
5
5
3
2
3
3
-
23
23
23
23
65
65
65
65
65
65
4
4
4
4
3
3
3
2
2
3
78
52
81
71
>95
>95
88
92
82
The MgBu2−HBpin catalytic system has been also tested for
the reduction of CO2 (Scheme 3). In this case, elevated
temperature and 10 mol % of the catalyst were necessary for
the full consumption of HBpin, leading exclusively to the Bpin-
protected methanol derivative.16,17
c
9
10
<5
a
1a (1 mmol), HBpin (3.1 equiv), MgBu2 (2−5 mol %, 0.5 M in
In agreement with the previous study, reaction of MgBu2
with HBpin gives the active species BuMgH, alongside with
BuBpin (Scheme 4a).11a For better understanding of the
reaction mechanism, we conducted several additional control
experiments. We reasoned that the hydroboration of
carbonates proceeds in three separated catalytic cycles with
the corresponding formate and formaldehyde as intermediates.
When the reaction with the plausible intermediate formate 4
was carried out (Scheme 4b), we found that only in the
presence of the Mg catalyst, the reduction was possible,
meaning that each catalytic step requires the presence of the
magnesium catalyst. Finally, when an equimolar mixture of
carbonate 1s and formate 5 was tested in a competitive
reaction, we found out that only formate 5 was reduced
quantitatively, whereas the carbonate 1s remained unreacted
(Scheme 4c). With this finding, we reasoned that the rate-
limiting step is the first Mg−H addition to the carbonate.
Based on the results and control experiments, we propose a
mechanism for the magnesium-catalyzed hydroboration of
carbonates (Scheme 5a). Reaction of MgBu2 with pinacolbor-
ane gives the active magnesium hydride species A. Carbonate 1
first reacts with magnesium hydride to form reactive
b
1
heptane), solvent [1 M] at a given temperature. Determined by H
NMR using 1,3,5-trimethoxybenzene as an internal standard. With β-
c
diketiminate−MgBu I.
Analysis of the crude reaction mixture by 1H NMR
spectroscopy revealed that 2a and MeOBpin were the only
products formed during the reaction. However, under these
conditions, the reactions were not complete within 4 h and
significant amounts of carbonate were still present in the crude
mixtures. Further optimization was carried out with toluene as
a solvent. Increasing the reaction temperature allowed the
achievement of full conversion (Table 1, entry 5).
Furthermore, we decided to lower the amount of the catalyst
(Table 1, entries 6−7), and thus the optimal catalyst loading of
3 mol % was established. It is worth mentioning that use of β-
diketiminate−MgBu complex I also provided the product;
however, lower yield was observed (entry 8 vs 9). In the
absence of the catalyst, no conversion of 1a was observed
(Table 1, entry 10).
Under the optimized reaction conditions, we investigated
the scope and limitations of this transformation. In general,
11635
DOI: 10.1021/acscatal.9b04086
ACS Catal. 2019, 9, 11634−11639

ACS Catalysis
Research Article
a
Table 2. Magnesium-Catalyzed Hydroboration of CO2-Derived Carbonates
a
b
1
1 (1 mmol), HBpin (3.1 equiv), MgBu2 (3 mol %, 0.5 M in heptane), solvent [1 M] at 65 °C for 3 h. Determined by H NMR using 1,3,5-
c
d
e
f
trimethoxybenzene as an internal standard. 4 equiv of HBpin was used. Mesitylene was used as an internal standard. Reaction time 8 h. Reaction
time 6 h at 85 °C. C6D6 was used as a solvent. Reaction time 4 h. Average Mn of starting material: ∼50 000.
g
h
i
magnesium alkoxide intermediate C, which subsequently
undergoes boron exchange with HBpin, regenerating the
active species A. Thus, formed formate D enters the second
catalytic cycle and reacts with the recovered active species A,
leading to the reduction of ester bond and the formation of
formaldehyde F and the corresponding intermediate E. The
11636
DOI: 10.1021/acscatal.9b04086
ACS Catal. 2019, 9, 11634−11639

ACS Catalysis
Research Article
second molecule of HBpin consumed in the reaction is
necessary for the formation the final product 2 and the
recovery of the active magnesium hydride species A, which
closes the catalytic cycle. Finally, formaldehyde F is reduced to
form magnesium methoxide G, which reacts with the third
molecule of HBpin liberating MeOBpin. Given that β-
diketiminate−MgBu I catalyzes the reaction (Table 1, entry
9), the formation of LMgH2Bpin II in the reaction12b (Scheme
5b) cannot be fully excluded.
Scheme 2. Large-Scale Transformation
In summary, we report the first earth alkaline metal-
catalyzed reduction of carbonates using a readily available
magnesium catalyst. The new protocol shows a broad substrate
scope, including various cyclic and linear carbonates.
Furthermore, polycarbonates can be successfully recycled
with the formation of the valuable diols and methanol. The
procedure is characterized by its mild reaction conditions, fast
reaction times, low catalyst loading, use of a readily available
catalyst based on earth abundant magnesium, and avoidance of
costly ligands as well as broad scope and competes favorably
with the transition metal-catalyzed protocols. Mechanistic
studies showed the formation of active BuMgH, which acts as a
reducing catalyst and is involved in three sequential reduction
cycles.
Scheme 3. MgBu2-Catalyzed Hydroboration of CO2
Scheme 4. Control Experiments for Mechanistic Studies (a)
Formation of BuMgH Active Species; (b) Reduction of
Plausible Intermediate; (c) Competing Reaction Rate of
Equimolar Mixture of Carbonate and Formate
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.9b04086.
Experimental procedures, characterization, and NMR
spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: magnus.rueping@Kaust.edu.sa.
ORCID
Marc Magre: 0000-0002-5950-4129
Magnus Rueping: 0000-0003-4580-5227
Notes
The authors declare no competing financial interest.
Scheme 5. (a) Proposed Mechanism for MgBu2-Catalyzed Hydroboration of Carbonates Involving Three Sequential Steps
Which Require the Mg−H Species; (b) Formation of the Alternative MgH Species When β-Diketiminato−Magnesium
Complex Is Used
11637
DOI: 10.1021/acscatal.9b04086
ACS Catal. 2019, 9, 11634−11639

ACS Catalysis
REFERENCES
Research Article
Malakhov, V.; Knochel, P. Regio- and Chemoselective Metalation of
Arenes and Heteroarenes Using Hindered Metal Amide Bases. Angew.
Chem., Int. Ed. 2011, 50, 9794−9824; Angew. Chem. 2011, 123,
9968−9999. (d) Knochel, P.; Benischke, A.; Ellwart, M.; Becker, M.
Polyfunctional Zinc and Magnesium Organometallics for Organic
Synthesis: Some Perspectives. Synthesis 2016, 48, 1101−1107.
(8) For recent reviews on the use of organomagnesium hydride
complexes as catalysts, see: (a) Crimmin, M. R.; Hill, M. S.
Homogeneous Catalysis with Organometallic Complexes of Group
2. Top. Organomet. Chem. 2013, 45, 191−241. (b) Revunova, K.;
Nikonov, G. I. Main Group Catalysed Reduction of Unsaturated
bonds. Dalton Trans. 2015, 44, 840−866. (c) Pellissier, H.
Enantioselective Magnesium-Catalyzed Transformations. Org. Biomol.
Chem. 2017, 15, 4750−4782.
■
(1) (a) Aresta, M. Carbon Dioxide as Chemical Feedstock; Wiley-
VCH: Weinheim, 2010. (b) Maeda, C.; Miyazaki, Y.; Ema, T. Recent
Progress in Catalytic Conversions of Carbon Dioxide. Catal. Sci.
Technol. 2014, 4, 1482−1497.
(2) For the reviews on transformation of epoxides to carbonates, see:
(a) North, M.; Pasquale, R.; Young, C. Synthesis of Cyclic Carbonates
from Epoxides and CO2. Green Chem. 2010, 12, 1514−1539. (b) He,
Q.; O’Brien, J. W.; Kitselman, K. A.; Tompkins, L. E.; Curtis, G. C. T.;
Kerton, F. M. Synthesis of Cyclic Carbonates from CO2 and Epoxides
Using Ionic Liquids and Related Catalysts Including Choline
Chloride−Metal Halide Mixtures. Catal. Sci. Technol. 2014, 4,
1513−1528. (c) Martín, C.; Fiorani, G.; Kleij, A. W. Recent Advances
in the Catalytic Preparation of Cyclic Organic Carbonates. ACS Catal.
2015, 5, 1353−1370. (d) Comerford, J. W.; Ingram, I. D. V.; North,
M.; Wu, X. Sustainable Metal-Based Catalysts for the Synthesis of
Cyclic Carbonates Containing Five-Membered Rings. Green Chem.
2015, 17, 1966−1987. (e) Cokoja, M.; Wilhelm, M. E.; Anthofer, M.
(9) For illustrative examples of the β-diketiminate−magnesium
complexes as catalyst precursors, see: (a) Green, S. P.; Jones, C.;
Stasch, A. Stable Adducts of a Dimeric Magnesium(I) Compound.
Angew. Chem., Int. Ed. 2008, 47, 9079−9083. (b) Arrowsmith, M.;
̈
Hill, M. S.; Hadlington, T.; Kociok-Kohn, G.; Weetman, C.
H.; Herrmann, W. A.; Kuhn, F. E. Synthesis of Cyclic Carbonates
̈
from Epoxides and Carbon Dioxide by Using Organocatalysts.
Magnesium-Catalyzed Hydroboration of Pyridines. Organometallics
2011, 30, 5556−5559. (c) Harder, S.; Spielmann, J.; Intemann, J.
Synthesis and Thermal Decomposition of a Pyridylene-Bridged Bis-β-
diketiminate Magnesium Hydride Cluster. Dalton Trans. 2014, 43,
14284−14290. (d) Garcia, L.; Dinoi, C.; Mahon, M. F.; Maron, L.;
Hill, M. S. Magnesium Hydride Alkene Insertion and Catalytic
Hydrosilylation. Chem. Sci. 2019, 10, 8108−8118.
(10) β-Diketiminate−magnesium complexes have also been
succesfully applied in C−H and C−F activations. For the leading
examples see: (a) Davin, L.; McLellan, R.; Kennedy, A. R.; Hevia, E.
Ligand-Induced Reactivity of β-Diketiminate Magnesium Complexes
for Regioselective Functionalization of Fluoroarenes via C−H or C−F
Bond Activations. Chem. Commun. 2017, 53, 11650−11653.
(b) Davin, L.; McLellan, R.; Hernan-Gomez, A.; Clegg, W.;
Kennedy, A. R.; Mertens, M.; Stepek, I. A.; Hevia, E. Regioselective
Magnesiation of N-Heterocyclic Molecules: Securing Insecure Cyclic
Anions by a β-Diketiminate-Magnesium Clamp. Chem. Commun.
2017, 53, 3653−3656.
(11) For MgBu2-catalyzed hydroborations, see: (a) Magre, M.;
Maity, B.; Falconnet, A.; Cavallo, L.; Rueping, M. Magnesium-
Catalyzed Hydroboration of Terminal and Internal Alkynes. Angew.
Chem., Int. Ed. 2019, 58, 7025−7029. (b) Jang, Y. K.; Magre, M.;
Rueping, M. Chemoselective Luche-Type Reduction of α,β-
Unsaturated Ketones by Magnesium Catalysis. Org. Lett. 2019, 21,
8349−8352.
(12) For leading examples of β-diketiminate−magnesium complexes
in hydroboration reactions, see: ref 9b and (a) Arrowsmith, M.;
Hadlington, T. J.; Hill, M. S.; Kociok-Kohn, G. Magnesium-Catalysed
ChemSusChem 2015, 8, 2436−2454.
̈
̈
̈
(3) Schaffner, B.; Schaffner, F.; Verevkin, S. P.; Borner, A. Organic
Carbonates as Solvents in Synthesis and Catalysis. Chem. Rev. 2010,
110, 4554−4581.
(4) For selected examples, see: (a) Balaraman, E.; Gunanathan, C.;
Zhang, J.; Shimon, L. J. W.; Milstein, D. Efficient Hydrogenation of
Organic Carbonates, Carbamates and Formates Indicates Alternative
Routes to Methanol Based on CO2 and CO. Nat. Chem. 2011, 3,
609−614. (b) Han, Z.; Rong, L.; Wu, J.; Zhang, L.; Wang, Z.; Ding,
K. Catalytic Hydrogenation of Cyclic Carbonates: a Practical
Approach from CO2 and Epoxides to Methanol and Diols. Angew.
Chem., Int. Ed. 2012, 51, 13041−13045; Angew. Chem. 2012, 124,
13218−13222. (c) Kim, S. H.; Hong, S. H. Transfer Hydrogenation
of Organic Formates and Cyclic Carbonates: An Alternative Route to
Methanol from Carbon Dioxide. ACS Catal. 2014, 4, 3630−3636.
́
́
̈
(d) vom Stein, T.; Meuresch, M.; Limper, D.; Schmitz, M.; Holscher,
M.; Coetzee, J.; Cole-Hamilton, D. J.; Klankermayer, J.; Leitner, W.
Highly Versatile Catalytic Hydrogenation of Carboxylic and Carbonic
Acid Derivatives using a Ru-Triphos Complex: Molecular Control
over Selectivity and Substrate Scope. J. Am. Chem. Soc. 2014, 136,
13217−13225. (e) Zubar, V.; Lebedev, Y.; Azofra, L. M.; Cavallo, L.;
El-Sepelgy, O.; Rueping, M. Hydrogenation of CO2-Derived
Carbonates and Polycarbonates to Methanol and Diols by Metal-
Ligand Cooperative Manganese Catalysis. Angew. Chem., Int. Ed.
2018, 57, 13439−13443. (f) Kumar, A.; Janes, T.; Espinosa-Jalapa, N.
A.; Milstein, D. Manganese Catalyzed Hydrogenation of Organic
Carbonates to Methanol and Alcohols. Angew. Chem., Int. Ed. 2018,
57, 12076−12080; Angew. Chem. 2018, 130, 12252−12256.
̈
Hydroboration of Aldehydes and Ketones. Chem. Commun. 2012, 48,
4567−4569. (b) Mukherjee, D.; Ellern, A.; Sadow, A. D. Magnesium-
Catalyzed Hydroboration of Esters: Evidence for a New Zwitterionic
Mechanism. Chem. Sci. 2014, 5, 959−964. (c) Anker, M. D.;
̈
(g) Kaithal, A.; Holscher, M.; Leitner, W. Catalytic Hydrogenation
of Cyclic Carbonates using Manganese Complexes. Angew. Chem., Int.
Ed. 2018, 57, 13449−13453; Angew. Chem. 2018, 130, 13637−13641.
̈
Arrowsmith, M.; Bellham, P.; Hill, M. S.; Kociok-Kohn, G.; Liptrot,
(5) Erken, C.; Kaithal, A.; Sen, S.; Weyhermuller, T.; Holscher, M.;
̈
̈
D. J.; Mahon, M. F.; Weetman, C. Selective Reduction of CO2 to a
Methanol Equivalent by B(C6F5)3-Activated Alkaline Earth Catalysis.
Chem. Sci. 2014, 5, 2826−2830. (d) Li, J.; Luo, M.; Sheng, X.; Hua,
H.; Yao, W.; Pullarkat, S. A.; Xu, L.; Ma, M. Unsymmetrical β-
Diketiminate Magnesium(I) Complexes: Syntheses and Application
in Catalytic Hydroboration of Alkyne, Nitrile and Carbonyl
Compounds. Org. Chem. Front. 2018, 5, 3538−3547. (e) Yadav, S.;
Dixit, R.; Bisai, M. K.; Vanka, K.; Sen, S. S. Alkaline Earth Metal
Compounds of Methylpyridinato β-Diketiminate Ligands and Their
Catalytic Application in Hydroboration of Aldehydes and Ketones.
Organometallics 2018, 37, 4576−4584.
́
Werle, C.; Leitner, W. Manganese-Catalyzed Hydroboration of
Carbon Dioxide and Other Challenging Carbonyl Groups. Nat.
Commun. 2018, 9, 4521.
(6) (a) Hill, M. S.; Liptrot, D. J.; Weetman, C. Alkaline Earths as
Main Group Reagents in Molecular Catalysis. Chem. Soc. Rev. 2016,
45, 972−988. (b) Rochat, R.; Lopez, M. J.; Tsurugi, H.; Mashima, K.
Recent Developments in Homogeneous Organomagnesium Catalysis.
ChemCatChem 2016, 8, 10−20.
(7) For illustrative examples using magnesium organometallics for
deprotonation/metalation reactions, see: (a) Dong, Z.; Clososki, G.
C.; Wunderlich, S. H.; Unsinn, A.; Li, J.; Knochel, P. Direct Zincation
of Functionalized Aromatics and Heterocycles by Using a Magnesium
Base in the Presence of ZnCl2. Chem.Eur. J. 2009, 15, 457−468.
(b) Piller, F. M.; Bresser, T.; Fischer, M. K. R.; Knochel, P.
Preparation of Functionalized Cyclic Enol Phosphates by Halogen−
Magnesium Exchange and Directed Deprotonation Reactions. J. Org.
Chem. 2010, 75, 4365−4375. (c) Haag, B.; Mosrin, M.; Ila, H.;
(13) For various magnesium complexes used in hydroboration
reactions, see: (a) Mukherjee, D.; Shirase, S.; Spaniol, T. P.; Mashima,
K.; Okuda, J. Magnesium Hydridotriphenylborate [Mg(thf)6]-
[HBPh3]2: a Versatile Hydroboration Catalyst. Chem. Commun.
2016, 52, 13155−13158. (b) Manna, K.; Ji, P.; Greene, F. X.; Lin,
W. Metal−Organic Framework Nodes Support Single-Site Magne-
11638
DOI: 10.1021/acscatal.9b04086
ACS Catal. 2019, 9, 11634−11639

ACS Catalysis
Research Article
sium−Alkyl Catalysts for Hydroboration and Hydroamination
Reactions. J. Am. Chem. Soc. 2016, 138, 7488−7491. (c) Ma, M.;
Li, J.; Shen, X.; Yu, Z.; Yao, W.; Pullarkat, S. A. Sterically Bulky Amido
Magnesium Methyl Complexes: Syntheses, Structures and Catalysis.
RSC Adv. 2017, 7, 45401−45407. (d) Rauch, M.; Ruccolo, S.; Parkin,
G. Synthesis, Structure, and Reactivity of a Terminal Magnesium
Hydride Compound with a Carbatrane Motif, [TismPriBenz]MgH: A
Multifunctional Catalyst for Hydrosilylation and Hydroboration. J.
Am. Chem. Soc. 2017, 139, 13264−13267. (e) Lampland, N. L.;
Hovey, M.; Mukherjee, D.; Sadow, A. D. Magnesium-Catalyzed Mild
Reduction of Tertiary and Secondary Amides to Amines. ACS Catal.
2015, 5, 4219−4226. (f) Barman, M. K.; Baishya, A.; Nembenna, S.
Magnesium Amide Catalyzed Selective Hydroboration of Esters.
Dalton Trans. 2017, 46, 4152−4156. (g) Falconnet, A.; Magre, M.;
Maity, B.; Cavallo, L.; Rueping, M. Asymmetric Magnesium-
Catalyzed Hydroboration by Metal-Ligand Cooperative Catalysis.
Angew. Chem., Int. Ed. 2019, DOI: 10.1002/anie.201908012.
(14) For the formation of BuMgH, see: ref 11a. For the formation of
magnesium hydride complexes using HBpin, see: refs.11−13 For the
formation of Mg−H complexes using hydrosilanes, see: ref 9d and
(a) Dunne, J. F.; Neal, S. R.; Engelkemier, J.; Ellern, A.; Sadow, A. D.
Tris(oxazolinyl)boratomagnesium-Catalyzed Cross-Dehydrocoupling
of Organosilanes with Amines, Hydrazine, and Ammonia. J. Am.
Chem. Soc. 2011, 133, 16782−16785. (b) Hill, M. S.; Liptrot, D. J.;
MacDougall, D. J.; Mahon, M. F.; Robinson, T. P. Hetero-
Dehydrocoupling of Silanes and Amines by Heavier Alkaline Earth
Catalysis. Chem. Sci. 2013, 4, 4212−4222. (c) Baishya, A.; Peddarao,
T.; Nembenna, S. Organomagnesium Amide Catalyzed Cross-
dehydrocoupling of Organosilanes with Amines. Dalton Trans.
2017, 46, 5880−5887. (d) Rauch, M.; Parkin, G. Zinc and
Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide. J.
Am. Chem. Soc. 2017, 139, 18162−18165.
(15) In some protocols for the transition metal-catalyzed direct
hydrogenation of organic carbonates, the scope is limited only to
cyclic carbonates. See for instance: refs.4a,e,g
(16) For the detailed experimental procedures, see Supporting
Information.
(17) Magnesium complexes have been successfully employed for
hydroboration of CO2. For more detailed information, see: refs. 12c,
13a
11639
DOI: 10.1021/acscatal.9b04086
ACS Catal. 2019, 9, 11634−11639