Two dimensional inorganic electride-promoted electron transfer efficiency in transfer hydrogenation of alkynes and alkenes

Article abstract of DOI:10.1039/c5sc00933b

A simple and highly efficient transfer hydrogenation of alkynes and alkenes by using a two-dimensional electride, dicalcium nitride ([Ca₂N]⁺·e^-), as an electron transfer agent is disclosed. Excellent yields in the transformation are attributed to the remarkable electron transfer efficiency in the electride-mediated reactions. It is clarified that an effective discharge of electrons from the [Ca₂N]⁺·e^- electride in alcoholic solvents is achieved by the decomposition of the electride via alcoholysis and the generation of ammonia and Ca(OⁱPr)₂. We found that the choice of solvent was crucial for enhancing the electron transfer efficiency, and a maximum efficiency of 80% was achieved by using a DMF mixed isopropanol co-solvent system. This is the highest value reported to date among single electron transfer agents in the reduction of C-C multiple bonds. The observed reactivity and efficiency establish that electrides with a high density of anionic electrons can readily participate in the reduction of organic functional groups.

Full text of DOI:10.1039/c5sc00933b

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DOI: 10.1039/C5SC00933B
Journal Name
ARTICLE
Two dimensional inorganic electride-promoted electron transfer
†
efficiency in transfer hydrogenation of alkyne and alkenes
Received 00th January 20xx,
Accepted 00th January 20xx
a,b‡
a‡
c
d
a*
Ye Ji Kim,
Sun Min Kim, Eun Jin Cho, Hideo Hosono, Jung Woon Yang, and Sung Wng
a,b*
Kim
DOI: 10.1039/x0xx00000x
A simple and highly efficient transfer hydrogenation of alkynes and alkenes by using a two-dimensional electride,
www.rsc.org/
+

dicalcium nitride ([Ca
2
N] ·e ), as an electron transfer agent is disclosed. Excellent yields in the transformation are
attributed to the remarkable electron transfer efficiency in the electride-mediated reactions. It is clarified that an effective
+

discharge of electrons from the [Ca
2
N] ·e electride in alcoholic solvents is achieved by the decomposition of the electride
i
via alcoholysis and the generating ammonia and Ca(O Pr)
2
. We found that the choice of solvent was crucial for enhancing
the electron transfer efficiency, and a maximum efficiency of 80% was achieved by using a DMF mixed isopropanol co-
solvent system. This is the highest value reported to date among single electron transfer agents in the reduction of C-C
multiple bonds. The observed reactivity and efficiency establish that electrides with a high density of anionic electrons can
readily participate in the reduction of organic functional groups.
the initiation mode: 1) electron transfer, 2) hydride transfer,
6
Introduction
and 3) hydrogen atom transfer. Among these reactions, the
electron transfer method using an electron donating agent is
currently the most classical and general reaction in synthetic
The development of simple and efficient chemical
transformation routes with maximal yields has been
chemistry.
a
For efficient electron transfer hydrogenation, it is essential to
use an agent with a high reduction potential to facilitate the
electron transfer. In this regard, many types of materials, such
as simple metals (Mg and Yb), stabilized alkali metal systems
continuously pursued challenge in synthetic chemistry as such
protocols can provide important benefits in the field of organic
synthesis such as saving starting materials, reagents, and
energy, thereby lowering production cost and environmental
1
(
(
Na in silica-gel and in ammonia liquid), and lanthanide iodides
SmI2 and TmI ), have been employed in electron transfer
impacts. Among fundamental reactions in synthetic organic
chemistry, the reduction of organic functional groups with
carbon-carbon (C-C) multiple bonds is one of the most
universally applied and crucial synthetic processes in academic
2
7
hydrogenation. Despite of their effectiveness in the reported
transfer hydrogenations, there are several drawbacks in the
methodology. Major disadvantages are the toxicity and the
cost of agents and rigorous reaction conditions. Furthermore,
the separation of products from the resultants is laborious and
inefficient, yielding pollutants. Above all these drawbacks, the
low electron transfer efficiency of the reaction is the most
critical issue to be addressed for efficient transfer
hydrogenation. For example, the simple metals show the
limited efficiency of 30%. Although moderate electron transfer
2
and in industrial circles. The reduction of C-C multiple bonds
has been widely used in the synthesis of natural products and
pharmaceutical compounds as well as in the petroleum
3
chemistry. The established methods for this are 1)
hydrogenation, where hydrogen gases are activated on a
transition metal catalysts such as Pt, Ru, Pd, Rh or Ni; and 2)
transfer hydrogenation, where hydrogen from source
4
,5
materials is added to a substrate. Transfer hydrogenation
can be further classified into three kinds of methods based on
efficiency in SmI -mediated reactions has been achieved, the
2
use of toxic and rare lanthanide element, and the requirement
of additives such as amines are drawbacks to be solved.
In order to achieve simple and efficient electron transfer
hydrogenation for C-C multiple bonds, it is necessary to use a
strong electron transfer agent and to simplify the reaction
process, satisfying the following characteristics. First, the
material must have a sufficient reduction potential to
effectively transfer the electrons to the C-C multiple bonds.
Second, the material must consist of abundant elements to
replace the high cost of lanthanides and circumvent the
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+
 a
View Article Online
DOI: 10.1039/C5SC00933B
2
Table 1. Optimization of transfer hydrogenation for alkyne utilizing [Ca N] ·e .
+

b
b
e
#
1
2
3
4
5
6
7
8
9
[Ca
2
N] ·e (equiv.)
Time
6 h
Solvent
Conversion
24%
1a′ : 2a : 2b ratio
70 : trace : 30
78 : 4 :18
Efficiency(%)
2
2
3
4
5
5
5
5
5
5
MeOH
41
71
66
66
66
69
72
59
80
-
2 h
THF:MeOH(1:1)
THF:MeOH(1:1)
THF:MeOH(1:1)
THF:MeOH(1:1)
THF:EtOH(1:1)
THF:iPrOH(1:1)
Toluene:iPrOH(1:1)
DMF:iPrOH(1:1)
DMF:iPrOH(1:1)
40%
2 h
54%
82 : 2 : 16
2 h
70%
88 : trace : 11
97 : trace : 3
85 : trace :14
79 : 9 :12
2.5 h
14 h
24 h
15 h
15 h
15 h
84%
93%
>99%
85%
74 : trace : 26
c
>99%(96%)
N.D
100 : trace : trace
N.D
d
1
0
a
+

b
c
Conditions: Diphenylacethylene (0.5 mmol, 0.125 M), [Ca
2
N] ·e [1 mmol ~ 2.5 mmol (2~5 equiv.)], r.t. Determined by gas chromatography. Isolated
d
4+

+

e
yield. [Ca24Al28
2
O64] ·4e electride was used instead of [Ca N] ·e . The electron transfer efficiency was calculated from the ratio of the participated
electrons in reactions to provided electrons from electride.
further purification process. Third, the reaction process must and Bouveault-Blanc reductions, organic electrides have not
be simple to directly transfer the electrons from source agents been used as functional materials due to their fatal drawback
11
to substrates. Fourth, the reaction must occur under very mild of thermal and chemical instability at room temperature.
conditions without forming side products, which consume the In contrast, the room temperature stable inorganic electride
4+


electrons of the agents in the transferring process. In this [Ca Al O ] ·4e (C12A7:e ) mitigates the drawback of
24
28 64
regard, the use of electrides in alcoholic solvents meets the organic electrides, providing access to a new frontier in
four requirements with respect to their potential for electron synthetic chemistry. Because the inherent electrons in
transfer based on low work functions and mild alcoholysis. electride can be directly participated in the chemical reactions,
Herein, we report a simple and highly efficient transfer the use of electride in synthetic chemical reactions would be
hydrogenation of alkenes and alkynes by using an inorganic favourable in energetically compared to the electrochemical
+

electride [Ca N] ·e , as an efficient electron transfer agent in reactions, which use an additional electrical energy with a low
2
alcoholic solvents. This protocol provided up to 80% of electron transfer rate, and indispensable noble metal
electron transfer efficiency, which is the highest value among electrodes and electrolytes. Moreover, as already
the electron transfer agents in the reductive reactions of C-C demonstrated, this electride-mediated methodology is feasible
multiple bonds. From the reactivity control of electride in for scalable reactions such as pinacol coupling reaction using
+

12
alcoholic solvents, we demonstrated that the choice of solvent [Ca N]
·
e electride.
2

in terms of both acidity and polarity was crucial for maximizing The C12A7:e electride was verified as an effective electron
the electron transfer efficiency. It is also suggested from the donor in chemical reactions such as ammonia synthesis and
working model based on the mechanistic studies through X-ray decomposition, N dissociation, and CO splitting. Recently, a
diffraction and ion chromatography that the alcoholysis played two-dimensional (2D) electride, [Ca N]
a key role in transferring the anionic electrons of electride. electron layers between cationic framework layers ([Ca N] ),
1
3
2
2
+

·
e , which has anionic
2
+
2
1
4
Electrides are ionic crystals in which loosely trapped interstitial was discovered.
This 2D electride also showed very
electrons in crystallographic empty space such as cavities, promising characteristics as a strong electron donor for
8
channels or interlayers behave as anions. The degree of synthetic chemistry in terms of its small work function (2.6 eV)
2
2

3
interaction between trapped anionic electrons, i.e., and high electron concentration (1.37ⅹ10 cm ) similar to
localization or delocalization, largely determines the chemical those of typical alkali metals. Because the opened layer
9
+

and physical properties of the system. The most characteristic structured [Ca N]
·
e electride provides better accessibility to
2
feature of the electrides is their low work function based on anionic electrons when compared to the closed cage
1
0

the loosely trapped anionic electrons. The first crystalline structured C12A7:e
+

a
C12A7:e electride to various organic
electride, Cs (18-crown-6)2·e , was synthesized by using transfer from
solvated electrons as precursors. The solvated electrons in an substrates bearing C-C multiple bonds and demonstrated
8c
alkali metal-amine solution were successfully separated from highly efficient electron transfer hydrogenation of alkynes and
12,15
the cations of alkali metals with the addition of crown ethers alkenes in alcoholic solvents.
and occupied the structural cavities in the grown crystals of
the electride. In synthetic chemistry, while solvated electrons
as a potent reducing agent were applied to the classical Birch
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Journal Name
ARTICLE
Results and discussion
transfer efficiency. Indeed, a transient solvated el
V
e
ie
c
w
tr
A
o
rt
n
icleinOn
the
line
DOI: 10.1039/C5SC00933B
water of the highest polar solvent is well-known phenomenon
in radiation chemistry, implying a possible existence of
transient solvated electrons in DMF:iPrOH solvent. In addition,
no reaction occurred when the C12A7:e electride was
employed under the optimized reaction conditions. This also
+
Table 2. Optimization of transfer hydrogenation for alkene utilizing [Ca
2
N] ·

a
e .
+

supports the gentle alcoholysis of layer structured [Ca N] ·e
2
electride in DMF:iPrOH solvent that allows the efficient
transfer of anionic electrons to substrates. However, we could
not achieve a selective hydrogenation during the reaction due
to the thermodynamic favour. The relative reactivity of
aromatic substrates increases in the following order:
b
#
1
2
3
Time
1 h
1 h
1 h
2 h
Solvent
MeOH
Toluene:MeOH(1:1)
DMF:MeOH(1:1)
DMF:EtOH(1:1)
DMF:iPrOH(1:1)
Conversion
62%
75%
>99%
79%
diphenylacethylene << cis-stilbene
 trans-stilbene, indicating
c
4
5
the difficulty of selective hydrogenation for cis- and trans-
c
14 h
35%
16
stilbene from diphenylacethylene.
a
+

2
Conditions: trans-Stilbene (0.5 mmol, 0.125 M), [Ca N] ·e (1.5 mmol, 3
c +
b

equiv.), r.t., solvent. Determined by gas chromatography. [Ca
remained after reaction.
2
N] ·e was Table 3. Scope of transfer hydrogenation for alkyne and alkene utilizing
+
 a
[Ca N] ·e .
2
Initially, we examined the effects of the solvent as an electride
activator and proton donor, and optimized the reaction
conditions (Table 1). The use of a methanol solvent showed a
low conversion of 1a (Table 1, entry 1). The reaction rapidly
b
#
1
2
3
4
Substrate
, R = C
, R = C
R = 3-OMe-C H R = C H (1c
Product Time Yield
R
1
= C
6
H
5
2
6
H
5
(
H
1a
)
1b
1a
1b
1c
1d
1e
1f
1g
1h








15 h
21 h
21 h
10 h
24 h
12 h
14 h
24 h
42 h
1 h
96%
89%
95%
98%
97%
96%
93%
94%
92%
99%
97%
92%
+

proceeded with the sudden decomposition of the [Ca N] ·e
2
R
1
= 2-Me-C
6
H
4
2
6
5
(
)
)
electride, forming a gel-type suspension and degrading the
homogeneity of the reaction mixture. Since the methanol
solvent reduces the reaction rate and the solubility of the
starting materials, we applied a THF:MeOH co-solvent for a
gentler reaction that can increase the solubility of the starting
material and improve the homogeneity of the reaction mixture,
leading to an increased conversion rate (Table 1, entry 2).
Further optimizations were investigated by increasing the
equivalents of the electride (Table 1, entries 2-5). Whereas the
1
6
4
2
6
5
R
1
= 4-OMe-C
= R = 4-Me-C
= 4-F-C , R = C
= 2,4-F-C , R = C
= 4-F-C = 4-F-C
= 4-Ph-C , R
= C , R , R
= C , R = C
= R = C = C
= 3,5-OMe-C = C
6
H
4
, R
2
= C
6
H
5
(
)
1f
1g
(
)
1d
)
c
5
R
1
2
6
H
4
(
1e
6
7
8
9
R
1
6
H
4
2
6
H
5
(
)
)
1h
R
1
6
H
4
2
6
H
5
(
R
1
6
H
4
, R
2
6
H
4
)
R
1
6
H
4
2
= H (1i
1i
1a
1a



d
d
d
1
1
1
0
1
2
R
3
6
H
5
4
= C
= H, R
, R
, R
2d
, R = 4-Me-C
H (2e
6
H
5
5
= H (2a
)
R
3
6
H
5
4
5
6
H
5
(
2c
, R
2b
)
)
1 h
1 h
R
3
4
6
H
5
5
6
H
5
(
1c

d
d
R
3
6
H
3
4
6
H
5
5
= H
1
1
3
4
2d

1 h
1 h
87%
85%
yield of 1a
′
increased as the equivalents of the electride
(
)
R
3
= 4-Me-C
6
H
4
4
6
H
4
, R
5
=
increased, the electron transfer efficiency was moderate. We
also postulated that both the acidity and the polarity of the
solvent may influence the reactivity of the electride. Indeed, a
1e

)
1
5
R
1
= C10H
20, R
2
= H (1j
)
1j
1j


N.D.
N.D.
N.D.
N.D.
d
1
6
R
3
= C10 20, R
H
4
5
= H, R = H (2f)
significant enhancement in the yield of 1a
switching the solvent from methanol to isopropanol (Table 1,
entries 5-7). The electron transfer efficiency was improved DMF:iPrOH (v/v=1:1). Conditions: Alkene derivatives (0.5 mmol, 0.125
′ was observed by
a
+

Conditions: Alkyne derivatives (0.5 mmol, 0.125 M), [Ca
2
N] ·e (2.5 mmol,
b
c
5
equiv.), r.t. Isolated yield. HMPA:iPrOH (v/v=1:1) was used instead of
d
+

with decreasing acidity of alcohols [pK ; methanol (15.54), M), [Ca
2
N] ·e (1.5 mmol), DMF:MeOH (v/v =1:1), r.t.
a
ethanol (15.9), isopropanol (17.1)], although the reaction
times were prolonged. This phenomenon is ascribed to the
hydrogen evolution consuming electrons by reacting with the
We also examined the electron transfer hydrogenation of
alkenes in this protocol using trans-stilbene 2a as a model
compound. Similar to the results of alkyne, the use of a co-
solvent improved the conversion of 2a (Table 2, entry entries
alcohols.
A relatively acidic alcohol readily reacts with
electrons to liberate hydrogen gas, consuming electrons and
then reducing efficiency. Thus, employing less acidic alcohols
1
-3). In addition, the relationship between the acidity of the
solvents and the electron transfer efficiency was identical
Table 2, entries 3-5), strongly suggesting that the rate of the
such as isopropanol is essential to achieve the high yield of 1a
′
(
owing to the enhanced electron transfer efficiency.
protonation step is an important parameter for enhancing the
electron transfer efficiency. It is notable that the reaction with
Finally, we used a polar solvent as a co-solvent to maximize
the yield of 1a and the electron transfer efficiency (Table 1,
′
DMF:MeOH was completed in one hour, providing
quantitative yield of 1a
a
entry 9). The use of the polar solvent DMF with iPrOH led to
significant enhancement in both the yield (>99%) and the
electron transfer efficiency (~80%). These results suggest that
a more polar environment might enhance the solvation effect
of anionic electrons from the electride in the co-solvent or the
dissociation of ionic bonds between the cationic layers and the
anionic electrons of the electride, leading to the high electron
′
.
With optimized conditions in hand, the substrate scope of
alkynes and alkenes was examined under the optimized
reaction conditions (Table 3). Both electron-rich and electron-
deficient aromatic alkynes were well converted into the
corresponding alkanes with excellent yield (Table 3, entries 1-
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ARTICLE
8
). Terminal alkyne 1i was also a suitable substrate for electron of radical scavengers under optimized conditionsVtioewcAlratircilefyOntlhinee
transfer hydrogenation, showing an excellent yield (Table 3, radical reaction that occurs by donationDoOfI:e1l0e.c1t0r3o9n/Cs5fSrCo0m09t3h3Be
entry 9). Reactions of stilbene derivatives also proceeded well, electride via the single electron transfer (SET) process. The
providing excellent yields of the corresponding alkanes, employment of various radical scavengers, such as TEMPO,
indicating a possibility for much broader application of the 1,4-dinitrobenzene and galvinoxyl, decreased the yields of 1a
′
present protocol (Table 3, entries 10-14).
to a level corresponding to the amount of scavengers (See
Supporting Information). On the basis of these results, we
proposed a plausible mechanism that involves the SET process
(Fig 1). In this process, the use of isopropanol eliminated the
need for an additional hydrogen source.
Fig 2. Comparison of electron transfer efficiency in the reduction of alkyne (red)
+
e electride.
and alkene (blue) using typical electron transfer agent and [Ca
The Na-SG is the sodium metal in silica gel.
2
N]
·
Fig 1. (a) Schematic illustration of electron transfer mechanism, (b) Proposed
mechanism of transfer hydrogenation utilizing electride as electron transfer
agent.
Finally, we compared the electron transfer efficiency of
electride-mediated reactions to the efficiency of reactions
using various electron transfer agents (Fig 2). The present
It was found that no reaction occurred in the use of aliphatic
substrates such as 1-dodecyne 1j and 1-dodecene 2f (Table 3,
entries 15-16). To clarify the different reactivity between
aromatic and aliphatic substrates in thermodynamic
consideration, we measured reduction potentials by using
cyclic voltammetry (CV), in which the potentials of
diphenylacetylene 1a and 1-dodecyne 1j were measured by
employing a glassy carbon electrode, an Ag/AgCl reference,
and a platinum wire auxiliary electrode in 0.5 M sulphuric acid
+

reaction using [Ca N] ·e electrides showed a higher electron
2
transfer efficiency than that of other agents [Yb, Mg, Na-SG,
SmI (H O) ], indicating that this protocol is simple and efficient
2
2
n
for the reduction of C-C multiple bonds. In comparison with
+

other agents, the electron transfer efficiency for the [Ca N] ·e
2
electride was significantly higher than that for Yb, Mg, and Na-
SG, and similar to the costly compound SmI (H O) .
2
2
n
Furthermore, we estimated the electron transfer efficiency of
all electride-mediated organic syntheses (Supporting
information). The present transfer hydrogenation showed
solution. It was revealed that the reduction potential (
1.54 V
vs Ag/AgCl electrode) of 1a was higher than that ( 1.70 V vs

Ag/AgCl electrode) of 1j, indicating that 1a is a better electron
acceptor than 1j. This result implies that the reduction
potential of electrons participating in the reduction of
+

better performance than the other reactions using [Ca N] ·e
2
12
and C12A7:e electrides such as pinacol coupling and
hydrotrifluoromethylations , supporting the simplicity and
effectiveness of the reaction protocol.
14
substrates is ranged between
1.54 and 1.70 V. This
thermodynamic consideration gives an important insight that
an appropriate choice of electron transfer media is a critical
factor to provoke the electride-mediated reactions via the
control of reduction potential since the electrons released
Conclusions
+

We have developed a simple and highly efficient transfer
from [Ca N] ·e electride are transferred to substrates through
2
+

hydrogenation process utilizing [Ca N]
·e
electride as an
2
transfer media.
electron donor in an alcoholic solvent as a hydrogen atom
donor. This methodology provides a new class of materials as a
promising alternative for reducing the processes of C-C
multiple bonds in terms of high electron transfer efficiency
using an electride composed of naturally abundant elements.
Next, we conducted additional experiments to verify the
validity of direct electron transfer via the alcoholysis of
+

[
Ca N] ·e . The decomposition of the electride was confirmed
2
by X-ray diffraction (XRD) measurements and ion-
i
chromatography (IC) that revealed the formation of Ca(O Pr)2
and ammonia, respectively (Supporting Information). The
reaction of diphenylacethylene was conducted in the presence
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Acknowledgements
View Article Online
DOI: 10.1039/C5SC00933B
This work was supported by the IBS-R011-D1, the National
Research Foundation (2013R1A1A1008025 and
012M3A7B4049652) of Korea and the Japan Science and
2
Technology Agency ACCEL program.
Notes and references
1
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A. Molná, A. Sárkány and M. J. Varga, J. Mol. Cat. A, 2001, 173, 185.
M. Hudlicky, Reductions in Organic Chemistry, Wiley-VCH, New York,
2
3
1
984.
4
J. G. de Vries and C. J. Elsevier, The Handbook of Homogenous
Hydrogenation, Wiley-VCH, Weinheim, 1984.
5
6
7
G. Brieger and T. J. Nerstric, Chem. Rev., 1974, 74, 567.
R. A. W. Johnstone and A. H. Wilby, Chem. Rev., 1985, 85, 129.
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