Copper-Catalyzed Difluoromethylation of Aryl Iodides with (Difluoromethyl)zinc Reagent

Article abstract of DOI:10.1021/acs.orglett.6b01733

The combination of difluoroiodomethane and zinc dust or diethylzinc can readily lead to (difluoromethyl)zinc reagents. Therefore, the first copper-catalyzed difluoromethylation of aryl iodides with the zinc reagents is accomplished to afford the difluorom

Full text of DOI:10.1021/acs.orglett.6b01733

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Difluoromethylation of Aryl Iodides with
(Difluoromethyl)zinc Reagent
Hiroki Serizawa, Koki Ishii, Kohsuke Aikawa, and Koichi Mikami*
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology,
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: The combination of difluoroiodomethane and zinc dust or
diethylzinc can readily lead to (difluoromethyl)zinc reagents. Therefore, the
first copper-catalyzed difluoromethylation of aryl iodides with the zinc reagents
is accomplished to afford the difluoromethylated arenes. The reaction proceeds
efficiently through the ligand/activator-free operation without addition of
ligands for copper catalyst (e.g., phen and bpy) and activators for zinc reagent (e.g., KF, CsF, and NaO-t-Bu). Moreover,
transmetalation of the CF2H group from zinc reagent to copper catalyst proceeds even at room temperature to form the cuprate
[Cu(CF2H)2]−.
rganic compounds possessing fluorinated functional
groups are of great interest in the field of pharmaceuticals
Scheme 1. Metal-Mediated and -Catalyzed
Difluoromethylations
O
and agrochemicals.1 In particular, trifluoromethylated arenes
have attracted much attention, and therefore, a variety of
trifluoromethylations to aromatic rings have been actively
explored.2 Interest in a difluoromethyl (−CF2H) group, which
is considered a lipophilic hydrogen-bonding donor with unique
characteristics in pharmaceutical and agrochemical applications,
is rapidly increasing.3 Difluoromethylated arenes can be conven-
tionally synthesized via the deoxofluorination reaction of
aldehydes with harsh reagents such as N,N-diethylaminosulfur
trifluoride (DAST) and its derivatives.4
Recently, direct and regiospecific difluoromethylations of aryl
halides have been desired as practical and reliable synthetic
methodsfordifluoromethylated arenes. However, reports haveso
far been quite limited,5 while synthetic methods for arenes with
functionalized difluoromethyl (−CF2R) groups are quickly
progressing.6,7 In 2012, Hartwig reported the first example of
difluoromethylation of aryl iodides employing an organosilicon
reagent, Me3SiCF2H (5 equiv), and a stoichiometric amount of
CuI along with CsF (3 equiv) at 120 °C and detected the cuprate,
[Cu(CF2H)2]−, in equilibrium with neutral and active CuCF2H
(Scheme 1, eq 1).8 This method is effective in aryl iodides with
electron-donating substituents but ineffective with electron-
withdrawing ones, giving the protonated arenes as the major
product. In contrast, Prakash has reported an alternative
difluoromethylation of aryl iodides with an organotin reagent,
n-Bu3SnCF2H (2−3 equiv) prepared from Me3SiCF3, with a
slight excess (1.3 equiv) of CuI and KF (3 equiv) at 100−120 °C.9
Interestingly, the same neutral and active CuCF2H species,
proposed independently, exhibited contrasting reactivity with
electron-withdrawing substituents to give high yields but with
electron-donating substituents in lower yields. Then Shen
succeeded in the first catalytic difluoromethylation of aryl iodides
and bromides by originally developing the cooperative dual
palladium/silver catalyst system with Me3SiCF2H (2 equiv)
(Scheme 1, eq 2).10 The drawback of this catalytic system is that
(1) (SIPr)AgCl is required as a mediator because the direct
transmetalation of the CF2H group to palladium from the silicon
reagent cannot take place and (2) NaO-t-Bu (2 equiv) possessing
a high nucleophilicity is also needed. Therefore, the development
of practical and reliable catalytic difluoromethylation of aryl
halides is still challenging.
As part of our research project based on (trifluoromethyl)- and
(perfluoroalkyl)zinc as organozinc reagents,11 we focused on the
preparation of (difluoromethyl)zinc reagent and its application to
catalytic aromatic difluoromethylation. Herein, we report the first
copper-catalyzed difluoromethylation of aryl iodides with
(difluoromethyl)zinc reagent through the ligand/activator-free
operation (eq 3).12 While this research was being finalized, Vicic
reported the nickel-catalyzed difluoromethylation of aryl halides
with a similar (difluoromethyl)zinc reagent.13
As described above, our laboratory has previously reported
synthetic protocols involving (trifluoromethyl)- and (perfluor-
oalkyl)zinc with DMPU or diamine as ligands.11,14 In these
Received: June 15, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01733
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Organic Letters
Letter
studies, the synthetic utility of the zinc reagents was improved on
thebasisoforiginalreportsinwhichL2Zn(CF3)BrandL2Zn(RF)2
were prepared from CF3Br/zinc dust15 and RFI/dialkylzinc in
solvent (L),16 respectively. Thus, in the preparation of desirable
(difluoromethyl)zinc reagents, we decided to employ difluor-
oiodomethane (HCF2I) as a difluoromethyl source. As a result, it
was found that the combination of zinc dust and difluoroiodo-
methane in DMF readily led to (DMF)2Zn(CF2H)I (1a) as a
major product, along with (DMF)2Zn(CF2H)2 (1b) as a minor
product (Scheme 2).17
did not observe HF2C−CF2H and HFCCFH formed via
generation of difluoromethyl radical and monofluorocarbene
species, only difluoromethane (CF2H2) via protonation of zinc
reagents byadventitious water inDMF. In sharp contrast, the zinc
reagent 2 in DMPU, which can serve as a stronger coordinating
solvent, was demonstrated to be more stable at 60 °C, and thus,
68% of 2 remained even after 24 h (Table 1c). Based on these
results, we expected that the zinc reagents prepared can be
adequately employed for the difluoromethylation which requires
heating, while the decomposition proceeds during the reaction at
60 °C.
The copper-catalyzed difluoromethylation of aryl iodides was
executed with the (difluoromethyl)zinc reagents (Table 2). A
Scheme 2. Preparation of Mono(difluoromethyl)zinc
Reagent
a
Table 2. Copper-Catalyzed Difluoromethylation of Aryl
a
Iodide
a
Yield was determined by 19F NMR spectroscopy.
Moreover, we gained the best protocols by various
investigations18 to obtain bis(difluoromethyl)zinc reagents with
L2Zn(CF2H)2 (b) as a major product.19 Consequently, treatment
of diethylzinc, difluoroiodomethane, and DMF (2 equiv) in
hexane underwent the reaction to provide (DMF)2Zn(CF2H)I
(1a) and (DMF)2Zn(CF2H)2 (1b) in a 18:82 ratio in 90% yield
(Scheme 3a). Subsequently, the ligand exchange between DMF
X
t
b
c
entry
1
Zn reagent
solvent
DMPU
(mol %) (°C) yield (%)
(DMPU)2Zn(CF2H)2
100
60
75
(2b)
d
2
(DMPU)2Zn(CF2H)2
DMPU
10
60
90 (85)
(2b)
e
3
(DMF)2Zn(CF2H)2 (1b) DMF
(DMF)2Zn(CF2H)l (1a) DMF
10
10
10
60
60
50
57
7
e
4
a
Scheme 3. Preparation of Bis(difluoromethyl)zinc Reagents
5
6
7
(DMPU)2Zn(CF2H)2
DMPU
81
(2b)
(DMPU)2Zn(CF2H)2
DMPU
DMPU
DMPU
DMPU
DMF
10
5
70
60
60
60
60
60
60
60
60
60
86
85
80
0
(2b)
(DMPU)2Zn(CF2H)2
(2b)
f
8
(DMPU)2Zn(CF2H)2
(2b)
2
9
(DMPU)2Zn(CF2H)2
0
(2b)
10
11
12
13
14
15
(DMPU)2Zn(CF2H)2
10
10
10
10
10
10
61
3
(2b)
a
Yield and ratio of a/b were determined by 19F NMR spectroscopy.
(DMPU)2Zn(CF2H)2
NMP
(2b)
and DMPU took place smoothly to produce (DMPU)2Zn-
(CF2H)2 (2b) as a major product (Scheme 3b). We also observed
a Schlenk equilibrium between (DMPU)2Zn(CF2H)2 (2b) and
(DMPU)2Zn(CF2H)I (2a) by addition of 1 equiv of ZnI2 at 60
°C.17
The thermal stability of (difluoromethyl)zinc reagents was
examined in DMF solution (0.1 M) at 60 °C by 19F NMR
spectroscopy, and 1 and 2 were found to almost decompose at 60
°C after 24 h (Table 1aand 1b). In thedecomposition process, we
(DMPU)2Zn(CF2H)2
DMSO
MeCN
THF
5
(2b)
(DMPU)2Zn(CF2H)2
2
(2b)
(DMPU)2Zn(CF2H)2
0
(2b)
(DMPU)2Zn(CF2H)2
toluene
0
(2b)
a
Conditions: 3a (0.3 mmol), zinc reagent (∼0.5 M DMPU solution:
0.6 mmol), CuI (X mol %) in solvent (2 mL). (DMF)2Zn(CF2H)I
b
and L2Zn(CF2H)2 (L = DMF, DMPU) prepared according to
a
methods shown in Schemes 2 and 3 were employed, respectively.
Table 1. Thermal Stability of Zinc Reagents at 60 °C
c
d
Yield was determined by 19F NMR spectroscopy. Gram-scale
e
(c) Zn reagent 2 in
experiment, isolated yield. Zinc reagents (∼0.5 M DMF solution)
were employed. Reaction time was 48 h.
(a) Zn reagent 1 in DMF (b) Zn reagent 2 in DMF
DMPU
f
time
(h)
ZnCF2H (%)
[1a:1b]
time
(h)
ZnCF2H (%)
[2a:2b]
time
(h)
ZnCF2H (%)
[2a:2b]
stoichiometric amount of CuI was first employed for the reaction
of ethyl 2-iodobenzoate (3a) as a model substrate with 2 equiv of
(DMPU)2Zn(CF2H)2 in DMPU at 60 °C for 24 h, providing the
difluoromethyl coupling product (4a) in 75% yield (entry 1).
Significantly, the reduction of the amount of CuI to 10 mol % led
to the higher yieldof product up to90% (entry 2). The gram-scale
reaction also proceeded smoothly. The (DMF)2Zn(CF2H)2
derivative afforded moderate yield, and (DMF)2Zn(CF2H)I
gave only a small amount of the product (entries 3 and 4).
1
6
12:75
12:31
7:5
1
6
12:77
8:40
4:8
1
6
11:81
10:75
7:71
12
24
12
24
12
24
3:<1
1:<1
7:61
a
Conditions: zinc reagents (0.1 mmol, a/b 18%/82%) in DMF or
DMPU (1 mL) at 60 °C for 24 h. Remaining zinc reagents a/b (%)
were determined by 19F NMR analysis using BTF (benzotrifluoride) as
an internal standard.
B
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Letter
Temperatures lower or higher than 60 °C gave slightly lower
yields, respectively (entries 5 and 6). Even a reduction in catalyst
amount up to 5 and 2 mol % also led to satisfactory yields (entries
7and 8). Control experiments clarified the requirement of copper
catalyst (entry 9). Additionally, a dramatic solvent effect was
observed in the present reaction. Except for DMPU and DMF
(entries 2 and 10), the difluoromethylation did not efficiently
proceed in NMP,8 DMSO,7a,13 MeCN,10 THF, and toluene
(entries 11−15). Therefore, the reaction conditions in entry 2
were set to be the best.
Scheme 5. Stoichiometric Experiments in
Difluoromethylation
a
Under the best reaction conditions, the substrate scope in the
reaction was investigated (Scheme 4). Aryl iodides with electron-
a
Scheme 4. Scope of Substrates
a
Conditions: CuI (0.1 mmol), zinc reagent (∼0.5 M DMPU solution:
0.1 mmol), 3a (0.1 mmol) in DMPU (1 mL). (DMPU)2Zn(CF2H)2
prepared according to the method shown in Scheme 3 was employed.
Yield was determined by 19F NMR spectroscopy.
neutral CuCF2H in equilibrium, which has been suggested as a
reactive species,8,9 was not observed due to the instability at room
temperature.17,20 After an additional 1 h at room temperature and
then at 60 °C, the increase of HF2C−CF2H and HCFCFH was
observed at the same time that [Cu(CF2H)2]− decomposed.
Furthermore, it was found that transmetalation of the CF2H
group by (DMPU)2Zn(CF2H)2 (2b) was faster than that by
(DMPU)2Zn(CF2H)I (2a) because 2a (85−90%) remained at
room temperature.
The reaction of aryl iodide 3a with the copper difluoromethyl
species after transmetalation was also investigated (Scheme 5b).
Even at room temperature for 1 h, the peak of [Cu(CF2H)2]−
almost disappeared and the formation of coupling product 4a was
observed in54% yield with complete conversion of (DMPU)2Zn-
(CF2H)2 (2b) to (DMPU)2Zn(CF2H)I (2a). Subsequently, the
reaction slowly ended after 24 h to afford 4a in 98% yield because
of the slower transmetalation of the CF2H group by 2a at room
temperature. Theratio ofstablecomplex[Cu(CF2H)4]−,20 which
is inert species in the reaction, did not change during the course of
reaction.
a
Conditions: 3 (0.3 mmol), zinc reagent (∼0.5 M DMPU solution:
0.6 mmol), CuI (10 mol %) in DMPU (2 mL). (DMPU)2Zn(CF2H)2
prepared according to the method shown in Scheme 3 was employed.
The yield was determined by 19F NMR spectroscopy, and the value in
parentheses is the isolated yield. (DMPU)2Zn(CF2H)2 (4 equiv) was
employed.
b
On the basis of these results by 19F NMR analysis, the
mechanism of the catalytic reaction is visualized in Scheme 6.
withdrawing substituents in the ortho-position of the ring, such as
nitro, cyano, and bromo, afforded the difluoromethylated
products 4b−d in moderate to good yields. The reaction of aryl
iodides bearing electron-withdrawing substituents in the para-
position of the ring also proceeded to give the desired products
4e−g in moderate to good yields. Furthermore, difluoromethy-
lation of heteroaromatics such as isoquinoline, pyrimidine, and
triazine provided high yields of the products (4h−j). The
nucleoside analogue and vinylic substrate also afforded the
products 4k,l in high yields. 1-Iodonaphthalene also led to the
corresponding product (4m) in moderate yield, but unfortu-
nately, aryl iodides with electron-donating substituents such as
tert-butyl and methoxy gave low yields (4n,o).
Scheme 6. Plausible Reaction Mechanism
In order to shed light on the mechanism of the present copper-
catalyzed difluoromethylation, transmetalation of the CF2H
group from the zinc reagent to CuI was monitored by 19F NMR
spectroscopy (Scheme 5a).18 After 15 min at room temperature,
theprogressofthetransmetalation wasobserved togivetwotypes
of (difluoromethyl)copper species, cuprates [Cu(CF2H)2]−
(−114.9 ppm, JF−H = 44.8 Hz)8,17,20 and [Cu(CF2H)4]−
(−116.3 ppm, JF−H = 49.6 Hz),20 in 27% and 1% yields,
respectively, along with HF2C−CF2H and HFCCFH. The
Initially, transmetalation of the CF2H group from (DMPU)2Zn-
(CF2H)2 to CuI triggers the catalytic reaction to produce neutral
CuCF2H and cuprate [Cu(CF2H)2]− in equilibrium that should
shift to the more stable [Cu(CF2H)2]− observed in 19F NMR
analysis. In fact, Hartwig has suggested that cuprate [Cu-
(CF2H)2]− can function as a stable reservoir for the unstable
and active species CuCF2H.8 The reaction of CuCF2H with aryl
C
DOI: 10.1021/acs.orglett.6b01733
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Organic Letters
Letter
(5) The radical difluoromethylation, see: (a) Fujiwara, Y.; Dixon, J. A.;
Rodriguez, R. A.; Baxter, R. D.; Dixon, D. D.; Collins, M. R.; Blackmond,
D. G.; Baran, P. S. J. Am. Chem. Soc. 2012, 134, 1494. (b) Fujiwara, Y.;
Dixon, J. A.; O’Hara, F.; Funder, E. D.; Dixon, D. D.; Rodriguez, R. A.;
iodide 3 bearing an electron-withdrawing substituent leads to
oxidative addition of the C−I bond with formation of the
corresponding copper(III) species. A final reductive elimination
in the copper(III) species closes the catalytic cycle to provide the
difluoromethylated arene 4 and to reproduce CuI. In sharp
contrast, the oxidative addition of aryl iodide with an electron-
donating substituent is relatively slow, and consequently, the
decomposition of CuCF2H mainly occurs to produce HF2C−
CF2H and HFCCFH.
́
Baxter, R. D.; Herle, B.; Sach, N.; Collins, M. R.; Ishihara, Y.; Baran, P. S.
Nature 2012, 492, 95. For Sandmeyer-type difluoromethylation, see:
(c) Matheis, C.; Jouvin, K.; Goossen, L. J. Org. Lett. 2014, 16, 5984. For
the difluoromethylation-containing difluorocarbene pathway, see:
(d) Feng, Z.; Min, Q.-Q.; Zhang, X. Org. Lett. 2016, 18, 44.
(6) For a review, see: Belhomme, M.-C.; Besset, T.; Poisson, T.;
Pannecoucke, X. Chem. - Eur. J. 2015, 21, 12836.
In summary, we have succeeded in the first copper-catalyzed
difluoromethylation of aryl iodides with the organozinc reagent
(DMPU)2Zn(CF2H)2 in DMPU at 60 °C. It was also
demonstrated that transmetalation of the CF2H group from the
zinc reagent to copper catalyst can proceed efficiently even at
room temperature, generating not neutral CuCF2H but cuprate
[Cu(CF2H)2]−. Moreover, aryl iodides bearing electron-with-
drawing substituents underwent the reaction to provide the
difluoromethylated products in moderate to high yields. The
reaction proceeded without use of any ligands for copper catalyst
(e.g., 1,10-phenanthoroline and 2,2′-bipyridine derivatives) and
activators for zinc reagent (e.g., KF, CsF, and NaO-t-Bu).
Development of Pd-catalyzed difluoromethylation of aryl halides
bearing not only electron-withdrawing substituents but also
electron-donating substituents is the topic of our following paper
(DOI: 10.1021/acs.orglett.6b01734).
(7) For selected examples of the direct introduction of functionalized
difluoromethyl (−CF2R) groups to arenes: (a) Fujikawa, K.; Fujioka, Y.;
Kobayashi, A.; Amii, H. Org. Lett. 2011, 13, 5560. (b) Ohtsuka, Y.;
Yamakawa, T. Tetrahedron 2011, 67, 2323. (c) Min, Q.-Q.; Yin, Z.; Feng,
Z.; Guo, W.-H.; Zhang, X. J. Am. Chem. Soc. 2014, 136, 1230. (d) Ge, S.;
Chaładaj, W.; Hartwig, J. F. J. Am. Chem. Soc. 2014, 136, 4149. (e) Xiao,
Y.-L.; Guo, W.-H.; He, G.-Z.; Pan, Q.; Zhang, X. Angew. Chem., Int. Ed.
2014, 53, 9909. (f) Arlow, S. I.; Hartwig, J. F. Angew. Chem., Int. Ed. 2016,
55, 4567. (g) Xiao, Y.-L.; Min, Q.-Q.; Xu, C.; Wang, R.-W.; Zhang, X.
Angew. Chem., Int. Ed. 2016, 55, 5837.
(8) (a) Fier, P. S.; Hartwig, J. F. J. Am. Chem. Soc. 2012, 134, 5524. Qing
has succeeded in improving Hartwig’s reaction conditions; see: (b) Jiang,
X.-L.; Chen, Z.-H.; Xu, X.-H.; Qing, F.-L. Org. Chem. Front. 2014, 1, 774.
(9) Prakash, G. K. S.; Ganesh, S. K.; Jones, J.-P.; Kulkarni, A.; Masood,
K.; Swabeck, J. K.; Olah, G. A. Angew. Chem., Int. Ed. 2012, 51, 12090.
(10) (a) Gu, Y.; Leng, X.-B.; Shen, Q. Nat. Commun. 2014, 5, 5405.
(b) Chang, D.; Gu, Y.; Shen, Q. Chem. - Eur. J. 2015, 21, 6074.
(11) (a) Mikami, K.; Nakamura, Y.; Aikawa, K. Japanese Patent
Application 2012-113898, May 18, 2012. (b) Mikami, K.; Nakamura, Y.;
Negishi, K.; Aikawa, K. Japanese Patent Application 2013-180007, Aug
30, 2013. (c) Nakamura, Y.; Fujiu, M.; Murase, T.; Itoh, Y.; Serizawa, H.;
Aikawa, K.; Mikami, K. Beilstein J. Org. Chem. 2013, 9, 2404. (d) Aikawa,
K.; Nakamura, Y.; Yokota, Y.; Toya, W.; Mikami, K. Chem. - Eur. J. 2015,
21, 96. (e) Aikawa, K.; Toya, W.; Nakamura, Y.; Mikami, K. Org. Lett.
2015, 17, 4996.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.6b01733.
Experimental procedures and compound characterization
data (PDF)
(12) (a) Serizawa, H. Doctor Thesis, Tokyo Institute of Technology,
Dec 21, 2015. (b) Mikami, K. Presented at Pacifichem 2015, Honolulu,
HI, Dec 15−20, 2015.
AUTHOR INFORMATION
Corresponding Author
■
(13) Xu, L.; Vicic, D. A. J. Am. Chem. Soc. 2016, 138, 2536.
(14) Recent examples of perfluoroalkylations using (perfluoroalkyl)
zinc reagents: (a) Kaplan, P. T.; Xu, L.; Chen, B.; McGarry, K. R.; Yu, S.;
Wang, H.; Vicic, D. A. Organometallics 2013, 32, 7552. (b) Kaplan, P. T.;
Chen, B.; Vicic, D. A. J. Fluorine Chem. 2014, 168, 158. (c) Kato, H.;
Hirano, K.;Kurauchi, D.;Toriumi, N.;Uchiyama, M. Chem. -Eur. J. 2015,
21, 3895. (d) Wang, X.; Hirano, K.; Kurauchi, D.; Kato, H.; Toriumi, N.;
Takita, R.; Uchiyama, M. Chem. - Eur. J. 2015, 21, 10993.
(15) Previous reports on the stable solid reagent mono(trifluor-
omethyl)zinc prepared from CF3Br and zinc dust: (a) Naumann, D.;
Tyrra, W.; Kock, B.; Rudolph, W.; Wilkes, B. J. Fluorine Chem. 1994, 67,
91. (b) Tyrra, W.; Naumann, D.; Pasenok, S. V.; Yagupolskii, Y. L. J.
Fluorine Chem. 1995, 70, 181. (c) Kremlev, M. M.; Tyrra, W.; Mushta, A.
I.; Naumann, D.; Yagupolskii, Y. L. J. Fluorine Chem. 2010, 131, 212.
(16) Previous reports on bis(trifluoromethyl)zinc and bis(perfuor-
oalkyl)zinc prepared from RFI and dialkylzinc: (a) Lange, H.; Naumann,
D. J. FluorineChem. 1984, 26, 435.(b)Schorn, C.;Naumann, D.;Scherer,
H.; Hahn, J. J. Fluorine Chem. 2001, 107, 159.
(17) The preparation of (difluromethyl)zinc has been reported by
Burton, viathereaction ofzincdustwithbromodifluoromethaneinDMF
to afford Zn(CF2H)Br and Zn(CF2H)2 in an 88:12 ratio in 73−85%
yield: Burton, D. J.; Hartgraves, G. A. J. Fluorine Chem. 2007, 128, 1198.
(18) See the Supporting Information.
(19) By treatment of HCF2I and diethylzinc in a 1:1 ratio, the formation
of both Zn(CF2H)Et and Zn(CF2H)2 was confirmed by Charette:
Beaulieu, L.-P. B.; Schneider, J. K.; Charette, A. B. J. Am. Chem. Soc. 2013,
135, 7819.
*E-mail: mikami.k.ab@m.titech.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Thisresearchwassupported byJST(ACT-C:AdvancedCatalytic
Transformation program for Carbon utilization) and JSPS
KAKENHI Grant No. 26620078.
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DOI: 10.1021/acs.orglett.6b01733
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