Cobaltaelectro-catalyzed oxidative allene annulation by electro-removable hydrazides

Article abstract of DOI:10.1039/c9cc09076b

An efficient C-H/N-H functionalization with allenes was enabled via versatile electro-oxidative cobalt catalysis. Thus, electrochemical C-H activations were accomplished with high levels of chemoselectivity and regioselectivity in an operationally simple

Full text of DOI:10.1039/c9cc09076b

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Cobaltaelectro-Catalyzed Oxidative Allene Annulation by Electro-
Removable Hydrazides
Received 00th January 20xx,
Accepted 00th January 20xx
Ruhuai Mei,*a,b Xinyue Fang,b Liang He,a Junmei Sun,a Liang Zou,c Wenbo Mab and Lutz
Ackermann*d
DOI: 10.1039/x0xx00000x
An efficient C−H/N−H functionalization with allenes was enabled of external oxidants and prevent the formation of undesired
via versatile electro-oxidative cobalt catalysis. Thus, byproducts.11
electrochemical C−H activations were accomplished with high
Allenes have been widely employed in molecular
levels of chemoselectivity and regioselectivity in an operationally assembling,12 functional material,13 medicines and natural
simple undivided cell setup. The user-friendly nature of this products.14 Recently, significant advance has been made in the
protocol was highlighted by excellent functional group tolerance, field of transition metal catalyzed C−H functionalization with
electro-reductive removable hydrazide directing group and easy allenes exploring, inter alia, iridium,15 rhodium,16 palladium,17
scalability. Experimental mechanistic studies were indicative of a ruthenium,18 manganese,19 cobalt20 and nickel21 catalyst. In
facile BIES C−H cobaltation event.
this regard, the Zhai group very recently disclosed a novel
cobalt-catalyzed trifunctionalization of allenes by a hydrazide
directing group (Figure 1a).20b Thus, various 3-acylquinolines
were accessed step-economically with Ag2CO3 as the chemical
co-oxidant. As a part of our program on sustainable cobalt-
catalyzed C−H activation,8b, 11c, 22 we have now reported here in
a novel electrochemical C−H annulation with allenes (Figure
1b). Notable features of our finding include (a) regioselective
electrocatalytic C−H/N−H annulation with allenes, (b)
inexpensive, Cp*-free cobalt catalyst, (c) operationally
convenient undivided cell setup under mild conditions, (d)
electro-removable hydrazide directing group,11c and (e)
experimental mechanistic studies towards electrocatalytic
allene functionalization.
Inexpensive cobalt-catalyzed oxidative C−H functionalization1
becomes as an increasingly competent tool for the
construction of C−C and C−Het bonds in modern synthetic
organic chemistry. In this context, Daugulis uncovered an
oxidative cobalt-catalyzed C−H/N−H functionalization with
alkynes with the aid of chemical oxidants.2 Thereafter, a
variety of oxidative C−H activations were achieved via cobalt
catalysis.3 Despite significant progress, these transformations
generally required excess of chemical oxidants, such as
silver(I),4 copper(II),5 cerium(IV),6 manganese(II/III),7 and
molecular oxygen,8 among others. Therefore, byproduct
formation was inevitable, which is jeopardizing the sustainable
nature of the C−H activation strategy. By contrast, the
Ackermann group devised the first electrochemical cobalt-
catalyzed C−H activation in 2017.9 Thus, electrons were
directly utilized as redox reagents10 to avoid the consumption
a) Cobalt-catalyze trifunctionalization of allenes with chemical oxidant (Zhai's work)
O
Me
N
Ag2CO3, O2, DMP
O
Me
N
N
2-Py
O
N
H
2-Py
+
R1
Co
R1
H
R2
R2
b) This work: Cobalt catalyzed electrochemical allene annulation
Pt
RVC
O
Me
N
O
Me
N
R2
N
2-Py
N
H
2-Py
+
R1
+
H2
Co
a. College of Pharmacy and Biological Engineering, Chengdu University, Chengdu
610106, P. R. China E-mail: rmei@cdu.edu.cn
R1
H
R3
R2
R3
b. Antibiotics Research and Re-evaluation Key Laboratory of Sichuan Province,
Sichuan Industrial Institute of Antibiotics, Chengdu University, Chengdu 610052,
P. R. China.
 Minimum byproduct
No Cu(II), No Ag(I)
 Electroreductive NN cleavage
 Electrooxidative CH activation
 Electricity as green oxidant
 Allene functionalization

c. Affiliated Hospital & Clinical Medical College of Chengdu University, Chengdu
610081, P. R. China.
Figure 1. Cobaltaelectro-catalyzed C−H/N−H functionalization
with allenes by electro-removable hydrazides
We started to investigate the cobaltaelectro-catalyzed
oxidative C−H/N−H annuation with allenes utilizing hydrazide
1a and diphenyl(propa-1,2-dien-1-yl)phosphine oxide 2a as the
model substrates(Table 1 and Table S1 in the Supporting
d. Institute for Organic and Biomolecular Chemistry, Georg-August-Universität
Göttingen, Tammannstr. 2, 37077 Göttingen, Germany. E-mail:
Lutz.Ackermann@chemie.uni-goettingen.de
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Information).23 To our delight, with an user-friendly undivided
cell setup, the desired regioselective C−H annulation product
3aa was obtained in good yield when MeOH or TFE was used
as solvent (entries 1-2). While other commonly used aprotic
solvents, H2O or HFIP led to no product formation or
O
Me
Pt
RVC
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Py
O
H
Me
N
N
N
Co(OAc)2 (10 mol %)
R DOI: 10.1039/C9CC09076B
N
H
Py
+
O
R
P
NaOAc (2.0 equiv)
TFE, 15 h, 40 °C
Ar, 2.0 mA
Ph
Ph
P
O
Ph
Ph
1
2a
3
O
Me
N
O
Me
N
R
O
Me
N
O
Me
N
R
24
N
Py
O
N
Py
O
N
Py
O
N
Py
O
diminished yields. The yield was further improved when the
R
reaction was performed at 40 °C (entry 3). Evaluation of
alternative additives revealed that NaOAc was slightly better
than NaOPiv and PivOH (entries 4-5). A lower yield was
observed when a higher current of 4 mA was applied (entry 7).
Control experiments demonstrated the key importance of the
carboxylate additive, the cobalt catalyst and the electricity
(entries 6, 8-9).
Ph
P
Ph
P
Ph
P
Ph
P
Ph
Ph
Ph
Ph
R = Me (3sa): 63%
R = OMe(3ta): 52%
R = F (3ua): 75%
R = Br(3va): 77%b
3wa: 41%b
R = H (3aa):
R = Me (3ba):
R = iPr (3ca):
R = Ph (3da):
91%a
67%
89%
89%
71%
65%
69%
79%
93%
62%
95%
67%
CCDC 1963954: 3aa
R = Me(3oa): 80%
R = CF3 (3pa): 66%
R = Ac(3qa): 68%
R = CN (3ra): 71%
R = OMe (3ea):
R = OBn (3fa):
R = F (3ga):
O
Me
N
O
Me
N
Me
O
Me
N
Me
O
O
N
Py
O
N
Py
O
N
Py
O
R = Cl (3ha):
R = I (3ia):
Me
Me
R = SMe (3ja):
R = CF3 (3ka):
R = CN (3la):
Ph
P
Ph
P
Ph
P
Ph
Ph
Ph
Table 1. Optimization of cobaltaelectro-catalyzed C−H
activation with allene 2a
3xa: 91%
3ya: 74%
3za: 67%
R = CO2Me(3ma): 50%
R = NHAc(3na): 82%
a 5 mmol scale: 67%; b 20 mol % Co(OAc)2.
O
Me
N
Scheme 1. Scope with respect to hydrazides 1
Pt
RVC
O
Me
N
N
2-Py
Co(OAc)2 (10 mol %)
N
H
2-Py
+
O
P
additive
solvent, 15 h,
T, Ar, 2.0 mA
Next, the scope for the differently substituted allenes 2 was
investigated for this transformation (Scheme 2). Generally, an
excellent regio- and chemo-selectivity was observed, thus the
annulation exclusively occurred at the terminal position of the
allenes 2. Apart from diphenylphosphine oxide substituted
allene 1a, diethyl phosphonate, arene and ester substituted
allenes 2b-2f also proved to be viable coupling partners,
furnishing the corresponding products in moderate to good
yields. In addition, the challenging cyclonona-1,2-diene 2g also
displayed good reactivity in this transformation, with two
regioisomers observed in a 2.7:1 ratio.23
Finally, the practical use of our electrocatalytic annulation
protocol was further illustrated with its easy scalability. Thus,
product 3aa was prepared in a comparable yield in a gram-
scale reaction with an operationally simple undivided cell
setup (3aa in Scheme 1). In addition, following our previous
reports,11c the N-methyl pyridine motif in annulation products
3 was smoothly removed by cathodic electro-reduction via
SmI2 catalysis in an operationally simple undivided cell
(Scheme 3).24
Ph
O
H
Ph
P
Ph
Ph
1a
2a
3aa
Entry
Solvent
MeOH
TFE
Additive t (℃)
Yield (%)a
81
1
2
3
4
5
6
7
8
9
NaOPiv
NaOPiv
NaOPiv
NaOAc
PivOH
---
NaOAc
NaOAc
NaOAc
23
23
40
40
40
40
40
40
40
84
87
91
81
TFE
TFE
TFE
TFE
TFE
TFE
TFE
48
69b
---c
Traced
aReaction conditions: 1a (0.55 mmol), 2a (0.50 mmol),
Co(OAc)2 (10 mol %), additive (2.0 equiv), solvent (3.5 mL),
undivided cell, CCE at 2.0 mA, 15 h, Pt-plate cathode (1.0 × 1.0
cm), RVC anode (1.0 × 1.5 cm), under Ar. 4.0 mA, 8.0 h.
Without cobalt. d Without electricity.
b
c
Thereafter, to verify the robustness and versatility of the
cobaltaelectro-catalyzed C−H annulation approach, a variety of
substituted hydrazides 1a were tested under the optimal
reaction condition being identified (Scheme 1). Thus, the
benzhydrazides bearing electron-donating (e.g. methyl, i-
propyl, methoxy, benzyloxy, methylthio) or electron-deficient
(e.g. fluoro, chloro, trifluoromethyl, benzyloxy) substituents in
the para position were efficiently transformed within the
allene annulation manifold (3aa-3na). A set of synthetically
useful electrophilic functional groups, such as iodo, bromo,
cyano, ketone, amide and ester, was fully compatible, thus
enabling further diversification of thus-obtained products.
Exclusive site selectivity was observed for the meta-
substituted hydrazides, thus only the less sterically hindered o-
C−H bond was exclusively functionalized (3oa-3ra). Moreover,
the ortho-substituted hydrazides also proved to be amenable
substrates, albeit gave slightly lower yields, compared with
their para- or meta- analogues (3sa-3wa). In addition, arenes
bearing multiple electron-rich substituents also furnished the
corresponding isoquinolin-1(2H)-one derivatives 3xa-3za with
high efficacy.
O
Me
N
Pt
RVC
O
Me
N
Co(OAc)2 (10 mol %)
N
Py
R1
N
H
Py
+
R1
NaOAc (2.0 equiv)
TFE, 15 h, 40 °C
Ar, 2.0 mA
R2
H
R2
1
2
3
O
Me
O
Me
N
O
Me
N
O
Me
N
O
Me
N
N
N
Py
O
N
Py
N
Py
N
Py
N
Py
AcHN
EtO
P
CO2Bn
3ac: 75%
CO2Et
3ad: 96%
CO2Et
Ph
OEt
3ab: 55%
3nd: 92%
3ae: 58%
O
Me
O
Me
O
Me
N
O
Me
N
Py
O
Me
N
N
N
N
Py
N
Py
N
N
Py
N
Py
+
nC9H19
tBu
nBu
CF3
3af: 66%
4ag': 50%
4ag: 19%
3ah: complex mixture
3ai: n.d.
Scheme 2. Scope with respect to allenes 2
Inspired by the outstanding versatility of the cobalt-
catalyzed electrooxidative C−H/N−H activation with allenes, a
2 | J. Name., 2012, 00, 1-3
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set of experiments were performed to rationalize its
mechanism(Scheme S1 in the Supporting Information).23 To
this end, electron-rich hydrazide 1b exhibited better reactivity
in an intermolecular competition experiment, which may
indicated that a base-assisted internal electrophilic type
substitution (BIES) metalation is operative.25 However, this
result can also be rationalized by preferential binding of the
electron-rich ligand to cobalt(III) species. Moreover, electron
withdrawing trifluromethyl substituted allene 2f displayed a
relatively lower reactivity as compared with its analogue 2e.
Interestingly, no H/D scrambling was observed in either the
product 3aa or the re-isolated hydrazide 1a when dueterated
methanol was used as a co-solvent. Furthermore, a minor
kinetic isotope effect (KIE) was obtained by a parallel
experiment, which is indicative of a facile C−H cleavage
process.
View Article Online
DOI: 10.1039/C9CC09076B
a. blank
b. 1a
c. 2a
d. [Co] + NaOAc
e. 1a + [Co] + NaOAc
f. 1a + 2a + [Co] + NaOAc
Figure 2. Cyclic voltammograms of n-Bu4NPF6 (0.1 M) solutions
in MeOH at 0.1 V/s. [Co] = Co(OAc)2.23
O
Me
N
O
Mg
Pt
SmI2 (10 mol %)
N
Py
NH
KI (2.0 equiv)
n-Bu4NPF6 (2.0 equiv)
DMF, 10 h, 23 °C
Ar, 5.0 mA
R1
R2
R1
R2
cathodic reduction
3
4
1a
+
NaOAc
1/2 H2
CoII(OAc)2
H
5
O
O
O
O
Me
N
NH
NH
anodic oxidation
NH
+
N
CoIII
HOAc
anodic oxidation
N
Ph
AcO Ln
6
4ae: 74%
4ag'
4ag
: 23%
: 45%
CCDC 1967120: 4ag'
Scheme 3. Electro-reductive removal of directing group
LnCoI(OAc)
C-H cleavage
O
10
HOAc
O
Me
N
Next, a series of cyclic voltammetric analyses was performed
to investigate the electrochemical C−H activation (Figure 2).
Hydrazide 1a exhibited oxidative peaks at 1.0 VSCE and 1.3 VSCE
with an onset potential at 0.76 VSCE (curve b, Figure 2), while
no obvious oxidation peaks were observed for allene 2a (curve
c, Figure 2). Notably, the onset potential for oxidation of
cobalt(II) precatalyst shifted from 0.77 VSCE (curve d, Figure 2)
to 0.60 V SCE and an obvious catalytic current was detected in
the presence of hydrazide 1a (curve e, Figure 2). These
observations clearly indicated substrate 1a being coordinated
to cobalt(II), which could facilitate the key anodic oxidation
toward cobalt(III) carboxylate complex. No significant
quenching was observed for the introduction of allene 2a into
the mixture (curve f, Figure 2).
Based on the above mentioned mechanistic findings and our
previous reports, a plausible catalytic cycle was proposed as
depicted in Scheme 4. The electrooxidative C−H activation was
initiated via the anodic oxidation, which was followed by a
carboxylated assisted BIES C−H cobaltation to furnish cobalt (III)
complex 7. Then, the regioselective allene insertion and
subsequent reductive elimination delivered cobalt(I) complex
10 and the exo-methylene isoquinolone 9, which could
transformed to the desired product 3 via isomerization. Next,
the catalytically competent cobalt(III) complex 6 was reformed
by the key anode oxidation to close the catalytic cycle. Overall,
the coboltaelectrocatalysis strategy avoided the consumption
of chemical oxidants and molecular hydrogen was released as
the only byproduct.
Me
N
2-Py
reductive elimination
N
N
CoIII
L
N
9
R
L
AcO
O
L
Me
N
insertion
7
OAc
isomerization
N
CoIII
L
N
R
OAc
2
3
R
8
Scheme 4. Plausible catalytic cycle
In conclusion, we have devised a highly atom-economical
electocatalytic C−H annulation with allenes. Thus, C−H/N−H
annulations were achieved with remarkable levels of regio-
and chemo-selectivity via an earth-abundant Cp*-free cobalt
catalyst. The synthetic power of this protocol was further
illustrated by the excellent functional group tolerance, electro-
reductive removable hydrazide directing group, and easy-
scalability. Detailed experimental mechanistic studies revealed
a facile carboxylate-assisted BIES C−H cleavage event.
This work was supported by National Natural Science
Foundation of China (Grant No. 21901023), “Thousand Talents
Program” of Sichuan Province (R. Mei) and the Deutsche
Forschungsgemeinschaft (DFG) (Gottfried-WilhelmLeibniz
award to LA). The program of 2018th Chengdu Key Medical
Subjects (laboratory) and Key Specialty Construction (L. Zou) is
also appreciated.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Conflicts of interest
There are no conflicts to declare.
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