Alkyl Carbazates for Electrochemical Deoxygenative Functionalization of Heteroarenes

Article abstract of DOI:10.1002/anie.202001571

The C?O bond cleavage for activation of alcohols is synthetically useful and practically challenging. This work describes carbazate as a new type of electrochemically activated alkylating agent derived from ubiquitous alcohols for direct functionalization of heteroarenes under mild electrolytic conditions. The simple undivided cell at low oxidative potentials with carbon/platinum electrode set-ups offers excellent substrate tolerance, affording a variety of primary, secondary and tertiary alkyl-decorated heterocycles in good chemical yields. Furthermore, the mechanism for this electrochemical deoxyalkylation reaction has been investigated.

Full text of DOI:10.1002/anie.202001571

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Accepted Article
Title: Alkyl Carbazates for Electrochemical Deoxygenative
Functionalization of Heteroarenes
Authors: Yongyuan Gao, Zhengguang Wu, Lei Yu, Yi Wang, and Yi
Pan
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001571
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10.1002/anie.202001571
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COMMUNICATION
Alkyl Carbazates for Electrochemical Deoxygenative Functionali-
zation of Heteroarenes
Yongyuan Gao, Zhengguang Wu, Lei Yu, Yi Wang*, and Yi Pan
Dedicated to the 100th anniversary of the School of Chemistry and Chemical Engineering, Nanjing University
Abstract: The C-O bond cleavage for activation of alcohols is
Decarboxylative fragmentation of carbazates
synthetically useful and practically challenging. This work describes
carbazate as a new type of electrochemically activated alkylating
agent derived from ubiquitous alcohols for direct functionalization of
heteroarenes under mild electrolytic conditions. The simple
undivided cell at low oxidative potentials with carbon/platinum
electrode set-ups offers excellent substrate tolerance, affording a
variety of primary, secondary and tertiary alkyl-decorated
heterocycles in good chemical yields. Furthermore, the mechanism
for this electrochemical deoxyalkylation reaction has been
investigated.
R1
O
Ph
O
R1
R2
R2
R3
Ph
Me
HO
O
Me
O
R3
O2
Acyl radical
Alkene Functionalization
Denitrogentive
acyl radical formation
Fe(III), O2
[Taniguchi]
-N2
O
R1
R1
R2
O
PhOCCl
R2
R3
OH
R3
O
NHNH2
Radical based concise synthesis of C-C bonds has emerged as
N2H4ꞏH2O
Alkyl carbazate
a
powerful transformation to rapidly increase molecular
1° 2° 3° alcohols as
complexity for the utility of biomedicine and materials science.1
In recent years, a myriad of alkyl radical precursors have been
developed, such as pyridinium salts,2 Hantzsch esters,3
haloalkanes,4 organoborons5 and sulfides.6 Along this line, the
effort in search of readily accessible radical precursors is still
highly appreciated. Alcohols are among the most available
feedstock in food and chemical industry. The delivery of alkyl
radicals through dehydroxylation of alcohols under mild
conditions is strategically appealing.7 However, due to the strong
C-O bonding energy (BDE∼95 kcal/mol) and high redox
potentials of unactivated alcohols, radical cleavage of hydroxyl
functionality requires harsh conditions and represent
alkyl radical precursors
-N2
-CO
2
No costly metal catalyst
No chemical oxidant
Release N2, CO2 and H2
[this work]
R1
R1
R2
R3
HET
R2
R3
Heteroarene Functionalization
Alkyl radical
Figure 1. Fragmentation of carbazates for radical formations
trol of radical reactivity and compatibility. However, no
electrochemical deoxygenative functionalization have been
reported up-to-date, mainly since alkyl radicals are prone to
over-oxidize to their carbocations at high potentials.5f,9,21
Whereas the documented oxalate salts are not compatible under
cell conditions, it is important to design a radical precursor
derived from alcohol that can be readily initiated at lower
oxidative potentials. In light of previous reports,20g,20h,22 we
speculate that by installing a hydrazine leaving group to the
carboxylic ester of alcohols, improved solubility and reactivity
tremendous difficulties8-9
. In the classic Barton-McCombie
framework, tertiary alcohols undergo radical deoxygenation via
the activation of thiocarbonyl moiety. However, inevitable high
temperatures and toxic tin reagents limited the application.10 In
this decade, alkyl oxalates have been developed as an
alternative for Barton deoxygenation. Overman and MacMillan
have reported the photocatalyzed deoxygenative alkylation of
conjugated alkenes.11-12. Gong has described an alkylation of
unactivated olefins using dialkyl oxalates and zinc as reducing
agent.13 Nevertheless, primary and secondary alcohols are
generally inapplicable to such fragmentation process and often
require metal catalysts or chemical oxidants.11-15 Therefore, a
more compatible and sustainable protocol for deoxygenative
alkylation represents unmet challenge and urgent demand.
could
be
realized
through
the
employment
of
hydrazinecarboxylates (carbazates) (Figure 1). Alkyl carbazates
have previously been recognized as a type of acyl radical
precursor. Under iron-catalyzed oxidative conditions, carbazates
undergo dehydrazinative acylation and release molecular
nitrogen.23 It is envisioned that under suitable cell conditions,
further anodic oxidative decarboxylation could occur to furnish
Electrochemically initiated radical reaction has drawn much
attention for the redox-efficiency, innate scalability and
sustainability of electrolytic process.16 Anodic oxidation methods
have been established for C-C,17 C-O,18 C-N19 and C-S20 bond
formations and display pronounced advances in the kinetic con-
alkyl radical. Herein, we describe carbazate as
electrochemically activated alkylating agent for direct
functionalization of heteroarenes with an oxidative
dehydrazination/decarboxylation sequence.
a new
We first employed quinoxalinone and t-butyl carbazate as model
substrates for the initial electrolytic examination. Using
platinum/graphite electrodes and n-BuNClO4 electrolyte, the
desired deoxygenative alkylation could take place at a constant
Yongyuan Gao, Zhengguang Wu, Lei Yu, Prof. Yi Wang* and Prof.
Yi Pan
State Key Laboratory of Coordination Chemistry, Jiangsu Key
Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023, China
E-mail: yiwang@nju.edu.cn.
current
of
6
mA
in
acetonitrile.
The
t-butyl
substituted quinoxalinone was afforded in 78% yield after 8 h
(Table 1, entry 1). Next, several catalysts including CpFe, NH4Br,
TBAB and NH4I were screened and only decreased yields were
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202001571
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COMMUNICATION
Altering the ratio of DMSO/MeCN to 3:1 afforded the product in
a satisfactory 94% yield (entry 16). Thus, the optimized
conditions were determined for the following investigations.
To explore the scope of this deoxygenative alkylation reaction, a
range of primary, secondary and tertiary alcohol-derived
carbazates have been treated with the electrochemical
conditions (Scheme 1). Tertiary alkylated carbazates bearing
branched (2-6) and cyclic hydrocarbons (7-8) were tolerated in
this process. In addition, linear and cyclic secondary alcohol-
derived carbazates were also applied to the reaction (9-13).
Tetrahydrofuran (14), pyrrolidine (15), indane (16), and
difluorocyclohexane (17) substituted carbazates could furnish
the corresponding alkylated quinoxalinones in good yields.
Restrained natural product borneol was also compatible under
the oxidative radical coupling conditions (18). Moderate
conversions were observed for primary alcohcol-derived
carbazates such as benzyl alcohol (19), thiophenylmethanol (20),
naphthalenylmethanol (21) and 2-phenylethanol (22). This could
due to the relative inertness of primary acyl and benzyl radicals.
entry
1
electrode
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
Pt/Pt
C/C
catalyst
solvent
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeOH
THF
electrolyte
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NPF6
LiClO4
yielda
78
62
41
43
38
58
52
66
72
82
72
67
43
28
88
94b
84
76
-
2
CpFe
3
NH4Br
4
TBAB
5
NH4I
6
-
-
-
-
-
-
-
-
-
-
-
-
-
7
8
C/ Ni
C/Fe
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
9
10
11
12
13
14
15c
16d
17e
18f
nBu4NPF6
nBu4NPF6
nBu4NPF6
nBu4NPF6
nBu4NPF6
nBu4NPF6
nBu4NPF6
DMSO
DMSO
DMSO
DMSO
Next, the reaction scope of heteroarenes was explored (Scheme
2). The reactions of electron-donating (23, 29) and electron-
withdrawing
group
(24-28)
substituted
quinoxalinones
Table 1. Optimization of the reaction conditions. [a] Yields determined by 1H
NMR spectroscopy using 1,3,5- trimethoxybenzene as internal standard; [b] 87%
isolated yield; [c] 1:1 DMSO/MeCN; [d] 3:1 DMSO/MeCN; [e] 3 mA for 16 h. [f]
12 mA for 4 h.
proceeded smoothly to furnish the corresponding alkylated
products. Different N-protected quinoxalinones were also
tolerated under the cell conditions (31-36). Furthermore, a
variety of heterocycles including benzoquinoxalinone (37),
pyrazinone (38), quinazolinone (39), isoquinoline (40),
phthalazine (41), quinazoline (42), and phenanthridine (43) were
susceptible to the reaction conditions to acheive the
corresponding alkylated products. Benzofuran (44) and
benzothiophene (45) were not compatible for the reaction.
Notably, bioactive caffeine (46) and prothioconazole (Provost,
47) are also compatible with this reaction.
obtained (entries 2-5). For the screening of electrode materials,
platinum/graphite electrodes were found superior to iron and
nickel (entries 6-9). Using different electrolytes for the undivided
cell, n-Bu4NBF4 was able to afford the product in a higher yield
(82%, entry 10). The choice of solvent was also proven crucial
for this reaction. With a 1:1 mixutre of DMSO and MeCN, the
reaction yield was significantly enhanced (88%, entry 15).
H
N
H
N
O
6 mA (2.1 mA/cm2)
O
H
O
R
O
H2N
R
+
N
O
nBu4NPF6, DMSO/MeCN (0.1 M)
undivided cell, 50 °C, Ar, 8 h
H 1
0.6 mmol
N
N
l
Me
Me
Me
Me
nBu
Et
Me
Ph
Me Me
Et
Me
Me Me
Et
64%
68%
77%
87%
72%
82%
3
4
5
6
7
8
ohol
nBu
Et
O
NBoc
iPr
58%a
86%a
91% a
56% a
73%a
83%a
10
11
12
13
14
15
Primary alcohol
H
S
F
F
Me
Me
Me
(from Borneol)
71%a (dr 1:1.3)
18
51%a
17
45%b
19
57%b
20
51%b
21
42%b
22
Scheme 1. Substrate scope of carbazates. a Reactions were performed at 80 °C for 8 h. b Reactions were performed at 85 °C for 14h.
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10.1002/anie.202001571
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COMMUNICATION
O
6 mA (2.1 mA/cm2)
H
R
H2N
R
Het
N
H
O
Het
+
nBu4NPF6, DMSO/MeCN (0.1 M)
undivided cell, 50 °C, Ar,8 h
1
0.2 mmol
0.6 mmol
H
N
H
N
H
N
H
N
H
N
O
O
O
Me
Me
O
R
O
F
81% 24
R = Cl 76% 25
Br 68% 26
Cl
N
86%
Br
N
62%
MeO
N
N
R
N
74%
29
88%
23
27
28
COOEt
O
CN
Ph
Me
N
H
N
Br
O
O
N
N
O
N
N
N
O
N
O
N
N
N
N
61%
30
71%
31
68%
32
45%
33
57%
34
62%
35
O
H
N
H
N
O
O
Ph
Ph
O
NH
N
N
O
N
N
N
N
N
N
53% a
39
42%a
40
72%
36
87%
37
71%
38
28% a
41
O
Cl
Me
N
S
OH
Me
O
O
S
N
N
N
N
N
Cl
N
HN
N
N
Ph
Me
68%a
42
(from Provost)
62% a (24h)
43
trace
44
trace
45
(from caffeine)
62%a
47
31% a
46
Scheme 2. Substrate scope of heteroarene. a The reactions were performed in MeCN (4 mL) with n-Bu4NClO4 (0.4 mmol) at a constant current (3 mA) and 80 °C
for 14 h.
C.
A.
Me
Me
tBu
N
H2NNHCO2tBu
Me
O
N
O
standard conditions
MeCN, 80 °C
Me 31%
H
N
B.
D.
H
N
O
O
O
H2NNHCO2Me
OMe
N
standard conditions
N
55%
O
+
O
O
O
- H+
NH
anode:
RO
NHNH2
anodic oxidation
anodic oxidation
NNH2
RO
NHNH2
- CO2
RO
RO
N
1
- H+
C
B
A
O
O
- N2
N
cathode:
2H
cathodic reduction
anodic oxidation
H2
R
OR
N
OR
- H+
F
E
D
Scheme 3. Control experiments and reaction mechanism. A) Cyclic voltammetry of related compounds in 0.1 M nBu4NPF6/MeCN using glass carbon working
electrode, Pt wire, and Ag/AgNO3 (0.1 M in MeCN) as counter and reference electrode: 2-quinoxalinone (5 mM, green); t-butyl carbazate (5 mM, red); 2-
quinoxalinone and t-butyl carbazate (5 mM, blue).
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To elucidate the reaction mechanism, control reactions have
been carried out (Scheme 3). The radical relay reaction of N-
phenylmethacrylamide proceeded to deliver the alkylated
oxindole in 31% yield (Scheme 3A). When subjecting methyl
carbazate to the standard conditions, no alkylation was
observed. Instead, the isolated product was the corresponding
methyl ester (Scheme 3B). This indicates that a stepwise
dehydrazination-decarboxylation sequence was involved in the
electrochemical fragmentation of carbazate. Furthermore, the
cyclic voltammetry results demonstrated that carbazate can be
easily oxidized under electrochemical conditions (E1/2ox = 1.44 V
vs AgNO3) for deoxyalkylation of quinoxalinone (Scheme 3C).
Based on the above experimental facts, a plausible reaction
mechanism is proposed. The first stage is consecutive anodic
oxidation of carbazate and deprotonation to generate
hydrazinecarboxylate radical B and diazenecarboxylate C.24
Further anodic oxidation cleaves diazene to form acyl radical E
and releases molecular nitrogen. The second step is
decarboxylation of acyl radical E to furnish alkyl radical F
(Scheme 3D).
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under electrochemical conditions. Primary, secondary and
tertiary alkyl radicals can be readily accessed via the sequential
anodic oxidative fragmentation for the direct functionalization of
N-heteroarenes. This sustainable method avoids the use of
chemical oxidants and only releases nitrogen, carbon dioxide
and hydrogen. Further study of carbazate is underway in the
laboratory.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 21772085 and 21971107). We also
thank Collaborative Innovation Centre of Advanced
Microstructures and Collaborative Innovation Center of Solid-
State Lighting and Energy-Saving Electronics of Nanjing
University.
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Keywords: carbazate
•
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•
deoxgenative
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10.1002/anie.202001571
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
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Yongyuan Gao, Zhengguang Wu, Lei
Yu, Yi Wang,* and Yi Pan
Page No. – Page No.
Alkyl Carbazates for Electrochemical
Deoxygenative Functionalization of
Heteroarenes
Carbazates as a new type of electrochemically activated alkylating agent for direct
functionalization of heteroarenes under mild cell conditions.
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