Electrochemical halogenation/semi-pinacol rearrangement of allylic alcohols using inorganic halide salt: An eco-friendly route to the synthesis of β-halocarbonyls

Article abstract of DOI:10.1039/c9gc01152h

An efficient and eco-friendly electrochemical method involving halogenation/semi-pinacol rearrangement of allylic alcohols using inorganic halide salt as the halogen source to synthesize various β-halocarbonyls bearing an all-carbon α-quaternary center under mild reaction conditions has been developed (X = Br, Cl). Stoichiometric oxidants, metal catalysts, and even external electrolytes were avoided in this method. It was found that the whole reaction system was compatible with various common interfering ions in water and exhibited good selectivity for the bromide ion. Furthermore, the halogen resource can be efficiently extracted from simulated wastewaters to construct key intermediates for the syntheses of natural products (±)-galanthamine and (±)-crinamine. Additionally, this strategy may be promising to treat oil and gas wastewaters and desalt seawater.

Full text of DOI:10.1039/c9gc01152h

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DOI: 10.1039/C9GC01152H
Journal Name
COMMUNICATION
Electrochemical Halogenation/Semi-pinacol Rearrangement of
Allylic Alcohols Using Inorganic Halide Salt: An Eco-friendly Route
to Synthesis of β-halocarbonyls
Received 00th January 20xx,
Accepted 00th January 20xx
Chao Chen, Jun-Chen Kang, Chen Mao, Jia-Wei Dong, Yu-Yang Xie, Tong-Mei Ding, Yong-Qiang Tu,
Zhi-Min Chen* and Shu-Yu Zhang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient and eco-friendly electrochemical method involving
halogenation/semi-pinacol rearrangement of allylic alcohols using
inorganic halide salt as the halogen source to synthesize various β-
halocarbonyls bearing an all-carbon α-quaternary center under
mild reaction conditions has been developed (X = Br, Cl).
Stoichiometric oxidants, metal catalysts, and even external
electrolytes were avoided in this method. It was found that the
whole reaction system was compatible with various common
R1
R2
HO
R1
C (+) Pt (-)

O
simulated wastewaters
(mixture of Br-, Cl-, etc.)
(X= Br, Cl)
X
R2
Electrochemical + brine
VS Classic halogen sources
 High cost and pollution
Green and efficiency
High chemical selective
 potential to treat oil and gas
wastewaters and desalt seawater
A) NCS ($25.4/100 g)
B) NBS ($31.2/ 100 g)
C) Br2 ($73.7/100 g, danger and toxic)
D) PhI(OAc)2($146/100 g)/NaX:
interfering ions in water and exhibited good selectivity for bromide Application to the synthesis of natural product
ion. Furthermore, halogen resource can be efficiently extracted
from simulated wastewaters to construct key intermediates for the
syntheses of natural products (±)-galanthamine and (±)-crinamine.
Additionally, this strategy may be promising to treat oil and gas
wastewaters and desalt seawater.
Oil and gas wastewaters containing high concentrations of bromide
Scheme 1 Approaches to β-halocarbonyls via electrochemical semi-
do not easily adsorb on sediments and could enter surface water
through a long-distance migration.1 Therefore, more and more
wastewaters around the world are becoming an urgent and severe
challenge to ecological environment and human health. Many
attentions have been paid to scientific research to treat the
wastewaters avoiding environmental pollution.2 The simplest
method is direct discharge of brine, however, it not only contributes
to serious threats to ecological environment, but also is enormous
waste of halogen resources.3 On the other hand, organohalo
compounds are particularly important and useful due to they are not
only widely present in bioactive natural products especially marine
natural products, but also reveal extensive synthetic utility.4
Accordingly, a number of efforts to develop the synthetic strategies
for the formation of organohalo compounds have been made.5
Among them, electrophilic halogenated reagents such as N-
chlorosuccinamide (NCS), N-bromosuccinamide (NBS), and bromine
pinacol rearrangement of allylic alcohols.
(Br2) have most frequently emerged as halogen sources and the
combination of halogen anion and stoichiometric strong oxidant
were also developed recently. For example, our group has been
focusing on halogenation/semi-pinacol rearrangement to synthesize
β-halocarbonyls bearing a quaternary center which broadly exist in
bioactive and pharmacological molecules, with different electrophilic
halogenated reagents (Scheme 1).6 Recently, a halogen-salt-involved
semi-pinacol rearrangement using stoichiometric PhI(OAc)2 as the
oxidant was also developed (Scheme 1).7 However, these two
strategies are not ideal from the perspective of the cost and
environmental protection. However, these two strategies are not
ideal from the perspective of the cost and environmental protection.
Considering the above two issues together, the development of a
protocol which could recycle halogenion from wastewaters to
environmentally friendly synthesize universal organohalo
compounds is in high demand.
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs & School of
Chemistry and Chemical Engineering, Key Laboratory for Thin Film and
Microfabrication of Ministry of Education, Shanghai Jiao Tong University, Shanghai
200240, P. R. China
E-mail: chenzhimin221@sjtu.edu.cn, zhangsy16@sjtu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Electrochemical method has attracted significant attentions
in organic synthesis due to environmentally friendly feature.
Stoichiometric chemical oxidants are avoided in the
electrochemical reaction while substrates are oxidized
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selectively at the anode in different potentials.8 Numerous yield (entries 3-4). Trace amount of desired product was
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transition-metal-catalyzed
electrochemical
C-H detected when acetonitrile was useDdOIa: 1s0.1s0o3l9v/eCn9tGC0a1lo15n2eH,
functionalization reactions have been reported by Ackermann, indicating that polar protic solvents may be crucial proton
Mei, and Lei etc., meanwhile radical transformations of alkenes transfer medium in this electrochemical reaction (entry 5).
initiated by electrocatalysis get great progresses, which are Subsequently, we tested other bromine salts as halogen source
accomplished by Lin, Lei, Zeng, Xu, Stahl, Baran, and other in the reaction but no better result was obtained (entries 6-8).
groups.9,10 Indeed, electrochemical reactions involving halogen Finally, control experiment indicates that electricity is
salt have been preliminarily investigated for the facile indispensable in the reaction (entry 9). Despite that a slightly
transformation of halogens to other function groups under higher yield was obtained when MeCN/MeOH were used as the
economic and eco-friendly reaction conditions. For example, Lin solvent, MeCN/H2O were identified as the promising solvent
group developed some Mn(II)-catalyzed electrochemical when economy and environmental friendliness were taken into
halogen-salt-involved alkene difunctionalizations in recent accounts.
years,11 while halogen salt can also be used as the catalyst in the
electrochemical reactions.12 Based on these advances and Table 1 Optimization.
combined our research interest, we are desired to realize
practical and economic electrochemical halogenation/semi-
pinacol rearrangement of allylic alcohols directly using inorganic
C (+)  Pt (-)
constant current (25 mA) Ph
HO
Ph
bromine salts
air, r.t, 1 h
undivided cell
O
Br
halide salts as the halogen source (Scheme 1b). To verify the
feasibility of the above idea, we first carried out cyclic
voltammetry studies, allylic alcohol 1a shows an oxidation peak
at Ep/2 = 1.59 V vs. SCE compared to MgBr2·6H2O Ep/2 = 0.95 V vs.
SCE or MgCl2 Ep/2 = 1.40 V vs. SCE, indicating that 1a is workable
to investigate a halogenation/semipinacol rearrangement
reaction (for the details, please see SI). Next, we roughly tested
the process for 1a, which involved constant-current electrolysis
using a carbon felt as the anode and a Pt plate as the cathode,
in an undivided cell containing MgBr2·6H2O in water at room
temperature. To our delight, the desired product 2a was
obtained in 59% yield (Figure 1). Encouraged by the preliminary
success, we further studied the rearrangement reaction in
detail. Herein, we are pleased to report electrochemical
bromination and chlorination/semi-pinacol rearrangement of
allylic alcohols with moderate to good yields. Furthermore, the
reaction also proceeded smoothly no matter using simulated
wastewaters or bittern, producing the desired β-halocarbonyls
in moderate to excellent yields.
2a
1a
entry
reaction conditionsa
MgBr2·6H2O, H2O
MgBr2·6H2O, MeCN/H2O (2:1)
MgBr2·6H2O, MeCN/MeOH (2:1)
MgBr2·6H2O, MeCN/AcOH (2:1)
MgBr2·6H2O, MeCN
NaBr, MeCN/MeOH (2:1)
LiBr, MeCN/MeOH (2:1)
MnBr2, MeCN/MeOH (2:1)
MgBr2·6H2O, MeCN/H2O (2:1), no
current
yieldb
59%
79%c
81%d
77%
<5%
34%
28%
78%
<5%
1
2
3
4
5
6
7
8
9
a1 (0.4 mmol), [Br] (8 equiv.), air, r.t, 1 h, carbon felt (1.0
cm × 1.0 cm) as the anode, and Pt plate (1.0 cm × 1.0 cm)
as the cathode, constant current (25 mA), undivided cell.
bisolated yield. cAnode potential starts from 1.42 V vs SCE,
Ecell range from 3.29 V to 3.65 V, Faradaic efficiency 85%
d
(2.35 F/mol). Anode potential starts from 1.42 V vs SCE,
Ecell range from 3.61 V to 4.02 V, Faradaic efficiency 86%
(2.33 F/mol).
C (+)  Pt (-)
constant current (25 mA)
rt, air, H2O, 2.5 h
OH
With the optimal conditions in hand, we examined a series
of allyl alcohols for this electrochemical reaction. In general, the
corresponding β-bromocarbonyls containing an all-carbon α-
quaternary center were produced with moderate to excellent
yields (Table 2). Initially, various tertiary alcohols were
subjected to the reaction for the synthesis of β-bromoketones
(Table 2a). By contrast to the model substrate 1a, methyl group
in the meta- or para-position obviously increased the yields,
while in the ortho-position did not affect the yield (2b-d). The
substrates with electron-donating substituents were well-
tolerated in this reaction and gave excellent yields (2f, g). It was
found that 2-naphthyl substrate 1h was also suitable for this
reaction delivering the corresponding product 2h in 93% yield.
To our delight, the moderate yields were obtained with
excellent diastereoselectivities (dr>20:1) when tri-substituted
alkenes were employed as the substrates (2i, 2j) and the
relative configuration of 2i was determined by X-ray
crystallography.13 It's worth mentioning that unactivated
O
Br
MgBr2 6H O

2
2a
1a
, 59% yield
, 0.4 mmol
Figure
1
The preliminary study of electrochemical
bromination/semipinacol rearrangement in water under open-air
conditions.
We speculated that the relatively poor solubility of allylic
alcohol 1a in water resulted in the moderate yield (Figure 1 or
Table 1, entry 1). To further improve the yield, the mixture of
acetonitrile and water (2:1) was first chosen as the solvent. To
our delight, a satisfying yield of desired product 2a was
obtained (79%, Table 1, entry 2). It should be noted that this
reaction proceeded without external electrolytes for that
MgBr2·6H2O was not only reactant but also electrolyte, and for
this reason the cell potential increased gradually in the reaction.
Next, other polar protic solvents (MeOH, AcOH) were also
screened as co-solvent. The use of methanol as a co-solvent
further increased the yield to 81%, while acetic acid gave 77%
2 | J. Name., 2012, 00, 1-3
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alkenes also reacted although resulting in some lower yields (2k,
COMMUNICATION
Considering high chloride ion content of seawaters and versatile
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DOI: 10.1039/C9GC01152H
2l). Substrate with simple cyclopentanol 1m led to an evident synthetic utility of organochlorides, we turned our attention to
decrease in yield due to the relatively weak expansion ability of investigate
electrochemical
chlorination/semi-pinacol
5-member ring, while 9-fluorenol worked well giving 81% yield. rearrangement of allylic alcohols. To our satisfaction, the desired
Nicely, 1,2-aryl and alkyl migrations were also accessible and chlorination product 3a was obtained with 64% yield using MgCl2 as
afforded the corresponding products in middle to good yields the chlorine source. Next, a number of allyl alcohols were subjected
(2o-q). We next examined some secondary alcohols as to the reaction, affording the corresponding products with moderate
substrates in the reaction. A variety of β-bromo-aldehydes were to good yields (3d-t). Overall, the scope of bromination was all
afforded through electrochemical semi-pinacol rearrangement suitable for the chlorination although the yield is some lower.
(2r-u). It is worth noting that the electron-rich phenyl migration
To gain insights into this electrochemical halogenation
group can result in a higher yield than electron-poor groups,
reaction mechanism, some additional experiments were
which indicates a cation migration process involving in the
conducted. Addition of radical scavengers such as BHT and
tandem reaction.
TEMPO results in the inhibition of the reaction, which indicates
that radical species is involved in the process. Meanwhile,
substrate with non-symmetric migration groups 1v was also
Table 2 Substrate Scope of Allylic Alcoholsa
R1
R2
tested and the product with electron-rich aryl group migration
HO
R1
(+) C | Pt (-) (25 mA)
( )n
( )n
[X]
2v was only obtained, which is identical to the product using
NBS as the electrophilic halogen source (for the detail, please
see SI). The outcome and the excellent diastereoselectivities of
2j and 2i conformably suggest that a cation migration process is
involved in the tandem reactions. On the basis of the above
results and previous works, a plausible mechanism was shown
in Scheme 2. Halogen anion was oxidized to molecular bromine
at the anode and then it promoted a classic electrophilic semi-
pinacol rearrangement6g, 14 to form β-halocarbonyls.
O
X
R2
(n= 1,2) MeCN/H2O (6:3)
rt, air, 1 h
(X= Br, Cl)
a) Synthesis of all-carbon -quaternary center -bromoketones
F
O
O
O
Br
) 2b
Br
Br
O
Br
(
(
2c
(
2d
 , 92%
(
2e
 , 79%

, 83%
 , 92%
MeO
O
O
Ph
Br
O
O
O
Br
Br
O
Br
(
2i
Scheme 2 Proposed mechanistic pathway
 , 79%
(
2f
 , 98%
(
2g
(
2h
 , 93%
 , 92%
dr > 20:1
OMe
O
O
Ph
O
Br
Br
Br
Standard conditions
O
Br
CF3
O
Br
OH
MgBr26H2O
Ph
(
2j
 , 45%
dr > 20:1
O
(
2l
 , 55%
(
2m
, 47%

(
2k
 , 66%
dr > 20:1
OMe
CF3
Br
2v
O
1v
, 95% yield
O
, 0.4 mmol
O
Ph
Ph
n-Pr
n-Pr
Br
Br
Ph
Ph Ph
Ph Ph
Br
(
2q
 , 56%
(
2n
 , 81%
(
2o
 , 79%
(
2p
 , 74%
Ph
Br
b) Synthesis of all-carbon -quaternary center -Bromoaldehydes
OH
 Br
Ph
O
C
A
T
H
O
D
E
O
O
A
N
O
D
E
OHC
OHC
OHC
OHC
2Br-
1/2 H2
H+
Br
Br
e
Br
e
Alkene
Br
Cl
MeO
2Br
Br2
(
2u
 , 52%
dr > 20:1
(
2r
 , 73%
(
2s
 , 58%
(
2t
 , 76%
c) Synthesis of all-carbon -quaternary center -chlorocarbonyls
F
It is well-known that bromine is described as “Ocean
element” for that more than 99% of bromine on the earth exists
in the ocean. Besides, there are abundant chlorine resources on
the earth, such as industrial effluent from soda ash factory and
O
O
O
O
Cl
Cl
Cl
Cl
(
3a
 , 64%
(
3d
 , 75%
(
3e
(
3h
 , 61%
 , 64%
O
Ph
O
OHC
Cl
Cl
sea water. The lower oxidation potential of MgBr2·6H2O than
Ph Ph
Cl
MeO
Cl
O
MgCl2 provides the priority of bromine ion to chlorine ion to
participate in the reaction, which could be used to split these
two different ions. It was found that only β-bromoketone 2a
was detected when the equal amounts of MgBr2·6H2O and
MgCl2 were added in the reaction system. Next, we assumed
that whether we can utilize bromine-rich brine as the halogen
(
3l
 , 48%
dr > 20:1
(
3m
(
3p
 , 58%
(
3t
 , 53%

, 40%
aReaction conditions: allylic alcohol (0.4 mmol), MgBr2·6H2O or
MgCl2 (4 equiv.), MeCN/H2O (6 mL/3 mL), air, r.t, 1 h, carbon felt
(1.0 cm × 1.0 cm) as the anode, and Pt plate (1.0 cm × 1.0 cm) as the
cathode, constant current (25 mA), undivided cell. Isolated yield.
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source, such as simulated dead sea water and bittern after Scheme 3 Using simulated wastewaters to syntheVsiiezweArbtiicoleaOcntilivnee
DOI: 10.1039/C9GC01152H
making salt with seawater. To verify the idea, simulated natural products
solutions were prepared according to relative literature and
were added into the reaction system (Table 3).15 Satisfyingly,
the electrochemical reaction system offered good compatibility
with common interfering ions in water, such as Na+, K+, Mg2+,
Ca2+, SO4 , HCO3 and OH-. For simulated dead sea water and
bittern after making salt with seawater, a good bromide-
selectivity (the ratio of [Br]/[Cl] is up to 5.1) was observed
although bromide was far less than chloride, which showed a
great efficiency to transform bromide ion from simulated brine
to readily convertible function group in the product.
2-
-
Table 3 Compatibility of reaction system for simulated
solutions
HO
Ph
C (+)  Pt (-) (25 mA)
Ph
Ph

simulated solutions
(A-D)
O
O
Cl
Br
1 h, air, MeCN
1a
2a
3a
scale of
1
(mmol)
simulated
solution
(Br :Cl)
Ratio
(2a:
3a)
calculation
standard
total
yield
entry
1
2
3
4
5
6
0.055
0.11
0.4
0.07875
0.4
0.21
A (1: 128)
A (1: 128)
B (1: 97)
B (1: 97)
C (0:100)
D (1:651)
Br, 4 equiv.
Br, 2 equiv.
Cl, 4 equiv.
Br, 2 equiv.
Cl, 4 equiv.
Cl, 8 equiv.
98%
85%
54%
96%
36%
62%
5.1:1
4:1
1:20
3:1
0:100
1:27
aReaction conditions: allylic alcohol (0.055 mmol), MeCN/simulated
solution A, air, r.t, 1 h, carbon felt (1.0 cm × 1.0 cm) as the anode,
and Pt plate (1.0 cm × 1.0 cm) as the cathode, constant current (3
mA), undivided cell. Isolated yield. bonly using simulated wastewater
based on Br concentration (1.7 g/L), isolated yield.
Components of simulated solution:
A (Bittern after making salt with seawater, g/L): Br 5~7, MgSO4
13.51, MgCl2 452.18, NaCl 2.18, KCl 2.47.
Conclusions
B (Dead sea water, g/L): Cl− (181.4), Br− (4.2), SO42− (0.4), HCO3− (0.2),
Ca2+ (14.1), Na+ (32.5), K+ (6.2), Mg2+ (35.2).
In summary, we have developed
a simple and facile
electrochemical method involving halogenation/semi-pinacol
rearrangement of allylic alcohols to synthesize β-halocarbonyls (X =
Br, Cl) with an all-carbon quaternary center using inorganic halide
salt as the halogen source. This method features open-air and eco-
friendly reaction conditions, moderate to excellent yields, high
selectivity for bromide ion, and synthetic application in the formal
syntheses of bioactive natural products (±)-galanthamine and (±)-
crinamine. Further studies are underway to efficiently treat the oil
and gas wastewaters.
C (Industrial effluent (producing Na2CO3), g/L): Cl- 99-115, OH- 1-2.7,
SO42- 0.1-1.2, Ca2+ 39-45, Na+ 18-25, NH3 0.01-0.03, Suspended solids
11-70, CaO 0.7-9.0, CaCO3 3.8-11, CaSO4 1.7-7.1, Others By
difference, pH 11-12.)
D (Seawater, mg/L): Na+ 11061.28, Mg2+ 1330.23, K+ 410.17, Ca2+
423.54, Cl- 19891.90, Br- 68.88, F- 4.73, SO42- 929.31.
As our initial goal was to synthesize useful organohalo
compounds eco-friendly, we next demonstrated the application
of this halogen-involved electrochemical semi-pinacol
rearrangement. Two allyl alcohols 1w and 1x were prepared
and employed in the reaction system while simulated bittern
after making salt with sea water was used as halogen sources
(Scheme 3). The desired β-bromoaldehydes 2w and 2x were
obtained in 35% and 42% yield, respectively, which were key
intermediates for the syntheses of Amaryllidaceae alkaloids,
such as (±)-galanthamine and (±)-crinamine.16 It's worth
mentioning that the desired 2x was also produced with 41%
yield only using simulated oil and gas wastewater without
adding MeCN as the co-solvent, which may be promising to
treat oil and gas wastewaters and afforded useful products.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the “Thousand Youth Talents Plan”,
NSFC (21672145, 51733007, 21871178 and 21702135), the
Shuguang program (16SG10) from SEDF
& SMEC, STCSM
(17JC1403700) and The Drug Innovation Major Project of the
Ministry of Science and Technology of China (2018ZX09711001-005-
002).
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(i) N. Sauermann, T. H. Meyer, Y. QiuDaOnId: 1L0..1A0c3Vk9iee/Cwrm9AGratiCncl0ne1,O1An5l2iCnHSe
Catal., 2018, 8, 7086; (j) Z.-W. Hou, Z.-Y. Mao and H.-C. Xu,
Synlett, 2017, 28, 1867; (k) S. Tang, Y. Liu and A. Lei, Chem.,
2018, 4, 27.
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