Radical-mediated alkoxypolyhaloalkylation of styrenes with polyhaloalkanes and alcoholsviaC(sp3)-H bond cleavage

Article abstract of DOI:10.1039/d1cc00400j

We have developed a new radical-mediated alkoxypolyhaloalkylation of styrenes with polychloroalkanes and alcohols for the facile synthesis of complex polyhaloalkanes. 4-Methoxybenzenediazonium tetrafluoroborate is a good radical initiator for this transformation. This protocol is well applied to the late-stage functionalization of complex molecules, including vitamin E, estrone and cholesterol derivatives.

Full text of DOI:10.1039/d1cc00400j

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Radical-mediated alkoxypolyhaloalkylation of
styrenes with polyhaloalkanes and alcohols via
Cite this: Chem. Commun., 2021,
57, 3684
C(sp3)–H bond cleavage†
Received 22nd January 2021,
Accepted 8th March 2021
Yun-Yan Liang,a Jing Huang,a Xuan-Hui Ouyang,*a Jing-Hao Qin,b Ren-Jie Songa
abc
and Jin-Heng Li
*
DOI: 10.1039/d1cc00400j
rsc.li/chemcomm
We have developed a new radical-mediated alkoxypolyhaloalkylation of ATRC, which enable the preferential cleavage of the C–H bonds
styrenes with polychloroalkanes and alcohols for the facile synthesis of in polyhaloalkanes, leading to the formation of polychloroalkyl
complex polyhaloalkanes. 4-Methoxybenzenediazonium tetrafluorobo- radicals, owing to the enthalpic and stereoelectronic effects.5,6
rate is a good radical initiator for this transformation. This protocol is Therefore, the transformation of alkenes into complex poly-
well applied to the late-stage functionalization of complex molecules, haloalkanes through polychloroalkyl radicals is an appealingly
including vitamin E, estrone and cholesterol derivatives.
straightforward tool in organic synthesis. To our knowledge,
however, such available examples of the polychloroalkyl
Polychlorinated hydrocarbons, especially those bearing di- or radical-initiated alkene difunctionalization reaction, especially
trichloromethyl groups, are prevalent in natural products, including the three component intermolecular mode, remain
pesticides and various bioactive molecules.1 Since Kharasch rare.6
reported the atom-transfer radical addition (ATRA) of terminal
Here, we report a new, general radical-mediated alkoxypoly-
alkenes with bromotrichloromethane as the polychloromethyl haloalkylation of alkenes with polychloroalkanes and alcohols to
unit,2 the radical addition reaction of alkenes, including produce various 1-alkoxyl-2-polyhaloalkyl alkanes via C(sp3)–H
difunctionalization of alkenes, has become one of the most bond cleavage (Scheme 1d). Using 4-methoxybenzenediazonium
powerful strategies for the rapid assembly of diverse complex tetrafluoroborate as a radical initiator enables the construction
polyhaloalkyl-containing molecules from abundant and readily of polyhaloalkyl radical intermediates, realizing the late-stage
available starting materials.3 Traditional radical-mediated synthesis of natural products.
alkene difunctionalization methods for the synthesis of
We initiated our investigations by exploring the alkoxypoly-
polyhaloalkyl-containing compounds usually focus on the haloalkylation of 1-methoxy-4-vinylbenzene (1a) with CHCl3
one-component atom-transfer radical cyclization (ATRC) and (2a) and ethanol (3a) in the presence of 4-MeOC6H4N2BF4
two-component atom-transfer radical addition (ATRA) pro- (Table 1). After evaluating various reaction parameters, the
cesses through C–halogen bond homolysis using photocatlysts
(e.g., ultraviolet light, sunlight), thermocatalysts, and/or highly
toxic organic initiators (e.g., azobisisobutyronitriles, organo-
tins) (Scheme 1a).4 To expand their applications, recent efforts
have been devoted to the polychloroalkyl radical-initiated two-
(Scheme 1b)5 and three-component (Scheme 1c)6 alkene difunc-
tionalization reactions with polyhaloalkanes and nucleophiles
(e.g., aryl C(sp2)–H bonds, OÀ, N3À) beyond the ATRA and
a Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources
Recycle, Nanchang Hangkong University, Nanchang 330063, China
b State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,
Changsha 410082, China
c State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, China. E-mail: xuanhuiouyang@163.com, jhli@hnu.edu.cn
Scheme 1 Difunctionalization of alkenes initiated by polychloroalkyl
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
radicals.
d1cc00400j
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Table 1 Optimization of the reaction conditionsa
Table 2 Variation of polyhaloalkanes (2) and nucleophiles (3)a
Entry
Variation from the standard conditions
Yieldb/(%)
1
2
3
4
5
6
7
8
None
91
0
Without 4-MeOC6H4N2BF4
Without Cu(MeCN)4BF4
CuCl instead of Cu(MeCN)4BF4
CuCl2 instead of Cu(MeCN)4BF4
Cu(OTf)2 instead of Cu(MeCN)4BF4
FeCl2 instead of Cu(MeCN)4BF4
Ni(acac)2 instead of Cu(MeCN)4BF4
Without Na2CO3
K2HPO4 instead of Na2CO3
2,6-Lutidine instead of Na2CO3
NaHCO3 instead of Na2CO3
KOH instead of Na2CO3
K2CO3 instead of Na2CO3
At room temperature for 24 h
At 70 1C
63
83
68
67
75
71
58
81
63
85
33
73
28
69
86
77
88
81
9
10
11
12
13
14
15
16
17
18b
19c
20d
At 90 1C
None
None
None
a
Reaction conditions: 1a (0.2 mmol), CHCl3 2a (2 mL), 3a (5 equiv.),
Cu(MeCN)4BF4 (10 mol%), Na2CO3 (2 equiv.), 4-MeOC6H4N2BF4
b
(2 equiv.), 80 1C, argon, and 6 h. 4-MeOC6H4N2BF4 (1.5 equiv.).
c
d
4-MeOC6H4N2BF4 (2.5 equiv.). 1a (1 mmol).
a
optimal reaction conditions were determined to be as follows:
10 mol% Cu(MeCN)4BF4, 2 equiv. 4-MeOC6H4N2BF4 and
2 equiv. Na2CO3 at 80 1C for 6 h (entry 1). The results showed
that 4-MeOC6H4N2BF4 was crucial since its omission led to no
reaction (entry 2). We were delighted to find that the reaction
could be promoted by Lewis acids (entries 4–8), such as CuCl,
Reaction conditions: 1a (0.2 mmol),
2 (2 mL), 3 (3 equiv.),
Cu(MeCN)4BF4 (10 mol%), Na2CO3 (2 equiv.), 4-MeOC6H4N2BF4
b
(2 equiv.), CHCl3 (2 mL), 80 1C, argon, and 6 h. CBrCl3 (2b, 2 mL).
c
d
e
f
CCl4 (2c, 2 mL). CHBrCl2 (2d, 2 mL). CH2Cl2 (2e, 2 mL). CBr4
(2h, 2 mL).
CuCl2, Cu(OTf)2, FeCl2 and Ni(acac)2, by comparison with the and aan in 66% and 31% yields, respectively. Regrettably,
results in the absence of Cu(MeCN)4BF4 (entry 3). It was noted 4-chlorobenzenethiol (3o) was inert to this transformation.
that bases could promote the reaction (entry 9). Replacement of
We next turned our attention to examine the scope of
Na2CO3 with other bases, including K2HPO4, 2,6-lutidine, polyhaloalkanes and found that a series of mono-, di- and tri-
NaHCO3, KOH and K2CO3, was adverse (entries 10–14). Reducing substituted polyhaloalkanes 2b–l were compatible with this
or increasing the reaction temperature resulted in decreasing reaction. Bromotrichloromethane (2b) and tetrachloride (2c)
yields (entries 15–17). The yield of the reaction did not improve were suitable substrates, providing the same product 4aaa in
when 1.5 or 2.5 equiv. of 4-MeOC6H4N2BF4 was used (entries 18 good yield via C–Cl and C–Br bond cleavage, respectively. The
and 19). Notably, this reaction was applicable to the reaction with same product 4aba was obtained by using bromodichloro-
a 1 mmol scale of alkene 1a, providing polychloroalkanes 4aaa in methane (2d) and dichloromethane (2e) as polyhalomethanes.
81% yield (entry 20).
By employing other polyhalomethanes, such as bromoform (2f)
With the optimized reaction conditions in hand, we set out to and dibromomethane (2g), mixed products were afforded.
investigate the substrate scope of the alkene alkoxypolychloroalkyla- When using perbromomethane (2h) as a substrate, the tribromo-
tion (Table 2). Firstly, a variety of alcohols 3b–l were investigated. alkane 4afa was smoothly obtained in 71% yield. Performing the
Primary alcohols 3b–3h bearing linear carbon chains, bromo, alkoxypolyhaloalkylation of alkenes with 1,2-dibromoethane (2i),
alkenyl, and bulky tert-butyl and phenyl smoothly gave the corres- 1,2-dichloroethane (2j), 1,1,2-trichloroethane (2k) and 1,1,2,
ponding products 4aab–aah in 51–82% yields. Secondary alcohols, 2-tetrachloroethane (2l) selectively cut C–H bonds, providing
including butan-2-ol (3i), DL-Menthol (3j) and adamantanol (3k), all products 4aia–ala in 41–77% yields, implying that the stability
reacted well, giving the corresponding polychloroalkanes 4aai–aak of polychloroalkyl radicals increases as the number of chlorine
in 77%, 44% and 56% yields, respectively. tert-Butanol (3l) was atoms increases.
compatible with the alkoxypolychloroalkylation reaction, but the
Subsequently, a series of alkenes were examined (Table 3).
activity decreased. Other nucleophiles including acetic acid (3m) and Styrenes 1b–e and 1g with electron-rich groups, such as OEt, OBn,
aniline (3n) were tested, providing the corresponding products 4aam SMe, Ph and OMe, efficiently realized the alkoxypolychloroalkylation
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Table 3 Variation of alkenes (1)a
Scheme 2 Control experiments and possible mechanism.
showed that the copper catalyst and base did not play any key role
(eqn (2), Scheme 2). Simultaneously, 4-vinylbenzonitrile (1f) and
oct-1-ene (1m) were inert to the alkoxypolyhaloalkylation, indicating
that a carbocation intermediate might be generated.
On the basis of our preliminary experiments and previous
reports,5–8 a plausible mechanism is depicted in Scheme 2.
Firstly, 4-MeOC6H4N2BF4 salt generates an aryl radical under
heating,7 which reacts with chloroform (2a) to generate the
trichloromethyl radical A and anisole. Then, radical A attacks
the CQC bond and affords radical B, which can be oxidized by
4-MeOC6H4N2BF4 salt to produce the cationic intermediate C
and aryl radical. The cationic intermediate C is captured by
nucleophile cation scavenger 3a to afford 4aaa.8
In summary, we have disclosed a new radical-mediated
alkoxypolyhaloalkylation of styrenes for the facile synthesis of
complex polyhaloalkanes via C(sp3)–H bond cleavage. Commer-
cially available polyhaloalkanes, including DCM and 1,2-DCE,
are suitable substrates for generating polyhaloalkyl radical
intermediates in the presence of 4-methoxybenzenediazonium
tetrafluoroborate. This reaction is also applicable to the late-
stage functionalization of complex molecules including vitamin
E, estrone and cholesterol derivatives. The further application
of this strategy for modifying bioactive molecules is currently
underway in our laboratory.
a
Reaction conditions: 1a (0.2 mmol),
2 (2 mL), 3 (3 equiv.),
Cu(MeCN)4BF4 (10 mol%), Na2CO3 (2 equiv.), 4-MeOC6H4N2BF4
(2 equiv.), CHCl3 (2 mL), 80 1C, argon, and 6 h.
reaction to obtain the target products 4baa–eaa and 4gaa in 51–85%
yields. Styrene 1f with a strong electron withdrawing group on the
benzene ring, such as CN, was inert; this may be due to the fact that
the cyano group is unfavorable for the stability of the carbocation
intermediate. Styrenes 1h–k bearing polysubstituted functional
groups on the benzene ring were suitable substrates, giving the
corresponding products 4haa–4kaa in 56–77% yields. Thiophene-
containing trichloroalkane 4laa was also provided in 57% yield by
this method. Unfortunately, alkyl alkene 1m was inert to this
transformation. 1-Methyl-1-aryl alkene 1n was a suitable substrate
for this transformation, giving the corresponding product 4naa in
65% yield. Nonsymmetric 1,1-diaryl alkene 1o was also efficient for
this transformation, offering product 4oaa in 46% yield. 1,
1-Diphenylethylene (1p) was converted to the corresponding product
4paa in 82% yield. Internal styrene 1q was reactive, giving the
corresponding product 4qaa in 67% yield. It is worth noting that
the alkene alkoxypolyhaloalkylation was suitable for the late-stage
modification of complex molecules, including vitamin E derivative
1r, estrone derivatives 1s and t and cholesterol derivative 1u,
providing the corresponding complex polychloroalkanes 4raa–uaa
in moderate to good yields.
We thank the National Natural Science Foundation of China
(No. 21901100 and 21625203) and Jiangxi Province Science and
Technology Project (No. 20204BCJ23010 and 20202ACBL216017)
for financial support.
Conflicts of interest
There are no conflicts to declare.
In order to explore the reaction mechanism, we performed
control experiments; the addition of radical inhibitors, including
TEMPO, 2,6-di-tert-butyl-4-methylphenol (BHT) and hydro-
Notes and references
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