Acylation of Alkenes with the Aid of AlCl3 and 2,6-Dibromopyridine

Article abstract of DOI:10.1021/acs.orglett.9b02688

Friedel-Crafts-type acylation of alkenes with acyl chlorides has been successfully conducted with a wide substrate scope by the combined use of AlCl₃ and 2,6-dibromopyridine. Trisubstituted alkenes afford allylketones or vinylketones depending on the presence or absence of hydrogen atom(s) at the β-position to the acylation site, while monosubstituted alkenes exclusively afford vinylketones.

Full text of DOI:10.1021/acs.orglett.9b02688

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Acylation of Alkenes with the Aid of AlCl3 and 2,6-Dibromopyridine
Shinya Tanaka,*,†,‡ Tsukasa Kunisawa,† Yuji Yoshii,† and Tetsutaro Hattori*,†
†Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku,
Sendai 980-8579, Japan
‡Environment Conservation Center, Tohoku University, 6-6-04 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan
S
* Supporting Information
ABSTRACT: Friedel−Crafts-type acylation of alkenes with acyl
chlorides has been successfully conducted with a wide substrate
scope by the combined use of AlCl3 and 2,6-dibromopyridine.
Trisubstituted alkenes afford allylketones or vinylketones depending
on the presence or absence of hydrogen atom(s) at the β-position to
the acylation site, while monosubstituted alkenes exclusively afford
vinylketones.
α,β-Unsaturated ketones are important building blocks used
for Michael addition, cycloaddition, Morita−Baylis−Hillman
reaction, etc. in organic synthesis.1−4 Aldol and Mukaiyama
aldol condensation are the most reliable methods for the
preparation of this class of compounds,5 though the intra-
molecular condensation of unsymmetrical diketones may cause
difficulty in product selectivity. Horner−Wadsworth−Emmons
olefination6 and its Still−Gennari modification7 of α-
acylphosphonate esters with aldehydes or ketones also provide
reliable access to this class of compounds, but multistep
reactions are necessary for the preparation of the starting
acylphosphonates.8 Direct carbonylation of alkenes is a desired
pathway to α,β-unsaturated ketones, as alkenes are easily
available feedstocks. Although carbonylative Heck reaction is
the most potent among this type of reactions,9 it requires
excess amounts of the alkene and toxic CO gas and is not
compatible with alkyl halides.
We recently found that alkenes can be carboxylated with
CO2 by the combined use of EtAlCl2 and 2,6-dibromopyr-
idine.17 Mechanistic studies suggested that EtAlCl2 electro-
philically substitutes an alkene with the aid of the base to give a
vinylaluminum ate complex, which is then carbonated with
CO2 (Scheme 1a). The pyridine base can abstract a proton
Scheme 1. Base-Assisted Electrophilic Substitution
Reactions of Alkenes
Friedel−Crafts-type acylation can provide an alternative
method to the carbonylative Heck reaction. Although there are
some reports on the acylation of alkenes with acid anhydrides,
acid chlorides, and other acylating agents, the yields of ketones,
especially in the case of alkyl alkenes, are unsatisfactory in most
cases.10 This is because the cationic intermediate generated by
the addition of an acylium ion to an alkene initiates
polymerization, unless proton elimination from the inter-
mediate is facilitated by an acyl group with a strong electron-
withdrawing nature, such as the trifluoroacetyl group.11−14 In
some cases, β-chloroketones are obtained under Friedel−
Crafts conditions by the addition of acyl chlorides to alkenes.15
Snider et al. prepared α,β-unsaturated ketones from simple
alkylalkenes through EtAlCl2-mediated acylchlorination and
subsequent dehydrochlorination using a base, although the
substrate scope was limited in this case.10d Recently, Huang et
al. reported a formal acylation of alkenes with secondary
amides. In this transformation, an alkene is iminated with a
nitrilium ion generated from an amide with the aid of Tf2O,
and the resulting enimine is hydrolyzed to the corresponding
enone.16
from the cationic intermediate generated by the addition of
EtAlCl2 to an alkene, without deactivating the Lewis acid, by
virtue of its bulkiness and weak basicity. We also succeeded in
the electrophilic borylation of alkenes with BBr3 using 2,6-
dichloropyridine or 2,6-lutidine, based on a similar concept
(Scheme 1b).18 Herein, we report that this base-assisted
Received: July 31, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02688
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
method for electrophilic alkene substitution can be successfully
extended to the Lewis acid mediated acylation of alkenes with
acyl chlorides (Scheme 1c).
decreased in the absence of B1 or in the presence of other
bases such as other 2,6-disubstituted pyridines, monosub-
̈
stituted pyridines, trimethylamine, and Hunig’s base (Table S1
in the Supporting Information).
First, the acylation of α,α-diphenylethylene (1a) with
octanoyl chloride was examined. The reaction was carried
out using 1 molar equiv each of EtAlCl2 and 2,6-
dibromopyridine (B1) in toluene at room temperature under
the optimized conditions used for our previously reported
carboxylation.17 The only exception is the temperature, which
was chosen considering the high reactivity of the acylating
agent. The reaction gave the desired enone 3a in a moderate
yield (61%), but this was accompanied by the alkylation of the
acyl chloride with EtAlCl2 (4a) (Table 1, entry 1). Increasing
Under the optimized conditions, the acylation was
investigated for α,α-diphenylethylene (1a) and its derivatives
1b−1d bearing substituents at the para-position of the benzene
rings, using benzoyl chlorides 2b−2e and phenylacetyl chloride
(2f) as the acylating agents (Table 2). The benzoylation of 1a
Table 2. Acylation of α,α-Diarylalkenes with Various Acyl
a
Chlorides
Table 1. Acylation of α,α-Diphenylethylene (1a) with
a
Octanoyl Chloride
b
yield (%)
entry
x
Lewis acid (y)
z
solvent
3a (4a)
1a
c
c
c
1
2
3
4
5
6
7
8
9
10
1.0 EtAlCl2 (1.0) 1.0 toluene−hexane
1.2 EtAlCl2 (1.0) 1.0 toluene−hexane
1.5 EtAlCl2 (1.0) 1.0 toluene−hexane
61 (23)
79 (28)
71 (27)
87 (15)
27
14
86
96
70
28
21
14
13
19
20
6
n.d.
n.d.
n.d.
c
1.2 EtAlCl2 (1.0) 1.0 CH2Cl2−hexane
1.0 AlCl3 (1.0)
1.5 AlCl3 (1.0)
1.0 AlCl3 (2.0)
1.0 AlCl3 (3.0)
1.0 AlCl3 (4.0)
1.0 AlCl3 (3.0)
1.0 CH2Cl2
1.0 CH2Cl2
1.0 CH2Cl2
1.0 CH2Cl2
1.0 CH2Cl2
2.0 CH2Cl2
quant
a
Reaction conditions: 1a (0.40 mmol), 2a (x molar equiv), EtAlCl2 (y
molar equiv, used as a 1.0 M hexane solution) or AlCl3 (y molar
b
equiv), B1 (z molar equiv), solvent (1.0 mL), rt, 3 h. Determined by
1H NMR analysis using CH2Br2 as the internal standard. The yield of
c
4a is based on the quantity of 2a. Hexane was incidentally
incorporated from the EtAlCl2 solution.
a
Reaction conditions: 1 (0.40 mmol), 2 (0.40 mmol), AlCl3 (1.20
b
mmol), B1 (0.80 mmol), CH2Cl2 (1.0 mL), rt, 3 h. Isolated yield.
the amount of the acyl chloride to 1.2 molar equiv improved
the yield of 3a to 79% (entries 2 and 3). The yield was further
improved to 87% when the solvent was changed to
dichloromethane (entry 4). The production of undesired
ethyl ketones by the alkylation of acyl chlorides with EtAlCl2
may render the purification of desired enones difficult, thus
restricting the practical application of this acylation. The
alkylation seems to be facilitated by the formation of ate
c
1
The yield shown in the parentheses is that calculated by H NMR
d
analysis of the crude product. 1d (0.40 mmol), 2b (0.60 mmol),
EtAlCl2 (0.60 mmol), B1 (0.40 mmol). Ethylketone 4b (30% based
e
f
on the amount of 2b) was formed. Obtained as a hardly separable
mixture with byproducts. The yield was calculated by 1H NMR
analysis of the mixture.
−
complex EtAlCl3 upon cleaving the C−Cl bond of an acyl
with 2b−2e proceeded to give the desired enones in high
yields, regardless of the electronic properties and steric
bulkiness of the substituents on the benzene rings of the
acylating agents (entries 1−4). Phenylacetyl chloride (2f) also
gave enone 3f, but the yield was moderate (entry 5). Other
diarylethylenes 1b and1c were acylated with benzoyl chloride
(2b) in high yields, regardless of the electronic properties of
the para-substituents in the alkenes (entries 6 and 7). The
exception was the reaction with methoxy derivative 1d, which
afforded enone 3i in a moderate yield (entry 8). This result
was probably due to demethylation caused by the Lewis acid
and/or deactivation of the Lewis acid by the coordination of
chloride with the Lewis acid, which increases the anionic
character of the ethyl group. We then attempted to use AlCl3
instead of EtAlCl2. The acylation using 1 molar equiv each of
AlCl3 and B1 gave 3a in a low yield (entry 5). However,
optimizing the molar equivalences of the reagents (1a/2a/
AlCl3/B1 = 1:1:3:2) improved the yield to 100% (entries 6−
10). Note that the acylation proceeded quantitatively with the
aid of the bulky and weak base B1 in the presence of AlCl3 and
HCl, the latter being generated by the acylation, although this
acid combination serves as a typical initiator for the
polymerization of alkenes.19 The yield of 3a was significantly
B
DOI: 10.1021/acs.orglett.9b02688
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Organic Letters
Letter
a
Table 3. Acylation of Various Alkenes with Octyanoyl Chloride or Benzoyl Chloride
a
b
c
Reaction conditions: 1 (0.40 mmol), 2 (0.40 mmol), AlCl3 (1.20 mmol), B1 (0.80 mmol), CH2Cl2 (1.0 mL), rt, 3 h. Isolated yield. E/Z =
d
e
f
1
76:24. E/Z = 92:8. In 1,2-dichloroethane, at 60 °C. Obtained as a mixture of the regioisomeric enones. The yield was calculated by H NMR
g
h
analysis of the mixture. Obtained as a hardly separable mixture with byproducts. The yield was calculated by 1H NMR analysis of the mixture. E/
i
j
Z could not be determined. E/Z = 71:29. E/Z = 70:30.
the methoxy groups. In accordance with this assumption, the
use of a weaker Lewis acid EtAlCl2 improved the yield of 3i
(entry 9).
that the isomerization would be mediated by the conjugate
acid of B1 with a pKa of −2.2.21 We attempted to suppress the
isomerization by the addition of a bulky and strong base, 2,6-
di-tert-butylpyridine (B4), which may capture the proton
without deactivating AlCl3.21 The isomerization could be
suppressed almost completely by using the reagents in a molar
ratio of AlCl3/B1/B4 = 3.0:1.0:1.5, although the desired
enones were obtained only in moderate yields (method B).
The yields and isomerization suppression could be balanced by
the combined use of EtAlCl2 and B1 in a molar ratio of
EtAlCl2/B1 = 1.5:1.0, which gave enones corresponding to the
original alkenes in good yields (method C).
We considered that the present acylation would proceed via
the addition of an acylium ion to alkene 1, followed by the
abstraction of a proton from the resulting cationic intermediate
A1 with base B1 (Scheme 3, path a). However, another path
might exist, that is, electrophilic metalation of 1 with AlCl3
aided by B1 and subsequent nucleophilic acylation of the
resulting aluminum ate complex with acyl chloride 2 (path b).
Intermediary ate complex A2 cannot be verified by trapping
with D2O, as reported for the alumination of 1-methylindole,22
because this metalation is in an equilibrium and the position of
which lies significantly to the reaction system. We hypothe-
sized that the reaction of 1a with an alkyl chloroformate may
provide useful insight into the reaction mechanism. We
previously found that ethyl chlroformate undergoes nucleo-
philic substitution with oganoaluminum ate complexes to
afford esters.22 On the other hand, it is known that
Various alkenes 1j−1r were acylated using octanoyl chloride
(2a) and benzoyl chloride (2b) as a representative alkyl- and
arylcarbonyl chloride, respectively (Table 3). Trisubstituted
alkenes 1k, 1l, and 1n having hydrogen(s) at the β-position to
the acylation site and those without such a hydrogen (1j and
1m) selectively gave allyl ketones and vinyl ketones,
respectively (entries 1−10). The formation of allyl ketones
can be explained by the abstraction of a proton at the sterically
less hindered γ-position to the carbonyl group, similar to the
case of our previously reported carboxylation.17 α-Methylstyr-
ene (1o) gave vinyl ketones 3oa and 3ob despite having
hydrogens at the β-position to the acylation site (entries 11
and 12). Cyclohexene (1p) gave a mixture of regioisomeric
cyclohexenyl ketones (entries 13 and 14).20 Monosubstituted
alkenes 1q and 1r also underwent this acylation to give (E)-
enones (entries 15−18). Acylation of internal disubstituted
alkenes 1s and 1t proceeded in low yields, accompanied by
hardly separable byproducts (entries 19−22). Biorenewable
monoterpenes, α- and β-pinene, gave complex mixtures with
no acylated products.
Terminal dialkylalkenes 1u and 1v had a marked tendency
to isomerize into internal alkenes, giving acylation products
corresponding to the internal alkenes exclusively (1v) or in
preference to those of the original alkene (1u) under the
standard conditions (method A in Scheme 2). We considered
C
DOI: 10.1021/acs.orglett.9b02688
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Letter
Scheme 2. Acylation of Terminal Alkenes with Benzoyl
a b
,
Chloride
prepared and treated with AlCl3/B1 to give indenone 9 by
intramolecular Friedel−Crafts acylation (eq 3). This observa-
tion indicates that 7b is readily converted into the acyl chloride
under the reaction conditions. These results support the
Friedel−Crafts-type mechanism (path a), though the ease of
participation of the chloroformate esters in dehalogenation as
compared to acyl halides must be taken into account.
In conclusion, we investigated the acylation of alkenes with
acyl chlorides by the combined use of AlCl3 and 2,6-
dibromopyridine (B1). Notably, various alkenes were acylated
in the presence of B1 without undergoing polymerization.
Terminal dialkylalkenes, which easily undergo isomerization,
could be acylated into enones corresponding to the original
alkenes by the addition of 2,6-di-tert-butylpyridine (B4) or by
the use of EtAlCl2 with B1. Further studies on the application
of the base-assisted method toward electrophilic alkene
substitutions are underway, and the results will be reported
in due course.
a
Reaction conditions: 1 (0.40 mmol), 2b (0.40 mmol), AlCl3 (1.20
mmol), B1 (0.80 mmol) for method A, 1 (0.40 mmol), 2b (0.40
mmol), AlCl3 (1.20 mmol), B1 (0.40 mmol), B4 (0.60 mmol) for
method B, 1 (0.40 mmol), 2b (0.80 mmol), EtAlCl2 (0.80 mmol), B1
b
(0.40 mmol) for method C. The products could not be separated by
column chromatography. The yields are those calculated by 1H NMR
c
d
e
analysis of the mixture. E/Z = >95/5. E/Z = 81/19. E/Z could not
be determined. Yield is based on the amount of 2b.
f
Scheme 3. Feasible Reaction Mechanism for the Acylation
of Alkenes
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02688.
Supplemental data, experimental procedures, character-
ization data, and NMR spectral charts (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: shinya.tanaka.d8@tohoku.ac.jp.
*E-mail: tetsutaro.hattori@tohoku.ac.jp.
ORCID
alkoxycarbonyliums generated from alkyl chloroformates by
dehalogenation with a Lewis acid rapidly decompose with the
release of CO2 as soon as they are generated.23 When 1a was
treated with ethyl chloroformate under the standard conditions
for the acylation, only a trace amount of ester 7a was obtained
(eq 1), which is in conflict with the alumination path (path b).
On the other hand, when the same reaction was carried out
using phenyl chloroformate, which is stable against decarbon-
ation, divinylketone 8 was obtained in a good yield (eq 2). The
formation of 8 can be explained by a stepwise reaction, that is,
the initial electrophilic substitution of 1a with phenoxycarbo-
nylium to give acrylate ester 7b, followed by its nucleophilic
acyl substitution with a chloride ion, and subsequent Friedel−
Crafts acylation of 1a with the resulting acryl chloride, though
Shinya Tanaka: 0000-0001-9741-6253
Tetsutaro Hattori: 0000-0001-8422-4674
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant Number
K17K14443.
■
REFERENCES
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D
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DOI: 10.1021/acs.orglett.9b02688
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