Site-Selective Linear Alkylation of Anilides by Cooperative Nickel/Aluminum Catalysis

Article abstract of DOI:10.1002/anie.201710520

We report meta- and para-selective linear alkylation reactions of anilides with alkenes by nickel/N-heterocyclic carbene (NHC) and aluminum catalysis. With a less bulky NHC, the alkylation reaction of N-methyl-N-phenylcyclohexanecarboxamides proceeded mainly at the meta position. In contrast, a bulky NHC ligand led to the para-selective alkylation of N-sec-alkyl anilides.

Full text of DOI:10.1002/anie.201710520

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Accepted Article
Title: Site-selective Linear Alkylation of Anilides by Cooperative Nickel/
Aluminum Catalysis
Authors: Yoshiaki Nakao, Shgogo Okumura, Kazuhiko Semba, Takuya
Komine, and Erika Shigeki
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710520
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10.1002/anie.201710520
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Site-selective Linear Alkylation of Anilides by Cooperative
Nickel/Aluminum Catalysis
Shogo Okumura, Takuya Komine, Erika Shigeki, Kazuhiko Semba, Yoshiaki Nakao*
Dedication ((optional))
Abstract: We report meta- and para-selective linear alkylation of
Table 1. Conditions for the meta-Selective Alkylation of Anilides
with 1-Tridecene.
anilides with alkenes by nickel/N-heterocyclic carbene (NHC) and
aluminum catalysis. With a relatively less bulky NHC, the alkylation
reaction of N-methyl-N-phenylcyclohexanecarboxamide proceeds
2a (0.22 mmol)
Ni(cod)2 (10 mol%)
C11H23
mainly at the meta-position. In contrast, a bulky NHC ligand results
in the para-selective alkylation of N-(sec-alkyl)-anilides.
R
N
C11H23
R
O
N
+
ligand (10 mol%)
Lewis acid (40 mol%)
O
O
N
R
N
3
5
4
O
toluene, 100 ºC, 18 h
N
R
R
0.20 mmol
R = Me: 1a
Cy: 1b
+
+
The site-selective C–H functionalization of readily available
mono-substituted benzenes is one of the most effective strategies to
synthesize a wide variety of poly-substituted benzenes.[1] Aniline
and its derivatives are important substrates for C–H
functionalizations owing to their availability and the applications
of substituted anilines in pharmaceuticals, agrochemicals, and
advanced materials. The C–H alkylation of anilines with alkenes as
alkylating agents represents an ideal alkylation reaction in terms of
atom economy. However, the ortho/meta/para selectivity, as well
as the linear/branched selectivity remain difficult to be controlled
precisely. The Friedel-Crafts alkylation of aniline with olefins
proceeds selectively at the ortho[2] or para[3] positions to afford
branched alkylanilines. Recently Ackermann has reported the
ruthenium-catalyzed meta-selective tertiary alkylation of N-
protected anilines with tertiary alkyl bromides,[4] whereas examples
of the meta-selective branch alkylation using alkenes have not yet
been reported. In contrast to the advances in branched alkylation
reactions, linear alkylation reactions of aniline derivatives remain
undeveloped. Although a few examples of ortho-selective linear
alkylation of anilines bearing directing groups[5] and several
methods for the direct meta-selective C–H functionalization of
aniline derivatives have been developed, [6] meta- or para-selective
linear alkylations of aniline derivatives with alkenes have not been
reported so far.
O
C11H23
C11H23
C11H23
6
R2
R2
R1
R1
N
R2
R2
N
N
N
O
O
Al
R3
R3
R2
R2
R2
R2
R1 = R3 = H, R2 = Me: IPr
MAD
IiPr
R1 = R2 = Me, R3 = H: IPrMe
R1 = H, R2 = Ph, R3 = OMe: IPr*OMe
entry
1
ligand
Lewis acid yield of
3 + 4 + 5
3/(4 + 6)/5[a]
(%)[a]
46[b]
48 (5)[c]
48 (8)[c]
26
21:66:13
4:82:14
3:84:13
<1:72:28
8:73:19
n.d.
1a
1b
1b
1b
1b
1b
1b
1b
IPrMe
IPrMe
IPrMe
1
MAD
MAD
MAD
MAD
MAD
MAD
AlMe3
none
2
3[d]
4
IPr*OMe
IPr
5
13
[e]
IiPr•HBF4
IPrMe
IPrMe
6
<1
7
5
10:75:15
n.d.
8
<1
[a] Determined by GC analysis using n-dodecane as an internal standard and
not corrected for response factors of minor isomers. [b] Dialkylation products
were observed, which were not considered when determining site-selectivity.
[c] Yield of 6. Other dialkylation products were observed in GC analysis in
~1% yield, which were not considered when determining site-selectivity.
[d] With 100 mol% MAD. [e] With 10 mol% NaOtBu.
Herein we report the meta- and para-selective linear
alkylation of anilides using a nickel/aluminum cooperative
catalysis system. We have previously reported the para-selective
effect of the N-acylamino groups. In order to accomplish a proof-of
principle for such a meta-selective alkylation of anilides, we chose
an NHC ligand whose %Vbur value[8] is probably smaller than the
one we used in the previous work (L1 in Table 3). [9] The reaction
of N-methylacetoanilide (1a) with 1-tridecene (2a) in the presence
of Ni(cod)2, IPrMe, and MAD in toluene at 100 ºC for 18 h afforded
a mixture of ortho- (3a), meta- (4a), and para-alkylation (5a)
products in 46% yield overall with a 21:66:13 selectivity (entry 1,
Table 1) and a small amount of dialkylation products. The
replacement of the acetyl group in 1a with a cyclohexanecarbonyl
group hampered the reaction at the ortho-position and enhanced the
site-selectivity to o/m/p = 4:82:14, in which m,m’-dialkylation
product 6b (5% yield) was also considered (entry 2). The use of
100 mol% of MAD slightly increased site-selectivity (entry 3). The
bulkier NHC ligand IPr*OMe generated the products in lower yield
and site-selectivity (entry 4). The less bulky ligands IPr and IiPr
were not effective in this reaction (entries 5 and 6) probably
because it might stabilized (NHC)Ni(alkene)2 complex, which
seemed to be a resting state in the catalytic reaction,[10a] or slowed
down the reductive elimination step. AlMe3 resulted in low yields,
under retention of the meta-selectivity (entry 7). Without Lewis
acid co-catalysts, no alkylation products were obtained (entry 8).
linear alkylation of benzamides and aromatic ketones by
[7]
nickel/aluminum cooperative catalysis.
As discussed in the
previous work, the para-selectivity was controlled by both
electronic and steric factors and bulky ligands for nickel, while
sterically unhindered and electron-poor C–H bonds worked best in
the reaction. Therefore, we expected that, should the steric factors
be neglected, the alkylation of anilides might proceed at the meta-
position, as the meta-position of anilides should be less electron-
rich than the ortho- and para-position on account of the resonance
[*]
S. Okumura, T. Komine, E. Shigeki, Dr. K. Semba, Prof. Dr. Y.
Nakao
Department of Material Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: nakao.yoshiaki.8n@kyoto-u.ac.jp
Supporting information for this article is given via a link at the end
of the document.
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10.1002/anie.201710520
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sterically
moderately
demanding
iso-propyl
group.
Table 2. Substrate Scope for the meta-Selective Alkylation of
Methylenecyclohexane (2e) participated in the para-alkylation
reaction albeit in low yield. The linear alkylation product was
obtained even with 2-octene (2f), possibly through its reversible
isomerization to 1-octene under the reaction conditions.[10a] Using
2d, the effect of a substituent at the ortho-position of the anilides
was studied. A 2-methyl-substituted anilide participated in the
alkylation reaction. Absolute C4-selectivity was observed for a 2-
methoxy-substituted anilide with low conversion, while a 2-
trifluoromethoxy group retarded the alkylation reaction. 2-Fluoro
group did not affect the alkylation reaction whereas a 2-chloro-
substituted anilide underwent no alkylation but the Heck-type
reaction (see Supporting Information). The presence of a 2-
methoxycarbonyl group lowered the para-selectivity. No alkylation
reaction was oberved with para-substituted anilides.
Anilides.
Ni(cod)2 (10 mol%)
IPrMe (10 mol%)
Cy
N
R2
Cy
N
MAD (100 mol%)
R2
+
O
O
toluene, 100 ºC, 18 h
R1
R1
1, 0.60 mmol
4, yield,[a] meta/para[b]
(yield of recovered 1)[c]
2, 1.8 mmol
tBu
Cy
N
Cy
Cy
N
Cy
N
C11H23
O
O
O
4b, 57% + m,m’-dialkylation 26%[d]
o/(m + m,m’)/p = 3:83:14
(4%)
4c,[e] 45%, 95:5[f]
4d, 43%, 94:6
(33%)
(42%)
Cy
N
[Si]
Cy
Table 3. Conditions for the para-Selective Alkylation of Anilides
N
C11H23
with 1-Tridecene.
O
O
2a (0.60 mmol)
OMe
Ni(cod)2 (10 mol%)
ligand (10 mol%)
Lewis acid (40 mol%)
[Si] = SiMe(OSiMe3)2
R
N
R
N
R
N
4e, 26%, 90:10
4f, 67%, m/o = 95:5[f]
tBu
tBu
tBu
+
C11H23
(64%)
(9%)
150 ºC, 18 h
O
O
C11H23
O
[a] Isolated yield of the mixture of 4 and its isomers. [b] Determined by GC
analysis. [c] Isolated yields. [d] Containing other dialkylation products (~3%
based on 1), which were not considered when determining site-selectivity.
[e] With 3.0 mmol of vinylcyclohexane. [f] Determined by 1H NMR analysis of
crude products.
5
0.20 mmol
4
Ar
Ar
Ar
Ar
R = Me: 1e
Et: 1f
N
N
iPr: 1g
3-Me-2-butyl: 1h
MeO
OMe
Ar
Ar
Ar
Ar
Ar = 3,5-Me2–C6H3: L1
Subsequently, we investigated the substrate scope of the
alkylation. N-Methyl-N-(3-methylphenyl)cyclohexanecarboxamide
(1c) was reacted with several alkenes. 1-Alkenes bearing
cyclohexyl (2b) or tert-butyl (2c) groups furnished meta-alkylation
products 4c and 4d, respectively, in moderate yield with high site-
selectivities. The use of bulky vinylsilane (2d) resulted in a
decreased yield of anilide 4e. 1,1-Disubstituted alkenes gave the
alkylation products in <10% yields (See Supporting Information).
2-Octene and cyclohexene did not participate in the meta-
alkylation reaction at all under these conditions. Methoxy group at
the para-position of 1b did not affect the meta-selectivity to give 4f.
Unfortunately, o-acetyl and o-dimethylamino groups did not
tolerate under the reaction conditions (see Supporting Information).
entry
1
ligand
LA
yield of
4/5[a]
4 + 5 (%)[a]
1[b]
2[b]
3[b]
4[b]
5
MAD
MAD
MAD
MAD
MAD
MAD
MAD
MAD
MAD
AlMe3
none
33[c]
74[c]
80[c]
66[c]
70[d]
65
51:49
37:63
28:72
24:76
11:89
10:90
44:56
56:44
40:60
22:78
24:76
1b
1e
1f
L1
L1
L1
1g
1h
1h
1h
1h
1h
1h
1h
L1
L1
6[e]
L1
IPr
7
7
IPrMe
IPr*OMe
L1
8
8
9
24
10
11
48
L1
15
[a] Determined by GC analysis using n-dodecane as an internal standard
and not corrected for response factors of minor isomers. [b] 0.60 mmol
scale. [c] Determined by 1H NMR analysis. [d] An unidentified product,
which could be an ortho- or branch-alkylation product was also observed
by GC and GC-MS in around 1% yield estimated by GC but was difficult to
characterize due to a small quantity. [e] with 100 mol% of MAD.
We further examined the reaction conditions for the para-
selective alkylation of anilides by using steric factors according to
our previous work (vide supra). The reaction of 1b with 2a in the
presence of Ni(cod)2, L1, and MAD at 150 ºC for 18 h afforded
meta- (4b) and para-alkylated anilides (5b) in 33% yield with a
m/p selectivity of 51:49 (entry 1, Table 3). The replacement of the
cyclohexanecarbonyl group to a pivaloyl group enhanced the yield
and the para-selectivity (entry 2). The introduction of bulkier N-
alkyl groups on the anilide, such as ethyl (1f) or iso-propyl (1g)
groups, increased the site-selectivity (entries 3 and 4). Using N-(3-
methyl-2-butyl)-N-pivaloyl-aniline (1h), the alkylation proceeded
in 70% yield with a m/p selectivity of 11:89 (entry 5). No
significant improvements of the yield and site-selectivity were
observed with the use of 100 mol% MAD (entry 6). We then
optimized the catalysis with 1h. While less bulky ligands such as
IPr, IPrMe, and IPr*OMe did not work well (entries 7–9), the steric
demand of the Lewis-acid co-catalyst affected the site-selectivity;
using AlMe3 instead of MAD lowered the para-selectivity (78:22;
entry 10). In its absence, the alkylation products were obtained in
only 15% yield with a m/p selectivity of 24:76 (entry 11).
Thereafter, we investigated the substrate scope. 2b and 2c
afforded para-alkylated anilides in 68% and 71% yield,
respectively. Using vinylsilane 2d, a high para-selectivity was
observed even though the N-substituent on the anilide was a
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The electronic effect of the anilides seems to be crucial for
1
the meta-selectivity. The H NMR spectrum of a mixture of 1b or
Table 4. Substrate Scope for the para-Selective Alkylation of
Anilides.
Ni(cod)2 (10 mol%)
L1 (10 mol%)
MAD (40 mol%)
1h and AlMe3 revealed that the meta-positions of 1-AlMe3 should
carry less electron density than the para-position, as the signal for
the meta-H appeared at lower magnetic field (Eq. 1 and 2). As
discussed in the previous report, electron-deficient positions are
privileged for C–H activations via the LLHT[10] mechanism.
R1
R1
R2
R2
tBu
tBu
+
N
R3
N
mesitylene, 120 ºC, 18 h
O
O
R3
5, yield,[a] para/meta[b]
(yield of recovered 1)[c]
1, 0.60 mmol
2, 1.8 mmol
Furthermore, the reductive elimination from
a
(meta-
aryl)(alkyl)Ni(II) intermediate may be more favorable than that
from a (para-aryl)(alkyl)Ni(II) species. Previous studies have also
shown that the rate of the reductive elimination from
(aryl)(alkyl)palladium(II) complexes to form C–C bonds depends
on the electronic properties of aryl groups.[11] The reductive
elimination from palladium complexes containing a more electron-
rich aryl group is slower than that from complexes containing an
electron-poor aryl group. Based on this knowledge, we speculate
that the reductive elimination from (meta-aryl)(alkyl)Ni(II)
intermediates could be faster than that from (para-
aryl)(alkyl)Ni(II) intermediates, when the aryl groups are based on
anilides. In contrast to the meta-selectivity, the para-selectivity of
the alkylation of 1g and 1h by Ni/L1 is probably governed by
steric effects. The combination of bulky substituents on the
nitrogen atom of the anilides (R in Table 3) and the bulky NHC
ligand is essential for the para-selectivity (Table 3), which would
suggest that the steric repulsion between the R group and NHC
hampers the reaction at the meta position and results in para-
selectivity. The significant enhancement of the para-selectivity in
the presence of MAD can be explained by the steric repulsion
between MAD and R, which may direct the alkyl group R to the
benzene ring and inhibit the C–H bond cleavage at the meta-
position.
tBu
tBu
N
tBu
N
N
O
O
O
tBu
C11H23
Cy
5j,[d] 44%, 88:12
5k,[d],[e] 68%, 90:10
5l,[f] 71%, 92:8
(52%)
(26%)
(29%)
tBu
N
tBu
tBu
N
N
O
O
[Si]
O
C6H13
5m, 77%, 93:7
5n,[d],[e],[g] 21%, 94:6
5o,[d],[g],[i] 37%, 89:11
(58%)[h]
(60%)
X
CO2Me
N
tBu
N
N
O
O
O
[Si]
[Si]
[Si]
5p,[j] 81%, 96:4
X = OMe: 5q,[k] 31%, >99:1, (68%)
= F: 5r, 95%, 96:4
5s,[l] 63%, 89:11
[a] Isolated yield of the mixture of 5 and its isomers. [b] Determined by GC
analysis. [c] Isolated yield. [d] at 150 ºC without mesitylene. [e] With 5.0
equiv of alkene. [f] In toluene instead of mesitylene. [g] 0.20 mmol scale.
[h] Estimated by 1H-NMR analysis of a crude product. [i] With 1.0 mmol of
2-octene. [j] 1.0 mmol scale. [k] With two portions of 1.8 mmol of 2d. [l]
With 100 mol% of MAD.
vial was charged with 1 (0.60 mmol), 2 (1.8 mmol), MAD (115 mg, 0.24
mmol or 0.29 g, 0.60 mmol), and solution A. The resulting mixture was
stirred for 18 h at 100 or 150 ºC. After the mixture was cooled to rt, ethyl
acetate was added to the mixture. The site-selectivity was determined by
GC analysis. The mixture was filtered through a short pad of silica gel.
The filtrate was purified by medium pressure liquid chromatography
(MPLC) using Biotage® SNAP Ultra to give the corresponding products.
For the reactions without solvents, a 4-mL vial was charged with 1 (0.60
mmol), Ni(cod)2 (17 mg, 60 µmol), L1 (70 mg, 60 µmol), MAD (115 mg,
0.24 mmol), and 2 (1.8 mmol). The resulting mixture was stirred for 18 h
at 150 ºC. The same purification as above gave the corresponding
products.
! 7.18
! 7.20
AlMe3 (0.50 mmol)
toluene, rt
Cy
N
Cy +N
O
(1)
! 7.42
! 7.35
! 7.52
! 7.47
O
Me3Al–
1b–AlMe3
1b, 0.50 mmol
AlMe3 (0.10 mmol)
hexane, rt
! 7.17
! 7.19
+N
(2)
tBu
tBu
N
! 7.38
–7.32
! 7.46
! 7.42
O
O
Me3Al–
1h, 0.10 mmol
1h–AlMe3
In summary, we have developed meta- and para-selective
linear alkylations of anilides by cooperative nickel/aluminum
catalysis. The use of IPrMe, i.e., an NHC with relatively low steric
demand, as a ligand for the nickel catalyst enables meta-selective
alkylations. The bulkier NHC L1 results in a para-selective
alkylation of N-(sec-alkyl)-anilides. The site-selectivity is probably
governed by both electronic and steric effects: the alkylation
reaction may electronically favor meta-C–H bonds, while it should
sterically favor para-C–H bonds, which could be the origin of the
switchable site-selectivity.
Acknowledgements
This work was supported by the JST CREST program Grant
Number JPMJCR14L3 in Establishment of Molecular
Technology towards the Creation of New Functions, the JSPS
KAKENHI Grant Number JP15H05799 in Precisely Designed
Catalysts with Customized Scaffolding, and the JSPS KAKENHI
Grant Number JP25708006.
Keywords: C–H functionalization • Cooperative catalysis •
Nickel • Alkenes
Experimental Section
[1]
[2]
G. Dyker, Handbook of C–H Transformation (WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim, 2005).
In a glove box, a 4-mL vial was charged with Ni(cod)2 (17 mg, 60
µmol), a ligand (60 µmol), and toluene or mesitylene (0.60 mL), and the
resulting mixture was stirred for 5–10 min at rt (solution A). Another 4-mL
a) G. G. Ecke, J. P. Napolitano, A. H. Filbey, A. J. Kolka, J. Org. Chem.
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Anderson, J. Arnold, R. G. Bergman, J. Am. Chem. Soc. 2005, 127,
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COMMUNICATION
14542; d) G.-Q. Liu, Y.-M. Li, Tetrahedron Lett. 2011, 52, 7168; e) M.
Beller, O. R. Thiel, H. Trauthwein, Synlett 1999, 243.
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which is structurally similar to L1, is estimated to be larger than that of
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[6]
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COMMUNICATION
COMMUNICATION
Shogo Okumura, Takuya Komine, Erika
Shigeki, Kazuhiko Semba, Yoshiaki
Nakao*
Page No. – Page No.
Site-selective Linear Alkylation of
We report meta- and para-selective linear alkylation of anilides with alkenes by
nickel/N-heterocyclic carbene (NHC) and aluminum catalysis. With a relatively less
bulky NHC, alkylation reaction of N-methyl-N-phenylcyclohexanecarboxamide
proceeds mainly at meta-position. In contrast, a bulky NHC ligand results in the
para-selective alkylation of N-(sec-alkyl)-anilides.
Anilides
by
Cooperative
Nickel/Aluminum Catalysis
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