Direct Allylic Amination of Allylic Alcohol Catalyzed by Palladium Complex Bearing Phosphine-Borane Ligand

Article abstract of DOI:10.1021/acs.orglett.7b03023

The direct electrophilic, nucleophilic, and amphiphilic allylations of allylic alcohol by use of a palladium catalyst and organometallic reagents such as organoborane and organozinc has been developed. The phosphine-borane compound works as the effective ligand for palladium-catalyzed direct allylic amination of allylic alcohol. Thus, with secondary amines, the reaction was completed in only 1 h, even at room temperature.

Full text of DOI:10.1021/acs.orglett.7b03023

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Direct Allylic Amination of Allylic Alcohol Catalyzed by Palladium
Complex Bearing Phosphine−Borane Ligand
Goki Hirata,† Hideaki Satomura, Hidenobu Kumagae, Aika Shimizu, Gen Onodera,*
and Masanari Kimura*
Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki
852-8521, Japan
S
* Supporting Information
ABSTRACT: The direct electrophilic, nucleophilic, and
amphiphilic allylations of allylic alcohol by use of a palladium
catalyst and organometallic reagents such as organoborane and
organozinc has been developed. The phosphine−borane
compound works as the effective ligand for palladium-catalyzed direct allylic amination of allylic alcohol. Thus, with secondary
amines, the reaction was completed in only 1 h, even at room temperature.
species and attacked by a nucleophile to give the product and
atalytic allylic substitution starting from the Tsuji−Trost
1
Creaction is one of the most useful processes for organic water. We think that the rate-determining step of this reaction is
an oxidative addition of allyl alcohol cleaving the C−O bond. We
have now thought that the modification of this intermolecular
oxidative addition to intramolecular process should be effective
for the acceleration of the direct allylic substitution. To execute
the intramolecular oxidative addition, we designed a palladium
catalyst bearing a Lewis acid moiety. The phosphine−borane
compound was expected to be a suitable ligand for this idea. Our
working hypothesis is shown in Scheme 1 (b). Boryl group of the
Pd/phosphine−borane catalyst acts as Lewis acid to activate the
hydroxy group of allyl alcohols. Successive oxidative addition
proceeds intramolecularly to give the π-allylpalladium inter-
mediate which can be attacked by a nucleophile.
synthesis. In the original Tsuji−Trost reaction, various allylic
electrophiles, such as allylic chlorides, acetates, carbonates, and
other types of allylic esters, could be used to give the allylated
products. These allylic electrophiles could be derived from allyl
alcohols via halogenation and esterification accompanied by the
formation of a stoichiometric amount of byproduct. From the
viewpoint of the step-economy synthesis and the reduction of
waste, the development of highly efficient direct allylic
substitution of allyl alcohols is in demand in the field of organic
synthesis.2,3 The direct allylic substitution gives the correspond-
ing allylic compound in one step along with only 1 equiv of water
as a byproduct. In our laboratory, we have been studying the
direct allylic substitution by use of a palladium catalyst and
organometallic reagents such as organoborane and organozinc.
In the course of our study, we have so far developed the direct
electrophilic allylation4 as well as the direct nucleophilic4i,5 and
amphiphilic4i,6 allylation using allyl alcohols. The electrophilic
direct allylic substitution catalyzed by a palladium species and
BEt3 is shown in Scheme 1 (a). In this reaction, allylic alcohol was
activated by Lewis acidic BEt3 to give π-allylpalladium species via
an oxidative addition. This π-allylpalladium was an electrophilic
The synthesis, structure analysis, and reactivity of various
transition-metal complexes bearing monophosphine−boranes7
and monophosphine−borates7e,8 as ligands have been reported
by many groups. Some of these phosphine−borane complexes
are useful for the catalytic reaction.9,10 Pd-catalyzed Suzuki−
Miyaura coupling11 and Rh-catalyzed hydroformylation12 using
phosphine−borane ligands were reported by Bourissou and co-
workers. In their initial works, o-(dimesitylboryl)phenylphos-
phine ligand behaved like a biaryl phosphine ligand. The weak π-
coordination of the mesityl group linked to boron to the metal
center was shown by X-ray crystal structure analysis, and this
coordination enhanced the catalytic activity. On the other hand,
we have tried to use the Lewis acidity of the borane moiety of
phosphine−borane ligands to catalyze the direct allylic
substitution. Herein, we report the palladium-catalyzed direct
allylic amination of allyl alcohols by using phosphine−boranes as
ligands.
Scheme 1. Plausible Reaction Pathway for the Direct Allylic
a
Substitution
Palladium-catalyzed allylic amination of cinnamyl alcohol (1a)
with N-methylaniline (2a) was investigated using various types of
phosphine−borane and phosphine−boronate ligands in tetrahy-
a
Received: September 27, 2017
Pathway b is our working hypothesis.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03023
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
was needed, and the reaction was completed after 24 h. This
result shows that the allylic amination was accelerated by using
the Pd/L1 catalyst system. As to the ratio of L1 to Pd, 3 or 2 equiv
of L1 to Pd was enough (entries 13 and 14), but an equimolar
amount of L1 to Pd was not effective for this reaction (entry 15).
These results suggest that 1 equiv of L1 is used as a reductant for
Pd(OAc)2 to form Pd(0) species, and another L1 coordinates to
Pd to form a 1:1 complex as a catalyst molecule.
Table 1. Reactions of Cinnamyl Alcohol (1a) with N-
Methylaniline (2a)a
b
entry
Pd catalyst
ligand
solvent
time (h)
yield (%)
c
1
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
PPh3
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
toluene
24
24
24
24
24
24
24
24
24
24
3
99
91
2
2
L1
L2
L3
L4
L5
L6
Reactions of cinnamyl alcohol (1a) with various secondary
amines 2b−m under the optimized reaction conditions (Table 1,
entry 13) are summarized in Table 2. The regioselectivity in all
3
4
0
5
64
33
2
6
Table 2. Reactions of Cinnamyl Alcohol (1a) with Various
7
a
Secondary Amines 2
d
8
L7
0
9
L1
L1
L1
0
e
10
11
12
13
14
15
Pd2(dba)3
2
Pd(OAc)2
99
98
99
89
5
b
e
d
f
temp
(°C)
time
(h)
yield
(%)
Pd(OAc)2
L1
toluene
toluene
toluene
toluene
1
entry
2
3
e
g
f
f
f
Pd(OAc)2
L1
1
e
h
1
2
3
HNEtPh (2b)
HNBnPh (2c)
HNBn2 (2d)
rt
rt
1.5
1
3ab
3ac
3ad
3ae
3af
99
Pd(OAc)2
L1
1.5
1.5
c
e
i
99
Pd(OAc)2
L1
rt
1.5
1.5
6
99
a
A mixture of 1a (1.2 mmol), 2a (1.0 mmol), Pd catalyst, ligand, and
d
e
b
c
4
HNPh2 (2e)
HNiPr2 (2f)
80
80
80
80
80
80
rt
80
solvent (5 mL) was stirred at rt under N2. Isolated yield. 30 mol % of
BEt3 was added. 10 mol % of ligand was used. 2.5 mol % of Pd
catalyst was used. 1 mL of toluene was used. 7.5 mol % of L1 was
used. 5 mol % of L1 was used. 2.5 mol % of L1 was used.
d
e
5
6
86
83
98
99
91
90
99
f
g
HNCy2 (2g)
1
3ag
3ah
3ai
h
i
f
7
pyrrolidine (2h)
piperidine (2i)
morpholine (2j)
N-phenyl-piperazine (2k)
indoline (2l)
12
12
1
8
9
3aj
10
11
12
12
3
3ak
3al
rt
g
dibenzazepine (2m)
rt
1
3am
97
a
A mixture of 1a (1.2 mmol), 2 (1.0 mmol), Pd(OAc)2 (0.025 mmol),
b
L1 (0.075 mmol), and toluene (1 mL) was stirred under N2. Isolated
c
d
yield. 3ac was obtained as a mixture of E/Z (85/15). The reaction of
1a with 2e in the presence of Pd(OAc)2 (2.5 mol %), PPh3 (7.5 mol
%), and BEt3 (7.5 mol %) gave 3ae (E/Z = 97/3) in 31% yield after
1.5 h. 3ae was obtained as a mixture of E/Z (93/7). The reaction of
1a with 2h in the presence of Pd(OAc)2 (2.5 mol %), PPh3 (7.5 mol
drofuran (THF) as solvent (Table 1). The complete
regioselectivity was observed in all cases to give the linear
allylamine product 3aa. The best ligand for this reaction was
revealed to be Ph2PCH2CH2(9-BBN) (L1) (entry 2). The yield
was the same level as the reaction using Pd/PPh3/BEt3 catalyst
system (entry 1). The ligands having dicyclohexylboryl group
and dicyclohexylphosphino group (L2 and L3) did not show the
catalytic activity (entries 3 and 4), showing that the combination
of the compound on phosphine and boron is important in this
reaction. Propylene- and o-phenylene-linked phosphine−borane
ligands (L4 and L5) were less effective than L1 (entries 5 and 6).
Almost no reaction was observed by use of phosphine-boronate
ligand L6 bearing a pinacol unit (entry 7). These results show
that the length and the moderate flexibility of ethylene linker in
L1 are well matched to this reaction. As bidentate diphosphines
seemed to be the useful ligands for the catalysis, we newly
prepared the diphosphine-borane L7 and used this ligand for the
direct allylic amination. However, unfortunately, the reaction did
not proceed at all (entry 8). In place of Pd(OAc)2, PdCl2 and
Pd2(dba)3 could not be used as a catalyst precursor in this
reaction (entries 9 and 10). We think that the Pd(0) did not form
from PdCl2 under these reaction conditions, and ligand exchange
did not proceed smoothly in the case of Pd2(dba)3. By use of
toluene as solvent instead of THF, the reaction catalyzed by Pd/
L1 was completed for 3 h to give allylated product in quantitative
yield (entry 11). Catalyst loading could be reduced under more
concentrated conditions (entry 12). In the case of the
Pd(OAc)2/PPh3/BEt3 catalyst system, longer reaction time
e
f
g
%), and BEt3 (7.5 mol %) gave no product after 12 h. 3am was
obtained as a mixture of E/Z (85/15).
reactions was perfect to give the corresponding cinnamyl amine
3ab−am as a single product. N-Ethylaniline (2b), N-benzylani-
line (2c), and dibenzylamine (2d) gave the products 3ab, 3ac,
and 3ad in quantitative yields (entries 1−3). The higher reaction
temperature was needed for sterically more hindered amines 2e−
g (entries 4−6). The reactions of 1a with various cyclic
secondary amines 2h−m afforded the corresponding amines
3ah−am in high yields (entries 7−12). For the purpose of
comparison of the phosphine−borane ligand with external BEt3,
two reactions of 1a with 2e and 2h in the presence of Pd(OAc)2,
PPh3, and BEt3 were investigated. The corresponding amines 3ae
d
f
and 3ah were obtained in lower yield (footnotes and ).
As an example of the reaction using a primary amine, the
reaction of cinnamyl alcohol (1a) with aniline (2n) was
investigated (Scheme 2). The reaction proceeded at room
Scheme 2. Reaction of Cinnamyl Alcohol (1a) with Aniline
(2n)
B
DOI: 10.1021/acs.orglett.7b03023
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 3. Reactions of Various Allyl Alcohols 1 with Dibenzylamine (2d)
b
c
d
entry
R1
R2
R3
1
time (h)
products
3bd
yield (%)
3/3′
E/Z
e
1
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Me
H
H
H
1b
1c
1d
1e
1f
0.5
1.5
1
99
88
80
95
99
99
95
76
95
99
97
99
97
87
91
2
4-ClC6H4
2-MeC6H4
3-MeC6H4
4-MeC6H4
4-MeOC6H4
1-naphthyl
2-naphthyl
2-furyl
2-thienyl
Me
H
3cd and 3′cd
3dd and 3′dd
3ed and 3′ed
3fd and 3′fd
3gd and 3′gd
3hd and 3′hd
3id and 3′id
3jd and 3′jd
3kd and 3′kd
3ld and 3′ld
3′ad and 3ad
3′ld and 3ld
3od
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
88/12
>99/1
93/7
3
H
4
H
1
5
H
1
6
H
1g
1h
1i
1
7
H
1
8
H
1
9
H
1j
1
10
H
1k
1l
1
f
11
g
H
1.5
0.5
3
76/24
1/>99
18/82
12
H
H
1m
1n
1o
1p
1q
13
14
15
16
H
H
H
Ph
Me
Me
1.5
12
3
H
3pd
Ph
3qd and 3′qd
88
>99/1
88/12
a
b
c
1
A mixture of 1 (1.2 mmol), 2d (1.0 mmol), Pd(OAc)2, L1, and toluene was stirred at rt under N2. Isolated yield. Determined by H NMR.
d
e
Determined by 1H NMR. The reaction of 1b with 2d in the presence of Pd(OAc)2 (2.5 mol %), PPh3 (7.5 mol %), and BEt3 (7.5 mol %) gave 3bd
f
g
in 18% yield after 0.5 h. E/Z mixture of 1l (E/Z = 83/17) was used. The reaction of 1m with 2d in the presence of Pd(OAc)2 (2.5 mol %), PPh3
(7.5 mol %), and BEt3 (7.5 mol %) gave 3ad in 22% yield after 0.5 h.
temperature to give N-cinnamylaniline (3an) in 90% yield along
with a small amount of N,N-dicinnamylaniline (4an) by use of 2
equiv of 2n. Although we also investigated the reaction of 1a with
n-butylamine, the complex mixture including a small amount of
n-butyl-bis(3-phenyl-2-propenyl)amine was obtained.
(1r) and 2-methyl-3-buten-2-ol (1s). By use of the Pd(OAc)2/
L1 catalyst system, prenylamine 3rd was obtained in a high yield
with a complete regioselectivity from both 1r and 1s (Scheme 3).
Scheme 3. Reactions of Prenyl Alcohol (1r) and 2-Methyl-3-
buten-2-ol (1s) with Dibenzylamine (2d)
Various allylic alcohols could be used in this allylic amination.
The results of the reactions with dibenzylamine (2d) are
summarized in Table 3. Allyl alcohol (1b) gave the
corresponding allylamine 3bd quantitatively (entry 1). The
reactions of cinnamyl alcohol derivatives 1c−g proceeded to give
the allylamines 3cd−gd in high yields with almost complete
regio- and stereoselectivities (entries 2−6). Some other 3-aryl-
and 3-heteroaryl-2-propen-1-ols 1h−k also reacted with 2d to
give the corresponding products 3hd−kd in high yields with
complete selectivities (entries 7−10). When crotyl alcohol (1l)
was used, the excellent yield of the mixture of allylamines 3ld and
3′ld was obtained in a ratio of 76/24 (entry 11). Cinnamylamine
3ad was obtained by the reaction of 1-phenyl-2-propen-1-ol
(1m) with 2d in a quantitative yield as a single product (entry
12). 3-Buten-2-ol (1n) gave the products as a mixture of 3ld and
3′ld in 97% yield (entry 13). The yields and the ratios of products
from α-substituted allylic alcohols 1m and 1n were nearly the
same as in the cases of cinnamyl alcohol (1a) and crotyl alcohol
(1l), suggesting that these allylation reactions proceeded via π-
allylpalladium intermediates. β-Substituted allylic alcohols 1o
and 1p could also be used in this reaction to give the
corresponding allylamines in high yields, respectively (entries
14 and 15). 2-Methyl-3-phenyl-2-propen-1-ol (1q) gave the
product 3qd in high yield with a complete regioselectivity, but
the E-selectivity was slightly lower (entry 16). We investigated
the reactions of 1b and 1m by using a Pd(OAc)2/PPh3/BEt3
catalyst system to show the beneficial influence of the ligand L1.
On the other hand, when EtPPh2 and B-n-hexyl-9-BBN were
used instead of L1, the yield of the desired products 3rd and 3′rd
was extremely low and the complex mixture was formed. In the
case of the Pd/PPh3/BEt3 system, almost no reaction was
observed after 24 h. These results indicate that the linkage
between phosphine and borane moieties is critical for obtaining
the high catalytic activity. We think that the activation of the
allylic C−O bond and successive intramolecular oxidative
addition are achieved by the catalysis of phosphine-borane
palladium complex as shown in Scheme 1 (b).
In summary, we have now found that the phosphine−borane
compound can be used as an effective ligand in the palladium-
catalyzed direct allylic amination of allylic alcohol. The structure
of the linker between phosphine and borane moieties in the
ligand is important for obtaining high catalytic activity. Further
studies on expanding the scope of nucleophiles are currently in
progress.
e
The lower yields of 3bd and 3ad were observed (footnotes and
g
).
The remarkable effect of linking the phosphine and borane
moiety was observed in the amination reaction of prenyl alcohol
C
DOI: 10.1021/acs.orglett.7b03023
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Horino, Y.; Mukai, R.; Tanaka, S.; Tamaru, Y. J. Am. Chem. Soc. 2001,
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03023.
■
S
Experimental procedures and detailed characterization
data for the compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: onodera@nagasaki-u.ac.jp.
*E-mail: masanari@nagasaki-u.ac.jp.
ORCID
Gen Onodera: 0000-0003-3082-1638
Masanari Kimura: 0000-0003-2900-9554
Present Address
†(G.H.) Department of Applied Chemistry, College of Life
Sciences Ritsumeikan University, Kusatsu 525-8577, Japan.
(6) (a) Mukai, R.; Horino, Y.; Tanaka, S.; Tamaru, Y.; Kimura, M. J.
Am. Chem. Soc. 2004, 126, 11138. (b) Kimura, M.; Tamaki, T.; Nakata,
M.; Tohyama, K.; Tamaru, Y. Angew. Chem., Int. Ed. 2008, 47, 5803.
(c) Yamada, N.; Hirata, G.; Onodera, G.; Kimura, M. Tetrahedron 2015,
71, 6541. (d) Hirata, G.; Yamada, N.; Sanada, S.; Onodera, G.; Kimura,
M. Org. Lett. 2015, 17, 600.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by JSPS KAKENHI Grant No.
JP15K17858 (to G.O.) and Grant-in-Aid for JSPS Research
Fellow JP16J00576 (to G.H.).
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K.; Krebs, B.; Lage, M.; Niemeyer, H. H.; Wurthwein, E. U. Z.
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DOI: 10.1021/acs.orglett.7b03023
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