Direct benzylation of amines with benzylic alcohols catalyzed by palladium/phosphine-borane catalyst system

Article abstract of DOI:10.1016/j.tetlet.2020.152537

Direct catalytic benzylation of amines with benzylic alcohols to give benzylamines has been newly developed by using palladium/phosphine-borane catalyst system. In this catalytic reaction, the linking between both phosphine and borane moieties in the ligand is very important. Hydroxy group of benzylic alcohols is activated by Lewis acidic borane to form a benzylpalladium intermediate which is attacked by amines to give benzylamine products.

Full text of DOI:10.1016/j.tetlet.2020.152537

Journal Pre-proofs
Direct benzylation of amines with benzylic alcohols catalyzed by palladium/
phosphine-borane catalyst system
Gen Onodera, Hidenobu Kumagae, Daiki Nakamura, Takuto Hayasaki,
Tsutomu Fukuda, Masanari Kimura
PII:
S0040-4039(20)31030-3
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152537
TETL 152537
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
25 August 2020
28 September 2020
4 October 2020
Please cite this article as: Onodera, G., Kumagae, H., Nakamura, D., Hayasaki, T., Fukuda, T., Kimura, M.,
Direct benzylation of amines with benzylic alcohols catalyzed by palladium/phosphine-borane catalyst system,
Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.152537
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Direct Benzylation of Amines with Benzylic
Alcohols Catalyzed by Palladium/Phosphine-
Borane Catalyst System
Leave this area blank for abstract info.
Gen Onodera a,*, Hidenobu Kumagae a, Daiki Nakamura a, Takuto Hayasaki a, Tsutomu Fukuda a, Masanari
Kimura a,*
cat. Pd(OAc)2
cat.
B
Ph2P
NR2R3
OH
R1
R1
NR2R3
+
H
MS4A

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Direct benzylation of amines with benzylic alcohols catalyzed by
palladium/phosphine-borane catalyst system
Gen Onodera a, , Hidenobu Kumagae a, Daiki Nakamura a, Takuto Hayasaki a, Tsutomu Fukuda a and
Masanari Kimura a, *
a Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
———
ARTICLE INFO
ABSTRACT
 Corresponding author. Tel.: +81-95-819-2679; fax: +81-95-819-2677; e-mail: onodera@nagasaki-u.ac.jp, masanari@nagasaki-u.ac.jp
Article history:
Received
Direct catalytic benzylation of amines with benzylic alcohols to give benzylamines has been
newly developed by using palladium/phosphine-borane catalyst system. In this catalytic
Received in revised form
Accepted
Available online
reaction, the linking between both phosphine and borane moieties in the ligand is very
important. Hydroxy group of benzylic alcohols is activated by Lewis acidic borane to form a
benzylpalladium intermediate which is attacked by amines to give benzylamine products.
2020 Elsevier Ltd. All rights reserved.
Keywords:
Phosphine-borane
Benzylation
Palladium
Benzylic alcohol
Benzylamine
Introduction
been known to be useful for the transition-metal-catalyzed
organic reactions [8,9]. Palladium-catalyzed Suzuki-Miyaura
Benzyl alcohol derivatives such as benzylic carbonates and
acetates are useful starting materials as benzylation reagents in
the presence of a suitable palladium-catalyst. Oxidative addition
of benzyl alcohol derivatives to a palladium complex gives a σ-
and/or π-benzylpalladium complex, which is a very useful
intermediate to form a variety of carbon–carbon and carbon–
coupling [10] and rhodium-catalyzed hydroformylation [11]
using phosphine-borane ligands were reported by Bourissou and
his co-workers. In our group, we found that a phosphine-borane
palladium complex is efficient for the direct allylation of amines
and active methylene compounds using allylic alcohols [12]. In
these reactions, Lewis acidic borane moiety of a phosphine-
borane ligand activates the hydroxy group of allylic alcohols and
the oxidative addition of allylic alcohols to palladium proceeds
smoothly to form an electrophilic π-allylpalladium intermediate.
We envisaged that this methodology might be applicable to the
formation of a π-benzylpalladium species via an oxidative
addition of benzylic alcohols. In this manuscript, we report the
palladium-catalyzed direct benzylation of amines with benzylic
alcohols in the presence of a phosphine-borane ligand.
heteroatom bonds [1].
In the early stage, α- and β-
naphthylmethyl and 1-(α- and β-naphthyl)ethyl esters were used
for obtaining benzylpalladium intermediates as electrophiles [2].
Kuwano and his co-workers reported that benzylic carbonates
could also be used as a benzylation reagent [3]. Nucleophilic
substitutions of benzylic esters were also catalyzed by some
palladium complexes [4].
Although benzylic esters and
carbonates derived from benzylic alcohols have so far been
mainly used as starting materials, less examples have been
reported about the direct transformation of benzylic alcohols.
One of the methods for activation of benzylic alcohols to form a
benzylpalladium intermediate is an addition of acid to protonate
Results and discussion
Palladium-catalyzed benzylation of N-methylaniline (2a) with
benzyl alcohol (1a) was investigated under a variety of reaction
conditions (Table 1). At first, the reaction was investigated in
toluene by using 2.5 mol% of Pd(OAc)2 and 2.5 mol% of L1, the
latter of which was revealed to be the best ligand for the so-far
disclosed allylation of amines. As a result, benzylamine 3aa
was obtained in 73% isolated yield (entry 1). The addition of
MS4A as a dehydrating agent was effective for this reaction to
improve the yield to 99% probably by removing water formed in
situ (entry 2). The catalyst loading can be reduced to 1 mol%
without decreasing the yield (entry 3). In the place of Pd(OAc)2,
the other Pd(II) species such as PdCl2 and Pd(acac)2 could not be
used as a catalyst precursor in this reaction (entries 4 and 5). On
the hydroxy group [5].
Hikawa and his co-workers have
developed the palladium-catalyzed benzylation by using benzylic
alcohols in neutral water, where water plays a crucial role to
activate the hydroxy group [6].
On the other hand, less
examples of reactions of benzylic alcohols in organic solvents in
the presence of a palladium catalyst have been reported [7].
Thus, the direct benzylic substitution reaction is still an attractive
method for the organic synthesis.
In the course of our study on a transition-metal-catalyzed
organic synthesis, we were interested in using phosphine-borane
compounds as ligands. Some phosphine-borane complexes have

2
Tetrahedron Letters
the other hand, Pd2(dba)3, one of a Pd(0) complex, gave the
Some other benzyl alcohols having a substituent (1) could be
used in this benzylation reaction. Results of the reactions with
N-methylaniline (2a) are summarized in Table 2. The reactions
of 2a with p- and m-tolylmethanol (1b and 1c) afforded the
corresponding amines 3ba and 3ca in high yields, while o-
tolylmethanol (1d) gave no benzylated product 3da. A slightly
larger amount of catalyst and a higher reaction temperature were
needed to obtain the amine 3ea in quantitative yield from 4-
corresponding product in good yield (entry 6) [13]. We found
that toluene was the best solvent for this reaction among some
other organic solvents (entries 7-10).
variety of phosphine-borane ligands was investigated.
Then, the effect of a
The
ligand having dicyclohexylboryl group L2 was also revealed to
be the good ligand to give 3aa in quantitative yield (entry 11).
Propylene-, butylene-, and o-phenylene-linked phosphine-borane
ligands (L3, L4, and L5) were less effective than L1 (entries 12-
14). No benzylamine 3aa was obtained by use of either ligand
L6 or ligand L7 (entries 15 and 16). The reaction in the co-
presence of both EtPPh2 and B-n-hexyl-9-BBN did not produce
any products (entry 17). These results show that the linkage
between phosphine and borane moieties is necessary and that the
length between phosphorus and boron atoms and its flexibility
are important for this reaction, the ligand L1 and L2 being well
matched.
methoxyphenylmethanol
(1e).
When
[4-
(dimethylamino)phenyl]methanol (1f) was used, the moderate
yield of amine 3fa was obtained, while 4-chlorophenylmethanol
(1g) did not react at all even if 2 mol% of catalyst was used in
refluxing p-xylene.
2-Naphthylmethanol (1h) gave the
corresponding amine 3ha in 43% yield, however, 1-
naphthylmethanol (1i) gave no desired product.
Table 2. Reactions of N-methylaniline (2a) with a variety of
arylmethanols 1.a,b
1 mol% Pd(OAc)2
Table 1. Reactions of N-methylaniline (2a) with benzyl
L1
1 mol%
alcohol (1a).a
+
H
NMePh
2a
Ar
Ar
NMePh
3
OH
MS4A
toluene
1 mol% Pd cat.
1
1 mol% ligand
NMePh
OH
NMePh
2a
+
H
reflux, 24 h
MS4A
solvent
reflux, 24 h
1a
3aa
NMePh
NMePh
NMePh
Entry Pd catalyst
Ligand
Solvent
Yield (%)b
3ba
3ca
3da
, 0%
, 82%
, 99%
1c,d
2c
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
EtPPh2
toluene
toluene
toluene
toluene
toluene
toluene
p-xylene
benzene
THF
73
99
99
0
NMePh
NMePh
NMePh
Me2N
MeO
Cl
3
c
c
c
3ea
3fa
3ga
, 0%
, 99%
, 51%
4
NMePh
5
Pd(acac)2
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
0
NMePh
6e
75
94
43
9
c
c
7
3ha
, 43%
3ia
, 0%
a A mixture of 1 (2.0 mmol), 2a (1.0 mmol), Pd(OAc)2 (0.01 mmol), L1 (0.01
mmol), and MS4A (20 mg) was stirred in refluxing toluene (1 mL) under N2.
8
9
b Isolated yield.
10
11
12
13f
14
15
16
17f,g
1,4-dioxane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
19
99
91
72
76
0
c 2 mol% of Pd(OAc)2 and 2 mol% of L1 were used in refluxing p-xylene
instead of toluene.
Different from primary arylmethanols 1, the reactions of 2a
with 1-phenylethanol and diphenylmethanol (secondary alcohols)
did not proceed at all, suggesting that this reaction does not
proceed via a carbocation intermediate.
Results of N-benzylation reactions of a variety of secondary
amines 2b-i with benzyl alcohol (1a) under the optimized
reaction conditions (Table 1, entry 3) are summarized in Table 3.
N-Ethylaniline (2b) and N-n-propylaniline (2c) gave the products
3ab and 3ac in high yields, respectively. The product yield of
3ad was lower from N-benzylaniline (2d). Because of the steric
hindrance, the reactivity of diphenylamine (2e) was low, and a
longer reaction time and a larger catalyst loading were needed to
achieve the moderate yield of 3ae. Dibenzylamine (2f) and di-n-
butylamine (2g) gave the products 3af and 3ag in 62% and 32%
0
0
a A mixture of 1a (2.0 mmol), 2a (1.0 mmol), Pd catalyst, ligand, MS4A (20
mg) and solvent (1 mL) was stirred under N2.
b Isolated yield.
c 2.5 mol% of Pd(OAc)2 and 2.5 mol% of L1 were used.
d In the absence of MS4A.
e 0.5 mol% of Pd2(dba)3∙CHCl3 was used.
f 2 mol% of Pd(OAc)2 and 2 mol% of ligand were used.
g 2 mol% of B-n-C6H13-9-BBN was added.
yields, respectively.
corresponding amine 3ah, morpholine (2i) reacted with 1a to
afford the corresponding amine 3ai in 64% yield.
While piperidine (2h) gave no
B
B
B
BCy2
Ph2P
Ph2P
PPh2
L3
PPh2
L4
L1
L2
O
B
B
B
P
PPh2
O
PPh2
PPh2 Ph
L5
L6
L7

3
a A mixture of 1a (2.0 mmol), 2 (1.0 mmol), Pd(OAc)2 (0.01 mmol), L1
(0.01 mmol), and MS4A (20 mg) was stirred in refluxing toluene (1 mL)
under N2.
Table 3. Reactions of a variety of secondary amines 2 with
benzyl alcohol (1a).a,b
1 mol% Pd(OAc)2
NR1R2
b Isolated yield.
L1
1 mol%
OH
NR1R2
2
+
H
MS4A
c 2 mol% of Pd(OAc)2 and 2 mol% of L1 were used.
toluene
reflux, 24 h
1a
3
The reaction of aniline (2j) (primary amine) with benzyl
alcohol (1a) proceeded to give N-benzylaniline (4aj) in 69%
yield along with a less amount of N,N-dibenzylaniline (5aj) by
use of 2 equivalents of 1a (Scheme 2).
NEtPh
NnPrPh
NBnPh
NPh2
c
3ab
3ac
3ag
3ad
3ah
3ae
, 57% (48 h)
, 99%
NBn2
, 91%
, 55%
NnBu2
N
N
O
c
3af
3ai
, 64%
, 62%
, 32%
, 0%
1 mol% Pd(OAc)2
L1
H
+
1 mol%
N
OH
N
+
NPh
H2
Ph
Ph
MS4A (20 mg)
toluene (1 mL)
reflux, 24 h
1a
2j
4aj
5aj
(2 mmol)
(1 mmol)
69%
22%
Scheme 2. Reaction of aniline (2j) with benzyl alcohol (1a).
In this benzylation reaction, three deferent reaction pathways
can be conceivable. First, the reaction proceeds via a benzyl
base is needed to produce the benzylamine in high yield. We
tentatively propose the reaction pathway for our catalytic reaction
as shown in Scheme 3. At first, benzyl alcohol reduces
cation intermediate.
However, as shown above, secondary
benzylic alcohols which can give more stable carbocation did not
react at all, omitting this possibility. The second pathway is a
hydrogen transfer strategy, including oxidation and reduction
Pd(OAc)2 to Pd(0). Then, boryl group of the phosphine-borane
ligand acts as a Lewis acid to activate the hydroxy group of
benzyl alcohol. The ethylene linker in L1 is competent to bring
Pd nearby to C–O bond. Successive oxidative addition proceeds
intramolecularly to give a π-benzylpalladium intermediate I
which can be attacked by a nucleophilic amine. The produced
ammonium salt II is deprotonated by the hydroxy group to give
the product amine and 1 equivalent of water as a sole byproduct.
The intramolecular oxidative addition may proceed smoothly by
using ethylene-linked phosphine-borane ligand L1.
process [14].
In this type of reaction, a starting alcohol is
oxidized to the carbonyl compound, which reacts with amine to
form an imine or iminium ion intermediate.
reduction gives the corresponding amine.
A successive
In a palladium-
catalyzed reaction system, the use of a base is necessary to form a
palladium-alkoxide intermediate giving the carbonyl compound
via a β-hydride elimination. In our reaction system, however no
P
B
B
P
NR
H
2
cat. Pd/
P
B
NR2
OH
NR2
Pd
OH
Pd
OH
–H2O
H

II
I
Ph2P
B
P
B
=
P
B
Pd HO
L1
Scheme 3. Plausible reaction pathway.
for supporting introduction of the new sharing system) Grant
Number JPMXS0422500320. This study was supported by the
Advanced Material Science Research Unit Sharing System of
Nagasaki University.
Conclusion
In summary, we have now found that the benzylation of
amines using benzylic alcohols can be catalyzed by
palladium/phosphine-borane catalyst system.
The linking
between phosphine and borane moieties in the ligand is very
important for this catalytic reaction.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.xxxxxx.
Declaration of Competing Interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
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This research was supported by JSPS KAKENHI Grant
Number JP19K05458 (to GO). This work was the result of
using research equipment shared in MEXT Project for promoting
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Direct benzylation of amines with benzylic alcohols
has been developed.
Palladium/phosphine-borane catalyst system was
effective.
The linking between phosphine and borane
moieties in the ligand was important.
A variety of benzylic alcohols and amines could be
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Declaration of interests