A silver triflate-catalyzed cascade of in situ-oxidation and allylation of arylbenzylamines

Article abstract of DOI:10.1016/j.tetlet.2016.06.076

A silver-triflate catalyzed cascade of in situ-oxidation and allylation of arylbenzylamines is reported. The 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate is employed as a mild oxidant which is compatible with both catalyst and ligand. Racemic BINAP is also utilized to assist with the catalyst in regulating the yields of products. Various homoallylic amines are obtained in 39–99% yields.

Full text of DOI:10.1016/j.tetlet.2016.06.076

Tetrahedron Letters 57 (2016) 3444–3448
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Tetrahedron Letters
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A silver triflate-catalyzed cascade of in situ-oxidation and allylation
of arylbenzylamines
b
Junjiao Wang a, , Shangdong Yang
⇑
a College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
b State Key Laboratory of Applied Organic Chemistry, Lanzhou University, 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A silver-triflate catalyzed cascade of in situ-oxidation and allylation of arylbenzylamines is reported. The
2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate is employed as a mild oxidant which is
compatible with both catalyst and ligand. Racemic BINAP is also utilized to assist with the catalyst in reg-
ulating the yields of products. Various homoallylic amines are obtained in 39–99% yields.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 25 April 2016
Revised 13 June 2016
Accepted 17 June 2016
Available online 29 June 2016
Keywords:
Silver triflate
In situ-oxidation
Allylation
Arylbenzylamines
Introduction
and convenient pathway for the synthesis of homoallylic amines.
The nodus of this method is to solve the compatibility between
Homoallylic amines are useful synthetic modules1 since the
double bond of the allylic group can be modified into various func-
tional groups in further manipulations2 toward synthesis of many
useful amines. Owing to the advantage of homoallylic amines, che-
mists devoted their efforts to the synthetic research on this kind of
compounds and have achieved fruitful results. So far as we know,
there are three pathways popular (as shown in Scheme 1) to
synthesize homoallylic amines: (1) Direct nucleophilic addition to
imines using allylic metal reagents. This kind of reaction is the most
commonly used pathway, which was studied thoroughly and
widely. It has been well developed no matter in the choice of
allylation reagents and catalytic systems,4e or the application in
synthesis of natural products.3 Moreover, considerable progress
has been achieved in substrate scope, reaction activity, and stereo
control.4 On the other side, there are a few shortages: some imine
substrates are unstable, which need to be prepared in situ; and a
few imine substrates are difficult to prepare or hard to purify. (2)
Using multicomponent reaction to make an aldehyde and an amine
condense into an imine in situ, which is then subjected to the attack
of allylation reagents. In this way, homoallylic amines can also be
obtained,5 and there are also numbers of reports and studies about
oxidant, metallic catalyst, and ligand. By now, there is only one case
using this method for the synthesis of homoallylic amine, it
was reported by Pitchaiah’s group in 2010 that FeCl3ꢀ6H2O
catalyzed C1-achiral oxidative addition to N-aryl tetrahydroiso-
quinoline by allyl-tin reagent can work in the presence of T-HYDRO
as oxidant.8 In this reaction, an iminium cation intermedi-
ate was involved and the double bond of imine was activated to
accept the attack of allyl-tin reagent. Consequently, homoallylic
amine was obtained as a product. Inspired by this work on C–H
oxidation, a silver triflate-catalyzed cascade of in situ-oxidation
and allylation of arylbenzylamines using allyl-Si reagent is reported
by our group.
Considered that allyl-Si reagents are less toxic and low priced,
we chose allyl trimethoxylsilane as allylation reagent at first to
carry out the study. Since we proposed to make the oxidation of
amine and allylation of the
a-position of amino group in one-pot
reaction, the key problem is to find an oxidant that can be compat-
ible with the catalytic system. Fortunately we find hydrogen
accepting oxidant of T+Yꢁ (TEMPO salts) type an ideal choice for
us.9 Employing N-phenylbenzylamine 2a as substrate, we first
tried some regular catalytic systems10 (Table 1, entries 1–3), but
no desired product was discovered. To simplify the reaction condi-
tion, we excluded ligand and additive from the reaction system and
screened Lewis acids directly. Results show that CuOTf, Cu(OTf)2,
Zn(OTf)2, AgOTf, and AgF can all catalyze our reaction and give
the corresponding allylation product 3a (Table 1, entries 4–8).
this method. (3) Recently, using C–H bond oxidation6 of
a-position
of the amino group to generate iminium cation in situ, which is
available to accept the nucleophilic attack,7 offers a brand new
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.tetlet.2016.06.076
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

J. Wang, S. Yang / Tetrahedron Letters 57 (2016) 3444–3448
3445
R1 NH2
R2 CHO
TsOH, we thus chose HOTf as the right additive for this reaction.
To nail down the necessary factor of the reaction, we carried out
control experiments11 based on the existing conditions before fur-
ther optimization. Results indicated that oxidant and AgOTf are
necessary for our reaction. Next, we investigated the loading of
AgOTf as well as reaction temperature. Observation showed that
room temperature and 40 mol % of HOTf is the better choice
(Table 1, entries 15 and 20). Additionally, prolonged reaction times
brought about a decline of yield (Table 1, entry 21). Screening of
solvents indicated that THF behaved better (Table 1, entries
11–19). At last, we tried to add some ligands into the reaction sys-
tem (see SI for details), although any of these ligands gave chiral
product, we pleasantly found that (S)-BINAP could improve the
yield to 99% and shorten the reaction time to 15 h. Given that,
we replaced (S)-BINAP with racemic BINAP and the yield remained
99%. In summary, we determined the optimal reaction conditions
as follows: 10 mol % of AgOTf, 12 mol % of BINAP, 40 mol % of HOTf,
1.1 equiv of T+BFꢁ4 , 2.0 equiv of allyltrimethoxysilane, keeping
stirring for 15 h in THF at room temperature.
We firstly introduced some substitutions on the phenyl group.
When the 4-position of the benzene ring was substituted, the
reaction performed good (Table 2, entries 2–5); besides, when
the 3-position or both the 3 and 4-position were substituted,
the yield was still good (Table 2, entries 6–7). Next, we intro-
duced different substitutions on the benzyl group and found that
when the para-position of amino was substituted by methyl,
halogens, or cyano groups, the reaction showed good to excellent
yields (Table 2, entries 8–11 and 13); however, when the para-
position was substituted by methoxyl or trifluoromethyl, the
yields turned poor (Table 2, entries 12 and 14). In order to further
expand the substrate scope, we also prepared a few substrates
containing substituents on other positions of the benzyl group
from available starting materials in our laboratory and obtained
3o, 3p, and 3q as corresponding products with moderate to good
yields (Table 2, entries 15–17). Generally, this reaction has good
functional group tolerance, for instance, the acid-sensitive cyano
group and unprotected phenolic hydroxyl group, they were not
affected in any way during the reaction (Table 2, entries 13 and
15). We also tried to use 2-pyridinecarboxaldehyde, furfural,
and 2-thenaldehyde as starting materials and synthesized the
aromatic heterocyclic substrates 2r, 2s, and 2t. Fortunately these
substrates were all able to complete the reaction under standard
condition and obtain the corresponding products 3r, 3s, and 3t in
moderate yields (Table 2, entries 18–20).
+
M
Ref. 5
Multicomponent
R2
M
M
R2
R1
N
R1
N
H
R2
R1
N
C-H oxidation
This work
imine addition
H
1
2
3
Scheme 1. Different strategies for the synthesis of homoallylamines.
Given that AgOTf is better than other Lewis acids, we further inves-
tigated some other silver salts (some selected cases are shown in
Table 1, entries 8–11), but AgOTf still gives better behavior. Sup-
pose that anion effect of OTfꢁ might be the key factor during the
reaction course, we took three acidic additives (HOTf, Tf2O, and
TsOH) into investigation (see Table 1, entries 12–14). As expected,
the yields of reactions have been improved further. Since H+ might
give assistance to the releasing of product, moreover, HOTf doesn’t
introduce any other anions into the reaction system compared to
Table 1
Screening of reaction conditionsa
conditions
N
Si(OMe)3
H
N
H
solvent, r.t.
2a
3a
Entry Catalyst
Oxidant
Solvent
Additive Ligandb
Yieldc
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
CuCl
CuF2
AgF
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
K2S2O8
Oxone
O2
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
TBAT
2,20-bipy N.D.
2,20-bipy N.D.
PPh3
N.D.
40%
47%
44%
25%
57%
N.D.
N.D.
N.D.
Trace
N.D.
N.D.
N.D.
21%
14%
54%
22%
27%
64%
30%
12%
39%
19%
27%f
59%g
99%h
d
Cu(OTf)2
CuOTfd
d
Zn(OTf)2
AgFd
AgOTfd
AgOTfd
AgOTfd
AgOTfd
AgOTfd
AgOTfd
AgOTfd
DDQ
TBHP
Furthermore, we found this reaction is also suitable for some
amino acid and peptides [Scheme 2(a) and (b)] and achieved corre-
sponding allylation products in moderate to good yields. For phos-
phoramide 2x, a trace of allylated product was also observed
[Scheme 2(c)].
With these conclusions in hand, we speculated a possible mech-
anism for the reaction (Fig. 1): initially, catalyst AgOTf coordinates
with the ligand, then the allyl-Si reagent B is activated by OTfꢁ
anion and a molecule of (MeO)3Si+OTfꢁ12 is released. Simultane-
ously, a transmetallation takes place between the allyl-Si reagent
and the Ag-ligand complex, giving allylic-silver complex C13 as
an active species. Next, C implements a nucleophilic addition to
imine A, which is oxidized from 2a in situ, and amino silver D is
formed as an intermediate. Finally, under the action of H+ provided
by HOTf, a protonation occurs to intermediate D and gives 3a as
the target product.
PhI(OAc)2 THF
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
T+BFꢁ4
THF
THF
THF
THF
THF
THF
THF
d
AgSbF6
AgNO3
AgBF4
d
d
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
HOTf
Tf2O
TosOH
HOTfe
HOTfe
HOTfe
HOTfe
CH2Cl2
Toluene
PhCl
1,4-Dioxane HOTfe
THF
THF
THF
HOTfe
HOTfe
HOTfe
BINAP
All the reactions were carried out using 0.2 mmol of 2a, 0.22 mmol of T+BF4ꢁ,
2 equiv of allylation reagent, 10 mol % of catalyst and 20 mol % of additives in 2 mL
solvent at rt.
a
b
The amount of ligand is 0.024 mmol.
c
Isolated yield.
d
20 mol % of catalyst was added.
Conclusion
e
40 mol % HOTf was added.
f
This reaction was carried out at 0 °C.
g
Our group have established a one-pot reaction system for
in situ-oxidation and allylation of arylbenzylamines. In this
Reaction time was 24 h.
Reaction time was 15 h.
h

3446
J. Wang, S. Yang / Tetrahedron Letters 57 (2016) 3444–3448
Table 2
Investigation of substrate scopea
-
1.1 equiv T+BF4
10 mol% AgOTf
R2
R2
Products
R1
12 mol% BINAP
N
H
N
H
40 mol% HOTf
R1
R1
Si(OMe)3
THF, r.t.
2a 2t
-
3a-3t
Entry
1
Products
Yields
99%
Entry
Yieldsb
N
H
N
H
3a
12
13
14
3l R1 = OMe
3m R1 = CN
39%
94%
41%
Me
3n R1 = CF3
N
H
3b
2
75%
X
HO
15
16
78%
52%
N
H
N
H
3o
3
4
3c X = F
3d X = Cl
73%
88%
91%
5
3e X = Br
N
H
OMe
3p
MeO
N
H
N
6
7
59%
85%
17
18
88%
61%
H
3f
OMe
3q
N
H
N
H
N
3g
3r
N
H
N
H
O
Me
X
3s
3h
8
89%
19
49%
N
H
9
10
3i X = F
3j X = Cl
99%
81%
11
3k X = Br
77%
20
73%
N
H
S
3t
a
All the reactions were carried out in the presence of 0.2 mmol of substrates in 2 mL solvent at rt.
Isolated yield.
b
reaction, the substrate is oxidized to imine in situ and thus avoids
some troubles of imine such as purification and deterioration, etc.
Besides, the oxidant we chose is mild and compatible with the cat-
alyst and ligand, and the allyltrimethoxysilane we employed as the
allylation reagent is inexpensive, non-toxic, as well as environmen-
tally benign. Moreover, this reaction is highly applicable for
various substrates, even for some heterocyclic substrates. The
resulting homoallylic amines can be transformed into variety of
useful building blocks through a series of modifications, addition-
ally, the aryls on the N atom are easy to remove. Although the
chirality control hasn’t been achieved by us yet, it will be an
objective for us in the future.

J. Wang, S. Yang / Tetrahedron Letters 57 (2016) 3444–3448
3447
-
1.1 equiv T+BF4
10 mol% AgOTf
12 mol% BINAP
40 mol% HOTf
H
N
H
N
(a)
CO2Et
CO2Et
Si(OMe)3
Br
Br
THF, r.t.
2u
3u
yield: 42%
(b)
1.1 equiv T+BF4
15 mol% AgOTf
15 mol% BINAP
-
O
O
H
H
N
N
Si(OMe)3
N
H
N
H
Br
THF, r.t.
Br
2v
3v
yield: 67%
-
1.1 equiv T+BF4
15 mol% AgOTf
15 mol% BINAP
O
O
H
N
H
N
OEt
OEt
N
H
N
H
Si(OMe)3
O
O
Br
Br
THF, r.t.
2w
3w
yield: 91%
-
1.1 equiv T+BF4
(c)
O
15 mol% AgOTf
15 mol% BINAP
O
N
Si(OMe)3
PPh2
H
N
H
PPh2
THF, r.t.
2x
3x
trace
Scheme 2. Screening of other types of substrates.
ligand
AgOTf
Si(OMe)3
B
(MeO)3Si+OTf-
L
Ag
N
O
C
Ph
BF4
Ph
Ph
N
H
N
N
H
3a
A
2a
N
OH
H+
Ph
Ag
L
N
D
Figure 1. Proposed catalytic cycle for the oxidation-allylation one-pot reaction.
evaporated and then the residue was purified on a silica gel column
using petroleum/ethyl acetate (v/v: 100/1) as eluent to afford the
desired product 3a.
Both the synthetic method for arylbenzylamine substrates and
the preparation of T+BF4ꢁ are described in detail in the supporting
information.
Experiment section
An oven-dried 10 mL screw-capped vial containing AgOTf
(0.02 mol), BINAP (0.024 mmol), were evacuated and purged with
argon three times. Then, THF (2.0 mL) was added via syringe. The
mixture was stirred at rt for 15 min and the solution became clear.
Then 36.7 mg (0.2 mmol) of arylbenzylamine 2a and 0.22 mmol of
T+BFꢁ4 were added to the system, and the reaction was stirred at the
same temperature for another 20 min. Next, 0.08 mmol of HOTf
was added, followed by addition of allylic trimethoxylsilane
(0.4 mmol). Kept the reaction stirring for 15 h and monitored by
TLC. After the starting material disappeared, the solvent was
Acknowledgments
We gratefully acknowledge all the coworkers in our lab for their
hard work. And we appreciate Professor Shangdong Yang for his
direction and full support for this work in earnest.

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J. Wang, S. Yang / Tetrahedron Letters 57 (2016) 3444–3448
J.; Kurusu, Y. Chem. Comm. 1999, 1075–1076; (n) Veenstra, S. J.; Schmid, P.
Supplementary data
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4005–4006; (p) Smith, G.; Miriyala, B.; Williamson, J. S. Synlett 2005, 839–841;
For allyl-tin reagent, see: (q) Saidi, M. R.; Azizi, N. Tetrahedron Asymmetry 2002,
13, 2523–2527.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.06.
076.
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Yamamoto, H. Eur. J. Org. Chem. 2009, 5129–5131.
11. Taking 2a as research object and allyltrimethoxysilane as
a necessary
condition, the necessity of AgOTf, T+BF4ꢁ and HOTf were investigated
respectively. Results showed that AgOTf and T+BF4ꢁ are essential for our
reaction, and, the reaction doesn’t work if there was only AgOTf or HOTf added;
if the reaction runs with T+BF4ꢁ only or without AgOTf we will just obtained an
imine which was oxidized from the substrate; excluding HOTf, the yield was
only 25%.
12. (a) Hoesl, C. E.; Maurus, M.; Pabel, J.; Polborn, K.; Wanner, K. T. Tetrahedron
2002, 58, 6757–6770; (b) Ichibakase, T.; Kaneko, T.; Orito, Y.; Kotani, S.;
Nakajima, M. Tetrahedron 2012, 68, 4210–4224.
13. Yanagisawa, A.; Nakashima, H.; Nakatsuka, Y.; Ishiba, A.; Yamamoto, H. Bull.
Chem. Soc. Jpn. 2001, 74, 1129–1137.