Radical addition of tetrahydrofuran to imines assisted by tert-butyl hydroperoxide

Article abstract of DOI:10.1016/j.tetlet.2020.152150

The TBHP-promoted selective radical addition of tetrahydrofuran to imines resulting in β-amino ethers has been developed. The presented radical method of β-amino ether synthesis is based on initiation by available TBHP and does not require metal catalysis

Full text of DOI:10.1016/j.tetlet.2020.152150

Journal Pre-proofs
Radical addition of tetrahydrofuran to imines assisted by tert-butyl hydroper‐
oxide
Vera A. Vil', Sergei S. Grishin, Elena P. Baberkina, Vera A. Kostyagina,
Alexey E. Kovalenko, Alexander O. Terent'ev
PII:
S0040-4039(20)30610-9
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152150
TETL 152150
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
25 April 2020
10 June 2020
11 June 2020
Please cite this article as: Vil', V.A., Grishin, S.S., Baberkina, E.P., Kostyagina, V.A., Kovalenko, A.E.,
Terent'ev, A.O., Radical addition of tetrahydrofuran to imines assisted by tert-butyl hydroperoxide, Tetrahedron
Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.152150
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Radical addition of tetrahydrofuran to
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imines assisted by tert-butyl hydroperoxide
Vera A. Vil’, Sergei S. Grishin, Elena P. Baberkina, Vera A. Kostyagina, Alexey E. Kovalenko, Alexander O.
Terent’ev
untrivial role of TBHP:
an initiator, not an oxidizer
R2
R2
HN
N
TBHP
O
R1
R1
O
12 examples
40-97% yield
metal-free
irradiation-free

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Radical addition of tetrahydrofuran to imines assisted by tert-butyl hydroperoxide
Vera A. Vil’ a, Sergei S. Grishin a,b, Elena P. Baberkina b, Vera A. Kostyagina b, Alexey E. Kovalenko b,
Alexander O. Terent’ev a,b,

a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991, Moscow, Russian Federation
b D. I. Mendeleev University of Chemical Technology of Russia, 9 Miusskaya square, 125047, Moscow, Russian Federation
———
ARTICLE INFO
ABSTRACT
 Corresponding author. Tel.: +7-916-385-4080; e-mail: terentev@ioc.ac.ru
Article history:
Received
The TBHP-promoted selective radical addition of tetrahydrofuran to imines resulting in β-amino
ethers has been developed. The presented radical method of β-amino ether synthesis is based on
Received in revised form
Accepted
Available online
initiation by available TBHP and does not require metal catalysis or light irradiation. Variety of
β-amino ethers are formed in good to excellent yields, despite the possible side oxidation
pathways: ether α-oxidation, the transformation of starting imines into amides, target β-amino
ethers dehydrogenation.
Keywords:
2009 Elsevier Ltd. All rights reserved.
Radical addition
tetrahydrofuran
imines
tert-butyl hydroperoxide
The addition of ethers to C=N bond37 was performed under the
Introduction
+ 39
action of Ti(III)/TBHP (aq.) system,38 Ti(III)/PhN2 ,
The use of radicals is a powerful tool for bond formation in
organic synthesis.1 In the last decades great advances have been
achieved in the field of organic radical chemistry that was largely
driven by the development of new radical precursors and
methodologies.2-12 The addition reactions of carbon–centered
radicals are in the focus of synthetic chemistry because they are
useful for the formation of C–C bonds considered as a basis of
organic chemistry.13, 14
41
TiO2/(NH4)2S2O8 under UV light irradiation,40,
Alk2Zn /
oxygen,42-46 AIBN, and allyl or benzyl chloride / air system.48
47
Kinetic studies were carried out using DTBP49 as radical initiator.
50, 51
Later, the addition of ethers to imines
compounds with C=N bonds
and to other
52-54
was conducted using UV-light
and ionizing irradiation. The Ir salt / thiol system55 and 1,3-
56
butanedione / NHS system were designed for the addition of
ethers to imines under visible light irradiation. In the present
work, we disclosed a selective TBHP-promoted radical addition
of tetrahydrofuran to imines in high yields (Scheme 1).
The radical addition to C=C bond was studied most
thoroughly and a lot of literature was amassed over the years.15-18
The addition of radicals to C=N bond is more challenged.14, 19-22
Even the widespread radical Minishi-type chemistry has
limitations related to the scope of heteroarenes, regioselectivity,
and moderate yields of products.23-25
Our study:
R2
R2
HN
N
TBHP
O
R1
R1
O
The radical addition to imine C=N bond is complicated by the
several factors. An intermediate aminyl radical is less stabilized
R2
R2
O
N
than a radical from oximes and hydrazones due to the absence of
HN
R1
neighboring heteroatom.26,
In addition, imines are easily
O
27
R1
O
hydrolyzed.26, 27 The aza-enolization of the imines with an acidic
α-hydrogen can reduce the content of the tautomer with the
reactive C=N bond.28 Thus, the effective radical addition to imine
C=N bond is the unsolved task, progress in which is limited by
scattered achievements.
O
ether
oxygenation
amine
dehydrogenation
amide
formation
Predictable side-processes do
not prevent the amino ethers synthesis
Scheme 1. Radical addition of tetrahydrofuran to imines and
possible side-processes.
Here we report a selective radical addition of tetrahydrofuran
to imines with the formation of β-amino ethers. This class of
compounds are of interest both as precursors in organic synthesis,
and as analgesic29 and antihistamine30-32 compounds in medicinal
chemistry. β-Amino ethers are also applied in the treatment of
neuromuscular disorders.33-36
Despite the possibility of various oxidative side processes, in
which starting ethers,57 imines and target secondary amines58, 59
can be involved, such as α-oxidation of ethers,60 oxidation of
imines into amides,61-63 oxidative self-coupling of amines,64 and

2
Tetrahedron
dehydrogenation of secondary amines,65 β-amino ethers are
amount up to 4.0 eq. and 6.0 eq. led to high yields of 3a up to
selectively formed under our conditions (Scheme 1).
92% and 89%, respectively (Table 1, entries 2-5). The use of 2.5
mL THF instead of 5.0 mL resulted in a dramatic decrease in 3a
yield to 43% (Table 1, entry 6). When the aqueous solution of
TBHP was applied, the yield of 3a was 30% (Table 1, entry 7). It
may be explained by the imine hydrolysis with the formation of
the aminal or aldehyde and aniline. When the reaction time was
Results and Discussion
To get a deeper insight into the radical addition of
tetrahydrofuran to imines, firstly we investigated the reaction of
(E)-N,1-diphenylmethanimine (1a) with tetrahydrofuran (2)
(Table 1). The influence of initiator nature, additives,
temperature, and the reaction time on the yield of β-amino ether
3a was studied.
°
reduced to 1 h or the temperature was lowered to 80 C, the yield
of the product decreased to 70% or 20% respectively (Table 1,
entries 8, 10). The increased reaction time did not affect the yield
of 3a product (Table 1, entry 9). Other peroxide initiators were
found to be less effective (Table 1, entries 11, 12). When the
metal salts were used for TBHP decomposition, the yield of 3a
was not higher than 60 % (Table 1, entries 13-18). The main side
products in the experiments 11, 14, 18 characterized by the low
yield of 3a and high conversion of 1a were benzaldehyde and
benzoic acid.
Table 1. Optimization of reaction conditions.a
Ph
Ph
HN
N
Initiator
Cond.
O
O
Ph
Ph
1a
2
3a
Yield
of
Convn
of 1a,
%
Initiator
(eq.)
Entry
Additives (eq.)
Cond.
d.r.d
With the optimized conditions in hand (Table 1, entry 4), we
next explored the substrate scope to demonstrate the generality of
3a,
%
β-amino ethers
tetrahydrofuran 2 to imines 1 (Table 2).
3 synthesis via the radical addition of
TBHP b
(0.1)
120 °C /
3 h
1
2
-
-
-
-
-
-
13
90
6
52:48
52:48
53:47
52:48
52:48
53:47
TBHP b
(1.0)
120 °C /
3 h
Table 2. Scope of the reaction.a
82
81
92
89
43
Ar2
Ar2
TBHP b
(2.0)
120 °C /
3 h
HN
Ar1
N
TBHP (4.0 eq.)
120 oC, 3 h
3
92
O
Ar1
O
TBHP b
(4.0)
120 °C /
3 h
4
>95
>95
95
1
2
3
TBHP b
(6.0)
120 °C /
3 h
5
HN
HN
TBHP b
(4.0)
120 °C /
3 h
6c
O
O
TBHP
(70%
aq.)
H3CO
3b, 92% (50:50) b
3a, 92% (52:48) b
120 °C /
3h
7
-
>95
30
52:48
(4.0)
TBHP b
(4.0)
120 °C /
1 h
8
-
-
-
-
-
85
>95
40
70
90
20
0
51:49
52:48
52:48
-
HN
HN
O
H3CO
O
TBHP b
(4.0)
120 °C /
5 h
9
3c, 93% (50:50) b
3d, 90% (51:49) b
TBHP b
(2.0)
80 °C /
3 h
10
11
12
13
14
15
16
17
18
BPO
(4.0)
120 °C /
3 h
HN
O
HN
>95
58
O
DTBP
(4.0)
120 °C/
3 h
53
60
0
51:49
52:48
-
F
Cl
Br
3f, 81% (52:48) b
3e, 86% (52:48) b
TBHP b
(4.0)
Co(OAc)2⸱
2H2O (0.1)
20 °C /
3 h
>95
85
TBHP b
(4.0)
CoCl2⸱6H2O
20 °C /
3h
HN
O
HN
O
(0.1)
TBHP b
(4.0)
CoSO4⸱7H2O
20 °C /
3 h
40
30
17
50
0
53:47
52:48
52:48
-
(0.1)
S
3g, 81% (51:49) b
3h, 40% (50:50) b
TBHP b Cu(OAc)2⸱H2O
(4.0)
20 °C /
3 h
83
(0.1)
F
Cl
TBHP b
(4.0)
Mn(OAc)2⸱
4H2O (0.1)
20 °C /
3 h
76
HN
HN
O
O
TBHP b
(4.0)
20 °C /
3 h
FeCl3 (0.1)
>95
a General reaction conditions: An initiator (0.1-6.0 mmol, 0.1-6.0 eq.) and
additives (0.1 eq) were added to a solution of (E)-N,1-diphenylmethanimine
(1a) (181.2 mg, 1.0 mmol, 1.0 eq.) in THF (5 mL). The reaction mixture was
stirred at 20-120 °C for 1-5 h. b 5.0 M solution of TBHP in decane c THF (2.5
mL). d d.r. (less polar/ more polar).
3j, 55% (54:46) b
3i, 67% (51:49) b
Br
HN
O
HN
O
The catalytic amount of TBHP did not promote this
transformation, the yield of 3a was 6% (Table 1, entry 1), which
is consistent with the previous data.47 Increasing of TBHP
3l, 97% (50:50) b
3k, 71% (55:45) b

3
t
NO2
t-BuO
OH
t-BuOOH
I
HN
NO2
HN
II
O
t-BuOH
O
O
2
3m, not detected
General reaction procedure: 5 M solution of TBHP in decane (4.0 mmol,
800 µL, 4.0 eq.) was added to a solution of imine 1 (1.0 mmol, 1.0 eq.) in
THF (5 mL). Reaction mixture was stirred at 120 °C for 3 h. b d.r. (less polar/
more polar)
3n, not detected
a
O
O
A
2
R2
N
IV
III
R2
R1
Various aryl-substituted imines 1a-g with electron-donating
and electron-withdrawing groups in aryl substituent Ar1 entered
the developed process resulting in the formation of the target β-
amino ethers 3a-g in high yields (Table 2). The thienyl-
substituted β-amino ether 3h was prepared in a moderate 40%
yield. β-Amino ether 3l with the electron-donating group in aryl
substituent Ar2 was obtained in a higher yield (97%) than β-
amino ethers 3i-k with electron-withdrawing groups in aryl
substituent Ar2 (55-71%) (Table 2). Imines 1m, 1n with strong
electron-withdrawing groups in aryl substituent Ar2 did not react
with tetrahydrofuran in these conditions (Table 2). Fine-tuning of
the reaction conditions allowed us to prepare target products with
tetrahydrofuran core in combination with amine moiety under the
action of TBHP. This selectivity is the advantage of the reaction.
1
R2
N
HN
R1
O
R1
O
B
3
V
t-BuOO
t-BuOOH
Scheme 3. Plausible mechanism of the reaction.
Conclusion
It was demonstrated that the radical addition of
tetrahydrofuran to imines could be performed under metal- and
irradiation-free conditions applying available TBHP as an
initiator. A broad scope of β-amino ethers was synthesized with
40-97% yields. Using experimental data and previous reports,
plausible radical mechanism of the reaction was proposed. A
remarkable feature of the work is the impressive selectivity of the
addition products formation in the presence of large excess of a
strong oxidant despite the lability to the oxidation of substrates
and products.
After that, we tried to confirm the radical mechanism of the
developed process. For this purpose, the reaction of
tetrahydrofuran with imine was carried out in the presence of the
radical scavengers BHT (2,6-di-tert-butyl-4-methylphenol) and
TEMPO (Scheme 2).
Ph
Radical trap (4.0 eq)
TBHP (4.0 eq.)
HN
Ph
Ph
N
O
120 oC, 3 h
O
Ph
1a
2
3a
Acknowledgment
solvent
Radical trap:
BHT
The study was supported by the Russian Foundation for Basic
Research (Grant № 20-33-70109). Authors are grateful to the
Department of Structural Studies, Zelinsky Institute of Organic
Chemistry for HRMS analysis.
36%
n.d.
TEMPO
O
N
O
Supplementary data
4
, 52%
1
Supplementary data (the experimental methods and the H
NMR, 13C NMR and HRMS data) associated with this article can
be found, in the online version, at http://........
Scheme 2. Control experiments.
The yield of 3a was reduced to 36% in the presence of BHT
(the yield of 3a in the optimal conditions is 90%). Addition of
TEMPO led to the complete inhibition of the 3a formation and
practically zero conversion of 1a. The radical-trapping product 4
was isolated in a 52% yield based on TEMPO (Scheme 2).
References and notes
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