Direct reductive coupling of secondary amides: Chemoselective formation of vicinal diamines and vicinal amino alcohols

Article abstract of DOI:10.1039/c4cc08330j

We report the first one-pot reductive homocoupling reaction of secondary amides and cross-coupling reaction of secondary amides with ketones to give secondary vicinal diamines and amino alcohols. This method relies on the direct generation of α-amino carb

Full text of DOI:10.1039/c4cc08330j

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ARTICLE TYPE
Direct Reductive Coupling of Secondary Amides: Chemoselective
Formation of Vicinal Diamines and Vicinal Amino Alcohols†‡
Pei-Qiang Huang,a* Qi-Wei Lang,a Ai-E Wanga and Jian-Feng Zhenga
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
formation has not been reported probably because of the
50 presence of a free N–H group in these amides.
We report the first one-pot reductive homocoupling reaction
of secondary amides and cross-coupling reaction of secondary
amides with ketones to give secondary vicinal diamines and
amino alcohols. This method relies on the direct generation of
10 α-amino carbon radicals from secondary amides by
activation with trifluoromethanesulfonic anhydride, partial
reduction with triethylsilane and samarium diiodide mediated
single-electron transfer. The reactions were run under mild
conditions and tolerated several functional groups.
As part of our goal of developing new C–C bond formation
reactions that employ stable amides as substrates,8,9b-c we now
report the generation of α-amino carbon radicals from secondary
amides and the application these reactive species for the
55 development of the first one-pot synthesis of vicinal diamines and
vicinal amino alcohols from secondary amides. Vicinal diamines
and vicinal amino alcohols are privileged scaffolds widely
present in synthetically useful chiral ligands, auxiliaries and
bioactive compounds.12 Although many methods have been
60 developed for the synthesis of these structural motifs,12b,13,14 it is
still highly desirable to develop methods that use stable and easily
available starting materials.
Our investigation was initiated by examining the reductive
homocoupling of benzamide 1a (Table 1). 1a was treated
65 sequentially with trifluoromethanesulfonic anhydride (Tf2O)15
(1.1 equiv) and 2-F-Py16,9 (1.2 equiv) at 0 °C for 30 min,
Et3SiH17,18 at 0 °C to RT for 5 h, and SmI219 (3.0 equiv) for 5 min.
To our delight, the desired diamine 2a was obtained in 86% yield
with a meso/dl ratio of 54:46 (Table 1, entry 1). No N-benzyl-
70 cyclohexamine as a result of unimolecular reduction was
observed. In the presence of a catalytic amount of NiI219d (1 %
15 Amides are easily available1 and highly stable carbonyl
compounds. These features make them excellent starting
materials and synthetic intermediates for a number of useful
transformations,2 including amide group directed C–H
functionalization.3 After these transformations, it is often
20 necessary to convert the amide group, which is at a high level of
oxidation state, to a functionality at lower oxidation state. In this
regard, chemoselective transformation of amides into amines and
ketones via C–C bond formation, as a class of redox economical
reactions,4 have attracted considerable attention.5-9 Although
25 significant progresses have been made recently after the
discovery of the well-known Kulinkovich–de Meijere reaction,5
most of the methods involve the chemoselective or controlled
generation of electrophilic intermediates such as iminoyl triflates,
iminium ions, or nitrilium ions, followed by capturing these
30 reactive intermediates with π-nucleophiles7 or organometallic
reagents.8-9 If reactive species other than electrophilic iminium
ions were generated directly from amides, many other subsequent
reactions other than nucleophilic addition could be anticipated.
Along these lines, few examples involving the direct generation
35 of α-amino carbenes10 or α-amino carbon radicals11 from tertiary
amides were reported. The direct generation of α-amino radicals
from the more challenging secondary amides for C-C bond
Table 1. Optimization of reaction conditions for the reductive
homocoupling of secondary amides.
one- o
p
t
.
e u v
-
-
,
.
e u v
2 F P 1 2 i
q
( )
Ph
N
Ph
N
1
Tf O 1 1
i
,
q
y
)
(
)
2
O
m n
i
CH2Cl2 0 °C 30
,
C
y
Ph
N
C
y
C
y
H
.
e u v
o
2
3
Et SiH 1 1
i
0 °C t RT 5 h
,
,
q
)
(
)
3
H
a
2
H
a
1
m
m n
n e u v
THF
a
ve
S
5
I2
i
i
dditi
RT
,
)
,
q
)
(
,
meso
/ dl
SmI2
(equiv)
% yielda
entry
additive
none
(meso:dl)b
1
2
3
4
5
6
7
3.0
3.0
3.5
2.2
3.0
3.0
3.0
86 (54 : 46)
88 (53:47)
88 (53:47)
74 (53:47)
86 (54:46)
89 (54:46)
88 (55:45)
NiI2 (1 mol%)
NiI2 (1 mol%)
NiI2 (1 mol%)
a Department of Chemistry and Fujian Provincial Key Laboratory for
40 Chemical Biology, Collaborative Innovation Centre of Chemistry for
Energy Materials, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen, Fujian 361005, P.R. China.
Tel: 86-592-2182240; E-mail: pqhuang@xmu.edu.cn
† Dedicated to Professor Henri-Philippe Husson on the occasion of his
45 75th birthday
tBuOH (2 equiv)
HMPA (2 equiv)
Yb(OTf)3 (1 equiv)
‡ Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI:10.1039/b000000x/
75 a Isolated yield; b Determined by 1H NMR analysis of the benzylic protons
of the mixture obtained from a preliminary column chromatographic
separation.
This journal is © The Royal Society of Chemistry [year
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DOI: 10.1039/C4CC08330J
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Page 2 of 4
d
aromatic protons; The reaction was treated with Tf2O at −78 °C for 10
mol), a slightly improved yield of 88% was obtained. However
increasing the amount of SmI2 to 3.5 equiv produced no
additional benefit for the yield (entry 3), lowing its quantity to 2.2
equiv was shown to be detrimental (entry 4). On the other hand,
replacing NiI2 with other additives, including t-BuOH, HMPA or
Yb(OTf)3 failed to improve the diastereoselectivity (Table 1,
entries 5–7). Hence, the use of 3.0 equiv of SmI2 and 1 mol%
NiI2 was determined to be the optimal conditions for the
reductive coupling reaction.[20]
With the reaction conditions optimized, the scope of the one-
pot reductive homocoupling reaction was explored by varying the
substituents on the phenyl ring and the amidyl nitrogen (Table 2).
Electron-donating groups (entries 2, 3), halogens (entries 4–6),
and electron-withdrawing groups (entries 4–7) were shown to be
15 well tolerated on the phenyl ring. A cyano group and an ester,
often considered to be sensitive and labile under reductive
conditions, were found to be compatible with the current process,
furnishing the desired diamine products in moderate yields
(entries 8–9). Meanwhile, amides that bear primary (entry 10)
20 and secondary (entries 1–9 and 11–15) alkyl substituents were
suitable substrates. The introduction of a sterically hindered t-Bu
group (entry 16) or a phenyl ring (entry 17), however, completely
abolished product formation. Lastly, non-benzamides such as
thiophenyl amide 1r (entry 18) and cyclohexyl amide 1s (entry 19)
25 also underwent reductive coupling to produce 2r in 65% yield
and 2s in 54% yield, respectively. However, the homocoupling of
other aliphatic amides gave low yields.
e
min and then at
35 cyclohexylmethanamine was obtained in 20% yield.
0
°C for another 10 min;
A N-benzyl-1-
During our investigations, we found that the reductive
homocoupling reactions also proceeded smoothly in the absence
of NiI2 (see: Electronic Supplementary Information), although the
addition of a catalytic amount of this additive did lead to better
40 yields and more consistent results.
Our success with the reductive homocoupling prompted us to
turn our attention to the more challenging cross-coupling
reactions of secondary amides with ketones. Using the reaction
conditions established for the homocoupling reaction, 1n was
45 subjected to amide activation and controlled reduction before
mixing with 2.0 equiv of cyclopentanone (Table 3). Under these
conditions, the desired cross-coupling product 3a was obtained in
27% yield, along with the homocoupling product 2n in 63% yield
(entry 1). Attempts to increase the yield of 3a by varying the
50 amount of SmI2 or ketone used, or by altering the reaction
temperature, were unsuccessful.
Gratifyingly, the addition of 1.5 equiv of Et3N to the reaction
before introducing cyclopentanone dramatically increased the
yield of 3a to 66%, and concomitantly, limited the formation of
55 2n to 25% yield (Table 3, entry 2). Increasing the amount of
ketone used to 3.0 equiv (entry 3) further tilted the reaction
toward cross-coupling (76% of 3a and 8% of 2n). Using even
higher amount (6.0 equiv), however, did not result in additional
yield increase (entry 4). Conversely, changing the amount of
60 SmI2 to above or below 2.5 equiv invariably lowered the amino
alcohol formation (entries 5–7). Hence, the best conditions for the
cross-coupling reaction consisted the using of 1.5 equiv of Et3N,
3.0 equiv of ketone, and 2.5 equiv of SmI2.
5
10
Table 2. One-pot reductive homocoupling of sec-amides.
one- o
t
-
p
R1
R1
.
e u v
-
.
e u v
2 F P 1 2 i
q
( )
1
)
Tf O 1 1
i
,
)
q
y
(
2
O
m n
0 °C 30
i
,
l
CH2C 2
,
R2
R2
N
H
N
H
R2
Table 3. Optimization of reaction conditions for the cross-coupling of
65 sec- amides with ketones.
R1
N
.
e u v
o
2
3
Et SiH 1 1
i
0 °C t RT 5 h
,
)
,
q
)
)
(
3
H
m
.
e u v
mo
1 l% NiI2
,
)
S
I2 3 0
m n
THF
,
i
q
(
1
2
RT 5
,
i
-
Tf O 2 F P DCM
, , ,
2
-
1
)
y
entry
substrate (R1, R2)
product (%)a
meso:dlb
.
0 °C 0 5 h
,
O
Ph
o
2
Et SiH 0 °C t RT 5 h
,
,
i
)
3
r
P
n
2
+
r
P
1
2
1a (Ph, c-hex)
2a (88)
2b (90)
2c (80)
2d (86)
2e (88)
2f (79)
2g (81)
2h (58)
2i (41)
2j (71)
2k (83)
2l (66)
2m (88)
2n (93)
2o (94)
2p (0)
53:47
55:45
61:39c
56:44
57:43
54:46
54:46
55:45
54:46
70:30
60:40
78:22
55:45
58:42
59:41
-
Ph
N
i
ase
c- en anone
p
.
3
b
0 °C 0 5 h
H
O
HN
H
,
t
,
;
I2
)
n
1
m
a
3
S
,
1b (4-MeC6H4, c-hex)
1c (4-MeOC6H4, c-hex)
1d (4-FC6H4, c-hex)
1e (4-ClC6H4, c-hex)
1f (4-BrC6H4, c-hex)
1g (4-CF3C6H4, c-hex)
1h (4-NCC6H4, i-Pr)
1i (4-MeO2CC6H4, i-Pr)
1j (Ph, n-Bu)
mo
l
1
NiI THF 0 °C
%
, ,
2
3
SmI2
yield (%)a
2n
ketone
(equiv)
4
entry
base
(equiv)
3a
5
1
2
3
4
5
6
6
none
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
2.0
2.0
3.0
6.0
3.0
3.0
3.0
2.5
2.5
2.5
2.5
2.0
3.0
3.5
27
67
76
72
60
71
68
63
23
8
6
7
8
8
9
10
12
14
10
11
12d
13
14
15
16
17
18
19e
1k (Ph, i-Bu)
1l (Ph, c-propyl)
1m (Ph, c-pentyl)
1n (Ph, i-Pr)
a isolated yield.
1o (4-MeC6H4, i-Pr)
1p (Ph, t-Bu)
The scope of the cross-coupling reaction was investigated
(Table 4). Cyclic ketones with ring sizes ranging from 4 to 8 all
70 reacted efficiently with good yields (products 3a–3e, 60–76%
yields). Acyclic ketones were also suitable coupling partners;
however, 8.0 equiv of the ketone were needed to ensure a good
yield of the amino alcohol. For the amide part, both secondary
and primary alkyl N-substituents were tolerated with the latter
75 being inferior. Reaction of cyclopentanone with the benzamides
bearing either electron-donating or electron-withdrawing groups
1q (Ph, Ph)
2q (0)
-
1r (2-thienyl, c-hex)
1s (c-hex, Bn)
2r (65)
2s (54)
60:40
52:48
a
b
1
30 Isolated yield; Meso:dl ratio, determined by H NMR analysis of the
benzylic protons of the mixture obtained from a preliminary column
c
chromatographic separation; Determined by 1H NMR analysis of the
2
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on the benzene ring afforded the expected vic-amino alcohols
3m–3p in 53–79% yields.
constructed efficiently from secondary amides through reductive
coupling reactions. The method relied on the generation of α-
amino carbon radicals from secondary amides through amide
activation, controlled reduction, and SmI2-mediated single-
30 electron transfer. The homocoupling of the α-amino radical or
Table 4. One-pot reductive cross-coupling of sec-amides with ketones.
- -
1 Tf O 2 F P CH Cl 0 °C 30
m n
i
,
,
y
,
2
,
)
2
2
R'
R''
R1
O
.
e u v
i
0 °C t RT 5 h
, ,
)
o
2 Et SiH 1 1
q
)
(
3
R2
cross-coupling with ketones afforded
a variety of vicinal
R1
N
HO
N
H
R2
.
e u v
i
0 °C 30
,
)
m n
H
3 Et N 1 5
i
,
;
q
)
(
3
diamines and vicinal amino alcohols, respectively. The more
challenging cross-coupling reaction required a careful control of
the reactivity of the imine intermediate. Studies that employ other
35 radical acceptors for the α-amino radicals to generate
functionalized amines are currently underway and will be
reported in due course.
The authors are grateful for financial support from the NSF of
China (21332007 and 21472153) and the Program for Changjiang
40 Scholars and Innovative Research Team in University (PCSIRT)
of Ministry of Education, China.
m
.
e u v
R'COR" S I2 2 5 i
,
q
)
,
(
1
3
mo
l
1
NiI 0 °C THF
%
,
,
2
Ph
Ph
Ph
NH
Ph
i
r
P
HO
NH
i
r
i
HO
NH
P
i
r
r
P
HO
HO
NH
P
3b (72%)a,b
3c (75%)a,b
3a (76%)a,b
3d (71%)a,b
Ph
Ph
Ph
HN
Ph
i
r
P
HO
NH
i
r
P
HO
NH
HO
3f (42)[a,b,c]
i
r
P
HO
NH
3g (61%)a,b
3h (68%)a,b
3e (60%)a,b
(75%)a,d
(76%)a,d
Notes and references
Ph
Ph
Ph
Ph
1. H. Lundberg, F. Tinnis, N. Selander, H. Adolfsson, Chem. Soc. Rev.,
2014, 43, 2714.
n
i
HO
HN
HO
HN
u
B
u
B
HO
HN
H
O
HN
45 2. Secondary amides: (a) L. Huang, Q. Wang, W. Wu, H. Jiang, J. Org.
Chem., 2014, 79, 7734; (b) N. Armanino, M. Lafrance, E. M.
Carreira, Org. Lett., 2014, 16, 572; (c) L. Song, K. Liu, C. Li, Org.
Lett., 2011, 13, 3434; Tertiary amides: (d) B. Peng, X. Huang, L.-G.
Xie. N. Maulide, Angew. Chem. Int. Ed., 2014, 53, 8718; (e) B. Peng,
3k (61%)a,b
3l (45%)a,b
3i (69%)a,b
3j (54%)a,b
F
CF3
e
r
M
e
OM
50
55
60
D. Geerdink, C. Farès, N. Maulide, Angew. Chem. Int. Ed., 2014, 53,
5462; (f) B. Peng, D. Geerdink, N. Maulide, J. Am. Chem. Soc., 2013,
135, 14968; (g) V. Valerio, D. Petkova, C. Madelaine, N. Maulide,
Chem. Eur. J., 2013, 19, 2606.
i
i
i
r
P
r
i
NH
HO
H
NH
P
O
r
HO
NH
P
HO
NH
P
3n (79%)a,b
3m (61%)a,b
3o (58%)a,b
3p (53%)a,b
3. (a) W. Song, S. Lackner, L. Ackermann, Angew. Chem. Int. Ed., 2014,
53, 2477; (b) L.-S. Zhang, K. Chen, G. Chen, B.-J. Li, S. Luo, Q.-Y.
Guo, J.-B. Wei, Z.-J. Shi, Org. Lett., 2013, 15, 10; (c) Q. Chen, L.
Ilies, E. Nakamura, J. Am. Chem. Soc., 2011, 133, 428; (d) M. Wasa,
K. M. Engle, J.-Q. Yu, J. Am. Chem. Soc., 2009, 131, 9886.
4. N. Z. Burn, P. S. Baran, R. W. Hoffmann, Angew. Chem. Int. Ed.,
2009, 48, 2854.
a
b
c
5
isolated yield; ketone/amide ratio = 3.0; homocoupling product 2n
was also obtained in 38% yield; d ketone/amide ratio = 8.0.
A plausible reaction mechanism for the coupling reactions is
depicted in Scheme 1. The treatment of secondary amide 1 with
Tf2O yielded reactive nitrilium ion A[16,9b-d], which is then
10 partially reduced with triethylsilane[17,18] to give the protonated
imine B. The highly reactive protonated imine B is then subjected
to the SmI2-mediated homocoupling reaction to give vicinal
diamine 2.
5. (a) V. Chaplinski, A. de Meijere, Angew. Chem. Int. Ed., 1996, 35,
413; (b) V. Chaplinski, H. Winsel, M. Kordes, A. de Meijere, Synlett,
1997, 111; (c) J. Lee, J. K. Cha, J. Org. Chem., 1997, 62, 1584; (d)
Ouhamou, Y. Six, Org. Biomol. Chem., 2003, 1, 3007.
65 6. For reviews, see: (a) O. G. Kulinkovich, A. de Meijere, Chem. Rev.,
2000, 100, 2789; (b) V. Pace, W. Holzer, Aust. J. Chem., 2013, 66,
507; (c) T. Sato, N. Chida, Org. Biomol. Chem., 2014, 12, 3147.
The predominance of 2 in the product profile of the reaction
15 of 1 with cyclcopentenone can also be attributed to the
predisposition of the highly reactive intermediate B to undergo
the reductive homocoupling reaction. This undesired
homocoupling reaction is suppressed by triethylamine to convert
B to its less reactive neutral form imine C. Having comparable
20 reactivity, the SmI2-mediated cross-coupling reaction between
imine C and a ketone is favoured to give vic-amino alcohol 3.
7. (a) G. Bélanger, R. Larouche-Gauthier, F. Ménard, M. Nantel, F.
Barabé, J. Org. Chem., 2006, 71, 704; (b) M. Movassaghi, M. D. Hill,
70
75
80
85
90
J. Am. Chem. Soc., 2006, 128, 4592; (c) H.-B. Zhou, G.-S. Liu, Z.-J.
Yao, J. Org. Chem., 2007, 72, 6270; (d) S.-L. Cui, J. Wang, Y.-G.
Wang, J. Am. Chem. Soc., 2008, 130, 13526.
8. (a) K.-J. Xiao, J.-M. Luo, K.-Y. Ye, Y. Wang, P.-Q. Huang, Angew.
Chem. Int. Ed., 2010, 49, 3037; (b) Y. Oda, T. Sato, N. Chida, Org.
Lett., 2012, 14, 950. (c) S.-Y. Huang, Z. Chang, S.-C. Tuo, L.-H. Gao,
A.-E Wang, P.-Q. Huang, Chem. Commun., 2013, 49, 7088; (d) P.-Q.
Huang, W. Ou, K.-J. Xiao, A.-E Wang, Chem. Commun., 2014, 50,
8761.
9. (a) W. S. Bechara, G. Pelletier, A. B. Charette, Nat. Chem., 2012, 4,
228; (b) K.-J. Xiao, A.-E Wang, Y.-H. Huang, P.-Q. Huang, Asian J.
Org. Chem., 2012, 1, 130; (c) K.-J. Xiao, A.-E Wang, Y.-H. Huang,
P.-Q. Huang, Acta Chim. Sinica, 2012, 70, 1917; (d) K.-J. Xiao, A.-E
Wang, P.-Q. Huang, Angew. Chem. Int. Ed., 2012, 51, 8314.
10. (a) A. Ogawa, N. Takami, M. Sekiguchi, I. Ryu, N. Kambe, N.
Sonoda, J. Am. Chem. Soc., 1992, 114, 8729; (b) A. Ogawa, N.
Takami, T. Nanke, S. Ohya, T. Hirao, Tetrahedron, 1997, 53, 12895;
(c) X.-L. Xu, Y.-M. Zhang, Tetrahedron, 2002, 58, 503.
R1
R1
R3
R1
O
R4
R2
one-pot
or
R1
N
H
R2
N
H
N
H
R2
N
H
R2
HO
1
3
2
3
step
Tf2O
2-F-Py
CH2Cl2
1
step
activation
cross-coupling
SmI2
THF
R3COR4
homocoupling
less reactive
reactants
2
step
reduction
OTf
OTf
R2
+
R1
R1
H
H
Et3SiH
NEt3
R1
N
N
N
R2
C
R2
A
TfOH
B
2-F-Py
2-F-Py
less reactive
intermediates
reactive intermediates
11. (a) S. Kashimura, M. Ishifune, Y. Murai, H. Murase, M. Shimomura,
T. Shono, Tetrahedron Lett., 1998, 39, 6199; (b) C. E. McDonald, A.
M. Galka, A. I. Green, J. M. Keane, J. E. Kowalchick, C. M.
Micklitsch, D. D. Wisnoski, Tetrahedron Lett., 2001, 42, 163; (c) K.
Scheme 1. A plausible reaction mechanism for the coupling reactions.
In summary, we have demonstrated for the first time that
25 secondary vicinal diamines and vicinal amino alcohols can be
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5
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20. For a comparison of more results on the reaction running in the
presence or in the absence of NiI2, see Table 1 in the Electronic
Supplementary Information (ESI).
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