Enantioselective oxidative cross-dehydrogenative coupling of tertiary amines to aldehydes

Article abstract of DOI:10.1002/anie.201109054

Cooperative catalysts: An enantioselective and direct oxidative coupling of aldehydes to tertiary amines to give β-amino alcohols is described. Catalyzed by copper(II), the reaction proceeds to give a racemic mixture of products; however, by using a combination of copper(II) and a chiral amine catalyst, the reaction gives the desired products with high enantioselectivity (see scheme).

Full text of DOI:10.1002/anie.201109054

Angewandte
Chemie
DOI: 10.1002/anie.201109054
Synthetic Methods
Enantioselective Oxidative Cross-Dehydrogenative Coupling of
Tertiary Amines to Aldehydes**
Junmin Zhang, Bhoopendra Tiwari, Chong Xing, Xingkuan Chen, and Yonggui Robin Chi*
The direct oxidative cross-dehydrogenative coupling (CDC)
amine catalyst.[15] An initial survey of solvents and catalysts
indicated that DMF is a good solvent and CuBr2 is an effective
metal catalyst (Table 1, entries 1–3, see also the Supporting
Information). The amino aldehyde product 3a is unstable
and, thus, was isolated as the corresponding amino alcohol
product 4a in 72% yield after reduction with NaBH4 in situ,
albeit with no diastereoselectivity (Table 1, entry 3). Under
these conditions, various amines, such as N-aryl tetrahydro-
isoquinolines 1 and N,N-dialkyl anilines, and aldehydes 2
were used to obtain the corresponding racemic amino
alcohols 4 in acceptable yields. Aldehydes with longer
carbon chains required elevated reaction temperatures. N,N-
dialkyl anilines gave the corresponding products in lower
yields (Table S6 and Scheme S3 in the Supporting Informa-
tion).
À
of two C H bonds can be an efficient and relatively clean
3
strategy in organic synthesis.[1] Various sp C H bonds, such as
À
[2,3]
À
À
benzylic and allylic C H bonds,
the a-C H bonds of
amines and ethers,[4,5] and the C H bonds of alkanes[6] can be
oxidized for direct reaction with carbon nucleophiles. Among
these reactions, the oxidation of tertiary amines to generate
iminium intermediates, as pioneered by Murahashi et al.,[7] Li
et al.[8] and others,[9–11] has received considerable attention.
Carbonyl compounds, such as 1,3-dicarbonyls,[8e,f,h,10b,g] a,b-
unsaturated ketones,[8f] and simple ketones[2c,11] have been
successfully coupled with tertiary amines by using metal
catalysis, acid catalysis, metal/organic cooperative catalysis, or
photoredox catalysis. In all of these reactions, the develop-
ment of enantioselective catalysis remains a challenge.[12]
Very recently, Wang and co-workers used metals together
with chiral bisoxazoline ligands to realize enantioselective
reactions of activated carbonyl nucleophiles (such as 1,3-
dicarbonyls and acetyl phosphates) to oxidatively generate
iminiums or their analogues.[4d] However, this approach with
chiral ligands was found to be unsuccessful for reactions
starting with simple ketone nucleophiles, as reported by
Klussmann and co-workers[11a] as well as Xie and Huang.[4c]
Approaches that use asymmetric enamine catalysis[13] for the
activation of carbonyl compounds have led to disappointing
results (for example, less than 20% ee) as well[4c,11a,c,d]. As part
of a larger program for developing sustainable oxidation
chemistry,[14] we herein report an enantioselective oxidative
coupling reaction of aldehydes and tertiary amines under
cooperative amine and metal catalysis. The racemic version of
this reaction was also realized by using metal catalysts alone
(without amine catalysts), on account of its potential utility
and the very different optimized conditions, relative to the
enantioselective reactions.
À
Table 1: Model reaction optimization.[a]
Entry Solvent
Amine/Metal t [h] Yield d.r.[c]
ee [%][d]
[%][b] [syn/anti] [syn/anti]
1[e]
2[e]
3[e]
4[e]
5
6
7
8
9
solvents[f]
DMF
DMF
DMF
DMF
DMF
CHCl3
Et2O
CHCl3/Et2O C/CuBr2
CHCl3/Et2O C/CuBr2
CHCl3/Et2O –/CuBr2
CHCl3/Et2O A/CuBr2
CHCl3/Et2O B/CuBr2
CHCl3/Et2O D/CuBr2
CHCl3/Et2O E/CuBr2
–/CuBr
–/CuBr
–/CuBr2
A/CuBr2
B/CuBr2
C/CuBr2
C/CuBr2
C/CuBr2
24
24
24
24
24
24
10
24
17
17
17
17
17
17
17
<5
65
72
63
60
65
52
56
60
n.d.
–
–
–
4/3
49:51
53:47
50:50
44:56
43:57
23:77
50:50
36:64
36:64
n.d.
35/37
48/51
70/67
97/94
96/91
96/92
n.d.
n.d.
70/67
n.d.
n.d.
We examined metal-catalyzed oxidative coupling reac-
tions between N-phenyltetrahydroisoquinoline (1a) and pro-
pionaldehyde (2a) in the presence of tBuOOH without an
10[g]
11[g]
12[g]
13[g]
14[g]
15[g]
66
[*] Dr. J. Zhang, Dr. B. Tiwari, C. Xing, X. Chen, Prof. Dr. Y. R. Chi
Division of Chemistry & Biological Chemistry
School of Physical & Mathematical Sciences
Nanyang Technological University, Singapore 637371 (Singapore)
E-mail: robinchi@ntu.edu.sg
<5
<5
42
<5
<5
n.d.
36:64
n.d.
n.d.
Homepage: http://chigroupweb.org
[a] Reaction conditions: amine (0.03 mmol), AcOH (0.03 mmol), CuBr2
(0.01 mmol), 1a (0.1 mmol), 2a (0.5 mmol), tBuOOH in decane
(0.15 mmol), and solvent (1.0 mL). [b] Combined yield of isolated 4 (syn
and anti). [c] Determined by 1H NMR spectroscopy and HPLC on a chiral
stationary phase. [d] Determined by HPLC on a chiral stationary phase.
[e] No AcOH was used. [f] DMSO, MeOH, toluene, THF, CH2Cl2, CHCl3
and Et2O. [g] 50 mol% AcOH was used. n.d.=not determined,
TMS=trimethylsilyl.
[**] We thank the Singapore National Research Foundation (NRF), the
Singapore Economic Development Board (EDB), GlaxoSmithKline
(GSK), and Nanyang Technological University (NTU) for generous
financial support. We also appreciate the thoughtful suggestions
from the referees.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201109054.
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Table 2: Asymmetric oxidative coupling reaction with metal/organic
We used a chiral amine combined with CuBr2 as
a cooperative catalytic system to develop the enantioselective
reaction between 1a and 2a. The reaction catalyzed by (S)-
proline/CuBr2 in DMF gave the desired product in 63% yield
and in a 1:1 diastereomeric ratio (d.r.), with virtually no
enantioselectivity (4%/3% ee) (Table 1, entry 4). The reac-
tion with diphenylprolinol-based silyl ether catalyst B, in the
presence of 30 mol% of AcOH, gave product 4a with low
ee values (35%/37%; Table 1, entry 5). The ee values were
improved by using catalyst C (48%/51% ee; Table 1, entry 6).
For reactions that were carried out in DMF (Table 1, entry 6),
the low ee values were largely caused by the racemic back-
ground reaction that is not catalyzed by the amine. To
suppress the racemic background reaction, we screened
solvents for this cooperative system with catalyst C (Table 1,
entries 7–9, see also the Supporting Information). The
reaction in CHCl3 was complete a in shorter time and gave
products with improved d.r. (syn/anti = 23:77), but with
moderate ee values (Table 1, entry 7). Fortunately, with
Et2O as the solvent, the reaction proceeded smoothly and
gave 4a with high enantioselectivity (97%/94% ee), moder-
ate yield (56%), but with poor diastereoselectivity (1:1,
Table 1, entry 8). Finally, we found that the mixture of
solvents CHCl3/Et2O (1:1, v/v) provided the coupling product
with an improved d.r., as well as good yields and high
enantioselectivity (Table 1, entry 9). Increasing the loading of
AcOH to 50 mol% led to a higher yield of the product
(Table 1, entry 10). Accordingly, a cooperative catalytic
system that consists of C, CuBr2, AcOH, and a solvent
mixture of CHCl3/Et2O (1:1, v/v) is the optimal system for this
reaction. The desired product 4a was be obtained in 66%
yield, with an improved d.r. (36:64) and in 96%/92% ee after
17 h at room temperature (Table 1, entry 10). Given the
sensitivity of the d.r. and ee values to the reaction conditions
(such as solvents and acid co-catalysts, Table 1, see also the
Supporting Inforation) and the high instability of the
aldehyde Mannich product 3a toward side reactions (such
as decompositions) and racemization, the d.r. and ee values
that were obtained are quite impressive for this class of
oxidative coupling reactions.
cooperative catalysis.[a]
Entry
R1, R2, R3
t [h]
4
Yield
[%][b]
d.r.[c]
[syn/anti]
ee [%][d]
[syn/anti]
1
2
3
4
5
6
7
8
9
H, H, Me
17
11
11
12
12
12
12
4a
4b
4c
4d
4e
4 f
4g
4h
4j
66
71
64
60
52
67
61
68
44
46
43
47
31
37
36:64
28:72
29:71
41:59
22:78
25:75
31:69
26:74
36:64
35:65
34:66
35:65
62:38
57:43
96/92
94/92
86/88
99/94
52/88
86/87
n.d./94
n.d./84
89/84
86/82
81/77
75/73
88/72
67/46
4-Me, H, Me
4-MeO, H, Me
4-Br, H, Me
2-MeO, H, Me
H, MeO, Me
4-Br, MeO, Me
4-Me, MeO, Me 12
H, H, Et
H, H, nPr
H, H, Bn
17
26
17
26
13
42
4j
10
11
4k
4k
4i
4i
[a] Reaction conditions: C (0.03 mmol), AcOH (0.05 mmol), CuBr2
(0.01 mmol), 1 (0.1 mmol), 2 (0.5 mmol), tBuOOH in decane
(0.15 mmol), and CHCl3/Et2O (1:1, v/v, 1.0 mL). [b] Combined yield of
isolated 4 (syn and anti; inseparable by chromatography on a silica gel
1
column for most substrates). [c] Determined by H NMR spectroscopy
and HPLC on a chiral stationary phase. [d] Determined by HPLC on
a chiral stationary phase; the absolute configuration was determined by
the Mosher method (see the Supporting Information).
because of product racemization (Table 2, entries 9–11).[16]
The CDC reaction is believed to involve a single-electron
transfer (SET) radical mechanism (see the Supporting
Information).[7b,17,18]
We also examined other tertiary amine substrates besides
tetrahydroisoquinolines (Scheme 1). Anilines substituted
with electron-donating functionalities gave the desired prod-
Under the optimized conditions, the scope of substrates
for the asymmetric oxidative coupling reactions was then
studied. N-aryl tetrahydroisoquinolines 1 with different R1
and R2 substituents were investigated in the reaction with 2a.
The reactions proceeded readily with both electron-with-
drawing and donating R1 substituents on the N-aryl ring of
1 at room temperature, to afford 4 with high enantioselectivity
(88–94% ee, for the major anti isomers) and in moderate to
good yields (52–71%, Table 2, entries 1–5). The R2 substitu-
ents on the tetrahydroisoquinoline ring did not influence the
reaction outcomes (Table 2, entries 6–8). Aldehydes that have
longer carbon chains, such as N-butyraldehyde and N-
valeraldehyde, were examined. The desired products were
obtained with good enantioselectivity, but in lower yields
(Table 2, entries 9 and 10). The use of 3-phenylpropionalde-
hyde as the aldehyde substrate also gave similar results (lower
yield and ee values). Prolonging the reaction time only slightly
improved the reaction yields (largely as a result of the
instability of 3 before reduction) and led to lower eevalues
Scheme 1. CDC reactions of anilines and aliphatic cyclic amines.
Reaction conditions are the same as used in Table 2. The stated yield
is the yield of isolated product. The ee values were determined by
HPLC on a chiral stationary phase. The d.r. was determined by
1H NMR spectroscopy and HPLC on a chiral stationary phase.
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14090 – 14091; d) D. J. Covell, M. C. White, Angew. Chem. 2008,
ucts 6a and 6b in low to moderate yields (Scheme 1), whereas
electron-deficient anilines were unsuitable substrates for the
oxidative reactions and products 6c and 6d were not obtained
(Scheme 1). With aliphatic cyclic amines, the size of the ring
was found to affect the outcome of the reaction. The reaction
of an N-aryl pyrrolidine with 2a gave the desired product 8a
with an excellent ee value (95% ee), albeit in low yield. No
detectable product was formed in the reaction with an N-aryl
piperidine (Scheme 1, 8b). Other amine substrates that were
examined but did not react well are summarized in the
Supporting Information. It appears that intrinsic limitations
on tertiary amine substrates remain a problem in nearly all of
the oxidative coupling reactions.[8–11]
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À
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In summary, we have developed an enantioselective,
direct, oxidative cross-dehydrogenative coupling reaction of
tertiary amines with aldehydes. With only a metal catalyst,
efficient racemic reactions were also realized. This method
allows optically active b-amino alcohols that contain tetrahy-
droisoquinoline[19] units to be synthesized. To our knowledge,
a highly enantioselective oxidative coupling reaction of
tertiary amines and non-activated carbonyl compounds that
is catalyzed by metal/organic cooperative catalysis has not
been reported previously. Additional studies to improve the
diastereoselectivity of such reactions and to develop other
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À
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Received: December 22, 2011
Revised: February 10, 2011
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Keywords: aldehydes · amines · copper · enantioselectivity ·
.
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[18] In the absence of amine catalysts in DMF, the aldehyde
prenucleophile is presumably activated as its enol form.
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