Highly stereoselective intramolecular Buchner reaction of diazoacetamides catalyzed by a Ru(ii)-Pheox complex

Article abstract of DOI:10.1039/c9cc06889a

This work reports the first efficient enantioselective intramolecular Buchner reaction of diazoacetamides. The Ru(ii)-Pheox catalyst was shown to be highly efficient in this transformation in terms of both the regio-and enantioselectivity (up to 99% ee) giving the desired products in quantitative yield.

Full text of DOI:10.1039/c9cc06889a

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Highly stereoselective intramolecular Buchner
reaction of diazoacetamides catalyzed by a
Ru(II)–Pheox complex†
Cite this: Chem. Commun., 2019,
55, 13398
Received 4th September 2019,
Accepted 9th October 2019
Nga Phan Thi Thanh,
Seiji Iwasa
Masaya Tone, Hayato Inoue, Ikuhide Fujisawa and
*
DOI: 10.1039/c9cc06889a
rsc.li/chemcomm
This work reports the first efficient enantioselective intramolecular
Buchner reaction of diazoacetamides. The Ru(II)–Pheox catalyst
was shown to be highly efficient in this transformation in terms of
both the regio- and enantioselectivity (up to 99% ee) giving the
desired products in quantitative yield.
Medium ring-containing organic molecules, such as seven-
membered rings, are the cornerstone of many bioactive natural
compounds such as guaiane sesquiterpenes, guaianolide
sesquiterpene lactones, and diterpene tiglianes.1 However,
there are few reports on their synthesis. Unlike five- and six-
membered rings, the synthesis of seven-membered rings is
more challenging and generally limited to multi-step processes
rather than direct intramolecular reactions.2 Thus, develop-
ment of an efficient method to prepare these scaffolds has
attracted a significant amount of research attention.
Scheme 1 Transition metal catalytic carbene transfer reaction of diazo-
acetamides.
Over the past few decades, the transition metal-catalyzed the asymmetric intramolecular Buchner reaction of diazoace-
intramolecular Buchner reaction3 has been reported by several tamides, whereby N-tert-butyl-N-(p-methoxybenzyl)enoldiazo-
research groups.4 This unique strategy toward seven-membered acetamide resulted in a mixture of the C–H insertion product
carbocycles has been utilized in natural product synthesis.5 and Buchner product in a total yield of 90% with moderate
However, the catalytic intramolecular reaction of diazoacetamides, enantioselectivities of 41 and 53% ee, respectively (Scheme 1c).8
diazoketones and diazoesters usually leads to competition between
Recently, we have developed a Ru(II)–Pheox9a complex,
the Buchner and C–H insertion reactions.6 Therefore, many reports which is efficient in carbene transfer reactions, in particular,
deal with controlling the regioselectivity of the reaction, which not asymmetric cyclopropanation and Si–H insertion reactions.9
only depends on the type of starting material used, but also the
nature of the reaction solvent.
Driven by our interests in the catalytic intramolecular Buchner
reaction of diazoacetamide, the efficiency displayed by the Ru(II)–
Moreover, when compared to the intramolecular C–H inser- Pheox catalyst, and the importance of the seven-membered ring
tion reaction of diazoacetamides, there are fewer reports on the scaffold in natural product synthesis,5 we started to study the
Buchner reaction (Scheme 1a and b).7 In particular, very few enantioselective reaction, which is much more challenging
examples have addressed the stereoselectivity of the Buchner (Scheme 2).
product from the corresponding diazoacetamide.8 To date, only
Herein, we report the development of an intramolecular
one research study by Doyle and co-workers (2015) has reported Buchner reaction of a variety of N-benzyl diazoamide derivatives
in the presence of a chiral Ru(II)–Pheox catalyst. The aromatic rings
are converted into the corresponding g-lactam ring fused seven-
membered ring system with high regio- and stereoselectivity.
Department of Applied Chemistry and Life Science, Toyohashi University of
Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan.
E-mail: iwasa@chem.tut.ac.jp
At the outset of this investigation, N,N-bis(4-methoxybenzyl)-
† Electronic supplementary information (ESI) available. CCDC 1944984. For ESI
2-diazoacetamide 1b was chosen as the substrate using 1 mol% of
catalyst to optimize the reaction conditions. Initially, well-known
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9cc06889a
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Table 2 Efficiency of the Ru(II)–Pheox catalyst
Scheme 2 Asymmetric intramolecular reaction of diazoacetamides catalyzed
by the Ru(II)–Pheox complex.
Entry X [%] Time [min] TONa TOFb [minÀ1] Yieldc [%] 2b eed [%]
1
2
3
4
5
6
1
0.1
0.01
0.005
0.003 30
0.002 60
2
2
2
2
99
49.5
99
99
99
99
99
20
99
99
99
99
99
99
990 495
9900 4950
19 800 9900
33 000 1100
10 000 167
carbene transfer catalysts were screened and the results summarized
in Table 1.
Extensive studies on the reaction conditions indicated that
after 48 h, product 2b was obtained in 52% yield with no chirality
using Ru(II)–Pybox10 (Table 1, entry 1). When Cu(II)–Box11 was used,
product 2b was formed in 87% yield and 15% ee (Table 1, entry 2).
a
b
TON = moles of desired product (2b)/moles of catalyst (Ru(II)–Pheox).
TOF = TON/reaction time (min). Isolated yield. Determined using
c
d
chiral HPLC analysis.
12
In the case of the Rh2(S-TBPTTL)4 complex, the yield of 2b
increased dramatically to 95%. However, the enantioselectivity
was relatively low (21% ee) (Table 1, entry 4). Screening the various
Ru(II)–Pheox catalyst derivatives developed by our group showed
that the chiral Ru(II)–Pheox complex (Cat. 3) was the most effective
catalyst (Table 1, entry 3 and Table S1, ESI†).9 The reaction
proceeds rapidly to give 2b in excellent yield (99%) with almost
perfect enantioselectivity (99% ee). For more details on the catalyst
screening process, see the ESI.†
We next focused on the efficiency of the Ru(II)–Pheox catalyst
and the results shown in Table 2. We found that decreasing the
catalyst loading from 1 to 0.002 mol% showed no change in the
enantioselectivity (99% ee) of product 2b, while the TON and
TOF values increased (Table 2). Using a very small amount of
the Ru(II)–Pheox catalyst (0.005 mol%) gave product 2b within 2
min in 99% yield with excellent TOF (9900 minÀ1) (Table 2,
entry 4). When 0.003 mol% of catalyst was used, the TON
increased dramatically to 33 000 (Table 2, entry 5).
In Table 2, we found that decreasing catalyst loading from
1 to 0.005 mol% Ru(II)–Pheox, the reaction kept proceeding
rapidly with excellent regio- and stereoselectivities. Therefore the
Ru(II)–Pheox catalyst is extremely efficient for synthesis product 2b
from the corresponding diazoacetamide 1b. However, to avoid
substrate dependency for various diazoamides, we decided to use
1 mol% Ru(II)–Pheox as the standard condition for screening
solvent and substrate scope of the reaction.
In addition, the influence of various solvents on the decom-
position of diazoacetamides was examined and the results shown
in Table S2 (ESI†). DCM was found to be the best solvent for the
Ru(II)–Pheox catalyzed reaction. For more details on the optimiza-
tion of the reaction conditions, see the ESI.†
Table 1 Catalyst screening experiments
Using the optimized reaction conditions, we decided to
explore the substrate scope of the reaction (Table 3). Various
diazoacetamides of N,N-bis(aryl)-2-diazo-acetamides were examined
(Table 3, entries 1–7). Substrates bearing either electron-withdrawing
or electron-donating groups (R = H, F, Cl, Br, CH3, and OCH3) on the
N-benzyl ring were tolerated in the reaction, giving the desired
products (2a–g) in 69–99% yield and 74–99% ee.
Substitution with an electron-donating group (e.g., 4-OCH3,
3-OCH3, and 4-CH3) on the N-benzyl ring moiety has a strong impact
on the reaction (Table 3, entries 2, 3 and 7). The corresponding
Buchner reaction products were obtained in excellent yield (up to
99%) and enantioselectivity (up to 99% ee). In the case of substrates
bearing an electron-withdrawing group (namely 4-Cl, 4-Br and 4-F),
the rate of the Buchner reaction slightly decreased and formation of
the C–H insertion product was observed (Table 3, entries 4–6).
Nevertheless, the yield and enantioselectivity of the products (2d–f)
remained excellent (70–91% yield and 90–96% ee). In addition,
the bicyclic product 2d was prepared with the purpose of
growing crystals suitable for analysis. The structure of 2d was
confirmed and the absolute configuration was determined to
be the S configuration using single-crystal X-ray diffraction
(Fig. 1 and Fig. S1, ESI†).
Entry
Cat.
Time [min]
2b : 3ba
Yieldb [%]
2bc ee [%]
1
Cat. 1
Cat. 2
Cat. 3
Cat. 4
48 h
60
2
100 : 0
100 : 0
100 : 0
100 : 0
52
87
99
95
0
15
99
21
2d
3
4
2
a
The ratio of 2b : 3b was determined using 1H NMR spectroscopy.
Isolated yield. Determined using chiral HPLC analysis. The reaction
b
c
d
temperature was 40 1C.
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Table
3
Ru(II)-Pheox-catalyzed intramolecular Buchner reactions of
diazoacetamides
Scheme 3 Asymmetric intramolecular reaction of diazoacetamide 1h
catalyzed by Ru(II)–Pheox.
Time
2 Yieldb 2 eec
Entry
1
R
R1
[min] 2 : 3a
[%]
[%]
dramatically changed the reaction, affording the corresponding
seven-membered ring product (2k) in 48% yield in 2 min
(Table 3, entry 10). Surprisingly, we found that the reaction
could afford product 2k in 67% yield over a longer reaction time
(4 h) (Table 3, entry 11). The dimerization reaction was prevented
and the reaction yield was improved upon slow addition of a
solution of the diazoacetamide to a stirred mixture of the Ru(II)–
Pheox catalyst in DCM over 4 h (Table 3, entries 8 and 11–15).
Furthermore, there is intense competition between the reactive
sites of the N-aryl-2-diazo-N-methylacetamide (Table 3, entries 8 and
12–14). Therefore, bicyclic products 2i and 2l–2n could be obtained
in moderate yield (40–61%). In the case of diazo compound 1o, the
corresponding product 2o was not obtained.
As a plausible explanation, the substituent changes the
electronic properties of the benzene ring and affects the regio-
selectivity. Nucleophilic substituents, such as 4-CH3 and 4-OCH3,
are regarded as electronic donating groups, which increase the
electropositivity of the aryl group and improve the reactivity in
the aromatic addition reaction. Electrophilic substituents, such
as –Cl, –Br, –F, and –H, are regarded as electron-withdrawing
groups, which decrease the electropositivity of the aryl group and
favor the C–H insertion reaction.
In summary, we have presented a highly stereoselective
intramolecular Buchner reaction of diazoacetamides using a
Ru(II)–Pheox catalyst. Specifically, a variety of g-lactam fused
5,7-bicyclic-heptatriene derivatives have been prepared from
diazoacetamides in up to 99% yield with high enantioselectively
(up to 99% ee) using a chiral Ru(II)–Pheox catalyst under mild
reaction conditions. The product containing diene can be used
for further transformation via the Diels–Alder cycloaddition
reaction.7a,13
1
2
3
4
5
6
7
8
1a
H
C6H5
2
2
2
2
2
2
2
4 h
2
75 : 25 69
100 : 0 99
97 : 3 96
83 : 17 80
80 : 20 70
78
99
97
96
95
90
74
71
99
99
99
92
96
92
—
1b 4-OCH3 4-CH3OC6H4
1c
1d 4-Cl
1e 4-Br
1f
1g 3-OCH3 3-CH3OC6H4
1i
1j
1k 4-CH3
1k 4-CH3
1l
4-CH3
4-CH3C6H4
4-ClC6H4
4-BrC6H4
4-FC6H4
4-F
92 : 8
91
100 : 0 87
45 : 55 40
100 : 0 76
90 : 10 48
H
H
H
H
H
H
H
H
H
9
4-OCH3
10
11
12
13
14
15
2
4 h
4 h
4 h
4 h
4 h
93 : 7
67
4-Cl
83 : 17 61
80 : 20 43
79 : 21 55
1m 4-Br
1n 4-F
1o 4-NO2
—
n.o.
a
The ratio was determined using 1H NMR spectroscopy of the reaction
b
c
mixture. Isolated yield. Determined using chiral HPLC analysis.
In short, entries 1–7 in Table 3 present an overview of the
decomposition of a series of N,N-bis(aryl)-2-diazo-acetamides
used to prepare the target seven-membered ring products
(2a–g) with excellent stereo- and regioselectivity.
Besides, a diazoamide bearing both electron-withdrawing
and electron-donating groups (1h) was also investigated as a
substrate, affording the desired Buchner reaction product (2h)
in high yield (84%) and excellent enantioselectivity (99% ee)
(Scheme 3).
Subsequently, a series of N-aryl-2-diazo-N-methylacetamides
was also investigated under the same reaction conditions
(Table 3, entries 8–15). Substrates with both electron-withdrawing
and electron-donating groups work well in the reaction and afford
their Buchner reaction products (2i–n) in moderate to good yield
(40–76%) and enantioselectivity (71–99% ee).
The reaction afforded the intramolecular Buchner product 2j
(R = 4-OCH3) in 76% yield with high enantioselectivity (99% ee)
(Table 3, entry 9). Switching the substrate to 1k (R = 4-CH3)
This work was supported by a Grant-in-Aid for Scientific
Research (B) (no. 26288087) and Ikeda Bussan Co. Ltd.
Conflicts of interest
There are no conflicts to declare.
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