Ag-Catalyzed ring-opening of tertiary cycloalkanols for C-H functionalization of cyclic aldimines

Article abstract of DOI:10.1039/d0cc07181a

We firstly describe a silver-catalyzed direct C-H functionalization of cyclic aldimines with cyclopropanols and cyclobutanolsviaa radical-mediated C-C bond cleavage strategy. The desired products were generated in decent yields with wide substrate scope under mild reaction conditions. In addition, a gram-scale reaction and synthetic transformation of the product were performed.

Full text of DOI:10.1039/d0cc07181a

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Ag-Catalyzed ring-opening of tertiary
cycloalkanols for C–H functionalization of cyclic
aldimines†
Cite this: Chem. Commun., 2021,
57, 1506
Received 1st November 2020,
Accepted 4th January 2021
Jingjing Wang,*a Xue Liu,b Ziyan Wu,a Feng Li,a Ming-Liang Zhang,a Yiman Mi,a
ab
Junhao Wei,a Yao Zhou *c and Lantao Liu
*
DOI: 10.1039/d0cc07181a
rsc.li/chemcomm
We firstly describe a silver-catalyzed direct C–H functionalization functionalization of heterocycles by utilizing strained cycloalka-
of cyclic aldimines with cyclopropanols and cyclobutanols via a nols as eminent building blocks, to our knowledge, the C–H
radical-mediated C–C bond cleavage strategy. The desired pro- functionalization of cyclic aldimines with strained cycloalkanols
ducts were generated in decent yields with wide substrate scope has never been documented to date.
under mild reaction conditions. In addition, a gram-scale reaction
and synthetic transformation of the product were performed.
Cyclic aldimines are one of the privileged core structural
frameworks in organic chemistry, and are recognized as
prevailing molecular entities in drug discovery.8 Therefore,
The elaboration of an inert C–C s-bond to accomplish other the establishment of mild and concise routes for incorporation
serviceable transformations has loomed as an intriguing yet of cyclic aldimines into the targeted molecules is of remarkable
challenging subject in organic synthesis.1 Due to the immanent significance. During the past few decades, a myriad of impress-
ring strain, unmasked cyclobutanols and cyclopropanols are ive strategies have been developed by organic chemists for
prone to undergo C–C bond scission, and have demonstrably the functionalization of cyclic aldimines.9 However, the b- and
served as prominent precursors for the assembly of b- and g-ketone functionalization of cyclic aldimines has yet to be
g- functionalized carbonyl compounds.2,3 Transition-metal-catalyzed explored. In this communication, we disclose the first example
and radical-mediated ring-opening of tertiary cycloalkanols could of b- and g-ketone functionalization of cyclic aldimines via
give rise to the formation of metal homoenolates2c,4 or ketone alkyl Ag-catalyzed ring-opening of tertiary cycloalkanols (Scheme 1c).
radical5 in situ, which could suffer from a plethora of transforma-
We commenced our study by selecting the readily available
tions via coupling with both electrophiles and nucleophiles. In spite cyclic aldimine (1a) and 1-phenylcyclobutanol (2a) as the bench-
of the undisputed advance on ring-opening of strained cycloalk- mark substrates for the optimization of the reaction conditions.
anols, C–H functionalization of heterocycles by using strained The ring-opening product 3aa was indeed gained in 40% yield
cycloalkanols as efficacious partners still remains to be achieved. when the reaction was carried out with the catalytic system of
In this vein, the elegant Minisci-type C–H functionalization of AgNO3/K2S2O8 at room temperature (Table 1, entry 1). The
electron-deficient heterocycles with strained cycloalkanols was
disclosed by Orellana and others (Scheme 1a).6,13 Alternatively,
transition-metal-catalyzed C–H activation could provide another
promising platform for C–H functionalization of heterocycles with
strained cycloalkanols (Scheme 1b).7 With respect to C–H
a Henan Engineering Laboratory of Green Synthesis for Pharmaceuticals,
College of Chemistry and Chemical Engineering, Shangqiu Normal University,
Shangqiu, Henan, 476000, China. E-mail: lifeng20150720@163.com,
liult05@iccas.ac.cn
b College of Chemistry, Zhengzhou University, Zhengzhou, Henan, 450052, China
c College of Chemistry and Chemical Engineering, Hubei Normal University,
Huangshi, Hubei, 435002, China. E-mail: yaozhou@hbnu.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1961173 and
Scheme 1 Transition-metal-catalyzed C–H functionalization of hetero-
cycles with strained cycloalkanols.
1972915. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d0cc07181a
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Table 1 Optimization of the reaction conditionsa
Table
2
Scope for C–H functionalization of cyclic aldimines with
cyclobutanolsa
Entry
Cat.
Oxidant
Solvent (1 : 1)
T (1C)
Yieldb (%)
Entry
1/R0
2/R
3
Yieldb (%)
1
2
3
4
5
6
7
8
AgNO3
AgNO3
AgNO3
AgNO3
AgF
AgOAc
Ag2CO3
Ag2O
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
—
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
Na2S2O8
(NH4)2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
—
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
Acetone/H2O
CH2Cl2/H2O
DMSO/H2O
CH3CN
25
40
60
80
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
40
52
85
78
81
82
80
77
79
72
69
71
76
48
75
84
1
2
3
4
5
6
7
8
1a/H
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2a/C6H5
2b/4-MeC6H4
2c/4-MeOC6H4
2d/2-MeOC6H4
2e/3-MeOC6H4
2f/4-t-BuC6H4
2g/4-FC6H4
2h/4-ClC6H4
2i/4-PhC6H4
2j/4-CF3C6H4
2k/3,5-Me2C6H3
2l/3,5-(t-Bu)2C6H3
2m/2-thienyl
2n/Bn
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
85
75
68
65
66
64
77
68
83
95
70
76
78
68
80
95
90
82
87
80
70
61
89
75
67
1b/6-Me
1c/7-Me
1d/8-Me
1e/6-Et
1f/6-t-Bu
1g/6-MeO
1h/6-F
1i/6-Cl
1j/7-Cl
1k/6-Br
1l/7-Br
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
9
10
11
12
13
14
15c
17d
18e
19
20
9
10
11
12
13
14
15
17
18
19
20
21
22
23
24
25
26
3ja
3ka
3la
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
3aj
3ak
3al
3am
3an
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
80
Trace
Trace
AgNO3
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst (20 mol%),
b
c
oxidant (1.0 mmol), solvent (2 mL), 48 h. Isolated yield. 10 mol% of
catalyst was used. 30 mol% of catalyst was used. 3.0 equiv. K2S2O8
was used.
d
e
1a/H
1a/H
1a/H
1a/H
structure of 3aa was unambiguously confirmed by X-ray single
crystal diffraction.10 In order to enhance the yield of 3aa, we
attempted to increase the reaction temperature (Table 1, entries
2–4). To our delight, the yield of the desired product 3aa was
significantly increased to 85% when the reaction proceeded at
60 1C (Table 1, entry 3). Subsequently, the model reaction was
subjected to a battery of Ag salts to test the effect of the catalyst.
However, the yield of this reaction was decreased when other Ag
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), AgNO3 (20 mol%),
b
K2S2O8 (1.0 mmol), CH3CN/H2O (1 mL/1 mL), 60 1C, 48 h. Isolated
yield.
discrepancy for this Ag-catalyzed C–H functionalization,
enabling the generation of the targeted products (3ea–3la) in
64%–95% yields. Next, we focused on the substrate scope with
salts, such as AgF, AgOAc, Ag2CO3 and Ag2O were used as a respect to cyclobutanols. A range of substituted cyclobutanols
catalyst (Table 1, entries 5–8). We also investigated the effect of were proved to be good donors in this Ag-catalyzed ring-
persulfates (Table 1, entries 9 and 10) and it turned out that
K2S2O8 was still the optimal choice. As a follow-up majorization, a
sequence of solvents were assessed (Table 1, entries 11–13).
However, no superior results were gained and the yield of 3aa
was acutely decreased to 48% when CH3CN was utilized as the
sole reaction medium without H2O as a co-solvent (Table 1, entry
opening reaction. The desired g-ketone functionalized products
3ab–3aj were achieved in good to excellent yields under the
identified conditions. In addition to the mono-aromatic sub-
stituted cyclobutanols, the di-aromatic substituted cyclobuta-
nols 2k and 2l were amenable to this transformation as well,
delivering the expected products 3ak and 3al in 61% and 89%
14). The attempt at changing the loading of AgNO3 and the yields, respectively. Pleasingly, 1-(thiophen-2-yl)cyclobutanol
amount of K2S2O8 was ineffectual, and just lead to inferior results 2m was also a good candidate for C–H functionalization of
(Table 1, entries 15–18). A sluggish reaction was obtained
when the reaction was conducted without AgNO3 or K2S2O8
(Table 1, entries 19 and 20), which manifested that the Ag salt
and oxidant were both obligatory for this transformation.
Having identified the optimal reaction conditions, the scope
of this Ag-catalyzed ring-opening of cyclobutanols for C–H
cyclic aldimines and furnished the targeted product 3am in
75% yield. The treatment of 1-benzylcyclobutanol 2n with cyclic
aldimine 1a was also triumphant and the corresponding pro-
duct 3an could be isolated in 67% yield.
Inspired by the success for C–H functionalization of cyclic
aldimines with cyclobutanols, we then turned our attention
functionalization of the cyclic aldimines was evaluated. As to employing cyclopropanols as ring-opening donors for this
showcased in Table 2, 6-, 7- and 8-substituted groups on the Ag-catalyzed C–H functionalization. As described in Table 3, a
aromatic ring of cyclic aldimines had no influence on the
reactivity and delivered the desired products 3ba–3da in good
yields. Furthermore, electronic effects of substituents on the
aromatic ring of the cyclic aldimines had no significant
vast array of cyclic aldimines displayed good reactivity with the
catalytic system of AgNO3/K2S2O8 by using cyclopropanols as
ring-opening partners. A set of substituted cyclic aldimines
which bear either electron-donating (Me, Et, tBu, OMe) or
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Table 3 C–H functionalization of cyclic aldimines with cyclopropanolsa
Entry
1/R0
4/R
5
Yieldb (%)
1
2
3
4
5
6
7
8
1a/H
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
4a/C6H5
5aa
5ba
5ca
5da
5ea
5fa
5ga
5ha
5ia
88
57
88
75
67
82
94
70
68
72
65
66
50
96
72
69
87
73
85
90
95
92
60
87
1b/6-Me
1c/7-Me
1d/8-Me
1e/6-Et
1f/6-t-Bu
1g/6-MeO
1h/6-F
1i/6-Cl
1j/7-Cl
1k/6-Br
1l/7-Br
1m/6,7-OCH2OC6H3
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
1a/H
Scheme 2 Larger synthesis and synthetic transformations.
also conducted. The reducing product 6aa could be easily
obtained in high yield by using NaBH4 as a reductant. In addition,
the treatment of 3aa with LiAlH4 gave rise to the ring-opening of
cyclic sulfonyl imines, in which the 1,5-diketone derivative 7aa
was achieved in 55% yield (Scheme 2b). Interestingly, the ketone
groups were retained by using LiAlH4 as a reductant according to
analysis by 1H-NMR and 13C-NMR. The reason for this unfamiliar
selectivity still remains unknown.
9
10
11
12
13
14
15
17
18
19
20
21
22
23
24
25
5ja
5ka
5la
4a/C6H5
5ma
5ab
5ac
5ad
5ae
5af
5ag
5ah
5ai
4b/4-MeC6H4
4c/4-MeOC6H4
4d/4-t-BuC6H4
4e/4-FC6H4
4f/4-ClC6H4
4g/4-BrC6H4
4h/4-PhC6H4
4i/3,5-Me2C6H3
4j/2-naphthyl
4k/2-thienyl
4l/Bn
As a follow-up investigation, primary mechanistic studies
were performed to shed light on the reaction mechanism. Since
cyclic-strained alcohols were excessive in the current protocol,
when 1a and 2a were exposed to the standard conditions, low
yield of the byproduct 8 was also observed, which might
originate from the intramolecular radical cyclization of the ketone
alkyl radical.12 In addition, when 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) was adopted as a radical probe under the optimal condi-
tions, this Ag-catalyzed C–H functionalization of cyclic aldimines
was significantly restrained and compound 9 was also detected by
NMR (Scheme 3). These results revealed that a radical-type reaction
regime was involved in this Ag-catalyzed ring-opening reaction.
Based on the above experiments, a possible reaction mecha-
nism for this Ag-catalyzed C–H functionalization of cyclic
aldimines is depicted in Scheme 4. Firstly, Ag(I) is oxidized to
Ag(II) with the assistance of persulfate, which is translated into
sulfate anion radical A.13 The sulfate anion radical A abstracts
the H-atom of the cyclic-strained alcohols to provide the
oxygen-centered radical B, which undergoes the C–C bond
cleavage to afford the ring-opened alkyl radical C.5 The radical
intermediate C attacks the cyclic aldimine 1a to engender the
N-centered radical D. Subsequently, the Ag(II) species is con-
verted to a Ag(I) species via Single Electron Transfer (SET) from
N-centered radical D, which simultaneously goes through
5aj
5ak
5al
1a/H
1a/H
a
Reaction conditions: 1 (0.2 mmol), 4 (0.3 mmol), AgNO3 (20 mol%),
b
K2S2O8 (1.0 mmol), CH3CN/H2O (1 mL/1 mL), 50 1C, 48 h. Isolated
yield.
electron-withdrawing (F, Cl, Br) groups on the aromatic ring
were well tolerated in this transformation, giving rise to the
sb-ketone functionalized products 5aa–5ma in decent yields.
The structure of 5aa was unambiguously confirmed by X-ray
single crystal diffraction.11 Subsequently, we evaluated the
substrate scope with regard to cyclopropanols. A branch of
cyclopropanol derivatives having different electronic properties
were well compatible, affording the targeted products 5ab–5ah
in 69%–96% yield. The cyclopropanol 4i bearing 3,5-dimethyl
on the benzene ring exhibited excellent speciality in this reac-
tion, in which the product 5ai was isolated with 95% yield.
When the sterically demanding 1-(naphthalen-2-yl)cyclopropanol
4j was submitted to this transformation, the ring-opening product
5aj was achieved in 92% yield. To our delight, cyclopropanols
installed on thienyl and benzyl could be engaged in this
Ag-catalyzed ring-opening reaction as well, leading to the produc-
tion of 5ak and 5al in 60% and 87% yields, respectively. Unfortu-
nately, the reaction of the five- and seven-membered cyclic imines
is not suitable for the present catalytic system.
We also inspected the scalability of this Ag-catalyzed ring-
opening reaction for C–H functionalization of cyclic aldimines.
Pleasingly, the current protocol could be readily executed on a
gram scale. When 1a (1.1 g) and 2a (1.34 g) were subjected to
the standard conditions, the desired product 3aa could be
gained in 75% yield (1.48 g) without loss of the efficiency
(Scheme 2a). The further conversion of the product 3aa was Scheme 3 Mechanism studies.
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Scheme 4 Proposed reaction mechanism.
deprotonation with a sulfate anion to deliver the desired product
3 or 5.
In conclusion, we have demonstrated an expedient route to
streamline the synthesis of b- and g-ketone functionalized cyclic
imines, which represents the first example of b- and g-ketone
functionalization of cyclic aldimines through Ag-catalyzed ring-
opening of cyclobutanols and cyclopropanols. By analyzing the
reaction, a radical process is involved in this Ag-catalyzed ring-
opening of tertiary cycloalkanols for C–H functionalization of
cyclic aldimines. The desired products were achieved in good to
excellent yields with a wide substrate scope. Furthermore, the
current Ag-catalyzed C–H functionalization of cyclic aldimines
could be easily carried out on a gram scale without loss of the
efficacy.
This research was financially supported by the National Natural
Science Foundation of China (No. 21402116, 21502111, and
21572126), the Key Scientific and Technological Project of Henan
Province (No. 202102310003), the Key Science Research of Educa-
tion Committee in Henan Province (No. 21A150044), and the
Science and Technology Innovation Talents of Henan Province
(2018JQ0011).
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