Iron-catalyzed C-O bond functionalization of butyrolactam derivatives with various N-/C-nucleophiles

Article abstract of DOI:10.1039/d0nj04548a

An efficient iron-catalyzed C-O bond functionalization of butyrolactam derivatives with various N-/C-nucleophiles to enable the synthesis of pharmaceutically important butyrolactam derivatives has been developed here. The versatility of the present methodology is demonstrated for direct amination and carbonation by using a variety of sulfonamides, amines, amides, indoles, and 1,3-dicarbonyl compounds under mild conditions. The reaction also showed a wide substrate scope and synthetic simplicity. Additionally, only alcohol was produced as the by-product in all cases. This journal is

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Iron-catalyzed C–O bond functionalization
of butyrolactam derivatives with various
Cite this:DOI: 10.1039/d0nj04548a
N-/C-nucleophiles†
a
Danhua Ge,a Mao-Lin Wang,a Xin Wangb and Xue-Qiang Chu
*
An efficient iron-catalyzed C–O bond functionalization of butyrolactam derivatives with various N-/C-
nucleophiles to enable the synthesis of pharmaceutically important butyrolactam derivatives has been
developed here. The versatility of the present methodology is demonstrated for direct amination and
carbonation by using a variety of sulfonamides, amines, amides, indoles, and 1,3-dicarbonyl compounds
under mild conditions. The reaction also showed a wide substrate scope and synthetic simplicity.
Additionally, only alcohol was produced as the by-product in all cases.
Received 11th September 2020,
Accepted 15th November 2020
DOI: 10.1039/d0nj04548a
rsc.li/njc
nitrogen-containing intermediates, especially of gem-diamino
Introduction
derivatives12c,d,e,f,13 (Fig. 1), in drug design and discovery, the
Amination is a direct and powerful strategy for the construction exploration of new methods for the incorporation of this
of new C–N bonds, which feature in the synthesis of various important heterocyclic core into N,N-aminals has become
pharmaceuticals and fine chemicals.1 Generally, this transfor- highly desirable, and the typical structure can be depicted as
mation is performed with pre-activating agents, which are compound I. However, existing approaches (Curtius or
problematic due to over alkylation and the production of Hofmann-type rearrangements)13b,14 to aminal products of this
stoichiometric amounts of waste (Scheme 1, 1).2 Recently, type significantly lack efficiency (multistep and low overall
considerable achievements towards the direct catalytic amina- yield). Although Brønsted acid15 or transition-metal16 catalyzed
tion of alcohols have been made (Scheme 1, 2).3 However, this amidation of N-acylimines is known (Scheme 1, 4), the sub-
transition metal-catalyzed hydrogen autotransfer process suf- strate scope of imines is limited to carboxylic acid derivatives,
fered from several limitations such as extra additives, harsh and related works are rarely reported to date. To the best of our
reaction conditions, metal residue and narrow substrate scope.4 knowledge, the general intermolecular catalytic addition of
On the other hand, while most of the subclasses of stabilized N-/C-nucleophiles to N-acyliminium ions has rarely been reported.
carbenium ions (from benzyl,5 allyl alcohols,6 benzylic ethers7
and acetates8 etc.) have gained prominence for N-alkylation,
similar reactions using N-acyliminium ions remain less
explored (Scheme 1, 3). In addition, previous works focus on
the reaction of intra- or intermolecular N-acyliminium ions
(a-amidoalkylation),9 which results in the formation of new
C–C bonds; however, dealing with coupling with N-centered
nucleophiles which leads to the construction of C–N bonds will
be challenging.10,11
Butyrolactam has been found to be a privileged structural
unit in the field of natural products and other biologically
active molecules.12 Considering the wide applications of
a School of Chemistry and Molecular Engineering, Nanjing Tech University,
Nanjing 211816, China. E-mail: xueqiangchu@njtech.edu.cn
b Hubei Province Geological Experimental Testing Center, Wuhan, Hubei 430034,
China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Methods for the amination.
d0nj04548a
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Table 1 Optimization of the reaction conditionsa
Entry Catalyst (x mol%) Solvent Temp. (1C) Time (h) Yieldb (%)
1
2
3
4
5
6
7
8
Cu(OTf)2 (5)
In(OTf)3 (5)
Zn(OTf)2 (5)
AgOTf (5)
—
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
CH2Cl2 Reflux
24
24
24
24
48
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
44
64
o10
50
0
73
54
FeCl3ꢀ6H2O (5)
I2 (5)
NaHSO4 (5)
HCl (5)
Fe(acac)3 (5)
FeSO4ꢀ7H2O (5)
FeCl2 (5)
59
9
45c
o10
67
10
11
12
13
14
15
16
17
18
19
20
57
70
72
59
Trace
38
Trace
88 (85)de
70
Fe(NO3)3ꢀ9H2O (5) CH2Cl2 Reflux
Fig. 1 Butyrolactam containing drugs and pharmaceutically relevant molecules.
FeCl3ꢀ6H2O (5)
FeCl3ꢀ6H2O (5)
FeCl3ꢀ6H2O (5)
FeCl3ꢀ6H2O (5)
FeCl3ꢀ6H2O (5)
FeCl3ꢀ6H2O (10)
FeCl3ꢀ6H2O (10)
CH2Cl2 Reflux
Tol. Reflux
CH3NO2 Reflux
Herein, we report a simple, chromatography-free, and efficient
iron-catalyzed amination or carbonation of N-acyliminium ions
via benzylic C(sp3)–O activation of functionalized butyrolactam
derivatives for the convenient construction of gem-diamino
derivatives or other butyrolactam derivatives under simple iron-
catalyzed reaction conditions.
MeCN
EtOH
Reflux
Reflux
CH2Cl2 Reflux
CH2Cl2 30
a
Reaction conditions: 1a (1 mmol) and 4-methylbenzenesulfonamide
b
c
2a (1.2 mmol) in 2 mL of solvent. Isolated yield. HCl (37 wt%).
d
e
Yield of 3aa was obtained using simple filtration. In DCM (1 mL).
Results and discussion
With the optimal reaction conditions in hand, we continued
Initially, butyrolactam derivative 1a was used as the electrophi- to examine the scope of the reaction by using various
lic precursor with sulfonamide 2a (a unique pharmacophore)17 N-acyliminium ion precursors. As listed in Table 2, the influence
in the reaction, to access the gem-diamine compounds. In(OTf)3 of leaving groups on N,O-acetal 1a was first investigated. Besides
gave the best result after screening different metal triflate-based ethoxy aminal derivative, methoxy-, isopropoxy-, and tert-butoxy-
catalysts9c,e,11d,g and afforded 3aa in moderate yield (64%) substituted partners have also been proved to be suitable
(Table 1, entries 1–4). As expected, no reaction occurred in the substrates in the reaction, respectively (Table 2, 3aa). To our
absence of a catalyst, even after 48 h (entry 5). Other Lewis acids delight, functional groups on the aromatic ring such as nitro
and Brønsted acids could also activate the benzylic C(sp3)–O (3aa–3ca), cyano (3da), methylsulfonyl (3ea), halogens (3fa–3ha)
bond of 1a (entries 6–9). Among them, FeCl3ꢀ6H2O was the most and methyl (3ja) were compatible with the mild reaction conditions,
suitable to promote this amidoalkylation process, affording the giving the desired butyrolactam derivatives 3aa–3ja in moderate to
desired product 3aa in 73% yield (entry 6). When other iron salts excellent yields (44–90%). However, the ortho steric hindrance had
including Fe(acac)3, FeSO4ꢀ7H2O, FeCl2, and Fe(NO3)3ꢀ9H2O dramatic detrimental effects on the transformation (3ca, 44%). It
were used, no better results were obtained (entries 10–13). Thus, was found that the substrates with strong electron-donating groups
FeCl3ꢀ6H2O was selected as the optimal catalyst for the reaction, obviously reduced the reactivity presumably owing to the low
not only because it offers practical advantages from economic stabilities of these starting materials under acidic conditions
and environmental standpoints, but also due to its low biotoxi- (3ka–3la, 35–40% yields). In the meantime, the corresponding
city. Additionally, changing CH2Cl2 to other solvents, such as aldehydes (by-products) were observed in our system. Other
toluene, CH3NO2, MeCN, and EtOH, just led to decreased aromatic motifs, such as naphthalene-2-yl (1m) and thiophen-
yields (entries 15–18). N-Acyliminium ions generated in these 2-yl (1n), could be facilely incorporated into the skeletons of
cases were less prone to amination than decomposition; thus, the g-butyrolactam-based compound in 78% and 52% yield,
by-product 4-nitrobenzaldehyde was also detected. To our respectively (3ma and 3na). However, heterocyclic substrate 1o
satisfaction, the yield remarkably increased with an attempt was totally inert under the identical conditions, which may be
to increase the catalyst loading from 5 mol% to 10 mol%, and due to the relatively strong basicity arising from the pyridine
the final product could be easily isolated in 85% yield by simple part (3oa). Furthermore, the double functionalization of sub-
filtration without column chromatography (entry 19, in 1 mL strate 1p also worked well, giving rise to the desired amination
of DCM). Additionally, a relatively lower yield (70%) was product 3pa in 59% yield.
obtained when the reaction temperature was decreased to
To gain further insights into this novel iron-promoted
30 1C (entry 20).
amination of N-acyliminium species, a series of N-sources
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Table 2 Substrate scope of butyrolactam derivativesab
Table 3 Substrate scope of various N-nucleophilesab
a
Reaction conditions: 1a (1.2 mmol) and N-source 2 (1 mmol) in DCM
b
(1 mL) at 40 1C for 24–48 h. Yield of pure products obtained using
simple filtration. 1.2 equiv. of N-nucleophile was used. Isolated
yield using column chromatography. 2 mmol-scale reaction.
a
c
d
Reaction conditions: 1 (1 mmol) and 4-methylbenzenesulfonamide 2a
b
e
(1.2 mmol) in DCM (1 mL) at 40 1C for 24–48 h. Yield of pure products
obtained using simple filtration. Isolated yield using column chromato-
graphy. 2 mmol-scale reaction.
c
d
with respect to structurally varied indoles 4, and the results are
listed in Table 4. Generally, indoles containing different sub-
stituents of varying electronic character and steric hindrance
on the indole moieties smoothly reacted with 1-(ethoxy(4-
nitrophenyl)methyl)pyrrolidin-2-one (1a) to deliver the expected
products 5ab–5af in 54%–95% yields (the typical structure can
be depicted as compound II in Fig. 1).
including sulfonamides, aromatic amines, amides and other
N-heterocyclic compounds were explored and the results are
summarized in Table 3. It was found that the reactions of
sulfonamides with 1a proceeded smoothly to produce the
corresponding products with up to 81% yields (3ab–3ad).
However, electron-poor 2e was not a suitable substrate because
the electronic effect might have a dramatic detrimental influ-
ence on product formation (3ae). In addition, a synthetically
useful heterocycle 2f18 also participated in the reaction to give
the corresponding amination product 3af in 91% yield. Simi-
larly, arylamines (2g–i) showed good reactivities and CF3-
containing 2j was also tolerated under the standard conditions
(3ag–3aj, 32–99% yields). Cyclic amide 2k was also found to be a
good candidate, furnishing bis-pyrrolidin-2-one product 3ak in
68% yield. By contrast, benzamide (2m) was unusual in this
regard, only N,N0-((4-nitrophenyl)methylene)dibenzamide 3am
was obtained unexpectedly. Especially noteworthy was that 1H-
benzo[d][1,2,3]triazole (2l) could also react well to give butyr-
olactam derivative 3al12c,d in 61% yield. Further examination
also revealed that other heterocyclic compounds 2n–2q were
reluctant to participant in the reactions.
Table 4 Substrate scope of various indolesa
Interestingly, a C3-alkylated product 5aa was formed in 89%
yield when indole (4a) was introduced into the reaction. Next,
a
Reaction conditions: 1a (1.2 mmol) and indole 4 (1 mmol) in DCM
we set out to study the scope of this C–O bond cleavage protocol (1 mL) at 40 1C for 24 h; isolated yield using column chromatography.
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Scheme 4 Radical trapping experiment.
Scheme 2 Testing 1,3-dicarbonyl compounds and 1,3,5-trimethoxy-
benzene.
The present method could also accomplish the coupling of
N,O-acetal 1a with 1,3-dicarbonyl compounds 6 to give the
corresponding products 7aa–7ac in 34–40% yields under
slightly modified reaction conditions (Scheme 2, 1). Further-
more, 1,3,5-trimethoxybenzene (8) was also a suitable substrate
for the present reaction, leading to product 9 in 43% yield
(Scheme 2, 2). In addition, a large-scale (100 mol) synthesis of
3aa (35.0 g, 90% yield) was also successfully performed based
on the present optimized protocol (Scheme 3).
Scheme 5 Proposed reaction mechanism.
units in biologically active molecules. Extension of the method
to other kinds of C-nucleophiles also improved the synthetic
potentials of the present method in the context of diversity-
oriented synthesis. Moreover, this C–N/C–C bond formation
reaction is highly efficient and practical since it works under
simple reaction conditions in which the products can even be
obtained using simple filtration without the need for organic
extraction or column chromatography. Rewardingly, the usage of
N-acyliminium ions as N-alkylating agents complements amination
approaches and makes headway in catalytic N-acyliminium ion
chemistry.
To gain insights into the putative mechanism of the above
reaction,
a radical trapping experiment was performed
(Scheme 4). When the model reaction was treated with radical
scavengers of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO, 2
equiv.) under standard reaction conditions, the product 3aa
was obtained in 82% yield. This result suggested that a radical-
type mechanism could be ruled out.
On the basis of the control experiment and a literature
survey,5–11 a proposed mechanism is depicted in Scheme 5.
Initially, the coordination of butyrolactam derivative 1 with iron
catalyst followed by C–O bond cleavage gives the N-acyliminium
ion B and FeIII-OR1 species. Subsequently, the intermediate B
reacts with nucleophiles to produce intermediate C. Finally, the
catalytic cycle closes with a proton transfer process between
intermediate C and FeIII-OR1 species and generation of product,
alcohol (HOR1), and iron catalyst.
Experimental
General procedures for the iron-catalyzed C–O bond
functionalization of butyrolactam derivatives with N-/C-
nucleophiles
A solution of butyrolactam derivative 1 (1.2 mmol), N-/C-
nucleophile (1.0 mmol), and FeCl3ꢀ6H2O (16 mg, 0.1 mmol)
in DCM (1.0 mL) was stirred under air at 40 1C for 24–48 h.
Upon completion of the reaction (detected using TLC), the free-
flowing solid was filtered and washed with ethyl acetate/petro-
leum ether (1 : 4) to afford the desired products as pale white
solids. The product thus obtained was recrystallized from
ethanol or ethyl acetate to get pure compounds as white
crystals. Once the final product can’t be separated by filtration,
the solvent of the resulting mixture was removed with the aid of
a rotary evaporator, and the pure product was obtained after
purification of the residue using column chromatography
(silica gel, ethyl acetate/petroleum ether).
Conclusions
In conclusion, a novel, efficient, and chromatography-free Fe-
catalyzed amination of N-acyliminium species via benzylic
C(sp3)–O activation of functionalized butyrolactam derivatives
has been developed. This method provides extremely inexpen-
sive and mild access to various gem-diamino equivalents in
moderate to excellent yields, which are important structural
General procedure for the large-scale synthesis
1a (100 mmol, 26.4 g), 4-methylbenzenesulfonamide 2a
(110 mmol, 18.8 g), and FeCl3ꢀ6H2O (1.62 g, 10 mmol) in 100 mL
Scheme 3 Large-scale direct amination of butyrolactam derivative 1a.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We gratefully acknowledge the financial support from Nanjing
Tech University (Start-up Grant No. 39837146), National Natural
Science Foundation of China (22001121), and Natural Science
Foundation of Jiangsu Province (BK20180690).
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