Ni-catalyzed direct alcoholysis of N-acylpyrrole-type tertiary amides under mild conditions

Article abstract of DOI:10.1007/s11426-019-9665-5

N-Acylpyrrole-type amides are a class of versatile building blocks in asymmetric synthesis. We report that by employing Ni(COD)₂/2,2′-bipyridine (5 mol%) catalytic system, the direct, catalytic alcoholysis of N-acylpyrrole-type aromatic and aliphatic amides with both primary and secondary alcohols can be achieved efficiently under very mild conditions (rt, 1 h) even at gram scale. By increasing the catalyst loading to 10 mol%, prolonging reaction time (18 h), and/or elevating reaction temperature to 50 °C/80 °C, the reaction could be extended to both complex and hindered N-acylpyrroles as well as to N-acylpyrazoles, Nacylindoles, and to other (functionalized) primary and secondary alcohols. In all cases, only 1.5 equiv. of alcohol were used. The value of the method has been demonstrated by the racemization-free, catalytic alcoholysis of chiral amides yielded from other asymmetric methodologies.

Full text of DOI:10.1007/s11426-019-9665-5

SCIENCE CHINA
Chemistry
•
ARTICLES•
https://doi.org/10.1007/s11426-019-9665-5
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Ni-catalyzed direct alcoholysis of N-acylpyrrole-type tertiary
amides under mild conditions
*
Hang Chen, Dong-Huang Chen & Pei-Qiang Huang
Department of Chemistry, Fujian Provincial Key Laboratory of Chemical Biology, Collaborative Innovation Center of Chemistry for Energy
Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
Received November 6, 2019; accepted December 13, 2019; published online February 20, 2020
N-Acylpyrrole-type amides are a class of versatile building blocks in asymmetric synthesis. We report that by employing
Ni(COD) /2,2′-bipyridine (5 mol%) catalytic system, the direct, catalytic alcoholysis of N-acylpyrrole-type aromatic and ali-
2
phatic amides with both primary and secondary alcohols can be achieved efficiently under very mild conditions (rt, 1 h) even at
gram scale. By increasing the catalyst loading to 10 mol%, prolonging reaction time (18 h), and/or elevating reaction temperature
to 50 °C/80 °C, the reaction could be extended to both complex and hindered N-acylpyrroles as well as to N-acylpyrazoles, N-
acylindoles, and to other (functionalized) primary and secondary alcohols. In all cases, only 1.5 equiv. of alcohol were used. The
value of the method has been demonstrated by the racemization-free, catalytic alcoholysis of chiral amides yielded from other
asymmetric methodologies.
amide transformation, C–N bond activation, esterification, catalysis, nickel
Citation:
Chen H, Chen DH, Huang PQ. Ni-catalyzed direct alcoholysis of N-acylpyrrole-type tertiary amides under mild conditions. Sci China Chem, 2020, 63,
https://doi.org/10.1007/s11426-019-9665-5
1
Introduction
stoichiometric amount of triflic anhydride. In 2015, Garg and
Houk et al. [6a] reported the first nickel-catalyzed activation
of amide C–N bonds for the direct conversion of tertiary
benzanilides to aromatic esters (Scheme 1(a)). Despite the
exciting breakthrough [6b–6d], the method is restricted to
(hetero)aromatic amides. The efforts to extend the substrate
scope to aliphatic amides remained unsuccessful [7]. Sub-
sequently, a two-step protocol to allow the alcoholysis of
secondary aliphatic amides was developed [7]. Nevertheless,
the catalytic, direct alcoholysis of common tertiary aliphatic
amides via C–N activation under mild conditions remains
unconquered [8].
N-Acylpyrroles (A) [9], N-acylpyrazoles (B) [10], N-acy-
lindoles (C) [11,9e], and N-acylindolines (D) [12,9g]
(Scheme 1(b)), a class of heterocyclic tertiary amides, have
been proved to be versatile building blocks in organic
synthesis in general [9–12], and in asymmetric synthesis [9–
Amides (N-monoacylamines) are a class of highly stable and
versatile building blocks in organic synthesis. The transfor-
mation of amides into other functional groups is in high
demand in both organic chemistry and pharmaceutical in-
dustry [1]. However, due to the high stability of amides,
synthetically useful methods are either multi-step transfor-
mations or required harsh conditions [1]. In recent years, the
direct transformation of amides has attracted considerable
attention, which resulted in many mild and useful C–C bond
forming methods [2,3] including the catalytic ones [4]. As
for the transformation of amides into esters, in 1998, Char-
ette et al. [5a] and Dossena et al. [5b] independently reported
a mild method for the one-pot transformation of amides into
esters, which features the in situ activation of amides with a
1
2] in particular. It was envisioned that this class of amides
*Corresponding author (email: pqhuang@xmu.edu.cn)
©
Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

2
Chen et al. Sci China Chem
1
h. The resulting mixture was concentrated under reduced
pressure, and the residue was purified by flash column
chromatography to yield the desired ester.
3
Results and discussion
On the basis of the above mentioned precedents, we opted for
the methanolysis of N-benzoylpyrrole (1a) as a model re-
action. In view of developing an economical method, the
employment of cheaper and earth abundant metal nickel [14]
on one hand, and of cheap and air-stable ligands on the other
hand, for C–N bonds activation was envisaged. At the outset
of our investigation, nickel (II) salts (NiI and NiCl )/2,2′-
2
2
bipyridine solely or in combination with Zn were attempted
Table 1, entries 1–4), but all failed to catalyze the ester-
(
ification reaction. Interestingly, the Ni(Cp) /2,2′-bipyridine
2
combination efficiently catalyzed the methanolysis of 1a to
give ester 3a in 97% yield (Table 1, entry 5). Moreover, Ni
Scheme 1 (a) The first nickel-catalyzed alcoholysis of amides; (b) the
useful heterocyclic tertiary amides; c) our plan (color online).
(
COD) /1,10-phenanthroline combination (Table 1, entry 6)
2
and Ni(COD) /2,2′-bipyridine combination (Table 1, entry 7)
2
effectively catalyzed the methanolysis reaction to produce
the desired ester 3a in almost quantitative yield, whereas
PPh and PCy (entries 8 and 9) were invalid as ligands.
Because 2,2′-bipyridine is much cheaper than 1,10-phenan-
throline, the former was selected for further investigation.
After screening divers reaction parameters including
are more reactive than common amides, and the catalytic
transformation would be feasible [13]. Indeed, recently we
reported a Ni-catalyzed cross-coupling reaction of N-acyl-
pyrrole-type amides with organoboron reagents for ketones
synthesis [13d]. As regarding the catalytic alcoholysis, three
isolated examples have been reported. However, it involved
either a N-acylpyrrole bearing an electron-withdrawing
group at C2 of the pyrrole ring [9e,9f], or a N-acylpyrazole
derivative [9h], and a large excess of alcohol (benzyl alcohol
or methanol) was used for the alcoholysis. The catalytic al-
coholysis of common N-acylpyrroles remains elusive. Thus,
there still remains a need for a versatile method for the al-
coholysis of N-acylpyrroles to esters that is general for dif-
ferent types of N-acylpyrroles, is chemoselective, and
without the need for using a large excess of an alcohol.
Herein, we report the Ni-catalyzed direct alcoholysis of
tertiary heterocyclic amides (A–C) into esters via C–N bonds
activation and cleavage.
3
3
equivalents of Ni(COD) and ligand (2,2′-bipyridine) (en-
2
tries 10–12), solvent used (entries 13 and 14), equivalents of
methanol (entries 15–17), and examining the control ex-
periments (entries 18 and 19), the optimized conditions for
the catalytic methanolysis of N-acylpyrrole 1a were defined
as those outlined in entry 16, namely, methanolysis in the
presence of 5 mol% of Ni(COD) /2,2′-bipyridine combina-
2
tion with 1.5 equiv. of methanol in toluene at rt for 1 h.
With the optimized reaction conditions in hand, the scope
of amide was first investigated and the results are displayed
in Table 2. The reaction tolerated both electron-donating
(p-Me, p-Ph, p-OMe, 3,4,5-tri-OMe, Table 2, entries 2–5)
and electron-withdrawing groups (p-Cl, p-F, p-CF , p-NO ,
3
2
p-CO Me, Table 2, entries 6–10). It is worth mentioning that
2
the high yielding access to esters bearing a F or a CF group
3
2
Experimental
is of value because F and CF are two important functional
3
groups for developing pharmaceuticals, agrochemicals, and
functional materials [15]. Moreover, the smooth methano-
lysis or ethanolysis (entry 11) of benzamides bearing func-
tional groups such as nitro and methyl ester groups (entries 9
and 11) to give the corresponding nitro ester 3i and diester
3jb in excellent yields (92% and 94%) reflect the good
functional group tolerance and chemoselectivity of the re-
action. Other aromatic amides such as 2-naphthamide (1k)
(entry 12) and heteroaromatic amides (1l and 1m, entries 13
and 14) also reacted to give the corresponding esters in 98%,
General procedure for the catalytic alcoholysis of N-acyl-
pyrrole type amides. To a vial was added a N-acylpyrrole-
type amide (0.24 mmol, 1.0 equiv.), 2,2′-bipyridine (1.9 mg,
0
.012 mmol, 5 mol%) and a magnetic stir bar. The vial was
then taken into a glove box. Ni(COD) (3.3 mg, 0.012 mmol,
2
−
1
5
mol%), toluene (0.48 mL, 0.5 mol L ), and an alcohol
(0.36 mmol, 1.5 equiv.) were added successively to the vial.
The vial was then sealed with screw cap, removed from the
glove box, and stirred vigorously at room temperature for

Chen et al. Sci China Chem
3
Table 1 Exploration of reaction conditions
a)
Entry
Catalyst (x mol%)
Ligand (y mol%)
n (equiv.)
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
2.0
1.5
1.0
1.5
1.5
1.5
1.5
1.5
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
Temp.
rt
Yield
1
2
3
4
5
6
7
8
9
NiI (10)
2,2′-bipyridine (10)
2,2′-bipyridine (10)
2,2′-bipyridine (10)
2,2′-bipyridine (10)
2,2′-bipyridine (10)
1,10-phenanthroline (10)
2,2′-bipyridine (10)
NR
2
NiCl (10)
rt
NR
2
NiI /Zn (10)
rt
NR
2
NiCl /Zn (10)
rt
NR
2
Ni(Cp) (10)
rt
97%
>99%
>99%
NR
2
Ni(COD) (10)
rt
2
Ni(COD) (10)
rt
2
Ni(COD) (10)
PPh (10)
rt
2
3
Ni(COD) (10)
PCy (10)
rt
NR
2
3
1
0
1
Ni(COD) (5)
2,2′-bipyridine (10)
2,2′-bipyridine (5)
2,2′-bipyridine (1)
2,2′-bipyridine (5)
2,2′-bipyridine (5)
2,2′-bipyridine (5)
2,2′-bipyridine (5)
2,2′-bipyridine (5)
2,2′-bipyridine (5)
–
rt
>99%
>99%
71%
92%
88%
>99%
>99%
85%
NR
2
1
Ni(COD) (5)
rt
2
12
13
14
15
16
17
18
19
20
21
22
Ni(COD) (1)
rt
2
Ni(COD) (5)
66
84
rt
2
Ni(COD) (5)
dichloroethane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
2
Ni(COD) (5)
2
Ni(COD) (5)
rt
2
Ni(COD) (5)
rt
2
–
rt
Ni(COD) (5)
rt
NR
2
Cu(OTf) (5)
2,2′-bipyridine (5)
2,2′-bipyridine (5)
2,2′-bipyridine (5)
rt
NR
2
Zn(OTf) (5)
rt
NR
2
BF ·Et O (5)
rt
NR
3
2
1
a) Yields determined by H NMR analysis of crude mixture using 1,3,5-trimethoxybenzene as an internal standard. NR=No reaction.
8
7% and 83% yield, respectively.
We next turned our attention to aliphatic amide substrates.
1w, (an amide analogue of progesterone) produced chemo-
selectively the desired ester 3w in 64% yield (entry 10).
The alcoholysis of pyrrole amide 1a with other alcohols
was next surveyed. As can be seen from Table 4, by em-
Under the standard conditions, the methanolysis of dodeca-
namide (1n) proceeded at rt for 1 h to give methyl ester 3n in
8
0% yield (Table 3, entry 1). The reaction of other aliphatic
amides 1o–1r produced the corresponding esters 3o–3r in
5%–91% yields (Table 3, entries 2–5). A longer reaction
ploying 10 mol% of the Ni(COD) /2,2′-bipyridine combi-
2
nation, the reaction can be extended to other primary
alcohols (EtOH, n-PrOH, BnOH, (adamantan-1-yl)methanol
(2k), entries 1–3, and 11) and secondary alcohol (cyclo-
hexanol, 80 °C, 18 h, entry 4). Various functionalized alco-
hols such as geraniol (2f), N-Boc-L-alaninol (2g), tryptophol
(2h), but-3-yn-1-ol (2i), and N-Boc (±)-3-hydro-
xypyrrolidine (2j) also reacted smoothly to yield the corre-
sponding esters 3ab–3ag in 75%–98% yields (entries 5a–
10). To our surprise, whereas the esterification of cyclo-
hexanol yielded a modest yield (60%, entry 4), that of N-Boc
3-hydroxypyrrolidine (2j) afforded the desired ester in an
excellent yield (98%, entry 10). This may be attributed to the
conformational flexibility of five-membered pyrrolidine ring
that renders the ring sterically less demanding. Nevertheless,
sterically hindered tertiary alcohols 1-adamantanol (2l) and
8
time (18 h) was necessary for the methanolysis of aliphatic
amide bearing an ester group (1s), which gave the corre-
sponding diester 3s in 76% yield (entry 6). As illustrated in
entries 7 and 8, α-branched aliphatic amide 1t and even
hindered adamantane-1-carboxamide 1u are viable sub-
strates for the methanolysis when increasing catalyst/ligand
loading to 10 mol% and prolonging time (rt, 5 h, 3t, 93%; or
at 50 °C for 3 h, 3u, 83%). The methanolysis of aliphatic
amide bearing a ketone group (1v, an amide derivative of the
nonsteroidal anti-inflammatory drug loxoprofen, rt, 18 h)
proceeded chemoselectively to produce the corresponding
keto-ester 3v in 93% yield (entry 9). Significantly, the me-
thanolysis of complex substrate containing an enone moiety

4
Chen et al. Sci China Chem
Table 2 Scope of aromatic N-acylpyrrole
Table 3 Scope of aliphatic N-acylpyrrole
a) Isolated yields; b) Ni (COD) /2,2′-bipyridine (10 mol%), rt, 18 h.
2
a) Isolated yields; b) Ni (COD) /2,2′-bipyridine (10 mol%), rt, 18 h; c)
TBSOH (2m), and more acidic phenol 2n failed to react
2
Ni (COD) /2,2’-bipyridine (10 mol%), rt, 5 h; d) Ni (COD) /2,2′-bipyridine
2
2
(entries 12–14) even at a higher temperature (100 °C, entry
(
10 mol%), 50 °C, 3 h.
1
2). As demonstrated by the alcoholysis of both aromatic
amide 1a and aliphatic amide 1o (entries 5b and 7b), the
reaction can be run at gram-scale and the yields were basi-
cally maintained. By elevating the reaction temperature, the
reaction time can be reduced whereas yield retained (Table 4,
entry 6b).
To further extend the scope of the catalytic alcoholysis
method, the methanolysis reactions of N-acylindole 1x, N-
acylcarbazole 1y, N-acylpyrazole 1z, and N-acylindazole
proceeded smoothly to yield methyl benzoate in excellent
yields (90%–96%).
To demonstrate the synthetic value of the method, we
turned our attention to the transformation of heteroaromatic
amide products of other synthetic methodologies. In 2010,
Fu’s group [9g] disclosed an asymmetric Suzuki cross-cou-
pling of racemic α-halo-N-acylindoles to yield the corre-
sponding α-aryl-N-acylindolines in high enantiomeric
1
aa were briefly examined. As can be seen from Table 5, all

Chen et al. Sci China Chem
5
Table 4 Catalytic alcoholysis of N-acylpyrroles: scope of alcohol
Table 5 Ni-Catalyzed alcoholysis of different types of amides and chiral
amides produced by other asymmetric methodologies
a) Ni(COD) /2,2′-bipyridine (10 mol%), rt, 5 h; b) Ni(COD) /2,2′-bi-
2
2
pyridine (5 mol%), rt, 1 h; c) Ni(COD) /2,2′-bipyridine (10 mol%), 80 °C,
2
1
2 h; d) Ni(COD) /2,2′-bipyridine (10 mol%), rt, 1 h; e) Ni(COD) /2,2′-
2
2
bipyridine (10 mol%), rt, 12 h.
racemic form (±)-4 by another method (cf. Supporting In-
formation online). Exposing the dehydration product (±)-1ab
(obtained from (±)-4) via oxidation with 2,3-dicyano-5,6-
dichlorobenzo- quinone (DDQ) to our catalytic methanolysis
conditions produced racemic ester (±)-3al in 75% yield.
Next, we addressed the alcoholysis of chiral N-acylpyrazoles
1
ac and 1ad, synthesized by Zhang’s rhodium/bisphosphine-
thiourea-catalyzed enantioselective hydrogenation of the
corresponding α,β-unsaturated N-acylpyrazoles [10b]. Ni-
Catalyzed methanolysis of 1ac and 1ad under standard
conditions proceeded smoothly to yield methyl esters 3am
and 3an in 88% and 72% yield, respectively. Similarly, the
catalytic esterification of chiral N-acylpyrazole 1ae, pre-
pared by Meggers’ method [10c] featuring catalytic, en-
antioselective addition of alkyl radicals to alkenes via
visible-light-activated photoredox catalysis with a chiral
rhodium complex, produced ester 3ao in 81% yield. It is
worth noting that no racemization was observed in the last
three methanolysis reactions, reflecting the mildness of our
method. In 2017, Hou’s group [9i] developed a highly dia-
a) Isolated yields; b) T=80 °C, 18 h; c) T=50 °C, 1 h; d) T=100 °C, 18 h.
excess. To showcase the feasibility of our method to trans-
form the products obtained by Fu’s methodology, we se-
lected one of his product (S)-4 (Table 5) and prepared its

6
Chen et al. Sci China Chem
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21931010), the National Key Research and
Development Program of China (2017YFA0207302), the Program for
Changjiang Scholars and Innovative Research Team in University of the
Ministry of Education, China.
stereo- and enantioselective palladium-catalyzed [3+2] cy-
cloaddition of vinyl epoxides and α,β-unsaturated ketones.
Subjecting one of his product (amide 1af) to the alcoholysis
with geraniol (2f) yielded the corresponding ester 3ap, a
potential substrate for the Ireland-Claisen rearrangement, in
8
0% yield.
Conflict of interest The authors declare that they have no conflict of
To probe a plausible Lewis acidic effect of Ni(COD) on
interest.
2
the reaction, Lewis acids Cu(OTf) , Zn(OTf) , and BF •EtO
2
2
3
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors
were employed to replace Ni(COD) , respectively. In all
2
cases, no expected ester 3a was detected (Table 1, entries 20–
2
2). Thus in the light of the computational study of Garg and
Houk [6a] on the nickel-catalyzed direct conversion of ter-
tiary benzanilides to aromatic esters, a plausible mechanism
for the Ni-catalyzed alcoholysis of N-acylpyrrole-type ter-
tiary amides is depicted in Scheme 2. The scenario involves
an oxidation addition of Ni(0) into amide C–N bond to
generate complex A. An exchange of ligand with an alcohol
forms complex B, which then releases ester 3 through re-
ductive elimination.
1
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3
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4
Conclusions
In summary, we have developed a versatile method for the
catalytic transformation of N-acylpyrrole, N-acylpyrazole,
N-acylindole, and N-acylindoline-type tertiary amides into
the corresponding esters using only 1.5 equiv. of an alcohol.
The method is amenable to both aromatic amides (with an
aromatic or heteroaromatic ring attached to the carbonyl) and
aliphatic amides on gram-scale. The observed good che-
moselectivity and functional group tolerance vis-à-vis sen-
sitive groups such as ketone, enone, and esters, as well as
amides bearing a chiral center reflect the mildness of the
reaction conditions. The synthetic applicability of this
method was demonstrated by the smooth alcoholysis of
chiral amides produced by several asymmetric methodolo-
gies developed by several groups. The ester formed from
geraniol produced a chiral ester that might be used as a
substrate for the Ireland-Claisen rearrangement.
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Scheme 2 A plausible mechanism for the Ni-catalyzed alcoholysis of N-
acylpyrrole-type tertiary amides (R=aryl; alkyl).

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7
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