Palladium-catalyzed carbonylation of benzylic ammonium salts to amides and esters: Via C-N bond activation

Article abstract of DOI:10.1039/c8ob00488a

An efficient palladium-catalyzed carbonylation reaction of readily available quaternary ammonium salts with CO is reported for the first time to afford arylacetamides and arylacetic acid esters via benzylic C-N bond cleavage. This protocol features mild reaction conditions under atmospheric pressure of CO, a redox-neutral process without an additional oxidant, and a broad substrate scope for various kinds of amines, alcohols and phenols.

Full text of DOI:10.1039/c8ob00488a

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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Palladium-Catalyzed Carbonylation of Benzylic Ammonium Salts
to Amides and Esters via C-N Bond Activation
Weijie Yua, Shuwu Yanga, Fei Xionga, Tianxiang Fana, Yan Fenga, Yuanyuan Huanga, Junkai Fu*b, Tao
Wang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient palladium-catalyzed carbonylation reaction of readily
available quaternary ammonium salts with CO is reported for the
first time to afford arylacetamides and arylacetic acid esters via
benzylic C-N bonds cleavage. This protocol is featured with mild
reaction conditions under atmosphere pressure of CO, redox-
neutral process without additional oxidant, and broad substrate
scope for various kinds amines, alcohols and phenols.
aminocarbonylation of styrene involving alkene
insertion into Pd–H species and subsequent migratory
insertion of CO into the forming metal–alkyl bond
(Scheme 1. B).7 However, both linear and branched
products can be obtained in most cases. Recently, the
palladiumꢀcatalyzed oxidative carbonylation of benzylic
C(sp3)−H bonds with stoichiometric amount of oxidants
provides an atomꢀeconomic approach to arylacetamides
and arylacetic acid esters (Scheme 1. C).8 Additionally,
most of the above methods require high pressures of CO
gas, which are hazardous for largeꢀscale preparation. In
this context, searching for pratical and redox neutral
method to promote carbonylation under mild reaction
conditions is highly desirable.
Introduction
Since the seminal work by Heck and coꢀworkers in
1974, the palladiumꢀcatalyzed carbonylation with CO as
inexpensive and readily available C1 building block has
attracted extensive attention in both scientific research
and chemical industries.1 A variety of carbonylꢀ
containing products such as esters, amides, ketones, and
aldehydes could be produced through carbonylation
reactions in the presence of suitable nucleophiles.2 In
this case, arylacetamides and arylacetic acid esters,
which are frequently encountered scaffold in nature
products and pharmaceutical agents,3 can be assembled
through palladiumꢀcatalyzed carbonylation with
inherent advantages over traditional amidation4 and
esterification5 of benzyl carboxylic acids suffering from
harsh reaction conditions or multistep operations. The
most common method is to use benzylic halides as the
palladium precursors (Scheme 1. A), which would result
in the formation of stoichiometric amount of halides as
the waste.6 Another strategy is palladiumꢀcatalyzed
A. Carbonylation reactions via carbon-halogen bond cleavage with CO
R
R
R
Nu
[Pd]
CO (2-50 bar)
X
[PdX]
O
NuH
- HX (stoichiometric)
NuH = amine, alcohol, phenol
B. Carbonylation reactions via CO insertion of alkenes
R2
N
PdCl2
R1
R1NR2
amine
+
CO (20 or 50 atm)
+
O
ꢀ
branched product
C. Carbonylation reactions via benzylic C(sp3)–H bond activation
Pd(Xantphos)Cl 2
Nu
+
CO (10 or 50 atm)
+
NuH
TBP (stoichiometric)
O
120 °C
NuH = amine, alcohol
D. Direct alkoxycarbonylation of benzyl amines and benzyl alcohols
OR′
[Pd(II)], ligand
R
R′OH
O
°
C
CO (1 bar or 20bar) , 140
R = NH2 or OH
E. Carbonylation reactions via C(sp3)–N bond cleavage with CO (this work)
Nu
PdCl2(dppf)
NMe
3 OTf
R
+
CO (1 atm)
+
NuH
R
O
100 °C
NuH = 1°, 2° (aryl, alkyl)amine, 1°, 2°, 3°alcohol and phenol
√ environment-friendly
√ redox neutral
√ mild reaction conditions (1 atm CO)
√ broad substrate scope
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Scheme 1. Synthesis of arylacetamides and arylacetic acid
naphthalenꢀ2ꢀamine
and
pyridinꢀ2ꢀamine
were
successfully transformed into corresponding products
3aw and 3ax in good yields. Furthermore, alkylamines
are suitable partners for this carbonylation reaction.
Primary alkylamines could well undergo the reaction to
afford amides 3ayꢀbd in good yields, while secondary
alkylamines just resulted in the formation of products
3beꢀbg in moderate yields. Notably, αꢀaminoꢀesters,
which can be easily obtained from corresponding
enantioꢀenriched natural amino acids, have proven to be
compatible with the reaction conditions. Commercially
available αꢀaminoꢀesters derived from glycine, valine or
phenylalanine could all be converted into the amide
products 3bhꢀbj in moderate yields with excellent ee
value.
esters via palladiumꢀcatalyzed carbonylation reactions
Quaternary ammonium salts have recently attracted
much attention as C–N bond containing electrophiles
with some remarkable features, such as nontoxic,
thermally stable and easy to handle. Benefited from
strong C−N bond polarity and release of electronically
neutral amine companions, the ammonium salts show
much higher C−N reactivity compared to the
corresponding amine precursors. Since the pioneering
work of Wenkert,9 transition metalꢀcatalyzed
borylation,10 Suzuki,11 Negishi,12 Kumada,13 Stille14 and
some others cross coupling reactions15 have been
successfully developed based on quaternary ammonium
salts. Watson16 reported a nickelꢀcatalyzed cross
coupling of benzylic quaternary ammonium salts and
boronic acids. Martin group17 disclosed a userꢀfriendly
nickelꢀcatalyzed reductive carboxylation of benzylic Cꢀ
N bonds with CO2. Very recently, the combination of
quaternary ammonium salts and carbonylation reaction
has got remarkable progress. Li utilized an in situ
prepared NaCo(CO)4 to promot carbonylation of
quaternary ammonium salts to tertiary amides at
200 °C.18 Huang reported a Niꢀcatalyzed carbonylation
through an amineꢀI2 chargeꢀtransfer complex.19 During
our preparation of this manuscript, Wu and coꢀ
workers20 disclosed an elegant palladiumꢀcatalyzed
direct alkoxycarbonylation of benzyl amines and benzyl
alcohols (Scheme 1. D). In continuing with our interest
in oxidative carbonylation of aryl hydrazines through Cꢀ
N bond activation,21 herein, we reported an efficient
palladiumꢀcatalyzed carbonylation with CO to
arylacetamides and arylacetic acid esters by utilizing
benzylic ammonium salts as acylpalladium precursors
under very mild conditions (Scheme 1. E).
Table 1. Substrate Scope of Aminocarbonylation.a
R2
PdCl2(dppf)
N
NMe3OTf
PPh3, Na2CO3
R3
R1
R2R3NH
R1
CO
PhMe/DMSO, 100°C
8-16 h
O
(1 atm)
1
2
3
H
N
R1
3al, R2= 4-CH3, 87%
3am, R2= 4-iPr, 96%
3an, R2= 4-OCH3, 89%
3ao, R2= 2-CH3, 99%
3ap, R2= 2-OCH3, 99%
3aq, R2= 2-Ph, 90%
3ar, R2= 4-F, 95%
3as, R2= 4-Cl, 87%
3at, R2= 4-CN, 95%
3au, R2= 3-CF3, 98%
3av, R2= 2-OH, 78%
O
3aa R1=H, 97%
3ab R1= 2-CH3 95%
3ac R1= 4-CH3 95%
3ad R1= 4-OCH3 96%
3ae R1= 4-F 81%
H
N
O
R2
3af R1= 4-Cl 89%
3ag R1= 4-CN 85%
3ah R1= 4-COOMe 94%
3ai R1= 4-CF3 98%
3aj R1= 3 4-difluoro 95%
H
N
H
H
N
N
N
O
O
O
3ak, 63%
3aw 76%
3ax, 89%
We started our studies by using benzyl quaternary
ammonium salt 1a and aniline 2a as the model
substrates. After screening of the reaction conditions
(see SI), the optimized conditions for the reaction were
PdCl2(dppf) (3 mol%) and PPh3 (30 mol%) in a mixture
solvent (Vtoluene : VDMSO = 5 : 1) under 1 atm CO at 100
oC in the presence of Na2CO3 (2 equivalent) as the base
for 12 h, affording the desired product 3aa in 97% yield.
As can be seen in Table 1, the scope of the
arylacetamides was firstly investigated. Various
ammonium salts including benzylic ammonium salts
with sterically hindered orthoꢀmethyl group (3ab),
electronꢀdonating (3ac, 3ad) or electronꢀwithdrawing
groups (3aeꢀaj) on the phenyl ring and 2ꢀnaphthyl
ammonium salt (3ak) could afford the desired products
in excellent yields. A variety of substituted anilines
were then tested to react with benzylic ammonium salt
to smoothly give corresponding amides 3alꢀau, and the
halogen atoms, such as F and Cl, did not affect the
carbonylation reactions. When 2ꢀaminophenol was
employed as the nucleophile, 3av was obtained in 78%
yield without the detection of alkoxycarbonylation byꢀ
product. Other types of aryl amines, for instance,
H
N
H
N
H
N
Ph
O
O
Ph
O
3ay 79%
3az, 72%
3ba, 81%
H
H
N
H
N
N
Ph
O
O
O
3bb, 91%
3bc, 63%
3bd
85%
S
O
N
O
N
O
N
O
3bg, 79%
3be 55%
3bf 55%
O
H
N
O
O
H
N
H
N
OEt
OEt
OEt
O
O
O
3bi, 42%[b], > 99% ee
3bj, 39%[b], 94% ee
3bh, 83%
aReaction conditions: 1 (0.40 mmol), 2 (0.20 mmol), PdCl2(dppf) (3
mol%), PPh3 (30 mol%), Na2CO3 (0.40 mmol) in mixture solvent
(Vtoluene : VDMSO = 5 : 1) under 1 atm CO at 100 oC for corresponding
times. b30 mol% dppf was used to replace PPh3.
The above encouraging results prompted us to
extend the reaction to alkoxycarbonylation (Table 2).
2 | J. Name., 2012, 00, 1-3
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(5ar), thiophene (5as), functionalized phenyl ring (5at)
and terminal alkene (5au) were all wellꢀtolerated for the
reaction. Later, the secondary alcohols were
investigated. All of the fourꢀmembered, fiveꢀmembered
and sixꢀmembered secondary alcohols were suitable
nucleophiles to give 5avꢀav in moderate to good yields.
However, cycloheptanol failed to give any desired
products. The tertiary alcohol, for example, 1ꢀ
methylcyclohexanol could also proceed smoothly to
afford the ester 5ay, albeit in a low yield of 20%.
Finally, the phenols were successfully employed in
the alkoxycarbonylation reaction under slightly
modified reaction conditions. As shown in Table 3,
benzyl ammonium salts bearing electronꢀneutral (7aaꢀ
ac), electronꢀdonating (7ad) or weak electronꢀ
withdrawing groups (7ae, 7af, 7aj) on the phenyl ring
all generated the desired esters in good yields, while
strong electronꢀwithdrawing groups (7agꢀai) would
slightly decrease the yields. The naphthaleneꢀcontaining
quaternary ammonium salt provided the product 7ak in
just 32% yield. Subsequently, a variety of phenols
bearing different substituents with diverse electronic
effects were tested, and gave the corresponding ester
products 7alꢀat in excellent yields except that the 4ꢀ
acetylphenol resulted in a moderate yield (7au).
Moreover, naphthalenꢀ2ꢀol, disubstituted phenols and
multifluoroꢀsubstituted phenols could all be transformed
into the corresponding products 7avꢀay in acceptable
yields.
Substituents on the phenyl ring of benzylic quaternary
ammonium salts were initially evaluated. The electronic
effects of the substituents made little difference for the
outcomes, and ester products 5aaꢀah were obtained in
good yields when using benzyl alcohol as the partner
under modified reaction conditions (for details, see SI).
It is worth noting that 4ꢀchloro/bromo benzylic
quaternary ammonium salts would undergo double
alkoxycarbonylation reactions to afford 5ai in 40%/45%
yields through simultaneous CꢀX (Cl, Br) bond cleavage
and CꢀN bond activation. For ammonium salt containing
naphthyl moiety, the alkoxycarbonylation reaction
offered the corresponding ester 5aj in 47% yield. Then
different kinds of alcohols were tested. Benzylic alcohol
with electronꢀwithdrawing group at paraꢀposition of the
phenyl ring (5al) gave a better yield of 79% compared
to that with electronꢀdonating group (5ak, 57%). Linear
Table 2. Substrate Scope of Alkoxycarbonylation with
Alcohols.a
O
R2
PdCl2(dppf)
NMe3OTf
R1
R1
R2OH
CO
(1 atm)
O
Na2CO3, PhMe
100°C, 10-16 h
1
4
5
[b]
5aa
, R1= H, 82% (94%)
, R1= 4-CH3, 86%
O
Bn
5ab
O
O
Bn
5ac, R1= 2-CH3, 86%
5ad, R1= 4-F, 82%
O
Bn
O
R1
O
5ai
, R1 =4-Cl/Br, 40%/45%
5ae
, R1= 3,4-difluoro, 81%
, R1= 4-CF3, 80%
5af
5ag
O
Table 3. Substrate Scope of Alkoxycarbonylation with
Bn
, R1= 4-OCH3, 62%
O
5ah, R1= 4-COOMe, 66%
Phenols.a
5aj, 47%
R2
O
O
Cl
O
PdCl2(dppf)
dppf
OH
O
n
5
NMe3OTf
O
R1
R2
O
CO
R1
O
O
Na2CO3, PhMe
100°C, 8-14 h
(1 atm)
R2
[c]
5ak
5am
n = 1, , 48%
n = 3, 5an, 54%
R2= OCH3,
R2= CF3, 5al, 79%
, 57%
5ao
, 61%
1
6
7
O
O
O
O
O
R1
7al, R2= 4-CH3, 83%
7am, R2= 4-iPr, 89%
7an, R2= 4-tBu, 84%
7ao, R2= 4-OCH3, 87%
7ap, R2= 2-CH3, 76%
7aq, R2= 4-F, 80%
O
O
O
N
O
O
7aa, R1= H, 81%
5ap
, 67%
5aq
, 65%
5ar,
54%
O
7ab, R1= 4-CH3, 88%
7ac, R1= 2-CH3, 70%
7ad, R1= 4-OCH3, 85%
7ae, R1= 4-F, 85%
O
O
S
O
O
O
O
O
R2
7ar, R2= 4-Cl, 89%
7as, R2= 4-CN 92%
7at, R2= 4-CF3 91%
O
O
7af, R1= 4-Cl, 80%
5as
, 63%
5at
, 74%
5au
, 80%
7ag, R1= 4-CN, 56%
7ah, R1= 4-CF3, 48%
7ai, R1= 4-COOtBu, 67%
5av
, 77%
O
7au
, R2= 4-COCH3, 58%
O
n = 0,
n = 1, 5aw, 61%
5ax
n
O
O
O
O
n = 2,
, 66%
F
O
O
O
5ay
, 20%
O
F
aReaction conditions: 1 (0.20 mmol), 4 (1.0 mmol), PdCl2(dppf) (10
mol%), Na2CO3 (0.20 mmol) in PhMe (2.0 mL) under 1 atm CO at
100 oC for corresponding times. bBenzyl alcohol as solvent. c20
mol% dppf was used.
7aj, 83%
7av, 58%
7aw, 71%
F
O
O
F
O
O
O
F
O
F
F
F
F
7ak, 32%
7ax, 73%
7ay, 48%
alcohols, such as ethanol and butanol, suffered from the
reaction conditions to provide 5am and 5an in 48% and
54% yields, respectively. Other primary alcohols with
diverse functionalities including chlorine atom (5ao),
oxygen atom (5ap), pyridine moiety (5aq), furan moiety
aReaction conditions: 1 (0.20 mmol), 6 (0.60 mmol), PdCl2(dppf) (10
mol%), dppp (20 mol%), Na2CO3 (0.20 mmol) in PhMe (2.0 mL)
under 1 atm CO at 100 oC for corresponding times.
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To prove the practicability of this methodology, the benzylic CꢀN bonds activation have been developed for
carbonylation reactions were carried out on gramꢀscale the synthesis of arylacetamides and arylacetic acid
(Scheme 2). Delightedly, when aniline, benzyl alcohol esters. This redoxꢀneutral protocol, with electronically
and phenol were employed as the partners to react with neutral amine as waste, could be smoothly performed
benzylic quaternary ammonium salts on 5 mmol scale, under atmosphere pressure of CO and compatible with a
the corresponding amide 3aa and esters 5aa, 7aa were wide range of nucleophiles including aryl/alkyl amines,
obtained in 68%, 86% and 70% yields, respectively.
primary/secondary/ tertiary alcohols and various
phenols. The successes for αꢀaminoꢀesters and gramꢀ
scale synthesis show its potential applications in protein
modifications and chemical industries.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the NSFC (21562026,
21762025, 21702027) and the Natural Science
Foundation of Jiangxi Province (20161BAB203084) for
financial support.
Scheme 2. Proposed mechanism.
On the basis of our experimental results and known
literatures,2,22 a plausible reaction mechanism is
depicted in Scheme 3. The Pd(0) species A reacts with
benzyl quaternary ammonium salt 1 via oxidative
addition to generate Pd(II) intermediate B and release
electronically neutral NMe3. Then CO insertion into the
metalꢀalkyl bond of intermediate B would form
acylpalladium C, which after nucleophilic attack by
suitable nucleophiles (amines, alcohols or phenols)
could deliver Pd(II) species D. Finally, reductive
elimination of D provides the carbonyl product (3, 5 or
7) and regenerate Pd(0) species.
Notes and references
1 (a) A. Schoenberg, I. Bartoletti, R. F. Heck, J. Org.
Chem. 1974, 39, 3318; (b) A. Schoenberg; R. F.
Heck. J. Org. Chem. 1974, 39, 3327; (c) A.
Schoenberg; R. F. Heck. J. Am. Chem. Soc. 1974, 96,
7761.
2
For select reviews on palladiumꢀcatalyzed
carbonylation reactions with CO, see: (a) G. Kiss,
Chem. Rev. 2001, 101, 3435; (b) R. SkodaꢀFöldes, L.
Kollár, Curr. Org. Chem. 2002, 6, 1097; (c) B.
Gabriele, G. Salerno, M. Costa, Top. Organomet.
Chem. 2006, 18, 239; (d) C. F. J. Barnard,
Organometallics 2008, 27, 5402; (e) A. Brennfuhrer,
H. Neumann, M. Beller, Angew. Chem. Int. Ed. 2009,
48, 4114; (f) A. Brennführer, H. Neumann, M. Beller,
ChemCatChem 2009, 1, 28; (g) R. Grigg, S. P.
Mutton, Tetrahedron 2010, 66, 5515; (h) X. F. Wu,
H. Neumann, M. Beller, Chem. Soc. Rev. 2011, 40,
4986; (i) X. F. Wu, H. Neumann, M. Beller, Chem.
Rev. 2013, 113, 1; (j) L. Wu, X. Fang, Q. Liu, R.
Jackstell, M. Beller, X.ꢀF. Wu, ACS Catal. 2014, 4,
2977; (k) Y. Bai, D. C. Davis, M. Dai, J. Org. Chem.
2017, 82, 2319. For some other recent reviews on
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Scheme 3. Proposed mechanism.
3 (a) B. Katzung, S. Masters, A. Trevor, Basic and
Clinical Pharmacology, 12th ed.; McgrawꢀHill: New
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Conclusions
In conclusion, a highꢀefficient palladiumꢀcatalyzed
aminocarbonylation and alkoxycarbonylation of readily
available quaternary ammonium salts with CO via
4 | J. Name., 2012, 00, 1-3
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