Synthesis of Amides and Esters by Palladium(0)-Catalyzed Carbonylative C(sp3)?H Activation

Article abstract of DOI:10.1002/anie.202007922

The 1,4-palladium shift strategy allows the functionalization of remote C?H bonds that are difficult to reach directly. Reported here is a domino reaction proceeding by C(sp³)?H activation, 1,4-palladium shift, and amino- or alkoxycarbonylation, which generates a variety of amides and esters bearing a quaternary β-carbon atom. Mechanistic studies showed that the aminocarbonylation of the σ-alkylpalladium intermediate arising from the palladium shift is fast using PPh₃ as the ligand, and leads to the amide rather than the previously reported indanone product.

Full text of DOI:10.1002/anie.202007922

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Accepted Article
Title: Synthesis of Amides and Esters by Pd0-Catalyzed Carbonylative
C(sp3)–H Activation
Authors: Tomáš Čarný, Ronan Rocaboy, Antonin Clemenceau, and
Olivier Baudoin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202007922
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Synthesis of Amides and Esters by Pd0-Catalyzed Carbonylative
C(sp3)–H Activation
Tomáš Čarný,† Ronan Rocaboy,† Antonin Clemenceau, and Olivier Baudoin*
T. Čarný
Slovak University of Technology, Department of Organic Chemistry
Radlinského 9, SK-81237 Bratislava, Slovakia
Dr. R. Rocaboy, Dr. A. Clemenceau, Prof. Dr. O. Baudoin
University of Basel, Department of Chemistry
St. Johanns-Ring 19, CH-4056 Basel, Switzerland
E-mail: olivier.baudoin@unibas.ch
[†]
These authors contributed equally.
Supporting information for this article is given via a link at the end of the document.
Abstract: The 1,4-Pd shift strategy allows the functionalization of
remote C–H bonds that are difficult to reach directly. We report a
domino reaction proceeding via C(sp3)–H activation, 1,4-Pd shift and
amino- or alkoxycarbonylation, which generates a variety of amides
and esters bearing a quaternary b carbon. Mechanistic studies
showed that the aminocarbonylation of the s-alkylpalladium
intermediate arising from Pd shift is fast using PPh3 as the ligand, and
leads to the amide rather than the previously reported indanone
product.
and amines or alcohols to generate amides and esters containing
a b-quaternary center (Scheme 1e).[13,14] Tuning the reaction
conditions favors the 1,4-Pd shift and disfavors the insertion on
palladacycle A leading to indanones (Scheme 1b).
a) principle
H
PdX
Pd0 cat.
H–X
base
or R–X
Pd
Br
H/R
A
B
b) trapping of A by carbonylation
R Me
R
Me
H
Pd0 cat.
In the past two decades, the palladium(0)-catalyzed activation
of C(sp3)–H bonds has proven to be a powerful method to
construct C(sp2)–C(sp3) bonds.[1,2] These methods rely on the
generation of a diorganopalladacycle such as A via oxidative
addition of a C(sp2)–X bond to palladium(0) and base-mediated
C(sp3)–H activation (Scheme 1a). The reductive elimination from
such palladacycles leads to a variety of useful carbo- and
heterocycles.[2] Alternatively, trapping the diorganopalladacycle
intermediate leads to interesting ring-expansion products. First,
the reaction of aryl bromides with diazo compounds furnishes
tetrasubstituted indanes via carbene insertion.[3] Indanes can be
also generated upon trapping of A with dibromomethane.[4] More
recently, the carbonylative C(sp3)–H arylation of aryl bromides,
giving rise to indanones via CO insertion was reported (Scheme
1b).[5] Alternative to direct trapping, the ring-opening of
palladacycle A by protonation or oxidative addition/reductive
elimination provides a s-alkylpalladium complex B (Scheme 1a),
hence leading to an overall 1,4-Pd shift,[6] as initially observed by
Dyker.[7] This alkylpalladium complex was shown to undergo
various reactions. First, b-H elimination furnishes olefins, hence
resulting in net alkyl desaturation.[8] Besides, trapping by boronic
acids and anilines leads to arylation and amination products
(Scheme 1c).[9] Moreover, intramolecular dearomatizing
carbopalladation (B, R = naphthol) provides spiroannulation
products.[10] The alkylpalladium intermediate B was also shown to
be a competent intermediate to perform a second intramolecular
C(sp3)–H activation, thus generating fused heterocycles[11] and
cyclopropanes (Scheme 1d).[12] In this paper, we show that the
trapping of this s-alkylpalladium complex by carbon monoxide is
feasible, and developed a carbonylative coupling of aryl bromides
CO
Br
O
c) trapping of B by arylation and amination
H
Ar/NHAr
Pd0 cat.
Ar–B(OH)2
or Ar–NH2
Br
H
t-Bu
t-Bu
d) trapping of B by C(sp3)–H activation
R
H
R
H
Pd0 cat.
PivOK
Br
H
e) this work: trapping of B by amino- and alkoxycarbonylation
1 Me
1 Me
R
O
R
H
Pd0 cat.
CO
NR2R3/OR2
+ R2R3NH
or R2OH
Br
Scheme 1. Examples of Pd0-catalyzed C(sp3)–H activation and trapping
through various reactions. The ligand is omitted for clarity.
We started out by exploring the reactivity of tert-
butylbromobenzene 1a under conditions similar to those
developed for the cyclopropanation reaction (see Scheme 1d),[12]
i. e. using Pd(PPh3)4 as the catalyst and stoichiometric cesium
pivalate as the base (Scheme 2). Under 1 atm of CO and using 2
equiv of water at 140 °C, carboxylic acid 2, presumably arising
from s-alkylpalladium complex B (see Scheme 1a) by CO
insertion and hydrolysis of the corresponding acylpalladium
1
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10.1002/anie.202007922
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intermediate, was formed in high yield.[15] Remarkably, the
protodehalogenated product was not formed despite the
presence of water. The reaction was successfully scaled up to 1
mmol without significant loss of yield (82%).
morpholine provided a 71% yield (3h). Of note, performing the
reaction on a five-fold (1 mmol) scale furnished a comparable
yield for amide 3b. Replacing one methyl group on the aryl
bromide substrate with various functional groups such as
sulfonamides (3i-l), a Boc-carbamate (3m) or a TIPS-protected
alcohol (3n) did not affect the efficiency of the reaction to a great
extent, to the exception of 3m for which degradation was
observed. It is interesting to note that the reaction furnishing
amide 3j could be conducted in a two-chamber (COware) system
using 3 equiv of COgen (COgen = 9-methylfluorene-9-carbonyl
chloride)[17] with a similar efficiency. In addition, electron-donating
(methoxy, methylene-dioxy, 3o-t) and electron-withdrawing
(fluoro, trifluoromethyl, 3u-x) substituents in meta or para position
to the bromine atom were well tolerated although reduced yields
were often observed. The reaction providing amide 3p was also
conducted on a 7.5 x scale without affecting the yield. For
selected examples (3j, 3r, 3s, 3u, 3w), using AdCO2Cs as the
base, formed in situ from Cs2CO3 and catalytic AdCO2H, provided
better results than CsOPiv. Of note, direct amidation was
observed for some other substrates (Table S6).
O
Pd(PPh3)4 (10 mol%)
CsOPiv (2 equiv)
H
OH
CO (1 atm), H2O (2 equiv)
o-xylene, 140 °C, 18 h
Br
1a
(0.2 mmol)
2
100%[a] (84%,[b] 82%[b,c]
)
Scheme 2. Formation of carboxylic acid 2 by hydroxycarbonylation of 1a. [a]
NMR yield using trichloroethylene as internal standard. [b] Yield of the isolated
product. [c] Performed on a 1 mmol scale.
Then, the same conditions were applied using benzylamine (2
equiv) instead of water, which provided the corresponding
aminocarbonylation product 3a in 62% yield (Scheme 3). To test
the generality of the current method, cyclic and acyclic primary
amines (benzylamine, cyclohexylamine, n-butylamine, tert-
butylamine), secondary amines (morpholine, piperidine,
diethylamine) and aniline were screened as nucleophiles. The
corresponding amide products 3a-h, bearing a quaternary b
carbon, were obtained in 45-71% yields, with an efficiency
mirroring the nucleophilicity of the amine.[16] For instance, the
weakly nucleophilic aniline delivered a 45% yield (3e), whereas
The synthesis of esters by alkoxycarbonylation required
further optimization of the reaction conditions, and potassium
benzoate was found to be a more efficient base (Scheme 4).[15]
Prolonging the reaction time to 27 h was also found necessary to
increase the yield, presumably due to the reduced nucleophilicity
of alcohols compared to amines.[18]
Scheme 3. Synthesis of amides via carbonylative C(sp3)–H activation. Reaction conditions: aryl bromide (0.2 mmol), amine (2 equiv), Pd(PPh3)4 (10 mol%), CsOPiv
(2 equiv), o-xylene (c 0.05 mol L–1), CO balloon, 140 °C, 18 h. [a] Performed on a 1 mmol scale. [b] Using Cs2CO3 (1 equiv) and AdCOOH (0.3 equiv) instead of
CsOPiv. [c] Using COgen (3 equiv) and a two-chamber system instead of CO balloon (NMR yield). [d] Performed on a 1.5 mmol scale. Ts = p-toluenesulfonyl; Boc
= tert-butyloxycarbonyl; TIPS = triisopropylsilyl.
2
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For the same reason, the reaction was more limited than the
preceding aminocarbonylation. Nevertheless, benzylic (4a-c),
primary (4d), secondary (4e-f) alcohols as well as phenol (4g)
were found to be competent coupling partners, delivering the
corresponding esters in 43-64% yield. The reaction with
cyclohexanol was scaled up to 1 mmol, furnishing ester 4e in 53%
yield.
aminocarbonylation step occurs with a low energy barrier. Next,
the known five-membered palladacycle 6b[21] was heated in the
presence of PPh3, CyNH2, CO, pivalic acid and cesium pivalate
(Scheme 6b). The formation of amide 3b, resulting from
palladacycle protonation[8d,12,22] and aminocarbonylation of the
resulting s-alkylpalladium complex, occurred from 100 °C, and
was quantitative at 120 °C. Finally, the oxidative addition complex
6c, prepared in 88% yield from 2-tert-butylbromobenzene 1a and
Pd(PPh3)4 in toluene at 120 °C,[12] was mixed with CsOPiv and
CyNH2 (Scheme 6c). The formation of amide 3b required a higher
temperature than from other complexes, with only 27% at 120 °C
and 75% at 140 °C. As expected, almost no product was obtained
in the absence of CsOPiv. These results indicate that the slowest
step in the current reaction is the C(sp3)–H activation leading to
the five-membered palladacycle intermediate. In the
cyclopropane-forming reaction occurring under similar conditions
(see Scheme 1d), similar studies concluded that the oxidative
addition was rate-limiting. The only difference in the current work
is the presence of CO and the amine, which likely modify the
catalyst by ligand displacement and therefore affect the relative
energy barriers.
a)
COD
O
PPh3 (2 equiv)
Cy-NH2 (2 equiv)
Pd
Cl
N
H
o-xylene, CO, 18 h
6a
3b
20 °C 90%
60 °C 99%
b)
Scheme 4. Synthesis of esters via carbonylative C(sp3)–H activation. Reaction
conditions: aryl bromide (0.2 mmol), alcohol (2.0 equiv), Pd(PPh3)4 (10 mol%),
PhCO2K (1.5 equiv), o-xylene (c 0.05 mol L–1), CO balloon, 140 °C, 27 h. [a]
Performed on a 1 mmol scale.
PPh3 (2 equiv), Cy-NH2 (2 equiv)
PivOH (1 equiv), CsOPiv (1 equiv)
3b
100 °C 70%
120 °C 99%
o-xylene, CO, 18 h
Pd
COD
6b
c)
Two of the products synthesized with the current method were
converted to g- and d-lactams (Scheme 5). First, amide 3p
underwent intramolecular C–H amidation to provide d-lactam 5a
upon treatment with NIS in 1,2-dichloroethane.[19] In addition,
ester 4e underwent cyclization to g-lactam 5b upon cleavage of
CsOPiv (2 equiv)
Cy-NH2 (2 equiv)
3b
o-xylene, CO, 18 h
120 °C
140 °C
27%
75%
PdBr(PPh3)2
6c
(formed in 88% at 120 °C)
140 °C[a] 5%
the tosyl group with Li/napthalene.
O
Scheme 6. Stoichiometric experiments with Pd complexes. NMR yields using
MeO
MeO
MeO
MeO
NIS
trichloroethylene as internal standard. [a] Without CsOPiv.
NHCy
C2H4Cl2, 130 °C
N
O
Cy
5a, 33%
Based on this and previous[12,22] mechanistic studies, the
following reaction mechanism is proposed (Scheme 7). The
oxidative addition of aryl bromide 1 leads to complex A
(corresponding to 6c in Scheme 6). Ligand exchange of bromide
with pivalate provides complex B,[12] which undergoes rate-
limiting C(sp3)–H activation via the concerted metallation–
deprotonation mechanism[22,23] to furnish five-membered
palladacycle C. Protonation of the latter[8d,22] with PivOH leads to
s-alkylpalladium complex D, which undergoes CO insertion and
nucleophilic attack by the amine to provide amide 3 via
intermediate E and close the catalytic cycle. The competence of
complexes C and D as reaction intermediates is indicated by the
experiments shown in Scheme 6b and 6a, respectively.
Interestingly, Wang and co-workers reported the formation of
indanone 7 from similar substrates and under similar conditions
except for the use of IMesMe, an N-heterocyclic carbene (NHC),
as the ligand (Scheme 1b).[5] Based on stoichiometric studies,
they proposed that 7 arises from palladacycle C by CO insertion
and reductive elimination. These authors also observed
3p
O
TsMeN
Me
N
Li, naphthalene
THF, –78 °C to 25 °C
OCy
O
5b, 40%
4e
Scheme 5. Application to the synthesis of lactams. Conditions are unoptimized.
NIS = N-iodosuccinimide.
To get further insight into the reaction mechanism, and in line
with our previous mechanistic work in the context of cyclopropane
synthesis,[12] we performed experiments with isolated Pd
complexes (Scheme 6). The known s-alkylpalladium complex
6a[20] was first heated in o-xylene to various temperatures for 18 h
in the presence of PPh3, cyclohexylamine and CO (Scheme 6a).
Amide 3b was formed in good yield at room temperature, and
quantitatively at 60 °C, thereby showing that the
3
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regioisomeric products arising from the reversible opening of C to
D. In our case, using PPh3 as the ligand and in the absence of
amine, carboxylic acid 2 (Scheme 2) was formed whereas
indanone 7 was not. In the presence of amine, the indanone was
also not observed. As shown in Scheme 5a, the CO insertion from
D and nucleophilic attack of the amine to give the amide product
3 occur at room temperature, and therefore are faster than the
pathway leading to the indanone 7 using PPh3 as the ligand.
Keywords: amides • C–H activation • carbonylation • domino
reactions • palladium
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Br
3
1
PivOH
nucleophilic attack
Pd0L2
fast
RNH2
reductive
oxidative
adition
[8]
[9]
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Org. Chem. 2014, 79, 11903.
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O
PdIILOPiv
PdIIL2Br
A
E
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ligand
exgange
CO insertion
[10] B. Tan, L. Bai, P. Ding, J. Liu, Y. Wang, X. Luan, Angew. Chem. Int. Ed.
2019, 58, 1474.
O
7
CO
CsBr + L
[11] R. Rocaboy, I. Anastasiou, O. Baudoin, Angew. Chem. Int. Ed. 2019, 58,
14625.
CO insertion
reductive elimination
slow
PdIILOPiv
H
[12] A. Clemenceau, P. Thesmar, M. Gicquel, A. Le Flohic, O. Baudoin,
ChemRxiv 2020, DOI : 10.26434/chemrxiv.12408839.v1
[13] For Pd-catalyzed intermolecular C(sp3)–H activation/amino- or
alkoxycarbonylations: a) P. Xie, Y. Xie, B. Qian, H. Zhou, C. Xia, H.
Huang, J. Am. Chem. Soc. 2012, 134, 9902; b) P. Xie, C. Xia, H. Huang,
Org. Lett. 2013, 15, 3370; c) H. Liu, G. Laurenczy, N. Yan, P. J. Dyson,
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Lei, Chem. Eur. J. 2016, 22, 14484; e) K. Tanaka, W. R. Ewing, J.-Q. Yu,
J. Am. Chem. Soc. 2019, 141, 15494.
PdIILOPiv
B
D
C(sp3)–H activation
protonation
rate-limiting
PdII
L
PivOH
PivOH
C
[14] For
Pd-catalyzed
intramolecular
C(sp3)–H
activation/amino-
Scheme 7. Mechanistic proposal. L = PPh3 or CO.
carbonylations: a) E. J. Yoo, M. Wasa, J.-Q. Yu, J. Am. Chem. Soc. 2010,
132, 17378; b) S. Li, G. Chen, C.-G. Feng, W. Gong, J.-Q. Yu, J. Am.
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In conclusion, the first carbonylative C(sp3)–H activation
reaction proceeding via 1,4-Pd shift was developed. A variety of
amides and esters bearing a quaternary b carbon was produced
by amino- or alkoxycarbonylation in moderate to good yields.[24]
Mechanistic studies showed that the aminocarbonylation of the s-
alkylpalladium intermediate is fast using PPh3 as the ligand,
thereby leading to the amide product at the expense of the
previously described indanone.
Acknowledgements
[15] See the Supporting information for details.
[16] T. Kanzian, T. A. Nigst A. Maier, S. Pichl, H. Mayr, Eur. J. Org. Chem.
2009, 6379.
This work was financially supported by the University of Basel and
National Scholarship Program of Slovak Republic. We thank Dr.
D. Häussinger for NMR experiments, Dr. M. Pfeffer for MS
analyses, P. Thesmar and S. Geigle for helpful suggestions.
[17] S. D. Friis, A. T. Lindhardt, T. Skrydstrup, Acc. Chem. Res. 2016, 49,
594.
[18] T. B. Phan, H. Mayr, Can. J. Chem. 2005, 83, 1554.
[19] L. Wu, Y. Hao, Y. Liu, Q. Wang, Org. Biomol. Chem. 2019, 17, 6762.
[20] E. Gutiérrez, M. C. Nicasio, M. Paneque, C. Ruiz, V. Salazar, J.
Organomet. Chem. 1997, 549, 167.
[21] J. Cámpora, J. A. López, P. Palma, D. del Rio, E. Carmona, P. Valerga,
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Conflict of interest
[22] M. Chaumontet, R. Piccardi, N. Audic, J. Hitce, J.-L. Peglion, E. Clot, O.
Baudoin, J. Am. Chem. Soc. 2008, 130, 15157.
The authors declare no conflict of interest.
[23] a) M. Lafrance, S. I. Gorelsky, K. Fagnou, J. Am. Chem. Soc. 2007, 129,
14570; b) C. E. Kefalidis, O. Baudoin, E. Clot, Dalton Trans. 2010, 39,
4
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10528; c) S. Rousseaux, S. I. Gorelsky, B. K. W. Chung, K. Fagnou, J.
Am. Chem. Soc. 2010, 132, 10692; d) S. Rousseaux, M. Davi, J. Sofack-
Kreutzer, C. Pierre, C. E. Kefalidis, E. Clot, K. Fagnou, O. Baudoin, J.
Am. Chem. Soc. 2010, 132, 10706.
[24] Attempts at developing an enantioselective version using chiral ligands
or acids were unsuccessful.
5
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Entry for the Table of Contents
R1
R1
O
Me
Me
Pd(PPh3)4 cat.
CsOPiv
H
NR3R4/OR3
R2
R2
CO, R3R4NH/R3OH
Br
31 examples
up to 84% yield
Making shift with CO. Amides and esters bearing a quaternary b carbon are synthesized by a domino reaction involving C(sp3)–H
activation, 1,4-Pd shift and amino/alkoxycarbonylation.
6
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