Cesium Carbonate Catalyzed Esterification of N-Benzyl- N-Boc-amides under Ambient Conditions

Article abstract of DOI:10.1021/acs.orglett.9b02513

We report a general activated amide to ester transformation catalyzed by Cs₂CO₃. Using this approach, esterification proceeds under relatively mild conditions and without the need for a transition metal catalyst. This method exhibits broad substrate scope and represents a practical alternative to existing esterification strategies. The synthetic utility of this protocol is demonstrated via the facile synthesis of crown ether derivatives and the late-stage modification of a representative natural product and several sugars in reasonable yields.

Full text of DOI:10.1021/acs.orglett.9b02513

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cesium Carbonate Catalyzed Esterification of N‑Benzyl‑N‑Boc-
amides under Ambient Conditions
Danfeng Ye,†,‡ Zhiyuan Liu,‡ Hao Chen,‡ Jonathan L. Sessler,*,‡,§ and Chuanhu Lei*,‡
†School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
‡Center for Supramolecular Chemistry & Catalysis and Department of Chemistry, College of Sciences, Shanghai University,
Shanghai 200444, China
§Department of Chemistry, the University of Texas at Austin, Austin, Texas 78712-1224, United States
S
* Supporting Information
ABSTRACT: We report a general activated amide to ester
transformation catalyzed by Cs2CO3. Using this approach,
esterification proceeds under relatively mild conditions and
without the need for a transition metal catalyst. This method
exhibits broad substrate scope and represents a practical
alternative to existing esterification strategies. The synthetic
utility of this protocol is demonstrated via the facile synthesis
of crown ether derivatives and the late-stage modification of a
representative natural product and several sugars in reasonable
yields.
mides are a fundamental functional group in organic
1
Achemistry.
They are an integral part of numerous
complex molecular architectures and present in a broad
range of molecules, including bioactive natural products,2
pharmaceutical agents,3 and fine chemicals.4 Amide linkages
are also present in numerous polymers, both natural and
synthetic, and are found in a host of other functional
materials.5 The ubiquity of amides makes them attractive as
feedstocks, and they continue to play important roles in the
late-stage modification of drugs and other materials.6 However,
the amide bond is relatively strong. This strength is ascribed to
resonance stabilization and is reflected in part in C−N rotation
barriers on the order of 15−20 kcal/mol.7 It also means that
the conversion of amides to other functional groups usually
requires harsh experimental conditions and a long reaction
time. In particular, the direct esterification of amides requires
strong acidic or basic conditions,8 which are incompatible with
sensitive functional groups. Efforts to overcome these classic
limitations are attracting increasing attention.
In this context, the pioneering work by Garg and co-workers
is of particular interest.9 In 2015, they reported a revolutionary
protocol for the esterification of amides based on a catalytic
Figure 1. Methods for esterification reactions of amides.
transition metal-based N-heterocyclic carbene (NHC) system
(Figure 1a).9a The key step of this reaction involves the
others14 developed a series of activated “twisted amides” in
the form of either N-acyl-N-Boc-carbamates or N-acyl-N-
tosylamides, which could be converted to esters by means of
transition-metal-catalyzed15 cross-coupling reactions (Figure
1a).
formation of a versatile acyl−nickel intermediate,10 enabled by
the facile oxidative addition of nickel(0) to N-alkyl-N-
phenylamides. The success of the reaction is thought to be
due to the unique conformation of the amide involved, which
is slightly twisted from a cis-configuration due to both steric
hindrance created by alkyl group and favorable electronic
effects (weak π−π interactions between the aryl and phenyl
groups).11 Nearly contemporaneously, Szostak12,13 and
Received: July 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02513
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Organic Letters
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Recently, the N-activation strategy was further expanded to
the transition-metal-free16 esterification of twisted amides. For
instance, Zeng et al. reported the fluoride-catalyzed ester-
ification of amides via an acyl fluoride intermediate that
proceeds at relatively high temperatures (Figure 1b).17
Separately, Szostak et al. reported the esterification of amides
using phenol and benzyl alcohol as nucleophiles promoted by
an excess of K3PO4 (Figure 1c).18 By using this method, a
broad array of aromatic esters could be obtained that
previously proved elusive in the context of transition metal-
catalyzed amide bond activation, and under mild conditions.
Despite these important advances, there remains a need for a
general and operationally straightforward synthetic method
that allows amides to be converted to the corresponding alkyl
esters under mild conditions. As an extension of Szostak’s
transition-metal-free transformation of amides to aryl/benzyl
ester, we report here progress toward achieving such a goal.
Specifically, we show that N-Boc-N-benzyl amides may be
converted to the corresponding esters by treatment with a
range of alcohols in the presence of substoichiometric
quantities of Cs2CO3 (Figure 1d). The reaction appears
general and proceeds in good yields at room temperature.
The starting point for the present study was the
consideration that the acyl fluoride intermediate that
presumably permits Zeng’s procedure to proceed in the
absence of a metal catalyst might be replaceable by a lower
energy species. Recently, we succeeded in trapping a carbonate
monoester within a supramolecular host.19 Carbonate
monoesters are elusive species that are unstable under ambient
conditions.20 They decompose to produce (bi)carbonate and
the constituent alcohol. As such, we hypothesized that they
might act as viable intermediates in the conversion of
appropriately chosen N-activated amides into esters, perhaps
under readily accessible and mild laboratory conditions.
Cesium carbonate was thus chosen as a potential catalyst to
test this hypothesis.
overall efficiency of the esterification reaction, although it did
reduce the apparent rate. A solvent screening study revealed
that DMSO constitutes the most effective medium for this
reaction at least among other readily available solvents
analyzed under identical conditions. For example, yields for
3a of 71%, 62%, and 52% were obtained when the reaction was
performed in CH3CN, DMF, and DMA, respectively (Table
1). Other common organic solvents (e.g., toluene, diethyl
ether, THF, DCM, and diglyme) gave little or no product (see
Supporting Information).
a
Table 1. Optimization for the Esterification of Amides 1a
b
entry
variation from the standard conditions
yield (%)
1
2
3
4
5
6
7
8
9
10
none
89
85
23
0.1 equiv of Cs2CO3
CsF instead of Cs2CO3
CsBr instead of Cs2CO3
CsOAc instead of Cs2CO3
without adding of base
CH3CN instead of DMSO
DMF instead of DMSO
DMA instead of DMSO
DCM instead of DMSO
c
nd
nd
nd
71
62
52
8
a
Conditions: 1a (1.0 equiv), 2a (1.5 equiv), base (0.2 equiv), solvent
b
c
(0.2 M), 23 °C, 12 h. Calibrated GC yield. nd indicates not
determined.
By applying these optimized reaction conditions, a number
of electronically and sterically crowded benzamides (imides)
including Boc amides, tosyl amides, glutarimide, and N-methyl-
N-phenylamide were tested. As summarized in Figure 2, it is
evident that the esterification reaction is sensitive to the choice
of N-substituents. Notably, N-benzyl-N-Boc-carbamate 1a
proved to be the most suitable electrophile for this
esterification reaction, under these specific reaction conditions.
Amide 1b bearing an N-methyl group gives rise to a slightly
reduced yield. However, when either the benzyl or methyl
group is replaced by the phenyl group, the yield of the reaction
dramatically decreases to 37% (1a and 1b vs 1c). The yield
decreased significantly with the introduction of a phenyl group
in the case of N-tosyl amides (1e vs 1f). Additionally, it was
observed that N-glutarimide, 1g, and N-methyl-N-phenyl
benzamide, 1h, were completely unreactive when the same
experimental conditions were employed. In fact, essentially
none of the desired ester product could be detected as
evidenced by GC analyses.
Initially, we performed a number of test reactions using N-
benzyl-N-Boc-carbamate (1a) as the amide electrophile and n-
hexanol (2a) as the nucleophilic partner. (This choice of amide
was also the result of more detailed screening studies as
detailed below.) Through experimentation, we found that the
reaction between 1a and 2a could be carried out in the
presence of 20 mol % of Cs2CO3 at 23 °C in DMSO to give
89% isolated yield of the esterified product 3a (Figure 2).
Reducing the Cs2CO3 loading to 10 mol % did not affect the
Next, the reaction scope was evaluated with respect to the
acyl component (Figure 3). It was found that a diverse array of
N-Boc-N-benzyl amides reacted smoothly to give the
corresponding ester. However, studies of benzoic acid
derivatives revealed that the reaction rate (as opposed to
yield per se) was influenced by the substituents attached to the
benzene ring. The presence of an electron-withdrawing group
on the phenyl moiety led to higher reactivity. For example, in
the case of the cyano-substituted amide (product 3l), the
reaction was deemed complete within 1 h. In contrast, a
reaction time of 12 h was required for the corresponding 4-
Figure 2. Evaluation of N,N-disubstituted benzamides. nd indicates
not determined.
B
DOI: 10.1021/acs.orglett.9b02513
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Letter
Figure 3. Substrate scope of the amides. Yields are isolated yields.
methylphenyl-substituted amide (see Supporting Information).
Substrates carrying aryl C−F, C−Cl, and C−Br bonds were
found to be compatible with this transformation (giving
products 3g−3i). Both ortho- and meta-substituted aromatic
amides showed comparable reactivity (products 3b and 3c).
This leads us to infer that the reaction was not particularly
sensitive to steric effects on the acyl moiety. Several substrates
with functional groups that are not compatible with Garg’s Ni-
catalyzed system, e.g., tert-butyloxylcarbonyl, cyano, and nitro
groups (products 3k−3m),9 could be converted to esters
readily using our methodology. The present method also
proved amenable to heteroaryl-derived amides, including those
containing furan, thiophene, and benzothiophene (giving
products 3o−3q), as well as cinnamyl amide (product 3r). It
also accommodates alkyl amides (e.g., products 3s and 3t).
Unfortunately, sterically hindered tert-butyl substituted sub-
strates failed to give the corresponding esters.
The scope of the amide-to-ester transformation was further
evaluated using a library of alcohols (Figure 4). Primary
alcohols with different carbon chain lengths (giving products
3v and 3w) and heteroatom-containing moieties give the
corresponding esters (e.g., products 3x−3z) in excellent yields.
An alcohol bearing a polyfluorinated alkyl chain (making the
substrate comparatively less nucleophilic than 2a) also
performed well, giving rise to the corresponding ester (product
3aa) in 82% yield. In addition, allylic alcohol can also be used
without modification of the reaction conditions. Similarly,
benzylic and heterobenzylic alcohols could be successfully
converted to the corresponding products (3ac−3ae) in
moderate to good yields. The present esterification procedure
could also be applied successfully to secondary alcohols. For
instance when benzhydryl alcohol was tested, the desired
product (3af) was obtained in 85% yield. Cyclobutanol and 3-
Figure 4. Substrate scope of the alcohol nucleophiles. Yield refers to
isolated yield. a Cs2CO3 (2.0 equiv) was used.
oxetanol also worked well, giving the desired ester in 89% and
84% yield, respectively. In contrast, in the case of cyclo-
pentanol we found that 2.0 equiv of Cs2CO3 were required to
promote the esterification reaction.
Using the original substoichiometric Cs2CO3 procedure, the
natural product citronellol was found to give the corresponding
product (3aj) in good yield. In this case, the chemistry could
be carried out readily on the gram scale (10 mmol of
substrate), illustrating its utility for the potential late-stage
functionalization of a representative natural product. Sugar-
containing complex alcohols were also tolerated. For instance,
as shown in Figure 4, the use of α-D-allofuranose containing a
sterically hindered secondary alcohol gave ester 3al in 73%
isolated yield using 0.2 equiv of Cs2CO3. This finding leads us
to propose that the present strategy could prove useful in the
modification of bioactive substrates.
However, an attempt at carrying out the esterification
reaction using a tertiary alcohol (e.g., t-BuOH) using our
standard reaction protocol failed to give an isolable quantities
of the desired ester product. In this case, only the starting
material was recovered, even when high temperatures (100
°C) were tested.
To demonstrate the synthetic utility of the present Cs2CO3-
based method, a series of crown ether derivatives, 5a−c
(Figure 5),21 were synthesized by treating N-activated amides
with polyethylene glycols (4a−c). Consistent with early
reports in the literature, the cesium cation acts as a template
to promote ring closure in lieu of linear condensation (thus
reducing polymerization).22 There is a clear ring size
C
DOI: 10.1021/acs.orglett.9b02513
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02513.
Figure 5. Synthesis of crown ether derivatives.
Experimental procedures, characterization of com-
pounds, mechanistic studies (PDF)
NMR spectra (PDF)
dependence on the yield. This dependence is reflected in the
lower yield of 5a (6%), a cyclic polyether-based product
wherein the ring size does not match well the ionic radius of
the Cs+ cation (rCs+ = 1.69 Å).23 However, in the case of the
polyether-diol, 5c, a better size complementarity between the
Cs+-template and the incipient ring is expected; in this case,
the overall yield for the formation of 5c is 51%. While beyond
the scope of the present study, the diester crown products
produced by the present methodology are expected to endow
unique molecular recognition features as compared to more
classic crown ethers containing only ethylene bridges between
the oxygen atoms.24
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chlei@shu.edu.cn.
*E-mail: sessler@cm.utexas.edu.
ORCID
Jonathan L. Sessler: 0000-0002-9576-1325
Notes
The authors declare no competing financial interest.
Detailed mechanistic studies are ongoing. However, two
findings discussed above are noteworthy. First, the best results
were obtained in the highly polar medium DMSO. Second,
higher reaction rates were seen for the 4-cyanophenyl amide as
compared to the corresponding 4-methylphenyl benzamide
(vide supra). This leads us to propose a mechanistic pathway
that involves initial nucleophilic attack of the carbonate anion
on the amide carbonyl group; this is a first reaction step that is
expected to be made more facile in the case of amides bearing
strongly electron-withdrawing groups. Once formed, attack by
the alcohol nucleophile and breakdown of the carbonate ester
ACKNOWLEDGMENTS
■
We thank Dr. Atanu Jana (Shanghai University) for valuable
discussions and helpful paper revisions. This work is supported
by Shanghai University (N.13-G210-19-214). C.L. would like
to thank the program for a Special Appointment Professorship
(Eastern Scholar) from the Shanghai Institutions of Higher
Learning, Shanghai Municipal Education Committee for
support.
REFERENCES
2−
−
■
(CO3 as the leaving group or HCO3 in the event the
intermediate is protonated) as the second reaction step would
then generate the observed ester products (Scheme 1).20 An
(1) (a) Zabicky, J. Amides; John Wiley & Sons: Chichester, 1970.
(b) Greenberg, A.; Breneman, C. M.; Liebman, J. F. The amide
linkage: selected structural aspects in chemistry, biochemistry, and
materials science; Wiley-Interscience: New York, 2000.
Scheme 1. Proposed Reaction Mechanism
(2) (a) Walsh, C. T.; O’Brien, R. V.; Khosla, C. Angew. Chem., Int.
Ed. 2013, 52, 7098−7124. (b) Kudo, F.; Miyanaga, A.; Eguchi, T.
Nat. Prod. Rep. 2014, 31, 1056−1073.
(3) (a) Carey, J. S.; Laffan, D.; Thomson, C.; Williams, M. T. Org.
Biomol. Chem. 2006, 4, 2337−2347. (b) Roughley, S. D.; Jordan, A.
M. J. Med. Chem. 2011, 54, 3451−3479.
(4) (a) Vasilakoglou, I. B.; Eleftherohorinos, I. G.; Dhima, K. B.
Weed Res. 2001, 41, 535−546. (b) Bretschneider, T.; Voerste, A.;
Fuβlein, M.; Kohler, A.; Gorgens, U. Amides and thioamides as
̈
̈ ̈
pesticides. U.S. Patent 8,536,204, Sep 17, 2013.
(5) (a) Deming, T. J. Prog. Polym. Sci. 2007, 32, 858−875.
(b) Marchildon, K. Macromol. React. Eng. 2011, 5, 22−54. (c) Guo,
X.; Facchetti, A.; Marks, T. J. Chem. Rev. 2014, 114, 8943−9021.
(6) (a) Cheung, C. W.; Ma, J.-A.; Hu, X. J. Am. Chem. Soc. 2018,
140, 6789−6792. (b) Larsen, M. B.; Herzog, S. E.; Quilter, H. C.;
Hillmyer, M. A. ACS Macro Lett. 2018, 7, 122−126. (c) Larsen, M. B.;
Wang, S.-J.; Hillmyer, M. A. J. Am. Chem. Soc. 2018, 140, 11911−
11915.
alternate reaction pathway involving direct nucleophilic
addition of the alcohol to the amide is also possible wherein
the cesium cation may act as a Lewis acid to activate the
amide.16f
(7) Kemnitz, C. R.; Loewen, M. J. J. Am. Chem. Soc. 2007, 129,
2521−2528.
In summary, we describe here the esterification of amides
using activated N-benzyl-N-Boc-carbamate and a variety of
alcohols substrates, with Cs2CO3 as the catalyst. The
effectiveness of this method and its wide tolerance for a
variety of functional groups lead us to propose it may have a
role to play in a range of practical synthetic applications. Initial
demonstrations of this potential have been made via the
esterification of citronellol and sugars, as well as the synthesis
of several crown ether derivatives.
(8) (a) White, E. H. J. Am. Chem. Soc. 1955, 77, 6011−6014.
(b) Nishimoto, S.-i.; Izukawa, T.; Kagiya, T. Bull. Chem. Soc. Jpn.
1982, 55, 1484−1488. (c) Keck, G. E.; McLaws, M. D.; Wager, T. T.
Tetrahedron 2000, 56, 9875−9883. (d) Glatzhofer, D. T.; Roy, R. R.;
Cossey, K. N. Org. Lett. 2002, 4, 2349−2352. (e) Toyao, T.; Nurnobi
Rashed, M.; Morita, Y.; Kamachi, T.; Hakim Siddiki, S. M. A.; Ali, M.
A.; Touchy, A. S.; Kon, K.; Maeno, Z.; Yoshizawa, K.; Shimizu, K.-i.
ChemCatChem 2019, 11, 449−456.
(9) (a) Hie, L.; Fine Nathel, N. F.; Shah, T. K.; Baker, E. L.; Hong,
X.; Yang, Y.-F.; Liu, P.; Houk, K. N.; Garg, N. K. Nature 2015, 524,
79−83. (b) Hie, L.; Baker, E. L.; Anthony, S. M.; Desrosiers, J.-N.;
D
DOI: 10.1021/acs.orglett.9b02513
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Senanayake, C.; Garg, N. K. Angew. Chem., Int. Ed. 2016, 55, 15129−
15132. (c) Weires, N. A.; Caspi, D. D.; Garg, N. K. ACS Catal. 2017,
7, 4381−4385.
(20) (a) Cronyn, M. W.; Jiu, J. J. Am. Chem. Soc. 1952, 74, 4726.
(b) Friour, G.; Alexakis, A.; Cahiez, G.; Normant, J. Tetrahedron
1984, 40, 683−693. (c) Moreau, J.-L.; Couffignal, R. J. Organomet.
Chem. 1985, 294, 139−144.
(10) Selected nickel-catalyzed transformations through an acyl-
nickel intermediate: (a) Weires, N. A.; Baker, E. L.; Garg, N. K. Nat.
Chem. 2016, 8, 75−79. (b) Baker, E. L.; Yamano, M. M.; Zhou, Y.;
Anthony, S. M.; Garg, N. K. Nat. Commun. 2016, 7, 11554. (c) Hu, J.;
Zhao, Y.; Liu, J.; Zhang, Y.; Shi, Z. Angew. Chem., Int. Ed. 2016, 55,
8718−8722. (d) Simmons, B. J.; Weires, N. A.; Dander, J. E.; Garg, N.
K. ACS Catal. 2016, 6, 3176−3179. (e) Medina, J. M.; Moreno, J.;
Racine, S.; Du, S.; Garg, N. K. Angew. Chem., Int. Ed. 2017, 56, 6567−
6571. (f) Boit, T. B.; Weires, N. A.; Kim, J.; Garg, N. K. ACS Catal.
2018, 8, 1003−1008.
(21) (a) Bradshaw, J. S.; Thompson, M. D. J. Org. Chem. 1978, 43,
2456−2460. (b) Piepers, O.; Kellogg, R. M. J. Chem. Soc., Chem.
Commun. 1978, 383−384. (c) Sharghi, H.; Sarvari, M. H. Synthesis
2003, 2003, 879−882. (d) Gokel, G. W.; Leevy, W. M.; Weber, M. E.
Chem. Rev. 2004, 104, 2723−2750.
(22) (a) Dijkstra, G.; Kruizinga, W. H.; Kellogg, R. M. J. Org. Chem.
1987, 52, 4230−4234. (b) Galli, C. Org. Prep. Proced. Int. 1992, 24,
285−307.
(23) Wang, Y.; Weinstock, I. A. Chem. Soc. Rev. 2012, 41, 7479−
7496.
(11) For a discussion of resonance energies of the amide bond and
how these could be responsible for the high reactivity associated with
ground-state destabilization mechanisms, see: (a) Szostak, R.; Shi, S.;
Meng, G.; Lalancette, R.; Szostak, M. J. Org. Chem. 2016, 81, 8091−
8094. (b) Szostak, R.; Meng, G.; Szostak, M. J. Org. Chem. 2017, 82,
6373−6378. (c) Meng, G.; Shi, S.; Lalancette, R.; Szostak, R.; Szostak,
M. J. Am. Chem. Soc. 2018, 140, 727−734. (d) Szostak, R.; Szostak,
M. Org. Lett. 2018, 20, 1342−1345.
(12) Pd-catalyzed amide activation based on N-Boc or N-
tosylamide: (a) Meng, G.; Shi, S.; Szostak, M. ACS Catal. 2016, 6,
7335−7339. (b) Lei, P.; Meng, G.; Ling, Y.; An, J.; Nolan, S. P.;
Szostak, M. Org. Lett. 2017, 19, 6510−6513. (c) Lei, P.; Meng, G.;
Ling, Y.; An, J.; Szostak, M. J. Org. Chem. 2017, 82, 6638−6646.
(d) Lei, P.; Meng, G.; Shi, S.; Ling, Y.; An, J.; Szostak, R.; Szostak, M.
Chem. Sci. 2017, 8, 6525−6530. (e) Lei, P.; Meng, G.; Szostak, M.
ACS Catal. 2017, 7, 1960−1965. (f) Meng, G.; Szostak, M. Org. Lett.
2018, 20, 6789−6793.
(24) (a) Bradshaw, J. S.; Baxter, S. L.; Scott, D. C.; Lamb, J. D.; Izatt,
R. M.; Christensen, J. J. Tetrahedron Lett. 1979, 20, 3383−3386.
(b) Bartsch, R. A.; Juri, P. N. J. Org. Chem. 1980, 45, 1011−1014.
(c) Lamb, J. D.; Christensen, J. J.; Oscarson, J. L.; Nielsen, B. L.; Asay,
B. W.; Izatt, R. M. J. Am. Chem. Soc. 1980, 102, 6820−6824.
(d) Lamb, J. D.; Izatt, R. M.; Swain, C. S.; Bradshaw, J. S.;
Christensen, J. J. J. Am. Chem. Soc. 1980, 102, 479−482. (e) Bradshaw,
J. S.; Baxter, S. L.; Lamb, J. D.; Izatt, R. M.; Christensen, J. J. J. Am.
Chem. Soc. 1981, 103, 1821−1827.
(13) Pd-catalyzed amide activation based on glutarimides: (a) Meng,
G.; Szostak, M. Angew. Chem., Int. Ed. 2015, 54, 14518−14522.
(b) Meng, G.; Szostak, M. Org. Lett. 2015, 17, 4364−4367. (c) Liu,
Y.; Meng, G.; Liu, R.; Szostak, M. Chem. Commun. 2016, 52, 6841−
6844. (d) Liu, C.; Szostak, M. Angew. Chem., Int. Ed. 2017, 56,
12718−12722. (e) Shi, S.; Szostak, M. Org. Lett. 2017, 19, 3095−
3098.
(14) (a) Li, X.; Zou, G. Chem. Commun. 2015, 51, 5089−5092.
(b) Bourne-Branchu, Y.; Gosmini, C.; Danoun, G. Chem. - Eur. J.
2017, 23, 10043−10047. (c) Chu, C. Q.; Dang, L. J. Org. Chem. 2018,
83, 5009−5018. (d) Zhou, P.-X.; Shi, S.; Wang, J.; Zhang, Y.; Li, C.;
Ge, C. Org. Chem. Front. 2019, 6, 1942−1947.
(15) For recent reviews on transition-metal-catalyzed C−N
activation, see: (a) Ouyang, K.; Hao, W.; Zhang, W.-X.; Xi, Z.
Chem. Rev. 2015, 115, 12045−12090. (b) Dander, J. E.; Garg, N. K.
ACS Catal. 2017, 7, 1413−1423. (c) Liu, C.; Szostak, M. Chem. - Eur.
J. 2017, 23, 7157−7173. (d) Liu, C.; Szostak, M. Org. Biomol. Chem.
2018, 16, 7998−8010. (e) Meng, G.; Szostak, M. Eur. J. Org. Chem.
2018, 2018, 2352−2365. (f) Shi, S.; Nolan, S. P.; Szostak, M. Acc.
Chem. Res. 2018, 51, 2589−2599. (g) Bourne-Branchu, Y.; Gosmini,
C.; Danoun, G. Chem. - Eur. J. 2019, 25, 2663−2674. (h) Buchspies,
J.; Szostak, M. Catalysts 2019, 9, 53. (i) Chaudhari, M. B.;
Gnanaprakasam, B. Chem. - Asian J. 2019, 14, 76−93.
(16) Transition-metal-free transamidation of twisted amides:
(a) Liu, Y.; Liu, R.; Szostak, M. Org. Biomol. Chem. 2017, 15,
1780−1785. (b) Liu, Y.; Shi, S.; Achtenhagen, M.; Liu, R.; Szostak, M.
Org. Lett. 2017, 19, 1614−1617. (c) Li, G.; Szostak, M. Nat. Commun.
2018, 9, 4165. (d) Liu, C.; Shi, S.; Liu, Y.; Liu, R.; Lalancette, R.;
Szostak, R.; Szostak, M. Org. Lett. 2018, 20, 7771−7774. (e) Liu, Y.;
Achtenhagen, M.; Liu, R.; Szostak, M. Org. Biomol. Chem. 2018, 16,
1322−1329. (f) Li, G.; Ji, C.-L.; Hong, X.; Szostak, M. J. Am. Chem.
Soc. 2019, 141, 11161−11172.
(17) Wu, H.; Guo, W.; Daniel, S.; Li, Y.; Liu, C.; Zeng, Z. Chem. -
Eur. J. 2018, 24, 3444−3447.
(18) Li, G.; Lei, P.; Szostak, M. Org. Lett. 2018, 20, 5622−5625.
(19) Mulugeta, E.; He, Q.; Sareen, D.; Hong, S.-J.; Oh, J. H.; Lynch,
V. M.; Sessler, J. L.; Kim, S. K.; Lee, C.-H. Chem. 2017, 3, 1008−
1020.
E
DOI: 10.1021/acs.orglett.9b02513
Org. Lett. XXXX, XXX, XXX−XXX