Synthesis of Sulfoxonium Ylides from Amides by Selective N-C(O) Activation

Article abstract of DOI:10.1021/acs.orglett.1c01535

The direct synthesis of sulfoxonium ylides from amides by selective N-C(O) cleavage is presented. The reaction proceeds through the nucleophilic addition of dimethylsulfoxonium methylide to the amide bond in acyclic twisted amides under exceedingly mild r

Full text of DOI:10.1021/acs.orglett.1c01535

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Letter
Synthesis of Sulfoxonium Ylides from Amides by Selective N−C(O)
Activation
Md. Mahbubur Rahman and Michal Szostak*
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ABSTRACT: The direct synthesis of sulfoxonium ylides from amides by selective N−C(O) cleavage is presented. The reaction
proceeds through the nucleophilic addition of dimethylsulfoxonium methylide to the amide bond in acyclic twisted amides under
exceedingly mild room temperature conditions. A variety of amides can be employed, and the protocol can be applied to the late-
stage derivatization of pharmaceuticals. Mechanistic studies outline the relative order of reactivity of amides.
mides represent one of the most fundamental functional
1,2
Agroups in chemistry and biology.
Although the
properties of amides are controlled by amidic resonance (nN
→ πC*=O conjugation, 15−20 kcal/mol, planar amides),3 the
selective activation of the N−C(O) bond has recently gained
significant attention as a means to functionalize the tradition-
ally unreactive amide bonds and open up new reactivity space
in organic synthesis and biology.4,5
In this respect, principally, two classes of reactions have been
developed, namely, (1) transition-metal-catalyzed oxidative
addition of the N−C(O) bond to furnish acyl-metal
intermediates, often followed by decarbonylation,4 and (2)
transition-metal-free manifold involving direct nucleophilic
addition to the amide bond to afford tetrahedral intermediates
(Figure 1A).5 Recent advances in the development of acyclic
twisted amides, wherein the amide N−C(O) bond is twisted
from planarity by steric means and often additionally activated
electronically by Nlp delocalization outside the amide bond,6
Figure 1. (a) Amide bond activation. (b) This work: Sulfoxonium
ylides from amides by selective N−C(O) cleavage.
engender one of the most promising and practical approaches
to uniformly facilitate the general use of amides as selective
acyl electrophiles in organic synthesis.
Simultaneously, recently, tremendous advances have been
in directing group enabled C−H activation reactions,11 where
made in the chemistry of sulfoxonium ylides as important
sulfoxonium ylides permit the historically challenging acyl-
building blocks in organic synthesis.7,8 In principle, after the
first preparation of sulfoxonium ylides by Corey and
Chaykovsky in 1962,9 and application in the synthesis of
Received: May 5, 2021
Published: June 7, 2021
cyclopropanes, epoxides and aziridines, these reagents have
emerged as practical metallocarbene precursors in N−H, O−
H, S−H, and C−H insertion reactions, where they act as safer
surrogates of diazo compounds.10 More recent studies
demonstrate the tremendous potential of sulfoxonium ylides
https://doi.org/10.1021/acs.orglett.1c01535
© 2021 American Chemical Society
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methylation reactions, including in a variety of tandem
annulation processes based on the installation of the carbonyl
group through this platform.12
selective N-tert-butoxycarboxylation, thus enabling to engage
generic acyclic amides in the reactivity platform.4,5 After
experimentation, we were delighted to discover that the
combination of KOt-Bu as a base and trimethylsulfoxonium
iodide as a precursor to dimethyloxosulfonium methylide
afforded the desired product at room temperature conditions
(Table 1, entry 1). As expected, the relative ratio of
trimethylsulfoxonium iodide and the base are crucial for the
reaction efficiency (Table 1, entries 1−9). Out of various bases
screened, KOt-Bu was found to be optimal (Table 1, entries
10−15). KOH and NaOtBu were less effective. Furthermore,
DMSO was identified as an effective solvent for the reaction
(Table 1, entries 16−19). Moreover, it is important to note
that the best results are obtained by nucleophile activation to
yield dimethyloxosulfonium methylide prior to the addition to
the amide (Table 1, entries 20−21). Overall, the optimized
process is highly efficient; the N-Boc deprotection deactivating
the amide bond is not observed. Furthermore, overaddition of
dimethyloxosulfonium methylide to give the epoxide7−9 as well
as direct displacement to give amino-epoxide18 (path b, Figure
1B) are not observed, consistent with the stability of the
tetrahedral intermediate and high selectivity of the process.
Likewise, overreaction product of sulfoxonium ylide with
amide has not been observed.
Having optimized conditions in hand, we next focused our
attention on evaluating the scope of this process (Scheme 1A).
As shown, the reaction displays broad scope and is successful
with a variety of amides. As such, electronically neutral (3a),
electron-donating (3b−3c), and electron-withdrawing sub-
stituents (3d−3g) on the aromatic ring are well-compatible
with the process. Of note, different fluorine substitution
important from the medicinal chemistry standpoint, such as
CF3, OCF3, F, or 3,4-F2 is well tolerated. Perhaps most notably
the method is fully compatible with sensitive functional groups,
such as bromo (3h), nitro (3i), cyano (3j), and ester (3k),
thus demonstrating significant practical advantage over metal-
catalyzed methods and providing handles for further
functionalization. Moreover, ortho-steric hindrance (3l), as
well as the presence of conjugated systems, such as biaryls
(3m) and differentially substituted naphthalenes (3n−3o), is
well accommodated. Likewise, the presence of heterocyclic
systems, such as furyl (3p) or thienyl (3q) are readily
tolerated. Finally, we were pleased to find that the method can
be extended to vinyl amides (3r) and aliphatic 1° and 2°
amides (3s−3u). At the present stage, 3° amides are not
tolerated because of the excessive steric hindrance. Unactivated
amides are recovered unchanged, consistent with resonance
destabilization as the driving force for the reaction.5,6
Aldehydes and ketones are not tolerated because of epoxide
formation.9
In this context, our laboratory has been focused on
developing new methods for amide bond activation.4a,b,5,6 In
continuation of studies, herein, we report the direct synthesis
of sulfoxonium ylides from amides by selective N−C(O)
cleavage (Figure 1B). The reaction proceeds through the
nucleophilic addition of dimethylsulfoxonium methylide to the
amide bond in acyclic twisted amides under exceedingly mild
room temperature conditions. The method advances the
growing portfolio of transition-metal-free reactions of activated
amides by tetrahedral intermediates, including amidation,13
esterification,14 thioesterification,15 and acylation,16,17 to the
synthesis of sulfoxonium ylides.
The method is notable in that the protocol provides the first
general approach to sulfoxonium ylides by amide N−C(O)
bond cleavage. The method displays a wide substrate scope
and can be applied to the late-stage derivatization of
pharmaceuticals. Furthermore, we present mechanistic studies
that outline the relative order of reactivity of amide derivatives
in this process.
Selected optimization experiments are summarized in Table
1. As shown, we selected monoactivated N-Boc/Ph amide as a
twisted amide precursor for this process (vide infra). These
amides feature decreased amidic resonance (RE = 7.2 kcal/
mol; τ = 29.1°; χN = 8.4°),6 while N-acyl-Boc-carbamates are
readily prepared from common 2° or 1° amides (vide infra) by
a
Table 1. Optimization of the Reaction Conditions
base (y
Me3SO+I− (x
yield
(%)
entry
1
2
base
equiv)
equiv)
solvent
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
Et3N
Et3N
Et3N
LiHMDS
LiHMDS
NaH
5.0
5.0
4.0
3.0
3.0
2.2
3.0
3.0
2.2
5.0
5.0
3.0
3.0
2.0
1.2
3.0
2.0
1.2
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
DMSO
DMSO
dioxane
toluene
THF
THF
76
65
61
95
63
62
81
61
47
<2
<2
<2
62
39
75
52
92
81
84
<5
<5
b
3
4
5
6
c
7
c
8
c
9
10
11
12
13
14
15
16
17
18
19
3.0
6.0
1: 1 vol/vol
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
Prompted by the broad scope of the process, we
demonstrated the utility of the reaction in the late-stage
derivatization of pharmaceuticals (Scheme 1B). As shown, the
reaction can be used for the preparation of the sulfoxonium
ylide from Febuxostat (antigout) (3v) and Probenecid
(antihyperuricemic) (3w), further demonstrating functional
group tolerance of the process and potential utility in
pharmaceutical settings.
NaH
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
d
20
e
21
We next studied the potential of various amides as
electrophilic precursors for this process (Scheme 2). As
shown, in addition to N-Boc/Ph amides, electronically tuned
N-acyl-Ts-sulfonamides (1x), atom-economic N-Ms amides
(1y), readily prepared from 1° amides N,N-Boc2 amides (1z),
a
Conditions: amide (1.0 equiv), Me3SO+I− (x equiv), base (y equiv),
solvent (0.10 M), nucleophile activation (65 °C, 2 h), then 23 °C, 15
b
c
d
h. Nucleophile activation (65 °C, 5 h). THF (0.2 M). Without
e
nucleophile activation, 23 °C. Without nucleophile activation, 65 °C.
See SI for details.
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a b
,
Scheme 1. Sulfoxonium Ylides from Amides by Selective N−C(O) Cleavage
a
Conditions: Amide (1.0 equiv), Me3SO+I− (3.0 equiv), KOt-Bu (3.0 equiv), THF (0.10 M), nucleophile activation (65 °C, 2 h), then 23 °C, 15 h.
b
c
d
Isolated yields. N,N-Boc2 amide was used. N-Ts/Ph amide was used. See SI for details.
and N-Boc/alkyl amides, such as N-Boc/Bn (1aa) and N-Boc/
Me (1ab), are perfectly accommodated in this process.
Next, studies were performed to gauge the relative order of
amides as acyl electrophiles (Scheme 4).
As shown, N,N-Boc2 amides are inherently more reactive
than N-acyl-Boc-carbamates (Scheme 4A, N,N-Boc2: N-Boc/
Ph = 86: 14), while the reactivity of N-acyl-Ts-sulfonamides is
in between these two amides (Scheme 4A, N-Ts/Ph: N-Boc/
Ph = 78: 22; N,N-Boc2: N-Ts = 69: 31). Furthermore,
intermolecular selectivity experiments using phenolic ester,
PhCO2Ph were conducted to gain insight into the relative N−
C(O) vs O−C(O) activation (Scheme 4B), where the
Next, mechanistic studies were performed to gain insight
into the selectivity of this new method (Schemes 3 and 4).
Intermolecular competition experiments between differently
substituted amides showed that electron-deficient amides are
inherently more reactive (Scheme 3A, 4-CF3: 4-MeO = 94: 6),
while sterically hindered amides are less reactive than
unsubstituted amides (Scheme 3B, 4-Me: 2-Me > 95: 5).
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rotation around N−C(O) and O−C(O) bonds (Figure 2)4a,6
and the nucleophilic addition pathway through tetrahedral
intermediate.
Scheme 2. Evaluation of Amides
Figure 2. Reactivity order of N−C(O) amide electrophiles.
In summary, we have reported the first general method for
the synthesis of sulfoxonium ylides from amides by selective
N−C(O) cleavage. The method allows for the straightforward
synthesis of sulfoxonium ylides under exceedingly mild
conditions with a broad substrate scope and excellent
functional group tolerance. The utility of the method has
been highlighted in the late-stage derivatization of pharma-
ceuticals. Mechanistic studies outline the relative order of
reactivity of amide electrophiles in this process. The high
reactivity achieved is due to ground-state destabilization in
acyclic amides. This general process is enabled by the
tetrahedral additional pathway, which typically is not accessible
from amides under mild and chemoselective conditions. Future
studies will focus on expanding the scope of sulfoxonium ylides
and achieving selectivity for amino-epoxide formation.
Scheme 3. Mechanistic Studies Amides
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01535.
Experimental details and characterization data (PDF)
Scheme 4. Mechanistic Studies N−C(O) Cleavage
AUTHOR INFORMATION
■
Corresponding Author
Michal Szostak − Department of Chemistry, Rutgers
University, Newark, New Jersey 07102, United States;
orcid.org/0000-0002-9650-9690;
Email: michal.szostak@rutgers.edu
Author
Md. Mahbubur Rahman − Department of Chemistry, Rutgers
University, Newark, New Jersey 07102, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01535
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSF (CAREER CHE-1650766) and Rutgers
University for support. The Bruker 500 MHz spectrometer
used in this study was supported by the NSF-MRI (CHE-
1229030). Additional support was provided by the Rutgers
Graduate School in the form of Dean’s Dissertation Fellow-
ship.
reactivity of PhCO2Ph was found to be comparable to that of
N-acyl-Ts-sulfonamides (OPh: N-Ts/Ph = 57: 43; N,N-Boc2:
OPh = 67: 33; OPh: N-Boc/Ph = 86: 14), demonstrating high
reactivity of the amide bond, consistent with barriers to
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Sulfoxonium Ylides with C-H Bonds. Angew. Chem., Int. Ed. 2017, 56,
13117−13121.
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