Gold-Catalyzed Oxidations of 1,3-Diynamides with C(1) versus C(3) Regioselectivity: Catalyst-Dependent Oxidative Cyclizations in the C(3) Oxidation

Article abstract of DOI:10.1021/acs.orglett.0c01480

This work reports two distinct paths in catalytic oxidations of 1,3-diynamides with 8-methylquinoline oxide. A typical C(1) regioselectivity was observed for aryl-substituted 1,3-diyn-1-amides, whereas an unexpected C(3) regioselectivty occurred for 5-hydroxy1,3-diyn-1-amides. We focused on the C(3) oxidations of 5-hydroxy1,3-diyn-1-amides because we observed two oxidative cyclizations of the same substrates to yield 2-aminomethylenefuran-3(2H)-ones and 2-amino-4H-pyran-4-ones using AuCl₃ and a cationic gold catalyst, respectively. Density functional theory calculations were performed to rationalize the C(3) regioselectivity on 5-hydroxy1,3-diyn-1-amides.

Full text of DOI:10.1021/acs.orglett.0c01480

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Letter
Gold-Catalyzed Oxidations of 1,3-Diynamides with C(1) versus C(3)
Regioselectivity: Catalyst-Dependent Oxidative Cyclizations in the
C(3) Oxidation
Manisha Skaria,§ Yu-Chen Hsu,§ Yan-Ting Jiang, Ming-Yi Lu, Tung-Chun Kuo, Mu-Jeng Cheng,*
and Rai-Shung Liu*
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ABSTRACT: This work reports two distinct paths in catalytic oxidations of 1,3-diynamides with 8-
methylquinoline oxide. A typical C(1) regioselectivity was observed for aryl-substituted 1,3-diyn-1-
amides, whereas an unexpected C(3) regioselectivty occurred for 5-hydroxy1,3-diyn-1-amides. We
focused on the C(3) oxidations of 5-hydroxy1,3-diyn-1-amides because we observed two oxidative
cyclizations of the same substrates to yield 2-aminomethylenefuran-3(2H)-ones and 2-amino-4H-
pyran-4-ones using AuCl3 and a cationic gold catalyst, respectively. Density functional theory
calculations were performed to rationalize the C(3) regioselectivity on 5-hydroxy1,3-diyn-1-amides.
Scheme 1. C(3) versus C(1) Regioselectivity
etal-catalyzed intermolecular oxidations of alkynes with
pyridine-based oxides prove to be powerful tools to
M
access various oxygen-containing molecules.1 The resulting gold
carbenes in these oxidations are versatile in reacting with
suitable functionalities to enable formal cycloadditions,2 X−H
insertions,3 cyclopropanations,4 enolate formations,5 Wolff
reactions,6 and skeletal rearrangement.7 With this broad scope
of useful applications, readily available alkynes are considered to
be effective surrogates of α-diazo carbonyls. In alkyne catalysis,
ynamides8 have been extensively studied because of their
remarkable C(1) reactivity toward nucleophiles using Au(I),
Pt(II), and other π-acid catalysts; such a regioselectivity is
attributed to the resonance structure of keteniminium (I).
Equation 1 in Scheme 1 shows eminent examples involving
Zn(II)-catalyzed oxidative functionalizations of ynamides with
nucleophiles, yielding 1,2-addition products III.9 Apart from this
example, gold-catalyzed oxidative cyclizations of ynamides via a
C(1) regioselectivity are useful tools to construct many bicyclic
rings.10
We sought new catalytic oxidations of ynamides beyond this
typical C(1) regioselectivity. We selected 1,3-diynes as targets
because the two alkynes can be simultaneously functionalized
via nucleophilic additions; this feature renders such alkynes very
appealing as four-carbon building units. With these useful
substrates, we report gold-catalyzed C(3) oxidations of 5-
hydroxy-1,3-diyn-1-amides 1,11 yielding α-oxo gold carbenes IV.
These gold carbenes delivered 2-aminomethylenefuran-3(2H)-
ones 3 with the AuCl3 catalyst (Scheme 1, eq 2), but they
afforded distinct 2-amino-4H-pyran-4-ones 4 with cationic gold
catalysts (Scheme 1, eq 3). In the case of aryl-substituted 1,3-
diyn-1-amides 5, their oxidations followed a typical C(1) path,
efficiently yielding 1,4-dioxo-2-butynes 6 (Scheme 1, eq 4).
In this work, density functional theory (DFT) calculations are
performed to confirm the feasibility of C(3) regioselectivity. In
our DFT calculations, gold−π-alkynes A and B are convertible
to each other via a novel 1,3-gold migration; species A is more
stable than B and becomes more readily transformed into α-oxo
gold carbenes IV with a small barrier.
Received: April 29, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01480
© XXXX American Chemical Society
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A

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Table 1. Chemoselectivity Varied with Gold Catalysts
Scheme 4. Chemical Functionalizations of Two Cyclized
Ketones
b
yield (%)
entry
catalyst (mol %)
t (h)
3a
4a
1
2
3
4
5
6
7
8
9
10
LAuCl/AgSbF6
L′AuCl/AgSbF6
L′′AuCl/AgSbF6
PPh3AuCl/AgSbF6
IPrAuCl/AgSbF6
(OPh)3PAuCl/AgSbF6
L′AuCl/AgNTf2
L′AuCl/AgOTf
L′AuCl/AgSbF6
AuCl3(10)
1.0
1.0
1.0
1.5
1.0
1.5
1.0
1.0
1.0
1.0
16
6
15
42
56
46
b
Scheme 5. 1,4-Oxidations of 1,3-Diyn-1-amides
18
12
40
35
74
a
b
1a (0.1 M 1.0 equiv) and 2a, 2b (3 equiv). Product yields were
obtained after purification from a silica column. L = JohnPhos, L′ =
CyJohnPhos, L′′ = t-Butyl MePhos.
a
Product yields were obtained after purification from a silica column.
1a (0.10 M, 1.0 equiv) and 2a (3.0 equiv).
b
Scheme 2. Selective Formation of 2-Aminomethylenefuran-
3(2H)-ones
We prepared 5-hydroxy-1,3-diyn-1-amide 1a to examine its
oxidations with 8-methylquinoline oxide 2a in dichloromethane
(DCM, 25 °C) using various gold catalysts. An initial test with
P(t-Bu)2(o-biphenyl)AuCl/AgSbF6 at 10 mol % loading yielded
two cyclic ketones 3a and 4a from two independent oxidative
cyclizations; their respective yields were 16 and 42%,
respectively (entry 1) (Table 1). A switch to CyJohnPho-
sAuCl/AgSbF6 improved the chemoselectivity toward 2-amino-
4H-pyran-4-one 4a with 56% yield (entry 2). Other cationic
gold catalysts including t-Bu MePhosAuCl/AgSbF6, Ph3AuCl/
AgSbF6, IPrAuCl/AgSbF6, and P(OPh)3AuCl proved to be less
effective or were inefficient12 and yielded the desired 4a in 0−
46% yield in these cases; the initial 1a was still completely
consumed (entries 3−6). For CyJohnPhosAuCl, the use of
AgNTf2 or AgOTf led to catalytic deactivations to yield
compound 4a in a very small amount (entries 7 and 8). A
switch of the oxidant with 8-isopropylquinoline oxide 2b gave
the desired 4a in only 35% yield (entry 9). Notably, the use of
AuCl3 altered the chemoselectivity to efficiently provide 2-
amino-4H-pyran-4-one 3a in 74% yield (entry 10). We studied
the production efficiency of 2-amino-4H-pyran-4-one 4a with
N-oxide 2a in various solvents including DCE, CH3CN, and
CH3NO2, but these efforts proved to be fruitless. (See the SI.)
For compound 3a, its 1H NMR spectrum is characterized with
two C(2)-enone protons at δ 7.56 and δ 8.13, respectively; this
proposed structure is confirmed by X-ray diffraction of its related
3e (Scheme 2, entry 4). In contrast, compound 4a has C(1)- and
C(2)-enone protons at δ 6.27 and δ 7.70, respectively; this
proposed structure is verified by X-ray diffraction of the related
4k (Scheme 2, entry 9). Notably, the two cyclic ketones 3a and
4a are not interconvertible to each other with the two gold
catalysts.
a
b
1a (0.10 M, 1.0 equiv) and 2a (3.0 equiv). Product yields were
obtained after purification from a silica column.
Scheme 3. Selective Formation of 2-Amino-4H-pyran-4-
ones
c
We assessed the substrate scope of this AuCl3-catalyzed 2-
aminomethylenefuran-3(2H)-one synthesis using various 5-
hydroxy-1,3-diyn-1-amides 1; the results are summarized in
Scheme 2. The same 1,3-diynamides were examined with
CyJohnPhosAuCl/AgSbF6 to deliver distinct 2-amino-4H-
pyran-4-ones 4 (vide infra). For unsubstituted alcohols 1b and
1c (R3 = H) bearing tosylamides (R1 = n-Bu and Ph), their
a
Product yields were obtained after purification from a silica column.
b
c
Yields in red ink correspond to the use of oxide 2b. 1 (0.10 M, 1.0
equiv) and 2a (3.0 equiv).
B
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Figure 1. (a) Gibbs free-energy profiles for the C(1) and C(3) oxidations of 2,4-diyn-5-ol 1c, (b) the further reaction of A3, and (c) the optimized 3D
structure for TS-A1B1. The reference point of the Gibbs free-energy surface is 1c plus one gold complex (AuL+). The bond lengths of the C−C−Au
three-membered ring for TS-A1B1 are marked (units are angstroms).
products more efficiently because it will not significantly reduce
the acidity of gold catalysts. To manifest the utility of 5-
hydroxyl-2,4-diynamides 1, the substrates 1b−l in Scheme 2
were used to test the generality of the 2-amino-4H-pyran-4-one
synthesis using CyJohnPhosAuCl/AgSbF6 (10 mol %) and 8-
methylquinoline oxide (Scheme 3). Herein 8-isopropylquino-
line oxide 2b was used as the oxidant for less basic PhNT amides
(entries 2, 7, and 8) because of its superior efficiency. Most of
these substrates were effective for generating the desired six-
membered ketones 4 in moderate or high yields, except for 1,3-
diynamides 1e, which gave a complicated mixture of products
(entry 4). Our initial tests on unsubstituted alcohol derivatives
(R3 = H) yielded 2-amino-4H-pyran-4-ones 4b and 4c in 53 and
33% yields, respectively (entries 1 and 2), but the latter was
produced in 78% yield using 8-isopropylquinoline oxide 2b. For
1,3-diynamides 1d and 1e bearing electron-rich 2-thienyl and 4-
methoxyphenyl, the former gave the desired product 4d in 57%
yield, but the latter was incompatible with this six-membered
ring cyclization.
We prepared various 1,3-diynamides 1f−l bearing various
sulfonamides; all of these amide variations were applicable to
these 4H-pyran-4-one syntheses, delivering our targets 4f−l in
moderate to good yields. With these amide variations, we again
observed that less basic amides such as TsNPh are more efficient
to yield such 4H-pyran-4-ones; this trend was also observed in
our previous reactions in Scheme 2.
Scheme 4 shows several functional group transformations of
these cyclized ketone products. For 2-aminomethylenefuran-
3(2H)-one 3b, its NaBH4 reduction afforded 2-aminomethyle-
Scheme 6. Formation of Three Observed Products
corresponding products 3b and 3c were obtained in satisfactory
yields (62−70%). We also tested these AuCl3-catalyzed on
various arylmethyl alcohol analogues 1d and 1e (Ar = 2-thienyl
and 4-methoxyphenyl), further yielding the desired products 3d
and 3e in 38 and 67% yield, respectively. The molecular
structure of compound 3e was clarified with X-ray diffraction to
confirm a 2-aminomethylenefuran-3(2H)-one framework. En-
tries 5−11 show additional 1,3-diynamides 1f−l bearing varied
sulfonamides NR1R2 = NTsMe, NTsPh, and NTsCH2Ph; these
amide variations were also suitable for these oxidative
cyclizations to afford compounds 3f−l in satisfactory yields
(72−86%). For an alkyl-substituted alcohol 1m (R3
cyclohexyl), no desired product was obtained in a tractable
amount in hot DCM (40 °C, 24 h).
The product yields of these sulfonamides indicate that a less
basic sulfonamide such as TsNPh tends to give cyclization
=
C
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netetrahydrofuran-3-ol 3b-OH. The treatment of a 4-phenyl
derivative 3h with EtMgBr/THF yielded E-configurated 2-
amino-2-en-1-one 3h-A as a single diastereomer; the yield was
86%. Particularly notable is the novel reductive rearrangement of
2-amino-4H-pyran-4-one 4c with aqueous NaOH (aq) solution,
forming 2-amino-5-hydroxymethylfuran 4c′ in 76% yield. The
molecular structure of this furan derivative 4c′ was confirmed
with X-ray diffraction. Compound 4c also underwent smooth
hydrogenation with Pd/C/H2 to yield compound 4c-H2 in 90%
yield.
For 1,3-diynamides, we sought the C(1) regioselectivity that
is a typical path for gold-catalyzed oxidations of ynamides with
pyridine-based oxides8−10 or nitrones.13 Indeed, these oxida-
tions occurred smoothly with aryl-derived 1,3-diynamides 1′ to
afford 1,4-dioxo-2-butynes 5; the results are provided in Scheme
5. For 1,3-ynamides 1′a,b bearing TsNPh, their Au-catalyzed
oxidations yielded double oxidation products 5a,b in 94 and
65% yields (entries 1 and 2). We prepared additional tosylamide
derivatives 1′c−f (R = n-Bu, Me and Bn); their alkyne oxidations
gave desired products 5c and 5e,f in 61−90% yields (entries 3
and 5−6), whereas diketone 5d was obtained in only 26% yield
(entry 4). We prepared thiophene-substituted 1,3-diynamides
1′g and 1′h that afforded the corresponding products 5g and 5h
in 37 and 53% yields respectively (entries 7 and 8).
1-amides 1 should be operable for other nucleophiles because of
its relatively low energy.
In summary, we report two distinct regioselectivities in gold-
catalyzed oxidations of 1,3-diyn-1-amides with pyridine-based
oxides. A typical C(1) regioselectivity is observed for 3-aryl-1,3-
diyn-1-amides, whereas an atypical C(3) regioselectivity is
found for 5-hydroxy-1,3-diyn-1-amides. In the C(3) oxidations,
the resulting gold carbenes yield 2-aminomethylenefuran-
3(2H)-ones with a AuCl3 catalyst, but a switch to a cationic
gold phosphine catalyst yielded distinct 2-amino-4H-pyran-4-
ones from the same substrates. We performed DFT calculations
to rationalize that the C(3) regioselectivity has low energy
profiles to favorably generate α-oxo gold carbenes kinetically
and thermodynamically.
ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01480.
Experimental procedures, characterization data, crystal-
1
lography data, computational details, and H NMR and
13C NMR for representative compounds (PDF)
Accession Codes
We performed DFT calculations (see the computational
details in the SI) to understand the nature of such C(1) and
C(3) regioselectivities, as depicted by the two paths (A1 → A2 →
A3 and B1 → B2 → B3, Figure 1a). In the complexation of 1,3-
diyn-5-ol 1c with a gold catalyst, two gold−π-alkynes A1 and B1
were obtained, with the former being more stable than the latter
by 4.7 kcal/mol (ΔG = −15.1 kcal/mol for A1 and −10.4 kcal/
mol for B1).
CCDC 1971798−1971799 and 1972980−1972981 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
In our calculations, we find that these two intermediates are
convertible to each other via a transition state TS-A1B1 with a
relative Gibbs free energy of −0.6 kcal/mol, slightly lower than
those for free diyne and the gold complex. The structure of TS-
A1B1 is shown in Figure 1c. In the nucleophilic addition of 8-
methylquinoline oxide 2a at these two π-alkynes, the formation
of the gold carbene A2 is more facile because the energy of the
transition state TS-A1A2 (+5.3 kcal/mol) is lower than that of
state TS-B1B2 (+7.8 kcal/mol). These two nucleophilic
additions turn out to be the rate-determining steps because
the remaining transition states TS-A2A3, TS-A3A4, TS-B2B3, and
TS-B3B4 have lower relative Gibbs free energy. Accordingly,
these DFT calculations support the notion that the C(3)
oxidation regioselectivity (Path A) is more feasible than the
other C(1) regioselectivity (Path B).
Following this C(3) regioselectivity, the preferable gold
carbenes A3 undergo a very facile 1,2-hydride migration to yield
the 3,5-dioxo-yn-1- amide intermediate A4 with a kinetic barrier
of only 1.8 kcal/mol (Figure 1b) that ultimately becomes free 3-
oxo-5-enol A5, which equilibrates with its enol form A5′ to
induce a ketone−alkyne cyclization.
Less alkynophilic AuCl3 is favorable for a 5-exo- dig path,
whereas a cationic gold catalyst tends to complex ynamide in a
keteniminium resonance I (eq 1) to leave a positive charge at the
C(1) carbon, thus enabling a 6-endo-dig cyclization. For 4-aryl-
1,3-diyn-1-amides, their corresponding gold carbenes B3′ seem
to undergo 1,3-carbene migration14 to form distinct gold
carbenes B4′ that are further oxidized by N-oxide 2a to form
observed products 5 (Scheme 6). The above calculation
indicates that the C(3) regioselectivity of 4-hydroxy-1,3-dien-
AUTHOR INFORMATION
Corresponding Authors
■
Mu-Jeng Cheng − Department of Chemistry, National Cheng
Kung University, Tainan City, ROC; orcid.org/0000-0002-
8121-0485; Email: mjcheng@mail.ncku.edu.tw
Rai-Shung Liu − Frontier Research Center on Fundamental and
Applied Science of Matters, Department of Chemistry, National
Tsing-Hua University, Hsinchu 30013, ROC; orcid.org/
0000-0002-2011-8124; Email: rsliu@mx.nthu.edu.tw
Authors
Manisha Skaria − Frontier Research Center on Fundamental and
Applied Science of Matters, Department of Chemistry, National
Tsing-Hua University, Hsinchu 30013, ROC; orcid.org/
0000-0002-6729-714X
Yu-Chen Hsu − Frontier Research Center on Fundamental and
Applied Science of Matters, Department of Chemistry, National
Tsing-Hua University, Hsinchu 30013, ROC; orcid.org/
0000-0001-8678-9943
Yan-Ting Jiang − Frontier Research Center on Fundamental and
Applied Science of Matters, Department of Chemistry, National
Tsing-Hua University, Hsinchu 30013, ROC
Ming-Yi Lu − Frontier Research Center on Fundamental and
Applied Science of Matters, Department of Chemistry, National
Tsing-Hua University, Hsinchu 30013, ROC
Tung-Chun Kuo − Department of Chemistry, National Cheng
Kung University, Tainan City, ROC
Complete contact information is available at:
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D
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Author Contributions
§M.S. and Y.-C.H. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Ministry of Science and Technology and the
Ministry of Education, Taiwan, for supporting this work.
■
(9) Li, L.; Zhou, B.; Wang, Y.-H.; Shu, C.; Pan, Y.-F.; Lu, X.; Ye, L.-W.
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