Gold(I)-Catalyzed Reactions between 2-(1-Alkynyl)-2-alken-1-ones and Vinyldiazo Ketones for Divergent Synthesis of Nonsymmetric Heteroaryl-Substituted Triarylmethanes: N-versus C-Attack Paths

Article abstract of DOI:10.1021/acs.orglett.0c02765

Gold-catalyzed synthesis of nonsymmetrical heteroaryl-substituted triarylmethanes using 2-(1-alkynyl)-2-alken-1-ones and vinyldiazo ketones is described. In this catalytic sequence, vinyldiazo ketones attack gold-containing 3-furylbenzyl cations to form the observed C(1)-addition products. We also note that vinyldiazo ketones can be thermally cyclized to yield pyrazole derivatives, which can react with 3-furylbenzyl cations to afford pyrazole-containing triarylmethanes, corresponding to a N(5)-addition path.

Full text of DOI:10.1021/acs.orglett.0c02765

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Letter
Gold(I)-Catalyzed Reactions between 2‑(1-Alkynyl)-2-alken-1-ones
and Vinyldiazo Ketones for Divergent Synthesis of Nonsymmetric
Heteroaryl-Substituted Triarylmethanes: N- versus C‑Attack Paths
Rahul Dadabhau Kardile and Rai-Shung Liu*
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ABSTRACT: Gold-catalyzed synthesis of nonsymmetrical heteroaryl-substituted triarylmethanes using 2-(1-alkynyl)-2-alken-1-ones
and vinyldiazo ketones is described. In this catalytic sequence, vinyldiazo ketones attack gold-containing 3-furylbenzyl cations to
form the observed C(1)-addition products. We also note that vinyldiazo ketones can be thermally cyclized to yield pyrazole
derivatives, which can react with 3-furylbenzyl cations to afford pyrazole-containing triarylmethanes, corresponding to a N(5)-
addition path.
inyldiazo carbonyl species serve as versatile all-carbon
Vbuilding blocks in numerous cycloaddition reactions,
forming valuable carbocyclic and heterocyclic rings of various
sizes.1,2 These diazo carbonyl species are nucleophilic so as to
react with electrophilic π-bond motifs to form products of five-
or six-membered rings; they serve as 2C- or 3C-building units.3
These diazo species can alternatively form electrophilic metal
carbenes that can be functionalized with nucleophilic π-bond
motifs, thus serving as 1C- or 3C-building units (Scheme 1a).
This umpolung nature is an appealing character for vinyldiazo
species as building units.4 We recently reported bicyclic
annulation5 between benzopyrilium and vinyldiazo ketones
wherein vinyldiazo ketones are first utilized as a five-atom (3C
+ 2N) building unit (eq 1); we postulated initial [5 + 4]-
annulations between benzopyrilium and vinyldiazo ketones.
Notably, such annulations were previously unattainable with
vinyldiazo esters. In diazo chemistry, few efforts have been
devoted to the development of vinyldiazo ketones as diazo
ketones and esters are believed to have the same reactivity. In
this work, we highlight two distinct approaches to employ
vinyldiazo ketones as 1-furyl and 1H-pyrazolyl units, whereas
vinyldiazo esters are inapplicable. We report the gold-catalyzed
divergent synthesis of highly nonsymmetric heteroaryl-
substtituted triarylmethanes using 2-(1-alkynyl)-2-alken-1-
ones and vinyldiazo ketones. In this system, vinyldiazo ketones
react with gold-containing 3-furylbenzyl cations (In-2) via
distinct C(1)-attack and N(5)-attack paths,6 leading to 2-
(furan-3-yl(phenyl)methyl)furan derivatives 3 and 1-(furan-3-
yl(phenyl)methyl)-1H-pyrazoles 5, respectively. The current
syntheses of triarylmethanes7,8 are limited mainly to benzene-
based triarylmethanes (TRAM) in symmetric patterns, with
only one report of their heteroaromatic analogues.9
Importantly, nonsymmetric heteroaryl-substituted triaryl-
methanes that are relevant in material and medicinal chemistry
can be accessed using these two reactions.10 Compounds I−V
are several representatives that exhibit potent biological effects
such as antitubercular,10e antiviral,10f anticancer,10g and anti-
inflammatory activity10h (Figure 1). Furthermore, nonsym-
metric triarylmethanes have found widespread applications as
leuco dyes11a and photochromic agents.11b,c They serve also as
building blocks to generate dendrimers11d and as substrates on
which to perform biological studies.12
The optimization of reaction of enynone 1a with vinyldiazo
ketone 2a (1.2 equiv) to form our target 3a is shown in Table
1. Our initial test was to use electron-deficient 5 mol % of
LAuCl/AgOTf (L = PPh3 and P(OPh)3) in dry dichloro-
Received: August 18, 2020
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© XXXX American Chemical Society
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compound 3a were 61% and 33%, respectively (entries 5 and
6). For P(t-Bu)2(o-biphenyl)AuCl/AgOTf, the yields of
compound 3a in specified solvents follow: DCE 74%, toluene
40%, THF 0%, and MeCN 21%. The X-ray diffraction studies
of related compound 3d confirmed the molecular structure of
compound 3a; the molecule contains one 1-furyl, one 2-furyl,
and one phenyl group linked to a methine moiety.
Scheme 1. Chemodivergence of Vinyldiazo Carbonyls and
This Work
The reaction scope of enynones 1 with vinyldiazo ketone 2a
under the optimized conditions is summarized in Table 2. We
Table 2. Reactions of Various 2-(1-Alkynyl)-2-alken-1-ones
a
b
[1] = 0.05 M. Isolated yields of 3. L = P(t-Bu)2(o-biphenyl).
tested substrates 1b−1d with variable alkynyl substituents 4-
XC6H4 (X = Me, OMe, and Br); their desired products 3b−3d
were isolated in 76−83% yields (entries 1−3). We examined
enynone 1e with a 3-chlorophenyl alkynyl substituent,
delivering analogous species 3e in 81% yield (entry 4). For
3-thienylalkyne moiety 1f, its resulting product 3f was isolated
in 84% yield (entry 5). We prepared enynones 1g−1h bearing
various alkyl-substituted alkynes (R2 = cyclopropyl and n-
butyl) that delivered desired compounds 3g−3h in 68−75%
yields (entries 6−7). We changed also the alkenyl substituents
(R1) of 2-(1-alkynyl)-2-alken-1-ones 1i−1j with R1 = 4-XC6H4
(X = Me and Cl), delivered corresponding products 3i and 3j
in 80−83% yields (entries 8 and 9). For 3-thienyl-substituted
alkene deivative 1k, this catalytic reaction furnished all-
heteroaryl-substituted compound 3k in 83% yield (entry 10).
Enynones 1l−1n bearing varied ketone substituents (R3 = Ph,
n-Bu, and Et) were also well tolerated under optimized
conditions, affording expected products 3l−3n in 81−90%
yields (entries 11−13). Notably, aldehyde substrate 1o (R3 =
H) was less efficient for this reaction, rendering product 3o in
58% yield (entry 14). We performed also a reaction on
enynone 1q with R1 = 4-NO2C6H4, R2 = Ph ,and R3 = Me,
yielding compound 3p in 48% yield (entry 15).
Figure 1. Representatives of bioactive heteroaryl-substituted TRAM.
Table 1. Catalyst Screening with Various Gold Catalysts
b
yield (%)
entry
catalyst
solvent
time (h)
3a
1a
1
2
3
4
5
6
7
8
9
(PhO)3PAuCl/AgOTf
Ph3PAuCl/AgOTf
LAuCl/AgOTf
IPrAuCl/AgOTf
LAuCl/AgNTf2
LAuCl/AgSbF6
LAuCl/AgOTf
LAuCl/AgOTf
LAuCl/AgOTf
LAuCl/AgOTf
DCM
DCM
DCM
DCM
DCM
DCM
DCE
toluene
THF
CH3CN
2
4
1
3
1.5
5
30
27
88
56
61
33
74
40
0
1
12
12
14
16
30
27
10
21
a
b
[1a] = 0.05 M. Isolated yields of 3a. L= P(t-Bu)2(o-biphenyl).
The scope of vinyldiazo ketones 2 with (E)-3-benzylidene-5-
phenylpent-4-yn-2-one 1a under optimized reaction conditions
is summarized in Table 3. We tested vinyldiazo ketones 2b−2f
bearing 4-substituted phenylketone (R4 = 4-XC6H4; X = Me,
OMe, tert-butyl, Cl, and Br); their expected compounds 4a−4e
were produced in satisfactorily yields (75−87%, entries 1−5).
The vinyldiazo species bearing 3-thienyl moiety 2g afforded
compound 4f in 84% yield (entry 6). For 2-naphthyl ketone
methane near 23 °C; these conditions afforded triarylmethane
derivative 3a in 27−30% yield (entries 1 and 2). To our
delight, the use of electron-rich LAuCl/AgOTf (L = P(t-
Bu)2(o-biphenyl) provided 3a in 88% yield (entry 3).
IPrAuCl/AgOTf gave desired compound 3a in 56% yield
(entry 4). For variation of silver sources as in P(t-Bu)2(o-
biphenyl)AuCl/AgX, (X = NTf2 and SbF6) the yields of
B
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Table 3. Reactions of Various Vinyldiazo Ketones
Scheme 2. Plausible Mechanism for C(1)- and N(5)-Attack
a
b
[1a] = 0.05 M. Isolated yields of 4. L= P(t-Bu)2(o-biphenyl).
derivative 2h, its corresponding product 4g was isolated in 81%
yield (entry 7). The alkylketone derivatives 2i and 2j (R5 = H,
R4 = cyclopropyl and n-butyl) were also suitable for this
transformation, yielding the desired products 4h and 4i in 70%
and 72% yield, respectively (entries 8 and 9). We also tested
the reaction on 3-alkyl-2-diazo-3-vinyl phenyl ketones 2k and
2l (R5 = Me and n-butyl, R4 = Ph); their resulting compounds
4j and 4k were isolated in 63−67% yields (entries 10 and 11).
We prepared vinyldiazo ester 2a′ to examine its chemical
reactivity, but it yielded products in a complicated mixture
under standard conditions (eq 4). Treatment of vinyldiazo
Table 4. Synthesis of N-Pyrazole-Based Triarylmethanes
a
b
[1] = 0.05 M. Isolated yields of 5. L= P(t-Bu)2(o-biphenyl), Np =
2- naphthyl.
form 1H-pyrazoles, which are subsequently treated with
enynones 1. We examined reaction of substrate 1b with a
tolylethynyl moiety (R2 = 4-MeC6H4); product 5b was
obtained in 80% yield (entry 1). For n-butyl substrate 1h
(R2 = n-butyl), its resulting product 5c was rendered with 71%
yield (entry 2). For species 1j bearing alkenyl substituent R1 =
4-ClC6H4, resulting product 5d was formed in 78% yield
(entry 3). For vinyldiazo ketones 2c and 2e with R4 = 4-
XC6H4; X = OMe and Cl, their reactions with 1a delivered
compounds 5e and 5f in 73% and 76% yields, respectively
(entries 4 and 5). The reaction maintained a high efficiency for
3-thienyl ketone derivative 2g (R4 = 3-thienyl), generating
compound 5g with 82% yield (entry 6). In the case of
cyclopropylketone derivatives 2i (R4 = cyclopropyl), its
corresponding product 5h was obtained in moderate yield
(61%, entry 7). We performed this catalytic reaction also on 3-
methyl-2-diazo-3-vinyl phenyl ketone 2k (R5 = Me); its desired
product 5i was obtained in 60% yield (entry 8). We prepared
also enynones 1p with R1 = 3-thienyl and R2 = 4-BrC6H4; its
reaction with 2-naphthylvinyldiazo ketone 2h delivered all
heteroaryl-substituted triarylmethane 5j in 83% yield (entry 9).
ketones 2a with a gold catalyst in DCM also led to a slow
decomposition of this diazo ketone, forming no furan
derivative 2a′′ at all. Instead, we found that compound 2a
itself underwent a thermal cyclization to form pyrazole 2α; the
yield was 93% (eq 5). This pyrazole has a hydrogen bond
because the NH proton resonance is down to δ 11.6 ppm,
whereas the other tautomer is reported to have N−H at δ 6.45
ppm.13b This information indicates that the 1-furyl ring is
constructed in the course of this catalytic reaction.
When pyrazole 2α was treated with enynone 1a under the
standard conditions, we obtained compound 5a in 81% yield;
the mechanism will be discussed in Scheme 2 (vide infra).
Notably, the availability of pyrazole 2α allows a new synthesis
of triarylmethanes bearing an N-pyrazole and a 2-furyl ring (eq
6).
We assessed also the reaction generality of these new 1H-
pyrazole syntheses using 2-(1-alkynyl)-2-alken-1-ones 1 and
vinyldiazo ketones 2; the results appear in Table 4. A general
procedure involves an initial heating of vinyldiazo ketones 2 to
C
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The X-ray diffraction studies confirmed the molecular structure
of compound 5j. Vinyldiazo ester 2a’ was also an applicable
substrate that reacted with enynone 1a to deliver its
corresponding product 5k in 77% yield (entry 10).
Among our triaryl products, the mechanism of the formation
of 1-furyl-containing compounds 3 and 4 is unclear; we thus
conducted an additional experiment. We prepared 2-
siloxyvinyldiazo ketone 2m that reacted with enynone 1a to
yield α-oxodiazo ketone 6 that was not convertible to our
target 7. A main reason for this unsuccessful reaction is that its
keto−enol tautomerization is unfavorable for enol form 6′,
rendering formation of its gold carbene difficult Herein, the
cationic gold catalyst coordinates with the 1,3-dicarbonyl
oxygens to form a stable chelation species.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02765.
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Experimental procedures, characterization data, crystal-
lography data, and 1H NMR and 13C NMR for
representative compounds (PDF)
Accession Codes
CCDC 2022577−2022578 contain the supplementary crys-
tallographic 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.
We postulate a plausible mechanism in Scheme 2 involving
key intermediate C that is inferred from isolation of α-oxo
diazo species 6 (eq 7). In the presence of a gold catalyst, gold
AUTHOR INFORMATION
Corresponding Author
■
Rai-Shung Liu − Frontier Research Center of Matter Science and
Technology, Department of Chemistry, National Tsing Hua
University, Hsinchu 30013, Taiwan, ROC; orcid.org/0000-
0002-2011-8124; Email: rsliu@mx.nthu.edu.tw
Author
Rahul Dadabhau Kardile − Frontier Research Center of Matter
Science and Technology, Department of Chemistry, National
Tsing Hua University, Hsinchu 30013, Taiwan, ROC
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02765
π-alkyne species forms gold-containing 3-furylbenzyl cation6 A.
An initial C(1)-attack of vinyldiazo species 2a on this cation A
generates C(1)-addition intermediate B that is deprotonated
with OTf− to generate 3-furylgold diazo species C and species
D sequentially. In the presence of the gold catalyst, diazo
species D forms gold vinylcarbenes that subsequently undergo
a known oxa-Nazarov cyclization to give furylinum species F,
ultimately yielding the observed product 3a.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Ministry of Science and Technology
(MOST 107-3017-F-007-002) and the Ministry of Education
(MOE 106N506CE1), Taiwan, for supporting this work.
To rationalize the N-attack of pyrazole intermediates, we
postulate two tautomers 2α and 2α′ as initial nucleophiles, as
depicted in Scheme 2. Tautomer 2α is the isolated form; its
reaction with 3-furylbenzyl cation A is expected to occur at the
N(5)-regioselectivity, so to retain a N−H---OC hydrogen
bond. In this path, resulting iminium intermediate G is highly
acidic and becomes readily deprotonated with TfO− to yield
observed product 5a through a N(5)-addition route.
Equation 4 shows the inapplicability of vinyldiazo ester 2a′
to this triarylmethane synthesis because the ester functionality
has less electron-withdrawing power. For ester 2a′, its ester
form is best described with resonance structure 2a′-I, whereas
vinyldiazo ketone 2a has important contributions from two
other forms 2a-II and 2a-III. Accordingly, vinyldiazo ketone 2a
is better than vinyldiazo ester as a nucleophile.
In summary, we report gold-catalyzed divergent syntheses of
nonsymmetric heteroaryl-substituted triarylmethanes using 2-
(1-alkynyl)-2-alken-1-ones and vinyldiazo ketones. Our mech-
anistic analysis indicates that vinyldiazo ketones attack gold-
containing 3-furylbenzyl cations6 to form the observed C(1)-
addition products. We found also that vinyldiazo ketones can
be thermally activated to yield pyrazoles; this process is also
elaborated into pyrazole-containing products using the same
reactants and gold catalyst.
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E
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