Thianthrenation-enabled α-arylation of carbonyl compounds with arenes

Article abstract of DOI:10.1021/acs.orglett.0c02913

The Pd-catalyzed α-arylation of carbonyl compounds with simple arenes enabled by site-selective thianthrenation has been demonstrated. This onepot process using thianthrenium salts as the traceless arylating reagents features mild conditions and a broad substrate scope. In addition, this protocol could also tolerate the heterocyclic carbonyl compounds and complex bioactive molecules, which is appealing for medicinal chemistry.

Full text of DOI:10.1021/acs.orglett.0c02913

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Letter
Thianthrenation-Enabled α‑Arylation of Carbonyl Compounds with
Arenes
Xiao-Xue Nie, Yu-Hao Huang, and Peng Wang*
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ABSTRACT: The Pd-catalyzed α-arylation of carbonyl compounds with simple
arenes enabled by site-selective thianthrenation has been demonstrated. This one-
pot process using thianthrenium salts as the traceless arylating reagents features
mild conditions and a broad substrate scope. In addition, this protocol could also
tolerate the heterocyclic carbonyl compounds and complex bioactive molecules,
which is appealing for medicinal chemistry.
Scheme 1. Transition-Metal-Catalyzed α-Arylation of
Carbonyl Compounds
he transition-metal-catalyzed α-arylation of carbonyl
Tcompounds with aryl halides and their variants is one
of the useful and powerful approaches for the construction of
the carbon−carbon bond1,2 and has been found widespread
application in the synthesis of pharmaceuticals and complex
natural products since the independent developments by
Buchwald,2a Hartwig,2b and Miura.2c However, an alternate
appealing approach with simple arenes as the arylating reagent
for α-C(sp3)−H of carbonyl compounds remains a paramount
challenge, probably due to both the low activities and the low
regioselectivity of simple arenes in the preactivation step.3
Certainly, the α-arylation of carbonyl compounds with simple
arenes via the cross coupling of α-C(sp3)−H and C(sp2)−H is
highly attractive and synthetically useful. In this context, the
Ku
̈
ndig4a and Taylor4b groups independently demonstrated a
Cu-mediated intramolecular α-C(sp3)-radical addition process
to access oxindole derivatives in 2009. Alternatively, Liu and
Li4c and Huo4d reported the elegant intermolecular Fe-
catalyzed coupling of three-substituted oxindoles and 3,4-
dihydro-1,4-benzoxazin-2-ones with electron-rich aromatics via
a dehydrogenative coupling strategy, respectively. With our
continuous interest in the transient mediator approach for the
functionalization of arenes,6 we envisioned that in-situ-
generated aryl sulfonium salts could serve as an efficient
arylating reagent, thus enabling the formal α-arylation of
carbonyl compounds with simple arenes. Herein we present
the site-selective thianthrenation-enabled α-arylation of car-
bonyl compounds with simple arenes with a broad substrate
scope and heterocycle compatibility. This process also
represents the first example of the Pd-catalyzed α-arylation
of carbonyl compounds using an aryl sulfonium salt as an
arylating reagent. Furthermore, this protocol provides an
alternative strategy for the synthesis of α-arylated carbonyl
compounds with high efficiency in comparison with the known
methods with aryl halides (Scheme 1c).
strategy for the regioselective functionalization of simple arenes
and complex bioactive molecules,5−7 in which the thianthre-
nation5b,6a and phenoxathiination6a of simple arenes are
noteworthy due to the remarkable regioselectivity gain in the
sulfonium formation process.5,6 Unlike aryl halides, normally
accessed via an electrophilic process with low functional group
tolerance and poor regioselectivity,8 the aryl thianthrenium
salts could be in situ generated in a highly regioselective
manner under milder conditions. Thianthrene or phenox-
athiine could be readily recovered after the reaction, which is
different fromaryl halides.
Received: August 31, 2020
Recently, the site-selective sulfonium salt formation and the
sequent transformation have been demonstrated as a versatile
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© XXXX American Chemical Society
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A

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a b
,
a b
,
Scheme 2. Evaluation of Reaction Parameters
Scheme 4. Scope of Arenes
a
Reaction conditions: (a) 2a (0.2 mmol), TTSO (0.24 mmol), Tf2O
(0.24 mmol), CH3CN (0.5 mL), N2; −20 °C for 20 min, then rt for
another 20 min. (b) K3PO4 (2.0 equiv) for 2 h, then Pd(OAc)2 (10
mol %), XPhos (15 mol %), CsF (3.0 equiv), 1 (3.0 equiv), DMF (0.5
b
mL), 80 °C, 11 h. Yields were determined by 1H NMR using
c
CH2Br2 as the internal standard. Phenoxathiine 10-oxide (POSO)
was used instead of TTSO.
Scheme 3. Reactivities of Aryl Sulfonium Salts
a
Reaction conditions: (a) 2 (0.2 mmol), TTSO (0.24 mmol), Tf2O
(0.24 mmol), CH3CN (0.5 mL), N2; −20 °C for 20 min, then rt for
another 20 min. (b) K3PO4 (2.0 equiv) was added for 2 h; then
Pd(OAc)2 (10 mol %), XPhos (15 mol %), CsF (3.0 equiv), 1 (3.0
We started our investigation using toluene-derived thian-
threnium salt 2h′ as an arylating reagent for the α-arylation of
acetophenone (Scheme 2). After systematically evaluating the
reaction parameters, we were glad to find that an 86% yield of
the desired monoarylated product could be obtained in the
presence of Pd(OAc)2 (5.0 mol %) and XPhos (5.0 mol %) in
DMF using CsF (3.0 equiv) as a base. However, the further
application of those conditions to the one-pot process led to
only a 10% yield using DCM as the solvent in the
thianthrenation step. After a quick investigation of the one-
pot process, a 99% yield of the desired product was obtained
using CH3CN as the solvent for the sulfonium salt formation
step and K3PO4 as the base to adjust the pH value of the
reaction system. Control experiments unveiled that all reaction
parameters are crucial to the formal α-arylation of ketone with
sample arenes. The reaction did not proceed in the absence of
palladium acetate and phosphine ligand. Both K3PO4 and CsF
are important for the high efficiency of this transformation.
The addition of K3PO4 and CsF in one portion also slightly
decreased the efficiency. Other solvents (entries 6 and 7),
bases (entries 8−11), and catalysts (entries 12−14) all led to
the drop in the yields. Notably, phenoxathiine 10-oxide
(POSO) was also an efficient mediator for this process with
high regioselectivity, albeit with a lower yield (entry 15).
Although DCM could be tolerated in several Pd-catalyzed
TTSO-mediated late-stage functionalizations of simple arenes
b
c
equiv), DMF (0.5 mL), 80 °C, 11 h. Isolated yield. CH3CN (0.2
d
mL) was used. Gram scale.
in one pot, this solvent showed insufficient compatibility with
our newly developed process (entry 16).
To gain more information regarding the reactivities of aryl
sulfonium salts in the Pd-catalyzed α-arylation of carbonyl
compounds, the control experiments with various aryl halides,
aryl triflates, and aryl sulfonium salts under our optimal
conditions were conducted (Scheme 3). Surprisingly, thian-
threnium salt 2h′ gave the best outcome, in which aryl chloride
2a′ and aryl bromide 2b′ gave slightly lower yields along with
the formation of a small amount of homocoupling byproducts.
Aryl iodide 2c′, normally a highly active coupling partner,
showed high activity for the homocoupling side reaction. Aryl
triflate 2d′ and phenoxathiinium salt 2g′ showed similar
reactivities, resulting in moderate yields. With regard to the
thianthrenium salt 2h′, the dimethyl-thioether-derived sulfo-
nium salt 2e′ provided the desired α-arylated product in
moderate yield and the demethylation byproduct methyl p-
tolyl sulfide in 29% yield. Interestingly, dibenzothiophene-
B
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a b
,
substrate. Predictably, the remarkable site selectivity for simple
arenes was dominated by electronics thanks to the
regioselective thianthrenation via the highly electrophilic
thianthrenium dication intermediate5e,6a (Scheme 4). Both
electron-rich and electron-deficient arenes were suitable
substrates for this protocol, albeit the electron-deficient
aromatics provided the desired products in relatively lower
yields (2a−q). This protocol could also apply to a wide range
of disubstituted (2r−y) and trisubstituted arenes (2z, 2aa),
heterocyclic arenes (2ab, 2ac), and complex bioactive
molecules (2ad−af) with remarkable regioselectivities. It is
noteworthy that the scalability of this process has been
demonstrated using acetophenone as the model substrate,
providing the desired product 3a in 90% yield.
Scheme 5. Scope of Ketones
Next, the scope of ketones was evaluated to further check
the breadth of this method. As depicted in Scheme 5, the
electronic properties of the substituents on aromatic ring of
acetophenone derivatives did not significantly affect the
reaction outcomes (5a−j). Substrates with the fluoro group
at the ortho, meta, or para position were all compatible with
this one-pot protocol, in which ortho-fluoro-substituted
acetophenone gave a slightly lower yield (5l vs 5d, 5k). In
addition, the reaction with dialkyl ketones could proceed at the
less steric hindrance site, providing the corresponding arylated
products in moderate yields (5m−r). The aryl alkyl ketones
(4s−t) and cyclic ketones (4u−x), containing a methylene
group, could also be arylated under our standard conditions in
moderate to good yields. Most importantly, heterocycle-
containing acetyl arenes, including pyridine, quinoline, 1,4-
benzodioxan, furan, thiophene, benzofuran, and benzothio-
phene, were all tolerated with this protocol. The compatibility
with heterocycles demonstrates that this methodology is highly
valuable in medicinal chemistry.
The generality of this protocol was further demonstrated by
the Pd-catalyzed α-arylation of phenylacetate derivatives using
aryl thianthrenium salts as the arylating reagent (Scheme 6).
Despite the fact that the one-pot process with arenes gave only
moderate yields (see the Supporting Information), the direct
arylation with the isolated aryl sulfonium salts could tolerate
both the electron-rich and electron-deficient phenylacetate
derivatives in moderate to good yields.
In summary, a Pd-catalyzed α-arylation of carbonyl
compounds with arenes via in-situ-generated sulfonium salts
was demonstrated for the first time. A series of ketones and
phenylacetate derivatives are tolerated with high efficiency.
This protocol is also compatible with heterocyclic substrates
and complex bioactive molecules.
a
Reaction conditions: (a) 2a (0.2 mmol), TTSO (0.24 mmol), Tf2O
(0.24 mmol), CH3CN (0.5 mL), N2; −20 °C for 20 min, then rt for
another 20 min. (b) K3PO4 (2.0 equiv) was added for 2 h, then
Pd(OAc)2 (10 mol %), XPhos (15 mol %), CsF (3.0 equiv), 4 (3.0
b
c
equiv), DMF (0.5 mL), 80 °C, 11 h. Isolated yield. 21% yield of
d
diarylated product (5m′) was obtained. Cs2CO3 (2.0 equiv) was
used instead of K3PO4.
a b
,
Scheme 6. α-Arylation of Phenylacetate Derivatives
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02913.
Experimental procedures, complete characterization
data, and copies of H and 13C NMR spectra (PDF)
1
a
Reaction conditions: 6 (0.2 mmol), 7 (3.0 equiv), Pd(OAc)2 (10
mol %), XPhos (15 mol %), CsF (5.0 equiv), DMF (1.0 mL), N2, 80
°C, 11 h. Isolated yield. Reaction was conducted in one pot.
AUTHOR INFORMATION
Corresponding Author
■
b
c
Peng Wang − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences (CAS), Shanghai 200032, P. R. China;
CAS Key Laboratory of Energy Regulation Materials, Shanghai
derived sulfonium salt 2f′ was less efficient under our standard
conditions.
With the optimal conditions, we first examined the
generality of arenes using acetophenone as the model
C
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Institute of Organic Chemistry, CAS, Shanghai 200032, P. R.
China; orcid.org/0000-0002-6442-3008;
Email: pengwang@sioc.ac.cn
Authors
Xiao-Xue Nie − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences (CAS), Shanghai 200032, P. R. China
Yu-Hao Huang − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences (CAS), Shanghai 200032, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02913
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge Shanghai Institute of Organic
Chemistry, State Key Laboratory of Organometallic Chemistry,
Shanghai Rising-Star Program (20QA1411400), and National
Natural Science Foundation of China (21821002) for financial
support.
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