F- Nucleophilic-Addition-Induced [3 + 2] Annulation: Direct Access to CF3-Substituted Indenes

Article abstract of DOI:10.1021/acs.orglett.8b02128

An efficient [3 + 2] annulation of (2,2-difluorovinyl)-2-iodoarenes and internal alkynes was developed for the synthesis of 1-(trifluoromethyl)-1H-indenes. The success of this strategy hinges upon a well-balanced process for the generation of two transient reactive species, specifically trifluoroethylsilver and alkenylpalladium intermediates, in the same molecule, as well as a smooth transmetalation step, which delicately joins together these two different metallic intermediates.

Full text of DOI:10.1021/acs.orglett.8b02128

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
F− Nucleophilic-Addition-Induced [3 + 2] Annulation: Direct Access
to CF3‑Substituted Indenes
Hai-Jun Tang, Yu-Feng Zhang, Yi-Wen Jiang, and Chao Feng*
Institute of Advanced Synthesis (IAS), College of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation
Center for Advanced Materials (SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, People’s Republic of
China
S
* Supporting Information
ABSTRACT: An efficient [3 + 2] annulation of (2,2-difluorovinyl)-2-
iodoarenes and internal alkynes was developed for the synthesis of 1-
(trifluoromethyl)-1H-indenes. The success of this strategy hinges upon a
well-balanced process for the generation of two transient reactive species,
specifically trifluoroethylsilver and alkenylpalladium intermediates, in the same molecule, as well as a smooth transmetalation
step, which delicately joins together these two different metallic intermediates.
he trifluoromethyl group is widely present in pharma-
advances, the frameworks constructed are relatively restricted,
Tceuticals, agrochemicals, and functional materials, mainly thus prompting us to further explore the synthetic potential of
because of its profound influence on the biological, chemical,
and physical properties of related structural motifs.1 As such,
diverse strategies aiming at its efficient incorporation have been
thoroughly investigated with the ever-increasing amount of
reports on nucleophilic, electrophilic, and radical trifluorome-
thylation being nicely devised.2 Although impressive, most of
the reported methods intrinsically rely on the elaborated
trifluomethylation reagents, thus suffering from problems such
as low atom economy,3 utilization of stoichiometric amounts
of transition-metal catalysts,4 or high cost of the CF3-
containing reagents. Recently, our group reported a concep-
tually novel protocol for the expedient synthesis of
homoallyltrifluoromethane derivatives triggered by fluoride
nucleophilic addition to gem-difluorolakenes. In the presence
of Pd-catalyst, the otherwise intractable α-CF3-carboanion,
generated in situ, underwent an allylation to afford
homoallyltrifluoromethanes in an efficient manner (see
Scheme 1a).5 Further expanding this strategy to arylation led
to a successful development of an intriguing synthetic protocol
for the construction of trifluoromethyl-diarylmethane motifs
(Scheme 1a).6 Note that, by taking advantage of easily
available gem-difluoroalkenes as reliable α-CF3-carboanion
precursors, the dilemma that conventional protocols may
experience can be elegantly ameliorated. Despite these
the strategy of fluoride nucleophilic-addition-induced trans-
formations.
Indene represents a molecular structure of considerable
interest in many research fields, including pharmaceuticals,7
materials,8 and organometallics.9 Therefore, many elegant
strategies for their construction were formulated,10 such as
intramolecular electrophilic substitution reactions,11 nucleo-
philic-attack-induced cyclizations,12 ring contraction and
expansion,13 and transition-metal-catalyzed cyclizations.14
Nevertheless, there is still a lack of general synthetic
approaches that permit the rapid assembly of 1-trifluorometh-
yl-1H-indenes. The introduction of the CF3 unit into indene
skeletons not only brings forth intriguing electronic perturba-
tion to the parent molecules but also exerts a positive influence
on the exploitation of specific architecture with desirable
bioactivities.15 While 1-trifluoromethyl-1H-indenes can be
accessed by nucleophilic trifluoromethylation of indenones16
or intramolecular cyclization with CF3-containing molecules,17
these strategies suffer from either the utilization of exquisitely
elaborated precursors or narrow scope with restricted
substitution patterns. Under these circumstances, we envi-
sioned an appealing strategy that engages the in-situ-generated
α-CF3-carboanion with an internal electrophilic vinylpalladium
species, thereby enabling an expedient synthetic protocol for
the construction of trifluoromethylated indenes. Herein, we
describe a palladium-catalyzed [3 + 2] annulation of (2,2-
difluorovinyl)-2-iodoarenes and internal alkynes (see Scheme
1b). This reaction provides a versatile and simple platform for
the construction of densely substituted 1-(trifluoromethyl)-
1H-indene derivatives that could not be easily accessed by the
reported methods. Since multisubstituted indenes are known
to be potent bioactive molecules and synthetically useful
Scheme 1. Methods for the Formation of Indene Derivatives
Received: July 7, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02128
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Reaction Scope of Alkynesa
ligands, the development of this novel protocol will further
expand the potential application of these prominent molecular
structures.
Our investigation was initiated with the reaction between 1-
(2,2-difluorovinyl)-2-iodobenzene 1a and diphenylacetylene
2a, using [allylPdCl]2 as the precatalyst and XPhos as the
ligand (see Table 1). We were delighted to find that the
a
Table 1. Optimization of Reaction Conditions
entry
catalyst
ligand
XPhos
XPhos
yield (%)
bc
,
1
2
3
4
5
6
7
[allylPdCl]2
Pd2(dba)3
22
c
NR
58
b
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
Pd(COD)Cl2
XPhos
b
d
d
BrettPhos
tBuBrettPhos
tBuXPhos
BrettPhos
85, 77
b
57
b
81, 75
trace, trace
e
f
a
Reactions performed with 1a (0.15 mmol), 2a (0.30 mmol), catalyst
(5.0 mol %), ligand (10 mol %), and AgF (1.2 equiv) in dioxane (1.0
b
c
mL) at 80 °C for 12 h. NMR yield. 2.5 mol % catalyst was used.
d
e
f
Isolated yield. 1.2 equiv of KF was used instead of AgF. 1.2 equiv of
CsF was used instead of AgF.
desired product 3aa was obtained in 22% yield at 80 °C with
AgF as the fluoride donor (Table 1, entry 1). An improved
result was achieved by changing the palladium catalyst to
Pd(COD)Cl2 (Table 1, entry 3). Further screening of
phosphine ligands demonstrated that BrettPhos was the
optimal choice, by which the product 3aa was obtained in
85% yield (Table 1, entries 3−6). After judiciously tuning the
reaction parameter, the combination of Pd(COD)Cl2,
BrettPhos, AgF, and dioxane was identified to be optimal.
Notably, replacing AgF with other alkali-metal-derived
fluorides, such as KF and CsF, had a deleterious effect on
the annulation reaction (Table 1, entry 7). Not surprisingly,
without either palladium catalyst or the phosphine ligand, no
desired product was observed (see the Supporting Information
for details).
With the optimized reaction conditions in hand, the reaction
generality, with respect to alkynes, were surveyed (see Scheme
2). Diverse symmetrical diarylacetylenes are viable substrates
to form 1-trifluoromethylindenes 3db−3dk. Different sub-
stituents on the aryl group were well-tolerated; for example,
Me- and OMe-bearing substrates provided the desired
products in good to excellent yields (70%−90%). Moreover,
halogen substituents, including F, Cl, and Br, were compatible
to the reaction conditions, giving rise to the desired products
in moderate to good yields. In addition, diarylacetylenes with
electron-withdrawing groups, such as CF3, CN, and COMe
were examined, and they afforded the corresponding 1-
trifluoromethylindenes in 38%−76% yields. The internal
alkynes substituted with two aliphatic groups were also
efficiently transformed to the desired products in moderate
yields by changing the ligand to tBuXPhos (3dl−3dn). As
expected, unsymmetrical internal alkynes participated in the
annulation successfully and the two regioisomeric products
could be isolated separately (3do/3do′ and 3dp/3dp′). In
these cases, the main reason for the major isomers formation
was attributed to the electronic perturbation of alkynyl
a
Reactions performed with 1d (0.15 mmol), 2 (0.225 mol),
Pd(COD)Cl2 (5.0 mol %), BrettPhos (10 mol %), and AgF (1.2
equiv) in dioxane (1.0 mL). 1.5 equiv of AgF was used. tBuXPhos
was used instead of BrettPhos. 2.0 equiv of 2 was used. NMR yield
was indicated. Major isomer was shown.
b
c
d
e
f
moieties, thus favoring the mode of triple-bond migration
insertion by generating C(aryl)−C(alkenyl) bonds at electroni-
cally more-positive carbon atoms. It is noteworthy that
activated alkyne substrates such as alkynyl ketone and
acetylenic acid derivatives were also amenable to this reaction
and delivered the products as a single regioisomer, which was
ascribed to the same reason for electronic polarization that
caused a highly selective alkyne migratory insertion (3dq−
3ds).
The scope of the reaction was further explored with various
(2,2-difluorovinyl)-2-iodoarenes (see Scheme 3). The sub-
strates bearing a range of functional groups, such as methyl,
methoxy, fluoride, chloride, bromide, or trifluoromethyl, were
well-tolerated. Electron-donating groups, such as Me and
OMe, on the phenyl ring had little influence on the reaction
efficiency, providing the corresponding products in 71%−91%
yields (3aa−3fa). To our pleasure, electron-deficient substrates
were also amenable in the reaction (3ga−3na), albeit with
somewhat diminished reaction efficiency. Remarkably, halo-
gen-containing substrates, especially those with Br, could
survive in this Pd-catalyzed reaction, thus providing the
opportunity for further synthetic elaboration of these structure
motifs (3ga−3la). Note that compound 1 was invariably
consumed completely, even in cases wherein the products were
obtained in low yields. To evaluate the scalability of this
B
DOI: 10.1021/acs.orglett.8b02128
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Organic Letters
Letter
intermediate IV affords the six-membered palladacycle V,19
which further undergoes reductive elimination to release the
desired product 3, along with the regeneration of active
catalyst. Path b represents an alternative reaction sequence, by
which the α,α,α-trifluoroethylsilver intermediate is performed
before the alkyne insertion. Analogously, vinylpalladium
intermediate IV is generated in the following step and
furnishes the product through the successive intramolecular
transmetalation and reductive elimination.
a
Scheme 3. Reaction Scope of o-iodo-difluorolefins
In summary, we have presented a Pd-catalyzed [3 + 2]
annulation of (2,2-difluorovinyl)-2-iodoarenes and internal
alkynes triggered by a fluoride nucleophilic addition. This
reaction is characterized by its operational simplicity and mild
reaction conditions, as well as wide functionality tolerance. By
leveraging on a smooth intramolecular transmetalation
between organosilver and palladium intermediates, the strategy
of fluoride nucleophilic-addition-induced transformations are
successfully expanded, thus providing a practical and efficient
synthetic route for the construction of highly functionalized 1-
trifluoromethyl-1H-indenes.
a
Reactions performed with 1 (0.15 mmol), 2a (0.225 mol),
Pd(COD)Cl2 (5.0 mol %), BrettPhos (10 mol %), and AgF (1.5
equiv) in dioxane (1.0 mL). 1.2 equiv of AgF was used. 3.0 mmol
scale reaction. NMR yield was indicated.
b
c
ASSOCIATED CONTENT
d
■
S
* Supporting Information
annulation reaction, a 3.0 mmol scale reaction between 1d and
2a was carried out, which gave rise to product 3da in 80%
yield.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02128.
Elegant works from Sanford and Hu’s groups suggested that
homolysis of organosilver intermediates could generate radical
species;18 however, such intermediates were not detected in
our previous fluoroallylation/arylation protocols. In order to
gain more insight into the reaction mechanism of the present
[3 + 2] annulation, control experiments with radical
scavengers, such as TEMPO, BHT, and 1,1-diphenylethylene
were performed. It was found that none of these additives
showed any impediment in the reaction efficiency, indicating
that a radical intermediate is not involved in the catalytic cycle
(see the Supporting Information for details). Therefore, we
proposed two plausible ionic reaction pathways, as shown in
Scheme 4. For path a, the reaction starts from oxidative
Detailed experimental procedures, and full spectroscopic
data for all new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: iamcfeng@njtech.edu.cn.
ORCID
Chao Feng: 0000-0003-4494-6845
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Scheme 4. Proposed Mechanism
We gratefully acknowledge the financial support of the
“Thousand Talents Plan” Youth Program, the “Jiangsu
Specially-Appointed Professor Plan”, the Natural Science
Foundation of Jiangsu Province (No. BK20170984), and
SICAM Fellowship by the Jiangsu National Synergetic
Innovation Center for Advanced Materials.
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