Highly Diastereoselective Synthesis of Trifluoromethyl Indolines by Interceptive Benzylic Decarboxylative Cycloaddition of Nonvinyl, Trifluoromethyl Benzoxazinanones with Sulfur Ylides under Palladium Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b00237

A highly diastereoselective synthesis of trifluoromethyl-substituted indolines under palladium catalysis is disclosed. The reaction proceeds by interceptive decarboxylative benzylic cycloaddition (IDBC) of nonvinyl, trifluoromethyl benzoxazinanones with sulfur ylides. The palladium-π-benzyl zwitterionic intermediates are suggested for this transformation, and this would be the first example of an IDBC reaction.

Full text of DOI:10.1021/acs.orglett.8b00237

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Highly Diastereoselective Synthesis of Trifluoromethyl Indolines by
Interceptive Benzylic Decarboxylative Cycloaddition of Nonvinyl,
Trifluoromethyl Benzoxazinanones with Sulfur Ylides under
Palladium Catalysis
Nagender Punna,† Pulakesh Das,† Veronique Gouverneur,‡ and Norio Shibata*,†,§
́
†Department of Nanopharmaceutical Sciences & Department of Life Science and Applied Chemistry, Nagoya Institute of
Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan
‡Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.
§Institute of Advanced Fluorine-Containing Materials, Zhejiang Normal University, 688 Yingbin Avenue, 321004 Jinhua, China
S
* Supporting Information
ABSTRACT: A highly diastereoselective synthesis of trifluor-
omethyl-substituted indolines under palladium catalysis is
disclosed. The reaction proceeds by interceptive decarbox-
ylative benzylic cycloaddition (IDBC) of nonvinyl, trifluor-
omethyl benzoxazinanones with sulfur ylides. The palla-
dium−π-benzyl zwitterionic intermediates are suggested for
this transformation, and this would be the first example of an
IDBC reaction.
as sulfur ylides 3,6c−e activated alkenes,6a,b,f enals,6g,h ketenes,6i
eterocyclic compounds containing a fluorine (F) or
Htrifluoromethyl (CF ) group on their core moieties are and isatins6j under palladium or other metal catalysis (Figure 1a).
3
Although the IDAC reactions of 1 have significant potential
with many reactive interceptive substrates, a diversity of
benzoxazinanone 1 are highly limited to “vinyl”-substituted
compounds because of the vital role of the vinyl group in
generating a palladium-stabilized π-allyl complex. Xiao expanded
the [4 + 1] interceptive decarboxylative cycloaddition reaction to
well studied in the field of pharmaceuticals and agrochemicals,
and some of them are successfully distributed in the drug
market.1 Among them, fluorinated indoles and oxindoles have
received a great deal of attention in the literature because of the
exceptional influence of the fluorine atom in terms of chemical
and physiological stability.2 On the other hand, the CF3-
substituted indolines are not developed, despite the attractive
biological properties of indolines, which are well distributed in
natural products and pharmaceuticals.3 We report herein the
highly diastereoselective synthesis of medicinally attractive CF3-
substituted indolines by interceptive decarboxylative benzylic
cycloaddition (IDBC) via a palladium−π-benzyl zwitterionic
intermediate.
The interceptive decarboxylative allylic cycloaddition (IDAC)
reaction of cyclic allylic carbamates under palladium catalysis4 is a
sophisticated extension of the Tsuji−Trost decarboxylative
allylation (DA) reaction.5 A zwitterionic π-allyl palladium
complex is a reactive intermediate that is then intercepted by a
sufficiently reactive substrate, providing a medium-sized ring
skeleton.4 Among the cyclic allylic carbamates in this trans-
formation, vinyl benzoxazinanones 1 are efficient multifarious
synthons for the IDAC reaction to afford biologically pertinent
heterocyclic molecules.6 The palladium−π-allyl zwitterionic
intermediate I generated from 1 was disclosed by Tunge et al.
and successfully trapped with benzylidene malononitriles,
producing tetrahydroquinolines via the [4 + 2] IDAC
reaction.6a,b This seminal work stimulated chemists into further
extensions of the IDAC reaction of 1 with other interceptors such
Figure 1. (a) IDAC reaction via zwitterionic π-allyl palladium complex
(previous work); (b) IDBC reaction via zwitterionic π-benzyl palladium
complex (this work).
Received: January 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00237
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Organic Letters
Letter
a
the propargylic analogues of 1 with sulfur ylides 3 under copper
catalysts, furnishing a series of alkynyl indolines.6e However, the
fundamental issue of the use of a nonvinyl substrate remains a
great challenge.6 In this context, we envisaged the generation of
palladium−π-benzyl zwitterionic intermediate II, an isoelec-
tronic variant of Tsuji−Trost allylic substitution, could be equally
possible, but in fact, there is no example of this type of benzylic
cycloaddition. The reaction involving π-benzyl−palladium
complex is rather limited to the acyclic reactions.7 The low
reactivity of benzylic substitution of this type is explained by the
aromaticity of the benzene ring, while allylic compounds are
inherently reactive. Recently, a Suzuki−Miyaura cross-coupling
reaction with secondary α-(trifluoromethyl)benzyl tosylates was
reported by Tredwell et al.7k Inspired by this result, we assumed
that the CF3 group at the benzylic position (C4) induces a strong
negative inductive effect on the neighboring carbon, allowing the
benzylic carbon to become electron deficient. Thus, a highly
electrophilic palladium−π-benzyl zwitterionic intermediate II
should be generated by oxidative addition of Pd(0) to amenable
IDBC reaction to produce biologically important CF3-indolines
(Figure 1b). These results are the first example of a palladium-
catalyzed IDBC reaction using nonvinyl benzoxazinanones 2.
The CF3 group plays an important role in this transformation.
First, we designed the previously unknown CF3-benzoxazina-
none 2a and investigated its suitability for IDBC reactions.4,5 We
rapidly discovered that CF3-benzoxazinanone 2a reacted with
sulfur ylide8 3a under palladium catalysis to provide the desired
[4 + 1] IDBC product 4aa in moderate yield. This reaction
performed with Pd(PPh3)4 (10 mol %) at 40 °C in THF gave
CF3-indoline 4aa in 59% yield with high diastereoselectivity (dr
>99:1). CF3 effect is very impressive as the methyl-substituted
benzoxazinanone 5a gave Me-indoline 6aa only in 17% yield (dr
>95:5) (Scheme 1). Encouraged by this result, we optimized the
Table 1. Effects of Pd, Ligand, and Temperature
b
c
run
Pd
PCy3
t (°C)
time (h)
yield (%)
dr
1
2
3
4
5
yes
yes
no
yes
no
40
40
40
60
rt
1
24
24
24
24
24
98
24
>99:1
>99:1
yes
no
NR
NR
91
no
yes
yes
yes
yes
>99:1
>95:5
d
e
6
40
42
a
Experiments were performed with 2a (0.15 mmol), 3a (0.225 mmol),
Pd2(dba)3·CHCl3 (0.0075 mmol or absence), PCy3 (0.015 mmol or
b
absence) in 1.0 mL of dry THF. Yields are 19F NMR yields with
c
internal standard PhCF3. Diastereomeric ratio was determined by 19
F
d
NMR analysis of the reaction mixture. Compound 5a was used
e
instead of 2a. Yield of 6aa.
With the optimized reaction conditions in hand, the generality
of the IDBC reaction was examined by reacting CF3-
benzoxazinanone 2a with sulfur ylides 3a−k. The results are
summarized in Scheme 2. Sulfur ylides 3b,d, which have electron-
a
Scheme 2. Scope of Sulfur Ylides 3
Scheme 1. Initial Attempts for IDBC Reaction of 2a or 5a with
3a Using Pd(PPh3)4 Catalyst
a
Experiments were performed with 2a (0.15 mmol), 3a−k (0.225
mmol), Pd2(dba)3·CHCl3 (0.0075 mmol), PCy3 (0.015 mmol) in 1.0
mL of dry THF with stirring at 40 °C for 1−8 h. Yields and 19F NMR
yields with internal standard PhCF3 are shown in parentheses. The dr
was determined by 19F NMR analysis of the reaction mixture.
b
Reaction performed with 1 mmol scale of 2a and 1.5 mmol of 3a (see
the Supporting Information for details).
reaction of 2a with 3a by screening different phosphine ligands
(dppe, dppf, PEt3, tBuXPhos, PCy3, and Binap) and solvents in
the presence of Pd(0) catalyst (Table 1 and Tables S1 and S2 in
Supporting Information). Gratifyingly, 4aa was obtained in
excellent yield (98%) and very high diastereocontrol (dr = 99:1)
using PCy3 (10 mol %) and Pd2(dba)3·CHCl3 (5 mol %) at 40
°C in THF for 1 h (run 1, Table 1). The amount of Pd catalyst
could be reduced to 1 mol % and 2.5 mol %, resulting in the
required product with moderate yields (Table S2). No reaction
occurred in the absence of the palladium catalyst (runs 3 and 4),
and the absence of PCy3 gave 4aa in low yield after longer
reaction time (run 2). Increasing the reaction temperature from
rt to 40 °C accelerated the reaction significantly from 24 to 1 h
(runs 1 and 5). The Me-substituted benzoxazinanone 5a
underwent IDBC and gave Me-indoline 5aa in 42% yield
under the same reaction conditions optimized for 2a, albeit only
after extended reaction time (run 6: 42%, 24 h vs run 1: 98%, 1
h).
donating groups (Me and OMe), afforded the desired CF3-
indolines in excellent yields (90% of 4ab and 78% of 4ad),
whereas the sulfur ylide 3c with an electron-withdrawing group
(NO2) resulted in moderate yield (54% of 4ac). Halogen-
substituted sulfur ylides 3e−g also underwent IDBC with
moderate to very good yields (55% of 4ae, 81% of 4af, and 84%
of 4ag). The heteroaryl systems 3h,i (2-furyl and 2-thiophenyl),
extended π-conjugate naphthalene-derived sulfur ylide 3j, and
aliphatic acyl-stabilized ylide 3k are also well tolerated to afford
the CF3-indolines 4ah (74%), 4ai (87%), 4aj (85%), and 4ak
(49%). All of these reactions proceeded with complete control
over diastereoselectivity (Scheme 2).
Next, a range of differently substituted CF3-benzoxazinanones
2b−f was examined to delineate the generality of the IDBC
reaction (Scheme 3). Various substituents on the benzene ring
with electronically dissimilar groups at different positions were
B
DOI: 10.1021/acs.orglett.8b00237
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Letter
a
substituted indole 10 with 40% yield in the presence of NaH in
THF (Scheme 4).
Scheme 3. Scope of Benzoxazinanones 2
A plausible catalytic cycle of the Pd-catalyzed IDBC reaction of
2a with 3a to 4aa is depicted in Figure 2, based on the reported
Figure 2. Proposed mechanism including stepwise formation of
zwitterionic intermediate II.
mechanism of the IDBC reaction of 2 with 3a via the Pd−π-
benzyl zwitterionic intermediate.6a−c The catalytic cycle is first
initiated by the oxidative addition of Pd(0) with 2a followed by
decarboxylation, which leads to the benzyl−Pd(II) zwitterionic
intermediate II. Intermediate II is very electrophilic due to the
CF3 group, and subsequently, a new C−C bond is formed by the
addition of sulfur ylide 3a to complex II, diastereoselectively
resulting in the zwitterionic complex III with the reductive
elimination of the Pd(0) catalyst. Finally, the zwitterionic
complex III undergoes an intramolecular SN2-type anti-
periplanar reaction from nitrogen to the stereogenic carbon
center attached to the sulfonium moiety in III, furnishing the
corresponding trans-CF3-substituted indoline 4aa with the
release of Me2S. The highly diastereoselective formation of the
trans-isomer of 4aa could be explained as follows. The
zwitterionic complex III can be formed via transition state TS,
due to the favorable steric repulsion between CF3 and SMe2
moieties (Figure 2).
In conclusion, we disclose a novel method to synthesize
diverse substituted trifluoromethyl indolines 4 from 2 under
palladium catalysis in a highly diastereoselective fashion. In this
transformation, a CF3 substitution at the benzylic position of
benzoxazinanones 2 is highly effective. Pentafluoroethyl (C2F5)
substitution was also suitable under the same reaction conditions,
whereas a nonfluorinated Me group gave a lower yield. The
electron-withdrawing nature of the perfluoroalkyl group played a
crucial role in this transformation. A series of CF3-benzox-
azinanones 2 was smoothly reacted with sulfur ylides 3 under
palladium catalysis in the presence of tricyclohexylphosphine
(PCy3) to provide pharmaceutically attractive CF3-subsituted
indolines 4 in high to excellent yields up to 91% with high
diastereoselectivities greater than 99:1 dr. This type of IDAC
reaction is belived to use vinyl-substituted substrates, thus, these
results are the first example of a palladium-catalyzed nonvinyl,
IDBC reaction, and CF3 is highly advantageous for the efficient
transformation. Moreover, the CF3-indolines 4 are prospective
synthetic building blocks for biologically attractive drug
candidates.1 Extension of this work is currently in progress,
including isolation of Pd−π-benzyl zwitterionic intermediates
and enantioselective variants of the reaction (for preliminary
studies of the asymmetric reaction, see Table S3).
a
Unless otherwise noted, the reaction was performed with 0.15 mmol
scale of 2 as mentioned in Scheme 2. The dr values and yields were
calculated as mentioned in Scheme 2. 10 mol % of Pd2(dba)3·CHCl3
and 20 mol % of PCy3 were used.
b
well tolerated in moderate to good yields with high
diastereoselectivities. The halogen-substituted CF3-benzoxazi-
nanones 2d and 2f (Cl and F) produced CF3-indolines 4 in
moderate to good yields (57% of 4da and 89% of 4fa). Substrates
2b,c with electron-donating groups on the benzene ring such as
Me and OMe were successfully transformed into corresponding
CF3-indolines in excellent yields (84% of 4ba and 86% of 4ca)
with high dr. An electron-withdrawing CF3 group on the benzene
ring of 2e reduced the yield of CF3-indoline 4ea by 28%. The
trans-configuration of 4 was confirmed by the X-ray crystallo-
graphic analysis of 4 cd (CCDC 1556229). The pentafluoroethyl
(C2F5)-benzoxazinanone 2g also underwent the IDBC reaction
smoothly to furnish C2F5-indoline 4ga with excellent yield of
89% and high selectivity (>99:1) in 4 h (Scheme 3).
To demonstrate the synthetic utility of CF3-indolines 4,
several reactions were carried out, as shown in Scheme 4. CF3-
Scheme 4. Derivatization of CF3-Indoline 4aa
indoline 4aa was successfully transformed to alcohols 7 and 8
having three consecutive stereogenic centers by reduction with
NaBH4 (70% yield) and by reaction with Me3SiCF3 (31% yield),
respectively (diastereoselectivities are >3:1). Indoline 4aa was
also converted to difluorinated derivative 9 with 50% yield by the
reaction with DAST. Furthermore, 4aa was converted into CF3-
C
DOI: 10.1021/acs.orglett.8b00237
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00237.
■
S
Syntheses and NMR spectra (PDF)
Accession Codes
CCDC 1556229 contains 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.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nozshiba@nitech.ac.jp.
ORCID
Veronique Gouverneur: 0000-0001-8638-5308
Norio Shibata: 0000-0002-3742-4064
Author Contributions
́
N.P. and P.D. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant No. JP
16H01142 in Middle Molecular Strategy, and ACT-C from the
JST Agency (JPMJCR12Z7).
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