Pd-catalyzed divergent C(sp2)-H Activation/Cycloimidoylation of 2-Isocyano-2,3-diarylpropanoates

Article abstract of DOI:10.1021/acs.orglett.8b00346

A Pd-catalyzed site-selective C(sp²)-H activation/cycloimidoylation of 2-isocyano-2,3-diarylpropanoates to construct diverse cyclic imine products has been developed. Six-membered 3,4-dihydroisoquinolines containing a C3 quaternary carbon cente

Full text of DOI:10.1021/acs.orglett.8b00346

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd-Catalyzed Divergent C(sp2)−H Activation/Cycloimidoylation of
2‑Isocyano-2,3-diarylpropanoates
Shi Tang,†,§ Sheng-Wen Yang,‡,§ Hongwei Sun,† Yali Zhou,† Juan Li,*,‡ and Qiang Zhu*,†
†State Key Laboratory of Respiratory Disease, Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences,
University of Chinese Academy of Sciences, 190 Kaiyuan Avenue, Guangzhou 510530, China
‡Department of Chemistry, Jinan University, Huangpu Road West 601, Guangzhou 510632, China
S
* Supporting Information
ABSTRACT: A Pd-catalyzed site-selective C(sp2)−H activation/cycloimidoylation of 2-isocyano-2,3-diarylpropanoates to
construct diverse cyclic imine products has been developed. Six-membered 3,4-dihydroisoquinolines containing a C3 quaternary
carbon center were generated dominantly by using bulky Ad2Pn-Bu as a ligand, while five-membered 1,1-disubstituted 1H-
isoindoles were formed preferentially in the presence of bidentate phosphine ligand DPPB. The selectivity for 1H-isoindole
formation was enhanced by using steric hindered aryl iodides. DFT calculations suggested that the experimentally observed
ligand-controlled selectivity was a result of trans effect.
the oxidative annulation occurred between the oxygen of the
directing group and the aromatic C−H on the main framework
to produce benzopyrans selectively (Scheme 1a). In 2015, an
uring the past decade, transition-metal-catalyzed direct
Dfunctionalization of inert C−H bonds has proven to be an
atom- and step-economic strategy in constructing new chemical
bonds.1 Chiral ligand-controlled enantioselective C−H activa-
tion,2 directing group-assisted regioselective functionalization of
long distance meta- or para-C(sp2)−H bonds over ortho ones,3
and reactions via selective cleavage of less reactive C(sp3)−H
bonds are major challenging topics in the area of C−H
activation.4 Continuous achievements in these topics empower
a C−H functionalization reaction as a practical and routine tool
in total synthesis of natural products, as well as in late-stage
modification of drug molecules.5 However, divergent C−H
functionalization reactions at different sites in the same substrate,
controlled merely by catalyst, ligand, or other reaction
conditions, are relatively undervalued.6 In 2013, during the
study of copper(II)-mediated aerobic C−H oxidation of N-(8-
quinolinyl)benzamide, Stahl and coauthors discovered that
directed methoxylation or chlorination of the benzamide group
and nondirected chlorination of the quinoline motif were
realized selectively under basic and acidic conditions, respec-
tively.7 Site-selective C−H functionalization of indole and
thiophene could be finely tuned by concise selection of catalyst
and solvent.8 In addition to site-selective C−H modification of a
parent scaffold, diverse frameworks could be constructed by
cyclization reactions via activation of different C−H bonds. In
2013, Lam reported a novel C−H oxidative annulation of 2-aryl
cyclic 1,3-dicarbonyl compounds with alkynes.9 When palla-
dium-N-heterocyclic carbene complex was used as the
precatalyst, alkyne insertion between the 2-aryl C−H bond and
the acidic α-C(sp3)−H took place to provide spiroindenes
exclusively. In contrast, when a ruthenium catalyst was applied,
Scheme 1. Divergent C−H Functionalization/Cyclization
elegant work by Yang demonstrated that the intermediate of
amino-palladation of unsaturated anilide could activate the
benzylic C−H or the amide α-C−H selectively, affording the
corresponding three- and five-membered ring fused indolines
under two optimized conditions (Scheme 1b).10
In recent years, applying functionalized isocyanide in C−H
cycloimidoylation for the synthesis of heterocycles has gained
ever-increasing interest.11 In 2012, Takemoto reported a seminal
work involving imidoylation and benzylic C(sp3)−H activation
to construct an indole skeleton (Scheme 2a).12 Later on, our
group used 2-isocyano-1,1′-biphenyls and tryptophan-derived
Received: January 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00346
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Organic Letters
Letter
a
Scheme 2. C−H Activation/Cycloimidoylation
Table 1. Optimization of the Reaction Conditions
t
total yield
b
entry
[Pd]/L
solvent
(°C)
(%)
3a/4a
1
2
3
4
5
Pd(OAc)2/L1
Pd(OAc)2/L2
Pd(OAc)2/L3
Pd(OAc)2/L4
Pd(OAc)2/L3
Pd(OAc)2/L3
Pd(OAc)2/L3
Pd(OAc)2/L4
Pd(OAc)2/L4
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
dioxane
PhMe
PhCF3
80
80
80
80
90
90
90
80
80
80
80
76
2.0/1
trace
74
3.6/1
1/5.4
>50/1
>50/1
>50/1
1/3.2
1/6.4
1/5.4
1/5.8
77
60
c
6
70
cd
,
e
e
7
8
9
83(80)
25
isocyanides for the preparation of phenanthridine and β-
carboline derivatives through palladium-catalyzed C(sp2)−H
activation and imidoylative cyclization (Scheme 2b,c).13
Recently, the first example of enantioselective C(sp2)−H
imidoylation was realized through desymmetrizing C(sp2)−H
activation of dibenzyl isocyanoacetates for the synthesis of 3,4-
dihydroisoquinolines containing C3 quaternary stereogenic
centers (Scheme 2d).14 However, discriminating two types of
C−H bonds in the same substrate to form different frameworks
after cycloimidoylation is unprecedented. In this Letter, 2-
isocyano-2,3-diarylpropanoates, which contain two different
ortho-C(sp2)−H bonds located 4 and 5 chemical bonds away
from the isocyano group, are studied as a special class of
functionalized isocyanides. Site-selective C(sp2)−H activation/
cycloimidoylation is realized by concise selection of ligands and
reaction conditions, leading to six-membered 3,4-dihydroisoqui-
nolines and five-membered 1,1-disubstituted 1H-isoindoles from
the common starting material (Scheme 2).
f
c
c
81
10
Pd(MeCN)2Cl2/L4 PhCF3
89
cd
,
11
Pd(MeCN)2Cl2/
L4
PhCF3
95(93)
a
Reaction conditions: 1a (0.1 mmol), 2a (0.12 mmol), [Pd] (0.01
mmol, 10 mol %), ligand (0.02 mmol, 20 mol %), CsOPiv (0.12
mmol), in solvent (1 mL), in Ar. A solution of 1a in solvent (1 mL)
was added via a syringe pump within 1 h. NMR yield with 1-iodo-4-
methoxybenzene as an internal standard. Cs2CO3 (0.12 mmol),
PivOH (0.06 mmol). 1a was added within 1.5 h. Isolated yield in
parentheses. Cs2CO3 (0.12 mmol). L1 = PPh3, L2 = P(t-Bu)3, L3 =
Ad2Pn-Bu, L4 = DPPB (1,4-bis(diphenylphosphanyl)butane).
b
c
d
e
f
the ratio of 3a/4a reached 1:5.8 in a total yield of 95% when the
reaction was catalyzed by Pd(MeCN)2Cl2 in PhCF3 as the
solvent (entry 11).
Having established the optimized reaction conditions for
selective synthesis of 3a, the scope and limitation for both aryl
iodide and functionalized isocyanide was investigated (Scheme
3). Aryl iodides bearing electron-donating or -withdrawing
substituents at the para- or meta-position afforded the desired
products in good to excellent yields (3a−3i, 71−85%), and the
possible five-membered side products 4 were not detected in all
of the cases. Disubstitued iodobenzene, such as 1,2-dichloro-4-
iodobenzene and 1-iodo-3,5-dimethylbenzene, also worked well
in this reaction, leading to the corresponding products in good
yields (3k, 81%; 3l, 71%). However, when 1-iodo-2-methyl-
benzene was tested in the reaction, the selectivity dropped
dramatically to provide 3j and 4b in 24% and 30% yields,
respectively. The results indicate that the steric hindrance on aryl
iodide favored the formation of 1H-isoindole derivative 4.
Heteroaromatic iodides were also tested, and the corresponding
pyridine- and thiophene-substituted 3,4-dihydroisoquinolines
3m and 3n were formed exclusively, although diminished yield
was observed for 3m (60%). Next, isocyanides 1 substituted with
a range of functional groups on the para-position of the benzyl
ring were studied. Both electron-withdrawing ester, halide, and
electron-donating methyl were well-tolerated, leading to the
desired products in moderate yields and excellent selectivity
(3o−3r, 69−74%). When chloride located at the meta-position,
activation of either neighboring C(sp2)−H bond took place,
delivering a mixture of 3s and its isomer 3s′ in 87% yield. It was
worthy of noting that, when the chloride substituent was on the
ortho position, 1H-isoindole 4c was also generated in 10% yield,
together with 44% yield of the desired product 3t. The result
indicated that the steric hindrance on the benzyl group also had
significant influence on the selectivity. 2-Isocyano-2,3-diary-
The site-selective C(sp2)−H cycloimidoylation of ethyl 2-
isocyano-2,3-diphenylpropanoate 1a was studied under palla-
dium catalysis with phenyl iodide 2a. The selection of phosphine
ligand was first investigated in the presence of Pd(OAc)2 as a
precatalyst and CsOPiv in toluene, as shown in Table 1. When
the common ligand PPh3 was used, two cyclization products 3a
and 4a, derived from respective activation of benzylic and
phenylic C−H bond, were obtained in 76% total yield favoring
the six-membered 3,4-dihydroisoquinoline 3a (3a/4a = 2.0:1,
entry 1, Table 1). However, neither 3a nor 4a could be efficiently
produced when changing the ligand to P(t-Bu)3 (entry 2). It was
intriguing that when a more bulky ligand Ad2Pn-Bu was applied
the efficiency for the cyclization was regained (74%) with
improved selectivity (3a/4a = 3.6:1, entry 3). To our delight, the
selectivity was reversed in the presence of bidentate ligand
DPPB, and five-membered 1,1-disubstituted 1H-isoindole 4a
was isolated as the major product (3a/4a = 1:5.4, entry 4).
Extensive screening of other mono- and bidentate ligands could
not improve the selectivity under otherwise identical conditions
(see the Supporting Information (SI) for details). Next, the
reaction conditions for the Pd(OAc)2/Ad2Pn-Bu catalytic system
were finely tuned. The formation of 4a was suppressed, and 3a
was produced in 60% yield at elevated temperature (90 °C).
Further optimization revealed that the combination of Cs2CO3
and PivOH was superior to CsOPiv (entry 6). Satisfyingly, 3a
could be generated as the sole product in 83% yield by changing
the solvent to dioxane and reducing the addition rate of 1a to the
reaction mixture (entry 7). However, to maximize the selectivity
for formation of 4a, palladium source, solvent, and other
conditions were optimized with the ligand DPPB fixed. Finally,
B
DOI: 10.1021/acs.orglett.8b00346
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Organic Letters
Letter
a
a
Scheme 3. Substrate Scope for Selective Synthesis of 3
Scheme 4. Substrate Scope for Selective Synthesis of 4
a
Reaction conditions: 1a (0.1 mmol), 2 (0.12 mmol), Pd(MeCN)2Cl2
(0.01 mmol, 10 mol %), DPPB (0.01 mmol, 10 mol %), Cs2CO3 (0.12
mmol), PivOH (0.06 mmol) in PhCF3, at 80 °C under Ar. A solution
of 1a in PhCF3 (1 mL) was added via a syringe pump within 1.5 h.
a
Reaction conditions: 1 (0.1 mmol), 2 (0.12 mmol), Pd(OAc)2 (0.01
mmol, 10 mol %), Ad2Pn-Bu (0.02 mmol, 20 mol %), Cs2CO3 (0.12
mmol), PivOH (0.06 mmol) in 1,4-dioxane (1 mL) at 90 °C, in Ar. A
solution of 1 in 1,4-dioxane (1 mL) was added via a syringe pump
within 1.5 h.
selective formation of five-membered 1H-isoindole. Following
this observation, the scope of functionalized isocyanide was
explored using 1-iodo-2-isopropylbenzene as the coupling
partner (Scheme 4). A variety of functional groups including
Me, F, Cl Br, and CO2Me on different positions of the benzyl
moiety were compatible with reaction conditions, affording the
corresponding products 4g−4q in 73−88% yields with excellent
selectivity. 2-Isocyano-2,3-diarylpropanoates bearing substitu-
ents on the 2-aryl ring, such as MeO, F, and Cl, underwent the
site-specific C(sp2)−H cycloimidoylation reaction smoothly to
give 1,1-disubstituted 1H-isoindoles 4r−4u exclusively in
excellent yields (87−98%).
The ligand effect on the selectivity of the reaction was
thoroughly investigated computationally.15 The free energy
barrier of the benzylic C−H cleavage (9.8 kcal/mol) is lower
than that of the phenylic C−H cleavage (12.1 kcal/mol) with the
sterically bulky Ad2Pn-Bu ligand (Scheme 5). However, the
energy barrier of benzylic C−H cleavage was determined to be
27.2 kcal/mol, which is 2.8 kcal/mol higher than that of phenylic
C−H cleavage for the bidentate phosphine ligand DPPB. The
steric influence of the bulky ligand Ad2Pn-Bu makes one of the
phosphine ligands dissociate automatically and thus enforces a
coordinatively unsaturated palladium center in the C−H
lpropanoate containing meta-Cl on the phenyl group was also
studied, and 3u was isolated in 84% yield as the sole product.
Next, the generality for selective formation of 1H-isoindole 4
was examined (Scheme 4). Compared with iodobenzene (entry
11, Table 1), 1-chloro-4-iodobenzene generated both products
with diminished yield and selectivity (74% yield in total, 4d/3c =
3.4:1) under the same reaction conditions. However, when the
reaction between 1a and 1-iodo-2-methylbenzene was per-
formed again under the Pd(MeCN)2Cl2/DPPB catalytic system
in PhCF3, the ratio of 4b/3j jumped to 13.2:1 from 1.3:1,
suggesting the significant influence of bidentate ligand on the
outcome of selectivity. Surprisingly, when increasing the size of
the ortho substituent from methyl to ethyl on iodobenzene, only
the 1H-isoindole product 4e (82% yield) was obtained, which
was also the case using even sterically demanding 1-iodo-2-
isopropylbenzene as an electrophile (4f, 83% yield); also the
preparation of 4f can be scaled up to 1 mmol with even higher
yield (91%). Those results indicated that steric hindrance of both
phosphine ligand and aryl iodide had synergistic influence on the
C
DOI: 10.1021/acs.orglett.8b00346
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Calculated C−H Cleavage Steps with Ad2Pn-Bu
ACKNOWLEDGMENTS
■
a
and DPPB Ligands
We are grateful to the National Natural Science Foundation of
China (NNSFC) (21472190, 21532009, 21573095, and
21672215) for financial support.
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a
All values are free energies in kcal/mol.
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of six-membered 3,4-dihydroisoquinolines was achieved by using
bulky Ad2Pn-Bu as a ligand, while the formation of five-
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phosphine ligand DPPB, and the selectivity was enhanced by the
steric hindrance of iodobenzene. DFT calculations agreed with
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first example of ligand-controlled divergent C(sp2)−H activa-
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00346.
General experimental procedure and characterization data
of the compounds (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
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D. A.; Smith, M. R., III Chem. Commun. 2010, 46, 7724.
(b) Santhoshkumar, R.; Mannathan, S.; Cheng, C.-H. J. Am. Chem.
Soc. 2015, 137, 16116. (c) Zhang, J.; Sha, S.-C.; Bellomo, A.; Tomson, N.
C.; Walsh, P. J. J. Am. Chem. Soc. 2016, 138, 4260. (d) Ding, D.; Mou, T.;
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Soc. 2013, 135, 9797.
*E-mail: zhu_qiang@gibh.ac.cn.
*E-mail: tchjli@jnu.edu.cn.
ORCID
Juan Li: 0000-0003-0693-3162
Qiang Zhu: 0000-0002-1243-2391
Author Contributions
§S.T. and S.-W.Y. contributed equally to this work.
Notes
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Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 8172. (c) Ueda, K.;
The authors declare no competing financial interest.
D
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(15) DFT calculations are detailed in the Supporting Information.
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