Enantioselective Synthesis of 5-Alkylated Thiazolidinones via Palladium-Catalyzed Asymmetric Allylic C-H Alkylations of 1,4-Pentadienes with 5 H-Thiazol-4-ones

Article abstract of DOI:10.1021/acs.orglett.8b01697

A palladium-catalyzed, enantioselective allylic C-H alkylation of 1,4-pentadienes with 5H-thiazol-4-ones has been developed. Under the cooperative catalysis of a palladium complex of chiral phosphoramidite ligand and an achiral Br?nsted acid, a broad range of substituted 5H-thiazol-4-ones bearing sulfur-containing tertiary chiral centers were accessed from the allylic C-H alkylation in high levels of yields and enantioselectivities. Alkyl and aryl 1,4-pentadienes led to linear and branched allylation products, respectively.

Full text of DOI:10.1021/acs.orglett.8b01697

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective Synthesis of 5‑Alkylated Thiazolidinones via
Palladium-Catalyzed Asymmetric Allylic C−H Alkylations of 1,4-
Pentadienes with 5H‑Thiazol-4-ones
Tian-Ci Wang,†,‡ Zhi-Yong Han,†,‡ Pu-Sheng Wang,†,‡ Hua-Chen Lin,†,‡ Shi-Wei Luo,*,†,‡
and Liu-Zhu Gong*,†,‡
†Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and
Technology of China, Hefei 230026, China
‡Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
S
* Supporting Information
ABSTRACT: A palladium-catalyzed, enantioselective allylic
C−H alkylation of 1,4-pentadienes with 5H-thiazol-4-ones has
been developed. Under the cooperative catalysis of a
palladium complex of chiral phosphoramidite ligand and an
achiral Brønsted acid, a broad range of substituted 5H-thiazol-
4-ones bearing sulfur-containing tertiary chiral centers were
accessed from the allylic C−H alkylation in high levels of yields and enantioselectivities. Alkyl and aryl 1,4-pentadienes led to
linear and branched allylation products, respectively.
ptically active sulfides are frequently found in natural
Oproducts and pharmaceuticals, playing key roles in many
biological structures and functions.1 They are also versatile
intermediates that are widely applicable in asymmetric
synthesis.2 The consequent desire for their preparation has
driven the development of abundant asymmetric synthetic
approaches to access optically active thiol derivatives.3
Although these reactions are well suited for the preparation
of secondary thiol derivatives, by contrast, few methods are
applicable to the enantioselective synthesis of tertiary thiol and
thioether derivatives.4 Significantly, 5-alkylated thiazolidinones
have been found in follicle-stimulating hormone (FSH)
receptor agonists as a core structural element (Figure 1).4f
As such, it is a remarkably valuable task to create asymmetric,
catalytic transformations yielding chiral tertiary thiol deriva-
tives.
phosphine-catalyzed γ-addition reaction with allenoates.5d
Jiang developed an organocatalytic, asymmetric [4 + 2]
reaction of 5H-thiazol-4-ones.5h Hartwig has established an
Ir-catalyzed allylation of 5H-thiazol-4-ones with cinnamyl
esters to produce highly enantioenriched tertiary thioether
derivatives (Scheme 1a).5b In comparison with traditional
Scheme 1. (a) Ir-Catalyzed Allylation of 5H-Thiazol-4-ones
with Cinnamyl Esters. (b) Allylic C−H Alkylation of 1,4-
Pentadienes with Pyrazol-5-ones. (c) Allylic C−H
Functionalization of 1,4-Pentadienes with 5H-Thiazol-4-
ones
Figure 1. Follicle-stimulating hormone receptor agonists.
Asymmetric functionalization of easily accessible prochiral
organosulfur compounds provides an ideal approach to access
chiral, tertiary thiol derivatives.3−5 5H-Thiazol-4-ones, first
introduced by Palomo in an organocatalytic, asymmetric
Michael addition to nitroalkenes,5a have been recognized as
excellent sulfur-containing pronucleophiles to prepare chiral
tertiary thiol derivatives. Subsequently, Lu reported a chiral
Received: May 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01697
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
transition-metal-catalyzed allylic alkylation reactions, the allylic
C−H activation-based alkylation reaction obviously holds
unique advantages, including the ability to use simple alkenes,
avoiding the installation of allylic leaving groups. In recent
decades, growing interest has been devoted to the develop-
ment and applications of allylic C−H functionalization
reactions.6 Although the first palladium-catalyzed allylic C−H
alkylation reaction was reported several decades ago,6g the
creation of highly enantioselective variants still represents and
will continue to be a formidable challenge, given only a limited
number of successful examples in C−O and C−N bond-
forming reactions.7 Even rarer examples regarding asymmetric
construction of C−C bonds have been reported.8 Unlike
traditional asymmetric allylic alkylations (AAA) reactions,9 to
date, only a few successful examples describe highly
enantioselective allylic C−H functionalization reactions with
carbon nucleophiles, including 2-acetyl-1-tetralones,8a,c pyr-
azol-5-ones,8d and in situ generated enamines.8b
1,4-Pentadienes, upon undergoing the allylic C−H function-
alization, will introduce a conjugated diene subunit to the
products, which will definitely have more synthetic trans-
formations of carbon−carbon double bonds. However,
simultaneously addressing issues in the activation of the C−
H bonds makes control of stereo- and regioselectivity in allylic
C−H functionalization of 1,4-pentadienes more challenging.
Indeed, the allylic C−H alkylation of 1,4-pentadienes had not
appeared until Trost reported a racemic variant.10 Very
recently, we established a highly enantioselective allylic C−H
alkylation of 1,4-pentadienes with pyrazol-5-ones cooperatively
catalyzed by a palladium complex of chiral phosphoramidite
ligand and an achiral Brønsted acid, favoring the branched
products (Scheme 1b).8d Herein, we describe the first
asymmetric allylic C−H functionalization of 1,4-pentadienes
with 5H-thiazol-4-ones (Scheme 1c).
Inspired by the successful allylic C−H alkylation reactions
reported previously,8d,10 some phosphorus ligands were
initially examined for the reaction of 1,4-pentadiene (2a)
with 5H-thiazol-4-one (1a) using 2,6-DMBQ as the oxidant
and Pd(dba)2 as a precatalyst in the presence of 2-
fluorobenzoic acid (OFBA) (Table 1, entries 1−3). However,
the PPh3 was unable to accelerate the palladium-catalyzed
reaction (entry 1). The chiral palladium complexes of
phosphoramidites L1 and L2 showed promising stereo-
selectivity but gave modest yields (entries 2 and 3). Thus, a
variety of other chiral phosphoramide ligands were screened
(entries 4−6 and Table S1) and indicated that H8-BINOL-
derived ligand L5 delivered the highest enantiomeric excess
(entries 4 and 5 vs 6). The presence of 2,5-dimethylquinone as
an oxidant also offered high enantioselectivity, but provided
lower yield than 2,6-DMBQ (entry 7). A brief screening of the
palladium source identified Pd2(dba)3 as a slightly superior
precatalyst (entry 6 vs 8 and 9). Previous reports7b,8d indicate
that the Brønsted acid cocatalyst might have an impact on the
control of stereoselectivity. Thus, a number of Brønsted acids
were investigated (entries 10−13 and entries 12−19 in Table
S1). Interestingly, carboxylic acids exerted very little effect on
the enantioselectivity but significantly affected the conversion
(entries 10 and 11). The diphenylphosphinic acid was also a
good cocatalyst, whereas a strong Brønsted acid, such as p-
methylbenzenesulfonic acid, somehow completely inhibited
the catalytic activity of the palladium complex (entries 12 and
13). Notably, dramatically diminished enantioselectivity and
yield were observed in the absence of Brønsted acid, implying
Table 1. Optimization of Reaction Conditions
b
c
yield
ee
b
entry
[Pd]
L
acid
OFBA
OFBA
OFBA
OFBA
OFBA
OFBA
OFBA
OFBA
(%)
3/4
(%)
1
2
3
4
5
6
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd(dba)2
Pd(OAc)2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
PPh3
L1
L2
L3
L4
L5
L5
L5
L5
L5
L5
L5
L5
L5
L5
trace
67
44
47
62
91
71
63
16
50
90
89
trace
77
13:1
15:1
10:1
17:1
18:1
17:1
18:1
−6
57
−58
78
88
88
87
87
87
86
d
7
8
9
OFBA
10
11
12
13
14
PhCOOH
CH3COOH
Ph2POOH
TsOH·H2O
none
17:1
18:1
18:1
87
6:1
20:1
54
90
e
15
OFBA
95
a
Reaction conditions: Unless indicated otherwise, reactions of 1a
(0.10 mmol), 2a (0.20 mmol), Pd (0.005 mmol), ligand (0.01 mmol),
OFBA (0.005 mmol), and 2,6-DMBQ (0.12 mmol) were carried out
in toluene (2 mL) for 12 h. The yields and ratios of 3aa/4aa were
determined by H NMR analysis of the crude reaction mixture. The
ee value was determined by chiral HPLC analysis. 2,5-DMBQ (0.12
mmol) was used instead of 2,6-DMBQ. The reaction was carried out
at 25 °C. 2,5(6)-DMBQ = 2,5(6)-dimethylquinone, OFBA = 2-
b
c
1
d
e
fluorobenzoic acid, dba = dibenzylidene acetone.
that the acid cocatalyst is crucial to catalytic activity and
stereochemical control of the chiral palladium complex (entry
14 vs 6 and 10−11). Product 3a could be isolated in 95% yield
and 90% ee by conducting the reaction at room temperature
(entry 15).
Under the optimized reaction conditions, we first explored
the generality for different 5H-thiazol-4-ones 1 (Table 2). A
broad range of 5H-thiazol-4-ones bearing various substituted
benzyl groups underwent the allylic C−H alkylation reactions
smoothly, providing the corresponding linear products in good
to excellent yields (up to 95%) and with high enantioselectiv-
ities of up to 93% ee (entries 1−15). The substrate scope with
regard to 1,4-pentadienes was then examined for the allylic C−
H alkylation reaction with 5H-thiazol-4-one 1d as the
nucleophile (entries 16−29). 1,4-Pentadienes bearing a linear
alkyl substituent were able to participate in clean reactions,
allowing the desired products to be isolated in excellent yields
(81−95%) and with high levels of enantioselectivity (89−93%
ee, entries 16−20). 1-Cyclohexyl-1,4-pentadiene also under-
went the reaction very smoothly, giving the linear product in
99% yield and 83% ee (entry 21). Moreover, 1-aryl-1,4-
pentadienes also tuned out to be good substrates to give high
B
DOI: 10.1021/acs.orglett.8b01697
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 2. Scope of 5H-Thiazol-4-ones and 1,4-Dienes
b
c
d
entry
R1
C6H5
R2
n-C6H13
3
yield (%)
3/4
ee (%)
1
2
3
4
5
6
7
8
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
95
56
91
93
37
89
90
87
43
52
66
66
89
92
94
95
91
90
91
81
94
81
54
50
86
73
99
79
92
20:1
38:1
17:1
23:1
14:1
13:1
12:1
12:1
10:1
19:1
11:1
13:1
15:1
20:1
30:1
20:1
23:1
38:1
20:1
21:1
34:1
24:1
22:1
17:1
40:1
5:1
90
86
84
93
90
89
91
87
85
86
87
88
89
91
83
92
91
93
93
89
83
90
91
92
93
84/70
84
83
90
4-FC6H4
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
n-C6H13
Me
3-MeOC6H4
4-CNC6H4
2-C10H7
2-ClC6H4
3-ClC6H4
4-ClC6H4
3-MeC6H4
2-MeC6H4
4-MeC6H4
2-BrC6H4
3-BrC6H4
4-BrC6H4
H
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
3ja
3ka
3la
3ma
3na
3oa
3db
3dc
3dd
3de
3df
3dg
3dh
3di
3dj
3dk
3dl
3dm
3dn
3do
n-Pr
n-C9H17
Ph(CH2)2
Cl(CH2)3
cyclohexyl
Ph
4-MeC6H4
4-MeOC6H4
3-ClC6H4
H
e
26
27
28
29
CO2Et(CH2)2
CONHBn(CH2)2
TsO(CH2)2
11:1
10:1
16:1
a
Reaction conditions: Unless indicated otherwise, reactions of 1 (0.10 mmol), 2 (0.20 mmol), Pd2(dba)3 (0.0025 mmol), L5 (0.01 mmol), OFBA
b
c
(0.005 mmol), and 2,6-DMBQ (0.12 mmol) were carried out in toluene (2 mL) for 72 h. Isolated yields with E/Z > 20:1. The ratios of 3/4 and
d
1
E/Z were determined by H NMR spectroscopic analysis of the crude reaction mixture. The ee value was determined by chiral HPLC analysis.
e
Ratios of E/Z.
levels of enantioselectivity (entries 22−25) Basically, the
presence of electron-rich aryl substituents led to considerably
lower yields than the 1,4-dienes bearing a slightly electronically
poor or electronically neutral aryl substituent, while similar,
excellent enantioselectivities were observed (entries 23 and 24
vs 22 and 25). In the case of a simple 1,4-diene, the allylic
alkylation product was obtained in moderate yield, good
enantioselectivity, and a 5:1 regioisomeric ratio (entry 26).
Ester, amide, and sulfonate functional groups could be
tolerated to give excellent yields and high levels of
enantioselectivity (entries 27−29). However, the allylbezene
is too unreactive to undergo the desired allylic C−H alkylation
reaction. The configuration of 3ha was assigned to be (R) by
X-ray analysis (see the Supporting Information).
the best ligand (see Table S2 for details and entry 1, Table 3).
The reoptimized conditions were applied to different
substrates, and the presence of an aryl group bearing either
electron-donating or -withdrawing substituents at the meta- or
para-position was found to proceed in good yields and
stereocontrol (entries 2−4). 1,4-Pentadienes bearing different
substituents were able to form the products in moderate yields
and enantioselectivities (entries 5−7).
To demonstrate the practicability of the current reaction, a
gram-scale reaction of 1d with 1,4-diene was performed
(Scheme 2a). A quantitative yield and a similar level of
enantioselectivity were observed. The chiral 5H-thiazol-4-one
3la obtained from this reaction can be transformed into α-thiol
ester derivatives P1 and P2, respectively (Scheme 2b). As
shown in Scheme 2c, the treatment of 6da with 91% ee with
NaBH4/MeOH and NsCl/NaH, readily generated 5-alkylated
thiazolidinones P3 in 69% yield and >99% ee after two steps
and recrystallization. The configuration of P3 was assigned to
Interestingly, when 2,5-diphenylthiazol-4(5H)-one 5a was
examined for the allylic C−H alkylation with a 1,4-pentadiene,
the branched product was preferentially generated, but with
poor yields and enantioselectivities. Thus, we reoptimized the
reaction conditions and identified the phosphoramidite L1 as
C
DOI: 10.1021/acs.orglett.8b01697
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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Table 3. Scope of 2,5-Diarylthiazol-4(5H)-ones and 1,4-
Dienes
Scheme 3. Experiments for Mechanistic Study
a
b
d
yield
(%)
ee
c
c
entry
R1
R2
6
6/7
dr
(%)
1
2
3
4
5
6
7
Ph
Me
Me
Me
Me
H
nBu
nPr
6aa
6ba
6ca
6da
6ab
6bc
6bd
90
70
87
80
92
57
75
11:1
10:1
13:1
10:1
5:1
6:1
4:1
5:1
13:1
3:1
87
91
88
88
89
91
91
3-MeC6H4
4-BrC6H4
4-FC6H4
Ph
3-MeC6H4
3-MeC6H4
e
10:1
10:1
9:1
a
Reaction conditions: Unless indicated otherwise, reactions of 5 (0.10
mmol), 2 (0.20 mmol), Pd2(dba)3 (0.0025 mmol), L1 (0.0075
mmol), OFBA (0.005 mmol), and 2,6-DMBQ (0.12 mmol) were
b
carried out in toluene (2 mL) for 16 h. Isolated yields of 6 with E/Z
c
> 20:1. The ratios of E/Z, 6/7, and diastereomeric ratio (dr) were
d
determined by 1H NMR analysis of the crude reaction mixture. The
e
ee value was determined by chiral HPLC analysis. Ratios of E/Z.
elementary reaction of allylic substitution. A kinetic isotope
effect of 0.99 (kH/kD) was observed for the reaction of D2-2h
(Scheme 3c), which revealed that the allylic C−H cleavage was
not the rate-determining step. Instead, the allylic substitution
might determine the reaction rate (see the SI).
Scheme 2. Applications in Synthesis
In conclusion, we have developed a palladium-catalyzed
asymmetric allylic C−H alkylation reaction with 5H-thiazol-4-
ones. The reaction features mild conditions, high yields, and
high enantioselectivities with broad substrate scope. The
regioselectivity of the product could be controlled by the
structure of the 1,4-pentadienes. The method provides a
simple and efficient route to access highly enantioenriched 2-
mercapto acid surrogates and 5-alkylated thiazolidinone
derivatives bearing a quaternary stereogenic center.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01697.
Complete experimental procedures and characterization
data for the prepared compounds (PDF)
Accession Codes
CCDC 1836280−1836281 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.
be (S,S,R) by X-ray analysis, and thus, that of 6ba was assigned
to be (S,R).
To understand the reaction, a typical Pd-catalyzed allylic
allylation of 1d in the presence of ligand L5 was conducted and
resulted in 23:1 l/b selectivity (Scheme 3a), which is relatively
close to the results observed in the small-scale reaction (entry
4, Table 2). Similarly, the Tsuji−Trost-type allylic substitution
of an aryl-substituted nucleophile 5b with 2b′ also favored the
branched selection (Scheme 3b). These results imply that both
the allylic C−H alkylation and Tsuji−Trost reactions share a
similar intermediate. In addition, ESI-MS identified the allyl Pd
intermediate, again suggesting that the protocol undergoes the
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: luosw@ustc.edu.cn.
*E-mail: gonglz@ustc.edu.cn.
ORCID
Zhi-Yong Han: 0000-0002-9225-5064
D
DOI: 10.1021/acs.orglett.8b01697
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
M.; Thaisrivongs, D. A. Angew. Chem., Int. Ed. 2012, 51, 4950.
(m) Howell, J. M.; Liu, W.; Young, A. J.; White, M. C. J. Am. Chem.
Soc. 2014, 136, 5750. (n) Kondo, H.; Yu, F.; Yamaguchi, J.; Liu, G. S.;
Itami, K. Org. Lett. 2014, 16, 4212. (o) Wang, P. S.; Lin, H. C.; Zhou,
X. L.; Gong, L. Z. Org. Lett. 2014, 16, 3332. (p) Tao, Z. L.; Li, X. H.;
Han, Z. Y.; Gong, L. Z. J. Am. Chem. Soc. 2015, 137, 4054.
Pu-Sheng Wang: 0000-0002-7229-3426
Liu-Zhu Gong: 0000-0001-6099-827X
Notes
The authors declare no competing financial interest.
̈
(q) Sharpless, K. B.; Teranishi, A. Y.; Backvall, J. E. J. Am. Chem. Soc.
ACKNOWLEDGMENTS
■
̈
1977, 99, 3120. (r) Backvall, J. E. Acc. Chem. Res. 1983, 16, 335.
We are grateful for financial support from the MOST (973
project 2015CB856600), the NSFC (21472180) and the
Chinese Academy of Sciences (Grant No. XDB20020000).
̈
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DOI: 10.1021/acs.orglett.8b01697
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