Selective CO2 adsorption and theoretical simulation of a stable Co(ii)-based metal-organic framework with tunable crystal size

Article abstract of DOI:10.1039/c8ce01942h

A three-fold interpenetrated metal-organic framework [Co₂(OBA)₄(PTD)·3DMF·CH₃CH₂OH·5H₂O]_n (1) has been synthesized by utilizing 4,4′-oxybis(benzoic acid) (H₂OBA) as the linker, 6-(pyridin-4-yl)-1,3,5-triazine-2,4-diamine (PTD) as the ligand, and CoCl₂·6H₂O via solvothermal method. Compound 1 exhibits not only a high uptake capacity for CO₂ molecules with an estimated high sorption heat (50.6 kJ mol^?1 at zero loading), but also a significant selective adsorption of CO₂ over CH₄, which may be ascribed to the presence of proper-sized pores with high polarity, amine groups and triazine rings of PTD linker decorating the pores. Meanwhile, the Grand Canonical Monte Carlo (GCMC) simulations of CO₂ adsorption of compound 1 demonstrate that CO₂ molecules are preferentially adsorbed around the PTD ligands. Furthermore, complex 1 displays a relatively high adsorption capacity of H₂ (101.7 cm³ g^?1 at 1 bar) under 77 K.

Full text of DOI:10.1039/c8ce01942h

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Chiral selenide-catalyzed enantioselective
synthesis of trifluoromethylthiolated 2,5-
disubstituted oxazolines†
Cite this: DOI: 10.1039/c8ob02575d
Received 17th October 2018,
Accepted 5th November 2018
Tian Qin, Quanbin Jiang,
Jieying Ji, Jie Luo and Xiaodan Zhao
*
DOI: 10.1039/c8ob02575d
rsc.li/obc
Chiral selenide-catalyzed enantioselective trifluoromethyl- Incorporation of the CF3S group into molecules can dramati-
thiolation of 1,1-disubstituted alkenes is disclosed. By this method, cally change the physical, chemical, and biological properties
a variety of chiral trifluoromethylthiolated 2,5-disubstituted oxazo- of these parent molecules.9 In this scenario, numerous
lines were obtained in good yields with high enantioselectivities. methods have been developed for the synthesis of various
This work not only provides a new pathway for the synthesis of CF3S-containing compounds by direct or indirect introduction
chiral oxazolines, but also expands the library of chiral trifluoro- of the CF3S group.10 However, in the field of direct trifluoro-
methylthiolated molecules.
methylthiolation, synthesis of chiral CF3S-containing com-
pounds is relatively rare compared with the preparation of
The oxazoline structural moiety is present in natural products,
bioactive molecules, and several chiral ligands.1,2 A number of
methods have been documented for the synthesis of oxazo-
lines.3 Among the developed methods, electrophilic cyclization
of alkenes is an efficient pathway to directly construct substi-
tuted oxazoline derivatives.4–7 In particular, organocatalytic
enantioselective electrophilic functionalization of alkenes has
been applied to construct chiral substituted oxazolines
(Scheme 1a).5–7 For example, Borhan demonstrated
(DHQD)2PHAL-catalyzed electrophilic chlorocyclization of
N-allylamides to synthesize chiral chloro-substituted oxazo-
lines in 2011.5 Toste reported an enantioselective electrophilic
fluorination of N-allylamides to construct chiral fluorinated
oxazolines using an anionic chiral phase-transfer catalyst in
the same year.6 Besides, Hamashima has developed
DTBM-BINAP-catalyzed bromocyclization of N-allylamides to
prepare chiral bromo-containing oxazolines.7 Despite these
great advances, developing new methods to construct chiral
oxazolines bearing a privileged functional group is still in
great demand.
achiral
CF3S-containing
molecules.11–14
Historically,
β-ketoesters and their analogues were utilized to construct
chiral CF3S compounds by organic small molecule- and tran-
sition metal-catalyzed enantioselective trifluoromethyl-
thiolation in the beginning.11 Recently, Wang developed
metal-catalyzed trifluoromethylthiolation of diazo compounds
via [2,3]-sigmatropic rearrangement of sulfonium ylides to
form chiral CF3S molecules bearing a quaternary stereogenic
center.12 More recently, Wang realized Cu-catalyzed tandem
1,4-addition/trifluoromethylthiolation of acyclic enones to
prepare α-CF3S-β-substituted ketones.13 Furthermore, Cahard
described the synthesis of chiral CF3S molecules by substrate-
controlled asymmetric induction.14
To expand the area for the synthesis of chiral CF3S mole-
cules, we have developed a new class of bifunctional chalco-
genide catalysts for enantioselective trifluoromethylthiolation
The trifluoromethylthio (CF3S) group is a privileged fluo-
rine-containing functional group due to its strong electron-
withdrawing effect and high lipophilicity (π
=
1.44).8
Institute of Organic Chemistry & MOE Key Laboratory of Bioinorganic and Synthetic
Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275,
P. R. China. E-mail: zhaoxd3@mail.sysu.edu.cn
†Electronic supplementary information (ESI) available: Experimental details,
characterization data and NMR spectra of products, and HPLC traces. CCDC
1873635. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8ob02575d
Scheme 1 Catalytic construction of chiral substituted oxazolines with
N-allylamides.
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Table 1 Optimization of reaction conditionsa
of alkenes.15 As a result, 1,2-disubstitued and trisubstituted
alkenes could be efficiently converted to the corresponding
chiral CF3S products in an intramolecular or intermolecular
way. In these transformations, the racemization of the thiira-
nium ion intermediate through olefin-to-olefin degradation
and the formation of an unstable carbocation could be over-
come because of the bifunctional binding or the relative stabi-
lity of the thiiranium ion intermediate. As a continuation of
our interest in the synthesis of chiral CF3S molecules, we ques-
tioned whether 1,1-disubstitued alkenes could undergo a
similar enantioselective trifluoromethylthiolation to give chiral
CF3S compounds in high enantioselectivity. Specifically, if sub-
stituted N-allylamides are used as substrates, valuable chiral
CF3S-functionalized oxazolines would be generated. This will
provide a new pathway for the synthesis of chiral functiona-
lized oxazolines. Herein, we demonstrate that N-allylamides
can be efficiently trifluoromethylthiolated to afford chiral CF3S
oxazolines by chiral selenide catalysis (Scheme 1b).
Yieldb
(%)
eec
(%)
We commenced enantioselective trifluoromethylthiolation
with N-(2-phenylallyl)benzamide (1a) as a model substrate
(Table 1). Different chiral selenide catalysts were first tested in
the presence of electrophilic CF3S reagent 2 and BF3·Et2O. To
our delight, amino acid-derived selenide catalysts were
effective for electrophilic cyclization of 1a to generate the
desired product 3a with up to 50% enantioselectivity (entries 1
and 2). On the basis of our previous findings,15 amino
indanol-derived selenide catalysts were superior to those
amino acid-derived for enantioselective trifluoromethyl-
thiolation of common alkenes. Consequently, several indane
Entry
Catalyst
Solvent
Acid
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C8
C8
C8
C8
C8
C8
C8
C8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
CH2Cl2/toluene (1 : 1)
CH2Cl2/toluene (1 : 1)
CH2Cl2/toluene (1 : 1)
CH2Cl2/toluene (1 : 1)
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
TMSOTf
Tf2NH
83
85
42
58
86
35
88
89
91
37
52
34
32
−50
54
53
80
47
86
87
85
84
75
81
78
—
9
10
11
12
scaffold-based selenide catalysts were screened (entries 3–10). 13
It was found that C8 with two methoxy groups on the phenyl
group delivered the highest enantioselectivity for this trans-
formation (entry 8). Next, different acids such as TMSOTf, 17e
TfOH
74
14
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
<10
73 (70)
18
71
93
15
93
91
92
88
16d
18f
Tf2NH, and TfOH were examined (entries 11–13). None of
a Conditions: 1a (0.04 mmol), 2 (1.5 equiv.), catalyst (20 mol%), acid
(1.5 equiv.), solvent (2.0 mL), −78 °C, 18 h. b Determined by 19F NMR
with trifluoromethylbenzene as the internal standard. The isolated
yield is in the parenthesis in 0.1 mmol scale. c Determined by HPLC
analysis. d N-CF3S-saccharin instead of (PhSO2)2N-SCF3. e 36 h.
f BF3·Et2O (3.0 equiv.).
them gave better results than BF3·Et2O. Other solvents such as
toluene led to the reaction being sluggish (entry 14). However,
a mixed solvent consisting of CH2Cl2 and toluene gave the
product in 93% ee (entry 15). In contrast, other electrophilic
CF3S reagents such as N-CF3S-saccharin, prolonging reaction
time, or increasing the amount of acid did not result in better
results (entries 16–18).
With the optimal conditions in hand, we evaluated the sub-
strate scope (Table 2). It was found that various N-allylamides for the reaction, the desired product 3j could still be obtained
could be efficiently transformed to the corresponding CF3S in high enantioselectivity. It is worth noting that when the
oxazolines under the standard conditions. When electron- naphthyl group was replaced by an alkyl group such as the
withdrawing and -neutral groups such as F–, Cl–, Br–, and Me– pentyl or heteroaryl group such as 3-thienyl, the desired pro-
were placed at the para position of the phenyl ring attached to ducts were obtained only in low and moderate enantioselectivi-
the double bond, the reactions proceeded well to form the pro- ties. Then, we turned our attention to the effect of different
ducts in high enantioselectivities (3b–3e, 83–90% ee). The aroyl groups on the nitrogen. It was found that electron-with-
functional groups such as the electron-withdrawing group (e.g. drawing, -neutral, and -donating groups such as Cl–, Br–, Me–,
Cl–) and electron-donating group (e.g. MeO–) at the meta posi- MeO–, and CF3O– at the para position of the phenyl ring of the
tion of the phenyl group had a positive impact on this aroyl group had a slight effect on this transformation (3k–3p,
trifluoromethylthiolation (3g, 94% ee; 3h, 91% ee). In general, 88–92% ee). In contrast, meta-substitution of the aroyl group
meta-substituted substrates gave the products in a little higher led to slightly lower enantioselectivities (3q, 87% ee; 3r,
enantioselectivity than those para-substituted. When electron- 85% ee). To determine the absolute configuration of chiral
rich N-allylamide bearing a naphthyl substituent was utilized products, 3a was converted to salt 4 under acidic conditions
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Table 2 Substrate scopea
Scheme 2 Proposed mechanism.
the enantioselectivity and reactivity of the reaction decreased
evidently compared with the optimal result (eqn (2)). For
example, using benzoyl-protected catalyst C11, the product was
formed only in 16% yield with 65% ee. Moreover, catalysts
with a stronger H-bonding donor such as sulfoamide (C12 and
C13) gave higher yields (46% and 57%) and better enantio-
selectivities (75% ee and 77% ee, respectively).
a Conditions: 1 (0.1 mmol), 2 (1.5 equiv.), C8 (20 mol%), BF3·Et2O (1.5
equiv.), CH2Cl2 (2.0 mL) + toluene (2.0 mL), −78 °C, 18 h. The yield
refers to the isolated yield. The ee value was determined by HPLC
analysis.
(eqn (1)).16 Compound 4 was determined to be (R)-configur-
ation by X-ray crystallographic analysis.
ð2Þ
In summary, we have developed chiral selenide-catalyzed
enantioselective trifluoromethylthiolation of N-allylamides.
The optically active 2,5-disubstituted oxazolines bearing a
CF3S group were obtained in good yields with high enantio-
selectivities. This work enables enantioselective trifluoro-
methylthiolation of 1,1-disubstitued alkenes.
ð1Þ
A plausible mechanism is proposed as shown in Scheme 2.
In the presence of BF3·Et2O, a cationic complex I is first
formed by the reaction of the selenide catalyst with the electro-
philic CF3S reagent based on our former studies.15 It then
reacts with substrate 1 to generate intermediate II, followed by
nucleophilic attack by the oxygen atom of the amide group to
form product 3. It is noted that acid BF3·Et2O not only helps
the selenide catalyst to activate the CF3S reagent, but also
serves as a hydrogen bond acceptor that links the catalyst and
the substrate.15d An anion bridge is proposed to set a suitable
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
chiral environment. In the entire cycle, H-bonding interaction We thank the “One Thousand Youth Talents” Program of
is essential for achieving high enantioselectivity of reaction. China, the National Natural Science Foundation of China
When catalysts with a weaker H-bonding donor were utilized, (Grant No. 21772239), the Natural Science Foundation of
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Guangdong Province (Grant No. 2014A030312018), and the
China Postdoctoral Science Foundation (Grant No.
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