Binuclear Double-Stranded Helicates and Their Catalytic Applications in Desymmetrization of Mesodiols

Article abstract of DOI:10.1021/acs.inorgchem.8b03643

The ligand L₁ of 4-methyl-2,6-diformylphenol and L₂ of 4-tert-butyl-2,6-diformylphenol are synthesized through Schiff base condensation with rac-, (R)-(+), or (S)-(-)-1,1′-binaphthyl-2,2′-diamine (BNDA). As a result, the racemic L₁^rac, L₂^rac, and enantiopure L₁^RR, L₁^SS, L₂^RR, and L₂^SS ligands are obtained incorporating Cu(II) and Zn(II) salts by a simple one-pot metal template method. The series of dinuclear complexes of [M₂LX₂] (here, M = Cu²⁺, Zn²⁺ X = acetate ion, chloride ion; L = L₁^RR, L₁^SS, L₁^rac, L₂^RR, L₂^SS, L₂^rac) formulas are obtained in common. Among them, the single crystal X-ray structures for [Zn₂L₁^rac(OAc)₂] and [Zn₂L₁^SSCl₂] complexes are obtained. The detailed crystal structure and the chiroptical studies performed on these complexes dictates a self-sorting behavior in their self-assembly process and illustrate a chirality transfer from the ligand to the metal center on the complexes. The enantiopure dinuclear complexes [M₂L^RRX₂] and [M₂L^SSX₂] generate enantiopure λλ and ΔΔ isomers, respectively, but the racemic complexes produce only homochiral λλ and ΔΔ assemblies. The detailed studies based on UFLC (Ultra Fast Liquid Chromatography), CD, and single crystal X-ray structure together show the absence of heterochiral λΔ mesocate. All these complexes are adapted as catalysts for desymmetrization of various mesodiols, and the enantiopure complexes are found to give efficient enantioselectivity in desymmetrization of mesodiols with benzoyl chloride to monobenzoylated ester providing 98% yield and 92% ee.

Full text of DOI:10.1021/acs.inorgchem.8b03643

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Binuclear Double-Stranded Helicates and Their Catalytic
Applications in Desymmetrization of Mesodiols
Eswaran Chinnaraja,†,‡ Rajendran Arunachalam,†,‡ Eringathodi Suresh,‡,⊥ Shovan K. Sen,‡,∥
Ramalingam Natarajan,‡,∥ and Palani S. Subramanian*,†,‡
⊥
†Inorganic Materials and Catalysis Division and Analytical and Environmental Science Division and Centralized Instrument
Facility, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Bhavnagar 364 002, Gujarat, India
‡Academy of Scientific and Innovative Research (AcSIR), Chennai 600113, Tamil Nadu, India
∥Organic & Medicinal Chemistry Division, CSIR-Indian Institute of Chemical Biology, Kolkata 700032, West Bengal, India
S
* Supporting Information
ABSTRACT: The ligand L1 of 4-methyl-2,6-diformylphenol and L2 of 4-tert-butyl-
2,6-diformylphenol are synthesized through Schiff base condensation with rac-, (R)-
rac
(+), or (S)-(−)-1,1′-binaphthyl-2,2′-diamine (BNDA). As a result, the racemic L1
,
SS
L2rac, and enantiopure L1RR, L1SS, L2RR, and L2 ligands are obtained incorporating
Cu(II) and Zn(II) salts by a simple one-pot metal template method. The series of
dinuclear complexes of [M2LX2] (here, M = Cu2+, Zn2+; X = acetate ion, chloride
ion; L = L1RR, L1SS, L1rac, L2RR, L2SS, L2rac) formulas are obtained in common. Among
them, the single crystal X-ray structures for [Zn2L1rac(OAc)2] and [Zn2L1SSCl2]
complexes are obtained. The detailed crystal structure and the chiroptical studies
performed on these complexes dictates a self-sorting behavior in their self-assembly
process and illustrate a chirality transfer from the ligand to the metal center on the
complexes. The enantiopure dinuclear complexes [M2LRRX2] and [M2LSSX2]
generate enantiopure ΛΛ and ΔΔ isomers, respectively, but the racemic complexes
produce only homochiral ΛΛ and ΔΔ assemblies. The detailed studies based on
UFLC (Ultra Fast Liquid Chromatography), CD, and single crystal X-ray structure together show the absence of heterochiral
ΛΔ mesocate. All these complexes are adapted as catalysts for desymmetrization of various mesodiols, and the enantiopure
complexes are found to give efficient enantioselectivity in desymmetrization of mesodiols with benzoyl chloride to
monobenzoylated ester providing 98% yield and 92% ee.
properties makes them potential candidates.16 Helicate
INTRODUCTION
■
possessing helical chirality (P/M) when constructed with
inbuilt chiral component (R/S) gains geometrical chirality (Δ/
Λ) and deserves additional advantages because of the
enhanced chiral components.17 Although such helicates are
rich in the literature, they are rarely explored in the asymmetric
catalysis.18 With this in mind, by adapting rac-, (R)-(+), or (S)-
(−)-1,1′-binaphthyl-2,2′-diamine (BNDA) with the appropri-
ate dialdehyde mentioned above, a series of binuclear double-
Desymmetrization of secondary diols can be achieved either by
kinetic resolution of secondary alcohols (KRSA) or by an
enantioselective desymmetrization of mesodiols (EDMD). For
enzyme-catalyzed asymmetric acylation,1,2 although known for
its high stereo- and regioselectivity, slow reactivity is its biggest
setback. The disadvantages in KRSA are its time consumption,
and the yields remain ≤50%. EDMD is found superior to the
KRSA, due to its many advantages such as (i) single step, (ii)
high yield, and (iii) enantioselectivity up to 100%. EDMD is
influenced by two different routes mediated through chiral (i)
stranded helicates of the type [M2LX2] (here M = Cu2+, Zn2+;
rac
X = OAc−, Cl−; L = L1RR, L1SS, L1rac, L2RR, L2SS, L2
)
nucleophilic pathway and (ii) metal catalytic pathway.3
A
incorporating Cu(II) and Zn(II) salts were synthesized as
shown in Scheme 1. The detailed chiroptical studies on these
complexes explore an interesting self-sorting behavior by
selectively forming homochiral ΔΔ and ΛΛ pairs. With
monoacylation of symmetric diols being a challenging reaction
in organic asymmetric transformations, these enantiopure
binuclear double-stranded helicates are intensively investigated
in the present work for their catalytic role in selective
number of efficient metal4 and organocatalysts5−10 have
already been applied to the synthesis of various natural
products, building blocks in the asymmetric synthesis of
antifungals,11 sugars,12 and so forth containing desymmetrized
alcohols. Metal complexes of mono- and binuclear Cu(II)13
and Zn(II)14 catalysts were also reported for the desymmet-
rization of mesodiols.15
Among the bimetallic complexes, the binuclear helicates
possessing attractive double helical architecture, its biochem-
ical relevance to DNA, and the associated chiroptical
Received: December 31, 2018
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of [M2LX2] Type Binuclear Terminally Closed Helicates by Metal Template Method
Figure 1. [ESI-MS]+ spectra of [Zn2L1rac(OAc)2] (3) (inset shows the isotopic distribution pattern).
RR
SS
rac
desymmetrization of mesodiols. The structure−reactivity
correlation between the helical structure of the complexes
and its role in favoring the desymmetrization is also discussed
in detail.
[Zn2L1RR X2] (3′), [Zn2L1 X2] (3″), [Zn2L2 X2] (4),
SS
[Zn2L2 X2] (4′), [Zn2L2 X2] (4″) were synthesized. Here
the X represents either acetate or chloride ions and L1 and L2
are two different Schiff base ligands of 4-methyl-2,6-
diformylphenol and 4-tert-butyl-2,6-diformylphenol, respec-
tively, synthesized by treating rac-, (R)-(+)-, or (S)-(−)-1,1′-
BNDA. Among these, complexes 1−4 are racemic containing
RR
RESULTS AND DISCUSSION
■
The binaphthyl moiety is known for its unique structural
rigidity and intrinsic axial chirality. By incorporating rac-, (R)-
(+)-, or (S)-(−)-1,1′-BNDA with 4-methyl-2,6-diformylphenol
L1 and L2 ligands, but the complexes 1′−4′ of L1RR, L2
rac
rac
and 1″−4″ of L1SS, L2 ligands are enantiopure in nature. All
these complexes are obtained by the one-pot subcomponent
approach as described in the Experimental Section and Scheme
SS
or 4-tert-butyl-2,6-diformylphenol and metal salts, a series of
rac
binuclear double-stranded helicates [Cu2L1
X2] (1),
rac
RR
SS
rac
1. A representative MS spectrum (Figure 1) for [Zn2L1
(OAc)2] (3) depicting a single dominant peak at m/z =
[Cu2L1RR X2] (1′), [Cu2L1 X2] (1″), [Cu2L2 X2] (2),
SS
[Cu2L2 X2] (2′), [Cu2L2 X2] (2″), and [Zn2L1racX2] (3),
B
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Figure 2. H NMR [Zn2L1SSCl2] or [3″.Cl2] recorded in DMSO-d6.
1
Figure 3. (a) CD spectra of [Cu2L1RR(OAc)2] 1′, [Cu2L2RR(OAc)2] 2′ and their enantiomeric counterparts (1″, 2″) recorded in THF (1 × 10−4
M). The insets show the d−d band (1 × 10−3 M). (b) CD spectra of zinc complexes were recorded in THF (1 × 10−4 M).
1011.34 along with isotopic peaks matches the calculated value
1011.17 of the monocationic species [3-(OAc)]+ or
[Zn2L1rac(OAc)]+. The positive ion ESI-MS of all the helicates,
depicting monocationic species [M2L(OAc)]+ in common,
confirms the formation of binuclear neutral complexes (Figures
S1−S12).
(CHN); a set of multiplets in the region δ = 6.75−8.50 ppm
correspond to an integral ratio of 28 hydrogens asserting the
aromatic protons. Similarly, the singlet at 2.06−2.16 ppm
corresponding to 3H of CH3 of aromatic substitution is
observed for 3−3″ complexes (Figures S15−S22), and a
similar singlet at 1.20−1.25 ppm corresponding to 18
hydrogens of −C(CH3)3 from 4−4″ (Figures S23−S28)
IR spectra of the complexes reveal the characteristic OH,
CN stretching frequencies at 3433 5 and 1629 5 cm−1
1
matches the respective complex. A representative H NMR
1
(Figure S13 and Figure S14). Both H and 13C NMR of
spectrum of [Zn2L1SSCl2] (3″) is shown in Figure 2.
zinc(II) dimers substantiate the formation of respective
Electronic spectra of the complexes (Figure S6) have shown
similar spectral patterns. The ligand centered bands at
approximately 264−270 and 285−290 nm are attributed to
the σ−π* and π−π* transitions. The peak at 421−431 nm
revealed the LMCT transition caused by the phenolate oxygen
1
complexes (Figure S5). The H NMR spectra of all zinc
helicates 3−3″ and 4−4″ (Figure S15−S29), depicting a
singlet at δ = 8.6−8.8 ppm in both, corresponding to four
hydrogens, confirm the existence of the azomethine peak
C
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Figure 4. Thermal ellipsoid plot depicting the neutral dimeric [Zn2L1rac(OAc)2] 3 with atom numbering scheme (45% probability factor for the
thermal ellipsoids; lattice water, acetonitrile, and hydrogen atoms are omitted for clarity).
Figure 5. Thermal ellipsoid plot depicting the neutral dimeric [Zn2L1SSCl2] or [3″.Cl2] complex with atom numbering scheme (45% probability
factor for the thermal ellipsoids; lattice water, acetonitrile, and hydrogen atoms are omitted for clarity).
to Cu(II) and Zn(II) center. Further, the position of the d−d
band at 660−670 nm in all the copper(II) helicates revealed a
pentacoordinated square pyramidal geometry of Cu(II)
(Figures S30 and S31).
and chloroform have also shown no change in their CD pattern
illustrating that the solvents did not show any change in
Cotton effect (Figure S32b). The CD and optical rotatory
dispersion (ORDE) in combination (Figure S8) confirm that
the Cu2L1RR exhibit negative cotton (ΛΛ) effect (Figure S33),
The CD spectra (Figure 3a) of the Cu(II) complexes 1′, 2′
against 1″, 2″ depicting exactly opposite Cotton effects; it is
obvious that the complexes are enantiopure. In addition, the
opposite optical sign in their d−d band (600−800 nm) in
Figure 3 representing the metal-centered chirality suggests ΛΛ
SS
while Cu2L1 exhibit positive cotton (ΔΔ) effect (Figure
S34).
The complex [Zn2L1rac(OAc)2] 3 was crystallized out by
slow evaporation from its acetonitrile solution, which yielded
crystals of yellow color suitable for X-ray analysis (Figure S9).
Complex 3, resolved in the monoclinic system with P21/n
space group, is composed of two Zn(II) atoms (Zn1 and Zn2),
one binaphthyl macrocyclic helically twisted ligand L1, two
coordinated acetate anions, one water molecule, and an
acetonitrile located as a solvent of crystallization in the lattice.
As depicted in Figure 4, both Zn atoms in the complex are
placed within the terminally closed helical ring and possesses a
N2O3 penta-coordination. In the dimeric unit, the ZnN2O3
RR
RR
in Cu2L1 (of 1′), Cu2L2 (of 2′) and ΔΔ chirality in
SS
SS
Cu2L1 (of 1″), Cu2L2 (of 2″). To understand its chiral
stability, the effects of temperature and solvent were
investigated by adapting a representative complex 3′ and
changing the temperature in the range −10 to 60 °C using the
CD spectral technique. This systematic study indicates that the
complexes are chirally stable in the studied temperature range
(Figure S32a). Similarly, the CD spectra recorded in different
solvents such as DMSO, THF, methanol, dichloromethane,
D
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Scheme 2. Self-Sorting Phenomena of Axially Chiral BNDA in Racemic and Enantiopure S-(−) and R-(+)-BNDA [M = Cu2+,
Zn2+, X = OAc−, Cl−, L = L1 or L2]
core of each metal center has distorted square pyramidal
geometry. The dinuclear and non-centrosymmetric structure of
3 (Figure 5) has thus constructed a Zn2N4O2 core. The
hexadentate macrocyclic ligand with N4O2 donor set shows a
helical twist upon coordination with two Zn(II) ions. Each
Zn(II) ion coordinating with two azomethine nitrogen is
further bridged by two phenolate oxygen atoms via μ2-bridge.
The distorted square pyramidal geometry is completed by
monodentate coordination of O atom of the acetate anion with
a tetrahedral bias (Zn1−O3 = 1.937(2) and Zn2−O5 =
1.954(2) Å) formed by the N2O2 core. The basal planes of
both metal centers are provided by two imino nitrogens and
two phenoxy bridging oxygens with Zn−N distances Zn(1)−
N(4) = 2.051(2), Zn(1)−N(1) = 2.082(2), and Zn(2)−N(2)
= 2.070(2), Zn(2)−N(3) = 2.104(2) Å. Intermetallic Zn(1)···
Zn(2) distance within the dimer is calculated as 3.1056(5) Å
with Zn(1)−O(1)−Zn(2) and Zn(1)−O(2)−Zn(2) angles
98.12(7)° and 98.13(4)°. In the dimeric complex rac-[Zn2L1]
3, the dihedral angles between the naphthyl-benzylideneamine
moieties coordinating to the respective metal centers as well as
between binaphthyl units of the ligand have shown differences.
Thus, the dihedral angle between the N-naphthyl-benzylidene-
amine moiety (i.e., between C1 to C10−N1−C11 and C46 to
C47−N4−C56) is 74.91°, while between the binaphthyl units
(i.e., C1 to C10 and C47 to C56), it is 70.48°. The dihedral
angles in the Zn2 geometry, between the N-naphthyl-
benzylideneamine moiety (i.e.,C19 to C28−N2−C18 and
C29 to C38−N3−C39) as well as between the binaphthyl
units (i.e., C19 to C28 and C29 to C38), are ∼86° and remain
almost perpendicular to make effective coordination with the
respective metal atoms. Interestingly, the apical coordination of
the square pyramidal geometry on each Zn(II) in the dimeric
unit is occupied by acetate anions. It is noteworthy to mention
that both acetate ions reside on the same side in the racemic
complex, and hence the opposite sites of the basal plane are
free. Possessing four molecules in the unit cell, the structure
analysis suggests that two are in ΔΔ form, while the other two
are in ΛΛ form reflecting their homochiral dimers in the
racemic complex, i.e., [ΛΛ-(Zn2L1RR(OAc)2] and [ΔΔ-
(Zn2L1SS(OAc)2]. The single crystal analysis of the racemic
complex did not show any heterochiral dimer ΛΔ-
[(Zn2L1RS(OAc)2] formation, although it was expected.19
Thus, the R and S−BNDA moieties follow the spontaneous
separation in the dimeric complexation. Thus, the helical
conformation favors the formation of self-sorting behavior and
promotes the formation of ΛΛ and ΔΔ homochiral pairs
instead of a heterochiral ΔΛ pair.
SS
The helicate [Zn2L1SSCl2] synthesized with enantiopure L1
has given a suitable single crystal by slow evaporation of its
acetonitrile solution at room temperature. The structure was
resolved in the monoclinic crystal system with chiral space
group C2 with five acetonitrile molecules in the lattice. The
crystal structure [3″.Cl2] illustrates that the ligand LSS in the
complex exists in helical chirality M, with metal centered
homochiral component ΔΔ. Possessing almost identical
dimeric units, the difference between these two structures of
3 and 3″ lies mainly on their chirality. Each Zn(II) atoms in
the dimeric unit possess a square pyramidal geometry and are
nonsuperimposable on each other. The Zn1 with N2O2Cl
chromophores are assembled in a square pyramidal geometry.
The basal plane formed by N2O2 exhibits coordination with
two azomethine nitrogens [Zn1−N1 = 2.021(17), Zn1−N2 =
2.137(19) Å] and two phenolate oxygens [Zn1−O1 = 1.94(2),
Zn1−O2 = 2.021(17)Å].
In a similar square base constructed by O1O2N3N4 with
Zn2 atom, the chloride ion Cl2 fulfilling the fifth site completes
a distorted square pyramidal geometry. Thus, both Zn(II)
atoms in the dimeric unit form a square base with the N2O2
chromosphere; the fifth site on the square pyramidal geometry
[Zn1−Cl1 = 2.266(5) and Zn2−Cl2 = 2.291(6)] is filled with
chloride ions. The respective geometries are inverting to each
other and are assembled in nonsuperimposable mirror images
(Δ, Δ). The Zn(II) atoms within the dimeric unit are
separated by a distance Zn1−Zn2 = 3.273 Å. The
coordinatively unsaturated sixth site in each zinc(II) center is
expected to play a crucial role in the asymmetric catalysis.
Selected bond distance and bond angles for both complexes of
[3.(OAc)2] and [3″.Cl2] are given in Table S1.
The racemic (RS) and enantiopure (RR, SS)-BNDA
moieties in the ligands L1 and L2 of the dimeric formulation,
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thus resulting in L1rac, L2rac, L1RR, L2RR, L1SS, and L2 ligands,
the self-assembly process followed in these respective complex-
ation inspired us to derive the self-sorting pathway based on
their chiroptical property. In general, helicates are intrinsically
chiral and exist as a diastereomeric mixture of homochiral pairs
(ΔΔ and ΛΛ)21 and heterochiral mesocate ΔΛ21 in the case
of achiral helicands. The homochiral pairs in the mixtures thus
possessing opposite chirality (ΔΔ and ΛΛ) cancel each other,
and hence the net CD activity is nullified. In the present case,
the racemic R and S−BNDA upon complexation follow a very
interesting pathway as illustrated in Scheme 2. The rac-BNDA
in Lrac have assembled in [M2LRR.X2] and [M2LSS.X2] mixtures
in which the R and S enantiomers are equally separated into
homochiral pairs constituting ΔΔ and ΛΛ chiral molecules in
the unit cell as represented in eq 1. In the other case, using
enantiopure R-BNDA and S-BNDA in complexes 1′−4′ and
1″−4″ results in self-assembly into [M2L1RR.X2], [M2L2RR.X2]
and [M2L1SS.X2], [M2L2SS.X2] in their respective complexes as
shown in eqs 2 and 3. On the other hand, the racemic ligands
generally lead to the formation of heterochiral complex (ΔΛ)
and/or mixture of homochiral pairs (ΔΔ and ΛΛ).22,23
Generally, racemic ligands are known to produce diastereo-
meric mixtures.20 Interestingly the present complex of Lrac
produces only homochiral dimers instead of a diastereomeric
mixture. This unique observation in the present complexes can
be explained by self-sorting (self/nonself) behavior in the
present chiral source of the ligands.24 The axially chiral25
BNDA in its different chiral forms, i.e., R-(+), S-(−), and
racemic, particularly exhibits this unique self-sorting behavior.
The BNDA in the racemic complexes 1−4 are self-assembled
and produce homochiral helicates (ΔΔ and ΛΛ) during
complexation and did not show the formation of any
heterochiral mesocate (ΔΛ) as shown in eq 1. However, by
incorporating enantiopure helicands, one can produce either
one enantiopure ΔΔ or ΛΛ helicates.26,27
existence of chirality with flack parameter value 0.05(2). As
expected, the racemic helicates with 50:50% ratio of
enantiomers nullify the chiral contribution in CD spectra
and give a flat line.
SS
In the UFLC, using phenomenox lux cellulose-1 column and
isopropyl alcohol/n-hexane (10/90%, 1 mL/min) as a mobile
phase, the enantiopure complexes are eluted as a single peak
suggesting that they are 100% ee (Figures S10, S37). When the
racemic helicates 1−4 were eluted, all of them have showed
two independent peaks corresponding to opposite enan-
tiomers. We did not find any third peak in the chromatogram
corresponding to mesocate confirming the absence of
heterochiral mesocate (ΔΛ). Thus, the chromatogram giving
only two separate peaks corresponds to ΛΛ and ΔΔ
homochiral compounds, supporting the observation in the
crystal structure of the racemic complex [3.(OAc)2]. The
absence of the third peak thus obviously concludes the absence
of mesocate ΔΛ. This observation further supports that the
[M2L1RR.X2].[M2L1SS.X2] homochiral pairs lead to CD
absence; thus, ruling out the formation of heterochiral
[M2L1RS.X2] in 1−4 is noteworthy. This gives an under-
standing that the racemic complex 1−4 generates ΛΛ and ΔΔ
homochiral dimers, but the respective enantiopure complexes
1′−4′ generate purely ΛΛ-[M2LRRX2], while 1″−4″ produce
only ΔΔ-[M2LSSX2] dinuclear complex selectively.
Among the 12 complexes shown in Scheme 1, eight (1′−4′
and 1″−4″) are enantiopure, while the remaining four (1−4)
are racemic. The detailed chiroptical study on these
enantiopure complexes has inspired us to investigate them
on enantioselective catalysis. Mesohydrobenzoin containing
two hydroxyl groups is known for its C2 symmetry.
Benzoylation of both hydroxyl groups is an easy task, but the
selective benzoylation of one of these two symmetric hydroxyls
of this mesodiol is of significant importance. Understanding
the challenge involved in the reaction, we have adapted these
complexes as catalysts to obtain enantiorich monobenzoylated
products and the results are summarized in Table 1.
Accordingly, the mesohydrobenzoin (5a) to monobenzoyla-
tion (5b) reaction was performed. Initially, the blank reaction
performed without catalyst provides a moderate conversion
without any enantioselectivity (Table 1, entry 1). Then, all
eight chiral catalysts were screened with 5 mol % catalyst
loading, using 2 equiv DIPEA (N,N-diisopropylethylamine),
benzoyl chloride (PhCOCl, 1 equiv), and chloroform as
solvent at RT. By adapting racemic complexes 1−4 as a
catalysts, the respective products (5b) are found to be racemic
as expected. Hence, by replacing the racemic complex with
respective enantiopure complexes 1′−4′ and 1″−4″, the
respective data are presented in Table 1 (entries 2−9).
Compared to the Cu catalysts (1′, 1″, 2′, 2″) (Table 1, entries
2−5), the Zn(II) catalysts 3′, 3″, 4′, 4″ (Table 1, entries 6−9)
generated better yield (92%) and enantioselectivity (78%),
with shorter reaction time (10 h). Thus identifying the zinc(II)
helicate as a suitable catalyst, we have optimized the reaction
conditions below.
rac
SS
4MX2 + L → ΛΛ‐[M2LRRX2] + ΔΔ‐[M2L X2] + 2X2
(1)
where Lrac = 50% ratio of R and S-BNDA.
2MX2 + LRR → ΛΛ‐[M2LRRX2] + X2
(2)
(3)
SS
SS
2MX2 + L → ΔΔ‐[M2L X2] + X2
Having used enantiopure R, S and rac-BNDA in the ligand
construction, the selective formation of optical isomers in the
dinuclear complexation was analyzed based on the single
crystal X-ray structure, circular dichroic cotton activity, and
UFLC (ultrafast liquid chromatography) results. The crystal
structure of [Zn2L1rac(OAc)2] 3 illustrates that the unit cell
contains a pair of homochiral helicates [Zn2L1RR(OAc)2] and
[Zn2L1SS(OAc)2] in which the metal centered chirality exists in
homochiral ΛΛ and ΔΔ forms. The unit cell contains four
molecules, two representing P-helicates with ΛΛ component,
and the other two M-helicate in the ΔΔ form. Thus, the self-
sorting behavior of this racemic ligand in [Zn2L1rac(OAc)2] 3
leads to formation of two pairs of molecules, i.e., a pair of (P)-
ΛΛ-[Zn2L1RR(OAc)2] and a pair of (M)-ΔΔ-[Zn2L1SS(OAc)2]
in the unit cell (Figure S35) as shown in eq 1. The crystal
structure analysis thus rules out the formation of heterochiral
ΛΔ-[Zn2L1RS(OAc)2] mesocate. However, the use of LSS in the
complex 3″ selectively leads to formation of enantiopure ΔΔ-
[Zn2L1SS.Cl2] (Figure S36). The single crystal X-ray analysis in
3″ suggesting the chiral space group further augments the
In our attempt to optimize the catalyst loading, we varied
the amount of catalyst from 0.5 to 10 mol % (Table 1, entries
10−16) and found 2 mol % of catalyst giving the highest yield
(92%) and enantioselectivity (85%) of monoester (Table 1,
entry 12).
As the solvent is known to play an important role, we next
performed a systematic screening of different solvents.
Considering the earlier reports, the main emphasis was given
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a
and carbon tetrachloride along with some nonhalogenated
solvents (Table 2, entries 4−6) like tetrahydrofuran, ethyl
acetate, and toluene (Table 2, entry 6) were tested. Among
these, DCM was found to give good yield, 96%, and better
enantioselectivity, 88% (Table 2, entry 2). For further increase
in the yield and enantioselectivity, the reaction temperature
was optimized by varying temperature such as 40, 20, 10, 0,
−5, and −10 °C (Table 2, entries 7−12). The highest yield,
98%, and enantioselectivity, 92%, at 0 °C prompted us to fix
this as optimized temperature. In a search to identify a suitable
base, various bases (Table 2, entries 13−16) such as
triethylamine, lutidine, sodium carbonate, and potassium
carbonate were tested. With the results being inferior
compared to DIPEA, all the follow-up reactions are conducted
using DIPEA as base.
Adapting the reaction conditions optimized above, we
further explored varying substrates of meso-1,2-diols (Table
3, 5b−15b) catalyzed by both 3′ and 4′. In the case of
hydrobenzoin (5a) both of them are behaving very similarly
(Table 3, 5b), i.e., 98% and 97% yield and 92% and 90% ee (3′
and 4′, respectively). The substrate 2,3-butanediol (6a), which
possess an additional methyl group, gives good yield 96% and
95%, but differs by its poor ee, i.e., 40% and 22% (Table 3, 6b).
Then, we moved from acyclic diol to cyclic diol substrates
containing five-membered, six-membered, and eight-mem-
bered rings. Although this attempt did not give encouraging
ee, the cyclopentane diol (7a) has converted to monobenzoy-
lated product (7b) with 97% yield and 44% and 35% ee (Table
3, 7b). Similarly, the monoester 8b obtained from cyclohexane
diol (8a), both catalysts 3′ and 4′ produce yield of 95% but
differ in their ee as 70% and 31% (Table 3, 8b). Finally, the
cyclooctane diol (9a) gives 94% and 95% yield and 30% and
42% ee of monoester product (9b) (Table 3, 9b). From this
substrate variation, we conclude that the phenyl substituted
1,2-diol (5a) only gave the best result (Table 3, 5b). A strong
π−π interaction between the phenyl moiety of substrate
mesodiol28 and the aromatic rings on the BNDA of the catalyst
might have influenced this enhanced activity.
Table 1. Screening of Catalyst
b
c
entry catalyst catalyst loading (mol %) yield
%
ee %(configuration)
1
2
3
4
5
6
7
8
blank
1′
1″
2′
2″
3′
3″
4′
4″
3′
3′
3′
3′
---
5
5
5
5
5
5
5
5
0.5
1
2
4
6
55
---
88
85
89
87
92
91
90
90
90
92
92
90
91
90
89
69(1S,2R)
68(1S,2R)
67(1S,2R)
67(1R,2S)
78(1S,2R)
78(1R,2S)
77(1S,2R)
77(1R,2S)
51(1S,2R)
62(1S,2R)
85(1S,2R)
78(1S,2R)
70(1S,2R)
68(1S,2R)
62(1S,2R)
9
10
11
12
13
14
15
16
3′
3′
3′
8
10
a
All the reactions were carried out with 0.2 mmol of mesohy-
drobenzoin, 0.2 mmol of PhCOCl, 0.4 mmol of DIPEA, and 2 mL of
solvent was used. Isolated yield. Determined by UFLC (Lux
cellulose-1 column). The reaction time for the Cu2-complex is 12 h
and Zn2 is 10 h.
b
c
to halogenated solvents (Table 2, entries 1−3). Hence, a series
of halogenated solvents like dichloroethane, dichloromethane,
a
Table 2. Screening of Reaction Condition
It is noteworthy to mention that the catalysts 3′ and 4′
favored the desymmetrization of 1,2-diphenyl substituted diols
over the cyclic substrates. The lack of π−π interaction between
all other substrates, resulting in less ee, is obvious to
understand. Thus, with the π−π interaction playing the crucial
b
c
entry
base
solvent
temp (°C)
yield
90
%
ee %
1
2
3
4
5
6
7
8
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
Et3N
CH3CH2Cl2
CH2Cl2
CCl4
THF
EtOAc
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
RT
RT
RT
RT
RT
RT
40
20
10
0
−5
−10
0
0
0
75
88
76
70
67
78
75
88
88
92
88
85
78
76
70
75
role, the P(ΛΛ) stereostructure in [Zn2L1RR] 3′ promotes
96
85
84
87
80
92
96
96
98
88
85
89
87
95
95
SS
(1S,2R) products, while M(ΔΔ) stereostructure in [Zn2L1
]
3″ promotes (1R,2S) enantiomeric product. Hence by keeping
mesohydrobenzoin as a choice of substrate, various aroyl,
cyclic, and acyclic acyl chlorides were treated with acylating
agent under the above optimized reaction conditions using 3′
as a catalyst (Table 3, 10b−15b). Consequently, various
electron donating and electron withdrawing substituted
benzoyl chlorides were tested. When 4-fluorobenzoyl chloride
(10a) was used, the corresponding monoester (10b) was
produced with 96% yield and 82% ee (Table 3, 10b). The 4-
nitrobenzoyl chloride (11a) gives monoester (11b) with 95%
yield, but the enantioselectivity was reduced to 10% (Table 3,
11b). In the case of 11b, the low ee might have been obtained,
because of the steric hindrance of the bulky nitro group at the
fourth position. The remaining 4-methylbenzoyl chloride
(12a) and 2-fluorobenzoyl chloride (13a) gives monoester
(12b and 13b) with good yield, 95% and 93%, respectively and
ee 88% and 78% (Table 3, 12b and 13b). Since the aroyl
chlorides are giving encouraging results, we also screened some
9
10
11
12
13
14
15
16
lutidine
Na2CO3
K2CO3
0
a
All the reactions were carried out with 2 mol % of catalyst, 0.2 mmol
of mesohydrobenzoin, 0.2 mmol of PhCOCl, 0.4 mmol of base, and 2
mL of solvent. Isolated yield. Determined by UFLC (Lux cellulose-
1 column). The reaction time for entries 1−9 is 10 h and for the rest
is 8 h.
b
c
G
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Inorganic Chemistry
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a
Table 3. Asymmetric Benzoylation of Meso-1,2-diols
a
All the reactions were carried out with 2 mol % of catalyst, and 0.2 mmol of mesohydrobenzoin, 0.2 mmol of PhCOCl, 0.4 mmol of DIPEA, and 2
mL of solvent were used. Yields presented here are isolated yields, and ee values were determined by UFLC (Lux cellulose-1 column).
nonaroyl cyclic (cyclohexanecarbonyl chloride, 14a) and
acyclic acyl chloride (acetyl chloride, 15a) which produced
monoesters (14b and 15b) with yield (94% and 96%,
respectively) and ee (34% and 18%, respectively) (Table
3,14b and 15b). These two nonaroyl chlorides, although giving
good yield, resulted in poor enantioselectivities. This detailed
study suggests that the benzoyl chloride gives the best results
(Table 3, 5b) among the acylating agents.
Scheme 3. Tentative Model of the Substrate−Catalyst
Interaction: (a) Racemic Complex; (b) Enantiopure
Complexes
In an attempt to understand the mechanism involved in the
catalytic process, based on the crystal structures of [3.(OAc)2]
and [3′.Cl2], we propose the mechanism in Scheme 3. In the
racemic crystal 3, both acetate ions coordinated to Zn(II)
metal centers reside on the same side. Hence, the diol is
approaching the Zn(II) ions through the sixth vacant site by
rearranging its geometry from square pyramidal to distorted
octahedral. The intramolecular M−M distance derived from
the crystal structure ranges 3.1−3.2 Å in the dimeric
association, and the distance between the hydroxyl groups of
the diol can comfortably make a six-membered chelate ring
involving Zn1 and Zn2 from the same side. The Zn1−Zn2
distance, 3.10 Å, derived from the crystal structure 3 and the
O···O distance in the hydrobenzoin29 ranging 2.69−3.59 Å
provide a strong possibility for chelate coordination involving
H
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Inorganic Chemistry
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Figure 6. 1H NMR for (i) [Zn2L1RR], (ii) addition of hydrobenzoin, (iii) addition of DIPEA, and (iv) addition of C6H5COCl. Here, Z − catalyst,
H − Hydrobenzoin, D − DIPEA, B − Benzoyl Chloride, P − monoester product, * − solvent CH2Cl2.
(Zn−O−C−C−O−Zn) of the same zinc dimer. Thus, the
diastereomeric dimers, i.e., ΛΛ and ΔΔ pairs in complex 3
might simultaneously promote the benzoylation on both the
hydroxyl group of the mesodiols. As a result, racemic complex
3 failed to break the symmetry (Scheme 3a) of mesodiols. The
P(ΛΛ) and M(ΔΔ) stereostructure of the chiral catalysts
[3′.OAc2] and [3′.Cl2] in their enantiopure form accelerates
the enantioselectivity. The π−π stacking interaction between
the phenyl rings of the BNDA moiety on the catalysts and
substrates in combination favors the enantioselective product
(Scheme 3). The absence of such π−π stacking interaction in
the case of nonaromatic cyclic (6b−9b) and acyclic products
(14b and 15b) failed to provide enantioselectivity and
supports the significance of the aromatic ring in the catalysis.
As shown in the crystal structure of the racemic complex 3,
both Zn(II) ions in the dimer projecting the apical site
(Scheme 3a) on the same side exists as ΔΔ and ΛΛ. Thus, the
stereostructure of catalyst 3 favors the substrate to approach
through the vacant side, and hence both hydroxyl groups get
equal opportunity for benzoylation, resulting in a symmetric
benzoylation. Thus, in the case of a racemic catalyst, both the
metal−metal distance and the geometry in the dimer favor
symmetric benzoylation.
ΔΔ, respectively, in the dimer; the accessible vacant site for
the substrate is available for only one hydroxyl group from
each side (Scheme 3b). Hence the other hydroxyl of the diol
remains unaffected, which favors the desymmetrization of the
C2 symmetric mesodiol.
To ascertain the mechanism in solution state, we have
1
recorded a series of systematic H NMR (Figure 6). Four
1
different experiments such as (i) H NMR for [Zn2L1RR], (ii)
addition of hydrobenzoin, (iii) addition of DIPEA, and (iv)
addition of C6H5COCl (here Z − catalyst, H − Hydrobenzoin,
D − DIPEA, B − Benzoyl Chloride, P − monoester product)
were carried out. The hydroxyl proton observed at δ = 2.46
ppm (H1) was found to shift to δ = 3.3 ppm upon adding
DIPEA. Subsequently, a further shift to δ = 4.0 ppm was
observed at the end of the catalytic run representing the
hydroxyl in the product (P1), showing a broad resonance.
The CH proton H2 of the mesohydrobenzoin at δ = 4.7
ppm corresponded to two hydrogens giving a strong singlet,
ensuring the existence of C2 symmetry in Figure 6(ii).
However, as the reaction progresses with benzoyl chloride,
the simultaneous decrease in intensity of H2 and appearance of
two new peaks at P2 and P3 at Figure 6(iv) illustrates the
symmetry-breaking leading to desymmetrized monobenzoy-
lated product formation.
In contrast, in the chiral complexes [Zn2L1RRCl2] and
[Zn2L1SSCl2], the geometry of Zn(II) exists either in ΛΛ or
I
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Inorganic Chemistry
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Scheme 4. Proposed Mechanism for Desymmetrization of Mesodiol by [Zn2L1RR] (3′) Catalyst
Figure 7. Recyclability of the catalyst 3′ and 4′ in mesohydrobenzoin with benzoyl chloride in an optimized condition.
Based on our understanding, we propose a probable
mechanism for desymmetrization of mesodiol in Scheme 4.
In the first step, the substrate interacts with the catalyst
through the sixth vacant site of the Zn(II) center, leading to an
octahedral geometry (TS-I). Then, the alcoholic protons of
diol get deprotonated by DIPEA to form a diisopropylethy-
lammonium ion. One of the two hydroxyl ions coordinates
with one Zn(II), leaving the other hydroxyl ion free to interact
with the carbonyl carbon of the benzoyl chloride as shown in
TS-II. The nucleophilic addition between the carbonyl carbon
of benzoyl chloride (Ph-OCδ+) and free hydroxyl ion of the
diol combine to form a stereoselective monoester product
leaving the chloride ion as hydrochloric acid. High stereo-
selectivity was observed only in mesohydrobenzoin compared
to the other substrates. As stated above, the phenyl moiety in
the mesohydrobenzoin, having a strong π−π interaction with
one of the binaphthyl groups of the catalyst, might promote
formation of the stereoselective product.
Based on the yield and enantioselectivity, the catalysts
RR
RR
Zn2L1 and Zn2L2 were found to be efficient. Hence, we
tested catalytic recyclability (Figure 7) using mesohydroben-
zoin and benzoyl chloride as substrate. Interestingly, both
Zn2L1RR and Zn2L2RR, with amazing yield and ee, had catalytic
1
efficiency that was retained up to seven cycles. The H NMR
J
DOI: 10.1021/acs.inorgchem.8b03643
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Inorganic Chemistry
Article
and were refined on F2 by the full-matrix least-squares technique.32 All
non-hydrogen atoms were refined anisotropically until convergence is
reached. The hydrogen atoms were either located from difference
Fourier map or fixed stereochemically. After the complete refinement
of the of neutral dimeric Zn complex with one water and one
acetonitrile molecule, diffuse peaks with residual electron density
ranging from ∼3.5 Å−3 to 3 Å−3 were observed in the difference
Fourier map, which can be attributed to a disordered solvent molecule
present in the crystal lattice. Attempts to model these disordered
peaks were unsuccessful since residual electron density was weak and
there were no obvious major site occupations for the lattice solvent
molecule. PLATON/SQUEEZE33 was used to correct the diffraction
data for the contribution from disordered lattice solvent (water)
molecule. The solvent accessible void volume and the corresponding
electron counts/unit cell estimated for [3.(OAc)2] was 1004.4 Å3 and
199 e Å−3, respectively. This electron count corresponds to
approximately nine disordered acetonitrile molecules present in the
unit cell as a solvent of crystallization. Final cycles of least-squares
refinements with the modified data set after subtracting the
contribution from the disordered solvent molecules using SQUEEZ
program significantly improved the R-values and Goodness of Fit of
the structural data. The molecular formula provided in the
crystallographic table is without incorporating the solvent molecules.
Crystallographic data of [3.(OAc)2] and [3″.Cl2] has been deposited
in the Cambridge Crystallographic Data Center as a supplementary
publication with CCDC No. 1874548−1874549.
recorded for fresh and recycled catalyst of [Zn2L1RR] after the
seventh cycle matching, confirming catalytic stability (Figure
S38).
CONCLUSIONS
■
In summary, we have synthesized and characterized a series of
terminally close-ended binuclear double-stranded helicates
incorporating cupric acetates, zinc(II) acetate and zinc(II)
chloride salts. The chirality transfer from ligand to metal
monitored by the CD and ORDE was correlated for
enantioselectivity on the asymmetric catalyst. The crystal
structure obtained for racemic complex 3 followed a self-
sorting behavior and revealed homochiral helicate pairs [ΛΛ-
(Zn2L1RR(OAc)2)] and [ΔΔ-(Zn2L1SS(OAc)2)] in the unit
cell. The enantiopure helicate ΔΔ-[Zn2L1SSCl2] and the
associated CD studies provide stereochemical compatibility
and promote the desymmetrized product via enantioselective
benzoylation. The self-sorting behavior of rac-BNDA leads to
homochiral dimeric helicates [(ΔΔ)] and [(ΛΛ)]. Formation
of (ΔΛ) heterochiral mesocate was found absent in the present
analysis. The enantiopure catalysts 1′−4′ and 1″−4″ after
synthesis were obtained as enantiopure compounds in ΛΛ and
ΔΔ forms and are successfully applied for desymmetrization of
C2-symmetric mesodiols to achieve unsymmetrical monoesters
with good yield (98%) and enantioselectivity (92%).
Identifying the efficiency of these catalysts on asymmetric
benzoylation of mesodiols, we have demonstrated the
formation of symmetrical diols to monobenzoylated unsym-
metrical diols through EDMD. The crystal structure and the
Synthesis of Helicate. Copper Helicates. 0.1 mmol of 4-methyl-
2,6-diformylphenol for L1 (16.4 mg) or 4-tert-butyl-2,6-diformylphe-
nol for L2 (20.6 mg) dissolved in acetonitrile (5 mL), 0.1 mmol of
triethylamine (14 μL), and cupric acetate monohydrate (0.1 mmol,
19.9 mg) were mixed together and the solution was stirred constantly
at room temperature for 30 min. Then, the rac-1,1′-binaphyl-2,2′-
diamine (0.1 mmol, 28.4 mg) was added drop by drop. After the
complete addition of the diamine, the solution slowly became a yellow
to green color and the reaction mixture was refluxed for 36 h. Finally,
the resulting dark brown solution was evaporated under rotavapor to
remove the solvent and the residue was washed with dichloromethane
(50 mL) and water (3 × 10 mL). The organic layer was separated and
dried. Yield 84−85%.
1
systematic H NMR studies in combination were used to
derive a feasible mechanism for the desymmetrization of
mesodiol. The recyclability and catalytic stability of the
complexes investigated were satisfactory even after the seventh
cycle.
[Cu2L1rac(OAc)2] (1). Yield 85%, UV−vis [(λ, nm) (ε/M−1)]: 245
(8414), 267 (7710), 286 (7270), 428 (4230), 662 (34) nm. FT-IR
(KBr): ν = 3449, 3060, 3027, 2954, 2906, 2362, 1610, 1518, 1425,
1387, 1325, 1263, 1212, 1125, 1057, 756, 696 cm−1. [ESI-MS]+:
Chemical formula for [M.OAc]+ C60H41Cu2N4O4, m/z Calc (found):
1007.17 (1007.85). Elemental Analysis: Mol. formula.
C62H46Cu2N4O7. Calc (found) C, 68.56 (68.13), H, 4.27 (4.78), N,
5.16 (5.62) %.
EXPERIMENTAL SECTION
■
Materials and Methods. 4-Methyl-2,6-diformylphenol, 4-tert-
butyl-2,6-diformylphenol, (R)-(+)-1,1′-binaphthyl-2,2′-diamine, (S)-
(−)-1,1′-binaphthyl-2,2′-diamine, and cupric acetate monohydrate
were purchased from Aldrich & Co. All these chemicals were used as
received without any further purification. Microanalyses were done
using a PerkinElmer PE 2400 series II CHNS/O elemental analyzer.
IR spectra were recorded using KBr pellets (1% w/w) on a
PerkinElmer Spectrum GX FT-IR spectrophotometer. Electronic
spectra were recorded on a Shimadzu UV 3101 PC spectropho-
tometer. Mass analyses were performed using the positive electron
spray ionization (ESI+) technique on a waters Q TOF-micro mass
[Cu2L1RR(OAc)2] (1′). The above procedure was adopted and R-
BNDA used instead of rac-BNDA. Yield 84%, UV−vis [(λ, nm) (ε/
M−1)]: 247 (8310), 267 (7394), 288 (6970), 426 (4007), 664 (35)
nm. FT-IR (KBr): ν = 3429, 3061, 3028, 2955, 2904, 2368, 1612,
1528, 1451, 1391, 1328, 1264, 1213, 1050, 833, 699 cm−1. [ESI-
MS]+: Chemical Formula for [M.OAc]+ C60H41Cu2N4O4, m/z Calc
(found): 1007.17 (1007.42). Elemental Analysis: Mol. formula.
C62H48Cu2N4O8. Calc (found) C, 67.44 (67.56), H, 4.38 (4.76), N,
5.07 (5.46) %.
1
spectrometer. H and 13C NMR spectra were recorded on a Bruker
Avance DPX 200, 500 MHz and JEOL Delta 600 MHz FT-NMR
spectrometer. Chemical shifts for proton resonances are reported in
ppm (δ) relative to tetramethylsilane (TMS) and the 13C is related to
solvents. The CD spectra were recorded on a JASCO 815
Spectrometer. The enantioselectivity of the monobenzoylated product
was determined by UFLC (Shimadzu SCL-10AVP) using chiral
columns (Phenomenox Lux cellulose-1 and Amylose-2 column).
X-ray Crystallography. Summary of crystallographic data34,35
and selected bond lengths and bond angles for 3.(OAc)2 and [3″.Cl2]
are given in Table S3. Single-crystal X-ray data collection for 3 was
performed on a Bruker SMART APEX CCD diffractometer equipped
with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å) at
150 K for [3.(OAc)2]. No crystal decay was observed during the data
collection. The SMART and SAINT software packages were used for
data collection and reduction, respectively.30 In all cases, absorption
corrections based on multiscans using the SADABS software was also
applied.31 The structures were solved by direct methods SHELXTL
[Cu2L1SS(OAc)2] (1″). Following the general procedure with S-
BNDA used instead of rac-BNDA. Yield 85%, UV−vis [(λ, nm) (ε/
M−1)]: 245 (8428), 267 (7459), 286 (6933), 428 (3879), 662 (33)
nm. FT-IR (KBr): ν = 3449, 3060, 3027, 2954, 2906, 2362, 1610,
1518, 1425, 1387, 1325, 1263, 1212, 1125, 1057, 756, 696 cm−1. [ESI-
MS]+: Chemical formula for [M.OAc]+ C60H41Cu2N4O4, m/z Calc
(found): 1007.17 (1007.43). Elemental Analysis: Mol. formula.
C62H46Cu2N4O7. Calc (found) C, 68.56 (68.12), H, 4.27 (4.60), N,
5.16 (5.48) %.
[Cu2L2rac(OAc)2] (2). Following the general procedure using 4-
methyl-2,6-diformylphenol and rac-BNDA. Yield 84%, UV−vis [(λ,
nm) (ε/M−1)]: 245 (8485), 267 (7981), 286 (7654), 416 (4326),
669 (41) nm. FT-IR (KBr) ν = 3438, 3053, 2956, 2360, 1613, 1541,
1391, 1196, 1065, 818, 746, 688, 498 cm−1. [ESI-MS]+: Chemical
K
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formula for [M.OAc]+,C66H53Cu2N4O4, m/z Calc (found) 1091.27
(1091.97). Elemental Analysis: Chemical Formula, C68H64Cu2N4O10.
Calc (found) C, 66.71 (66.92), H, 5.27 (5.65), N, 4.58 (4.12) %.
[Cu2L2RR(OAc)2] (2′). Following the general procedure using 4-tert-
butyl-2,6-diformylphenol and R-BNDA. Yield 85%, UV−vis [(λ, nm)
(ε/M−1)]: 246 (8532), 266 (8133), 288 (7907), 414 (4190), 667
(38) nm. FT-IR (KBr): ν = 3432, 3053, 2962, 2362, 1607, 1541,
1385, 1199, 1059, 814, 750, 688, 492 cm−1. [ESI-MS]+: Chemical
formula for [M.OAc]+, C66H53Cu2N4O4, m/z Calc (found) 1091.27
(1091.77). Elemental Analysis: Chemical Formula, C68H68Cu2N4O12.
Calc (found) C, 64.80 (64.35), H, 5.44 (5.103), N, 4.45 (4.43) %.
[Cu2L2SS(OAc)2] (2″). Adapting the above general procedure and
using 4-tert-butyl-2,6-diformylphenol and S-BNDA. Yield 84%, UV−
vis [(λ, nm) (ε/M−1)]: 245 (8089), 267 (7740), 286 (7372), 416
(4289), 669 (33) nm. FT-IR (KBr): ν = 3438, 3053, 2956, 2360,
1613, 1541, 1391, 1196, 1065, 818, 746, 688, 498 cm−1. [ESI-MS]+:
Chemical formula for [M.OAc]+, C66H53Cu2N4O4, m/z Calc (found)
1091.27 (1091.76). Elemental Analysis: Chemical Formula,
C68H64Cu2N4O10. Calc (found) C, 66.71 (66.28), H, 5.27 (5.383),
N, 4.58 (4.72) %.
Zinc Helicates. The 4-methyl-2,6-diformylphenol for L1 (16.4 mg,
0.1 mmol) or 4-tert-butyl-2,6-diformylphenol for L2 (20.6 mg, 0.1
mmol) dissolved in acetonitrile (5 mL), triethylamine (14 μL, 0.1
mmol), and zinc acetate dehydrate (21.95 mg, 0.1 mmol) were mixed
together, and the resultant solution was stirred constantly at room
temperature for 0.5 h. Then 0.1 mmol of 1,1′-binaphyl-2,2′-diamine
(28.4 mg) was added drop by drop. After the complete addition of
diamine, the reaction mixture was refluxed for 36 h. Finally, from the
yellow solution, the solvent was removed by rotavapor and the
resulting residue was washed with dichloromethane (50 mL) and
water (3 × 10 mL). The organic layer was separated and dried.
Recrystallization from the acetonitrile yields yellow crystal which is
suitable for single crystal XRD. Yield 84−85%.
1629, 1548, 1391, 1326, 1209, 1065, 994, 818, 746, 668, 554, 485
cm−1. 1H NMR (200 MHz, CD3OD, ppm): δ = 8.702(s, 4H, −CH
N), 7.989−7.834 (m, 8H, −CH), 7.435−7.233(m, 16H, −CH),
6.810−6.768 (m, 4H, −CH), 2.066 (s, 6H, −CH3) 1.688 (s, 6H,
−CH3). 13C NMR (125 MHz, DMSO-d6, ppm) δ = 174.968,
169.931, 165.628, 144.732, 144.055, 143.890, 142.638, 133.426,
131.708, 130.033, 128.296, 128.207, 127.102, 125.768, 125.339,
124.507, 123.402, 123.168, 122.539, 121.056, 118.563, 22.325,
19.322. [ESI-MS]+: Chemical formula for [M.OAc]+,
C60H41N4O4Zn2, m/z Calc (found) 1011.17 (1011.57). Elemental
Analysis: Chemical Formula, C62H48N4O8Zn2. Calc (found) C, 67.22
(67.50), H, 4.37 (4.52), N, 5.06 (4.99) %.
[Zn2L2rac(OAc)2] (4). 4-tert-Butyl-2,6-diformylphenol used instead
of 4-methyl-2,6-diformylphenol and rac-BNDA. Yield 84%. UV−vis
[(λ, nm) (ε/M−1)]: 246 (8465), 268 (6843), 289 (6420), 406 (4011)
nm. FT-IR (KBr): ν = 3429, 3060, 2923, 2369, 1629, 1548, 1391,
1326, 1209, 1065, 994, 818, 746, 668, 554, 485 cm−1. 1H NMR (500
MHz, CD3OD, ppm): δ = 8.893 (s, 4H, −CHN), 8.145−8.128 (m,
4H, −CH), 7.975−7.958 (m, 4H, −CH), 7.801−7.769 (m, 4H,
−CH), 7.444−7.398 (m, 6H, −CH), 7.234−7.205 (m, 6H, −CH),
6.777−6.760 (m, 4H, −CH), 1.708 (s, 6H, −CH3) 1.250 (s, 18H,
−C(CH3)3). 13C NMR (125 MHz, CD3OD, ppm): δ = 174.992,
170.591, 165.453, 144.802, 144.055, 139.831, 138.321, 133.483,
131.767, 130.073, 128.613, 128.293, 128.023, 127.084, 126.031,
125.786, 125.330, 123.519, 123.174, 122.222, 121.055, 118.563,
110.505, 33.657, 30.970, 30.866, 22.414. [ESI-MS]+: Chemical
formula for [M.OAc]+, C60H41N4O4Zn2. m/z Calc (found) 1093.27
(1093.49). Elemental Analysis: Chemical Formula, C68H66N4O11Zn2.
Calc (found) C, 65.55 (65.07), H, 5.34 (4.65), N, 4.50 (4.55) %.
[Zn2L2RR(OAc)2] (4′). Similar procedure adopted above except R-
BNDA instead of rac-BNDA. Yield 85%. UV−vis [(λ, nm) (ε/M−1)]:
244 (7607), 269 (7168), 289 (6839), 409 (4251) nm. FT-IR (KBr):
ν = 3387, 3060, 2923, 2368, 1623, 1528, 1388, 1329, 1208, 1061,
1002, 872, 820, 748, 676, 538, 480 cm−1. 1H NMR (200 MHz,
CD3OD, ppm): δ = 8.877 (s, 4H, −CHN), 8.144−8.101 (m, 4H,
−CH), 7.977−7.937 (m, 4H, −CH), 7.779 (m, 4H, −CH), 7.481−
7.373 (m, 6H, −CH), 7.247−7.172 (m, 6H, −CH), 6.797−6.754 (m,
4H, −CH), 1.705 (s, 6H, −CH3), 1.242 (s, 18H, −C(CH3)3). 13C
NMR (125 MHz, DMSO-d6, ppm) δ = 174.978, 170.607, 165.475,
144.818, 144.065, 139.840, 138.336, 133.493, 131.779, 130.088,
128.631, 128.305, 128.262, 128.039, 127.098, 126.049, 125.798,
123.528, 123.185, 122.237, 121.073, 118.576, 110.519, 33.670,
30.982, 30.879, 22.422. [ESI-MS]+: Chemical formula for
[M.OAc]+, C60H41N4O4Zn2, m/z Calc (found) 1093.27 (1093.46).
Elemental Analysis: Chemical Formula, C68H60N4O8Zn2. Calc
(found) C, 68.52 (68.37), H, 5.07 (5.02), N, 4.70 (4.75) %.
[Zn2L1rac(OAc)2] (3). Yield 84%. UV−vis [(λ, nm) (ε/M−1)]: 243
(8062), 269 (7616), 286 (7265), 412 (5204) nm. FT-IR (KBr): ν =
3429, 3060, 2923, 2369, 1629, 1548, 1391, 1326, 1209, 1065, 994,
818, 746, 668, 554, 485 cm−1. 1H NMR (500 MHz, CD3OD, ppm): δ
= 8.715 (s, 4H, −CHN), 7.996−7.987 (m, 4H, −CH), 7.875−
7.860 (m, 4H, −CH), 7.446−7.417 (m, 6H, −CH), 7.355−7.339 (m,
5H, −CH), 7.240−7.209 (m, 5H, −CH), 6.793−6.776 (m, 4H,
−CH), 2.085 (s, 6H, −CH3) 1.677 (s, 6H, −CH3). 13C NMR (125
MHz, CD3OD, ppm): δ = 175.136, 169.965, 169.464, 165.583,
144.713, 144.059, 143.94, 142.668, 133.452, 131.723, 130.075,
128.622, 128.306, 128.220, 128.033, 127.124, 126.045, 125.77,
125.344, 124.564, 123.392, 123.176, 122.516, 121.065, 118.571,
22.394, 19.327. [ESI-MS]+: Chemical formula for [M.OAc]+,
C60H41N4O4Zn2, m/z Calc (found) 1011.17 (1011.34). Elemental
Analysis: Chemical Formula, C62H48N4O8Zn2. Calc (found) C, 67.22
(67.50), H, 4.37 (4.517), N, 5.06 (4.99) %.
[Zn2L2SS(OAc)2] (4″). Similar procedure adopted above except S-
BNDA instead of rac-BNDA. Yield 84%. UV−vis [(λ, nm) (ε/M−1)]:
246 (8451), 268 (7713), 289 (7298), 406 (3791) nm. FT-IR (KBr):
ν = 3429, 3060, 2923, 2369, 1629, 1548, 1391, 1326, 1209, 1065, 994,
818, 746, 668, 554, 485 cm−1. 1H NMR (200 MHz, CD3OD, ppm): δ
= 8.883 (s, 4H, −CHN), 8.139−8.096 (m, 4H, −CH), 7.976−
7.936 (m, 4H, −CH), 7.810−7.786 (m, 6H, −CH), 7.477−7.365 (m,
6H, −CH), 7.245−7.134 (m, 4H, −CH), 6.796−6.750 (m, 4H,
−CH), 1.690 (s, 6H, −CH3) 1.238 (s, 18H, −C(CH3)3). 13C NMR
(125 MHz, DMSO-d6, ppm) δ = 174.884, 170.581, 165.455, 144.802,
144.051, 139.824, 138.291, 133.468, 131.756, 130.056, 128.600,
128.290, 128.013, 127.076, 126.023, 125.776, 125.316, 123.513,
123.165, 122.221, 121.040, 118.554, 110.487, 33.653, 30.968, 30.862,
22.387. [ESI-MS]+: Chemical formula for [M.OAc]+,
C60H41N4O4Zn2, m/z Calc (found) 1093.27 (1093.47). Elemental
Analysis: Chemical Formula, C68H64N4O10Zn2. Calc (found) C, 66.51
(66.37), H, 5.25 (5.67), N, 4.56 (4.83) %.
[Zn2L1RR(OAc)2] (3′). Similar procedure adopted above for
[(Zn2L1rac(OAc)2)] (3) and using R-BNDA instead of rac-BNDA.
Yield 85%. UV−vis [(λ, nm) (ε/M−1)]: 245 (8549), 265 (6741), 290
(8343), 411 (6939) nm. FT-IR (KBr): ν = 3387, 3060, 2923, 2368,
1623, 1528, 1388, 1329, 1208, 1061, 1002, 872, 820, 748, 676, 538,
1
480 cm−1. H NMR (200 MHz, CD3OD, ppm) δ = 8.651 (s, 4H,
−CHN), 7.916−7.783 (m, 8H, −CH), 7.459−7.387 (m, 4H,
−CH), 7.333−7.146 (m, 12H, −CH), 6.823−6.780 (m, 4H, −CH),
1.998 (s, 6H, −CH3) 1.661 (s, 6H, −CH3). 13C NMR (125 MHz,
DMSO-d6, ppm) δ = 174.936, 169.169, 165.636, 144.741, 142.637,
133.420, 131.707, 130.032, 128.605, 128.296, 128.207, 128.019,
127.094, 126.031, 125.772, 125.328, 124.486, 123.406, 123.166,
122.546, 121.048, 118.560, 110.491, 22.321, 19.326. [ESI-MS]+:
Chemical formula for [M.OAc]+, C60H41N4O4Zn2, m/z Calc (found)
1011.17 (1011.69). Elemental Analysis: Chemical Formula,
C62H52N4O10Zn2. Calc (found) C, 65.10 (65.81), H, 4.58 (4.41),
N, 4.90 (4.88) %.
[Zn2L2SSCl2] (4″.Cl2). Similar procedure adopted above except S-
BNDA and zinc chloride instead of rac-BNDA and zinc acetate. Yield
83%. UV−vis [(λ, nm) (ε/M−1)]: 247 (8321), 266 (7642), 288
(7341), 404 (3625) nm. FT-IR (KBr): ν = 3430, 3056, 2918, 2365,
1625, 1545, 1386, 1322, 1204, 1061, 992, 815, 744, 665, 552, 482
cm−1. 1H NMR (600 MHz, DMSO-d6, ppm): δ = 8.855(s, 4H,
−CHN), 8.176−8.162 (d, J = 9 Hz, 4H, −CH), 8.043−8.029 (d, J
[Zn2L1SS(OAc)2] (3″). Similar procedure adopted above for
[(Zn2L1rac(OAc)2)] (3) and using S-BNDA instead of rac-BNDA.
Yield 83%. UV−vis [(λ, nm) (ε/M−1)]: 243 (7372), 269 (6814), 286
(6392), 412 (4119) nm. FT-IR (KBr): ν = 3429, 3060, 2923, 2369,
L
DOI: 10.1021/acs.inorgchem.8b03643
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
= 8 Hz, 4H, −CH), 7.652 (s, 4H, −CH), 7.487−7.462 (t, J = 8 Hz,
4H, −CH), 7.395−7.381 (d, J = 8 Hz, 4H, −CH), 7.274−7.249 (t, J =
8 Hz, 4H, −CH), 6.676−6.662 (d, J = 9 Hz, 4H, −CH) 2.160 (s, 6H,
−CH3). 13C NMR (125 MHz, DMSO-d6, ppm) δ = 170.604,
144.556, 144.101, 139.523, 138.656, 133.231, 131.665, 130.012,
128.544, 128.222, 128.102, 127.978, 127.565, 126.415, 125.333,
125.103, 123.458, 123.159, 122.656, 121.858, 118.333, 110.942,
22.356. [ESI-MS]+ : Chemical formula for [M.Cl]+ ,
C58H38ClN4O2Zn2, m/z Calc (found) 987.12 (987.43). Elemental
Analysis: Chemical Formula, C58H42Cl2N4O4Zn2. Calc (found) C,
65.68 (65.89), H, 3.99 (3.54), N, 5.28 (5.62) %.
General Procedure for Desymmetrization of Mesodiols. A
dry and nitrogen flushed 5 mL flask, equipped with a magnetic stirring
bar, was charged with zinc(II) catalyst (2 mol %) and freshly distilled
dry DCM (2 mL). Corresponding diol (0.2 mmol), and 2 equiv N,N-
diisopropylethylamine (DIPEA) (0.4 mmol) were then added
successively, and the resulting mixture was stirred up to 10−15 min
and benzoyl chloride (0.2 mmol) was added slowly by syringe at 0 °C.
The mixture was stirred at the same temperature until completion of
the reaction. The reaction was monitored by TLC. After completion
of the reaction, the solvent was evaporated by rotavapor and washed
with hexane/EtOAc (90/10%) to separate catalyst from the mixture
(8 times). Finally, the crude product was purified by column
chromatography (silica gel: 100−200 mesh using ethyl acetate and
hexanes) to give corresponding monoester product. Enantiomeric
excess was determined by UFLC using Phenomenox Lux cellulose-1
and Amylose-2 column using isopropanol and hexane as eluting agent.
The absolute configurations of the products were assigned by
comparison HPLC profile with the reported literature.36
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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.inorg-
chem.8b03643.
■
S
Characterization of all the complexes such as ESI-MS,
NMR, UV−vis, FT-IR, crystal data; and characterization
of catalytic products (PDF)
Accession Codes
CCDC 1874548−1874549 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.
AUTHOR INFORMATION
Corresponding Author
*E-mail: siva@csmcri.res.in or siva140@yahoo.co.in.
ORCID
Eswaran Chinnaraja: 0000-0003-0617-2316
Eringathodi Suresh: 0000-0002-1934-6832
Ramalingam Natarajan: 0000-0002-8281-7158
Palani S. Subramanian: 0000-0002-9035-0973
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
CSMCRI communication number CSIR-CSMCRI 181/2018.
Author EC and RA acknowledge the CSIR for SRF grants. The
authors gratefully acknowledge DST-SERB New Delhi
(Project No. SR/S1/IC-23/2011) for funding. Members of
ADCIF of CSMCRI are acknowledged for their instrumenta-
tion support.
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Almena, J.; Kadyrov, R.; Borner, A. A Highly Tunable Family of
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DOI: 10.1021/acs.inorgchem.8b03643
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P21/n, a = 12.4323(12) Å, b = 24.785(3) Å, c = 19.267(2) Å, V =
5873.0(10) Å3, Z = 4, T = 150.0(2) K, μ(Mo Kα) (λ = 0.71073 Å)
mm−1, Dcalc = 1.279 g/cm3, 13258 reflections measured (1.35° ≤ 2Θ
≤ 27.59°), 34643 unique (Rint = 0.0446; Rσ = 0.0651) which were
used in all calculations. The final R1 was 0.0764 [I > 2σ(I)] and wR2
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(35) Complex 3″: CCDC 1874549. Crystal data for
C68H53Cl2N9O2Zn2 (M = 1229.83 g/mol): monoclinic, space group
C2, a = 21.467(2) Å, b = 27.811(3) Å, c = 10.7501(10) Å, V =
6418.2(10) Å3, Z = 4, T = 100.0 K, μ(Mo Kα) (λ = 0.71073 Å)
mm−1, Dcalc = 1.273 g/cm3, 46266 reflections measured (3.794° ≤ 2Θ
≤ 55.758°), 14702 unique (Rint = 0.1119; Rσ = 0.1561) which were
used in all calculations. The final R1 was 0.0935 [I ≥ 2σ(I)] and wR2
was 0.2756 (all data), Flack parameter = 0.03(2).
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DOI: 10.1021/acs.inorgchem.8b03643
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