CuCl/bpy-promoted unusual Z-stereoselective synthesis of trichloroacetic acid phenyl ester for Hirshfeld surface analysis and DFT study

Article abstract of DOI:10.1016/j.molstruc.2019.02.018

A trichloromethyl ester devoid of suitably substituted C[dbnd]C bond, any leaving group or a H-atom at the β-position to the radical, which are otherwise known for ATRA/ATRC, 1,2-rearrangement/fragmentation or simple 1,2-H shift respectively, in a reaction of trichloro-acetic acid phenyl ester with 2 mol equiv CuCl/bpy in reflux DCE or benzene under inert N₂ condition resulted to stereoselective synthesis of Z-2,3-dichloro-but-2-enedioic acid diphenyl ester and/or formation of reductive de-chlorination side product. The Z-stereochemistry of the product was confirmed by X-ray diffraction spectroscopy of Z-2,3-dichloro-but-2-enedioic acid dinaphthalen-1-yl ester which crystalizes in monoclinic system of P21/c symmetry elements. Hirshfeld surface analysis of experimentally established structure of Z-2,3-dichloro-but-2-enedioic acid diphenyl ester unrevealed the intermolecular interactions showing both H-bonding and short contacts. The stereochemistry was also interpreted computationally using Density Functional Theory at B3LYP/6-311G(d,p) level of theory. The calculated energies, energy gap and balance between the electrostatic potential on the molecular surface were computed which revealed unusual formation of less stable Z-isomer.

Full text of DOI:10.1016/j.molstruc.2019.02.018

Accepted Manuscript
CuCl/bpy-promoted unusual Z-stereoselective synthesis of trichloroacetic acid phenyl
ester for Hirshfeld surface analysis and DFT study
Ram Kumar Tittal, Ram Nath Ram, Ayushi Nirwan, Vikas D. Ghule, Satish Kumar
PII:
S0022-2860(19)30142-5
DOI:
https://doi.org/10.1016/j.molstruc.2019.02.018
MOLSTR 26175
Reference:
To appear in: Journal of Molecular Structure
Received Date: 31 December 2018
Revised Date: 2 February 2019
Accepted Date: 5 February 2019
Please cite this article as: R.K. Tittal, R.N. Ram, A. Nirwan, V.D. Ghule, S. Kumar, CuCl/bpy-
promoted unusual Z-stereoselective synthesis of trichloroacetic acid phenyl ester for Hirshfeld
surface analysis and DFT study, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/
j.molstruc.2019.02.018.
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ACCEPTED MANUSCRIPT
CuCl/bpy-Promoted Unusual Z-stereoselective Synthesis of Trichloroacetic
Acid Phenyl Ester for Hirshfeld Surface Analysis and DFT Study
Ram Kumar Tittal,a,b,* Ram Nath Ram,b Ayushi Nirwan,a Vikas D. Ghule,a
aDepartment of Chemistry, National Institute of Technology Kurukshetra Haryana-136119, India
bDepartment of Chemistry, Indian Institute of Technology Delhi, Hauz Khas New Delhi-110016, India
(e-mail: rktittaliitd@mail.nitkkr.ac.in)
A simple trichloromethyl ester underwent stereoselective synthesis of Z-2,3-dichloro-but-2-enedioic
acid diphenyl ester with 2 mol equiv CuCl/bpy in reflux benzene under inert N2 condition. The Z-
stereochemistry of the product was confirmed by X-ray diffraction spectroscopy of Z-2,3-dichloro-but-
2-enedioic acid dinaphthalen-1-yl ester which selectively formed from one of the two positional isomer
of trichloro-acetic acid naphthalen-1- and 2-yl ester. The stability and stereochemistry was interpreted
by Hirshfeld surface analysis and computationally using Density Functional Theory at B3LYP/6-
311G(d,p) level of theory. The study revealed that the thermodynamically stable E-isomer is
predominated by kinetically controlled Z-isomer.

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CuCl/bpy-Promoted Unusual Z-stereoselective Synthesis of Trichloroacetic
Acid Phenyl Ester for Hirshfeld Surface Analysis and DFT Study
Ram Kumar Tittal,a,b,* Ram Nath Ram,b Ayushi Nirwan,a Vikas D. Ghule,a Satish Kumarc
aDepartment of Chemistry, National Institute of Technology Kurukshetra Haryana-136119, India
bDepartment of Chemistry, Indian Institute of Technology Delhi, Hauz Khas New Delhi-110016, India
cDepartment of Chemistry, St. Stephen's College, University Enclave, Delhi, 110007
(e-mail: rktittaliitd@mail.nitkkr.ac.in)
Abstract: A trichloromethyl ester devoid of suitably substituted C=C bond, any leaving group or a H-
atom at the β-position to the radical, which are otherwise known for ATRA/ATRC, 1,2-rearrangement/
fragmentation or simple 1,2-H shift respectively, in a reaction of trichloro-acetic acid phenyl ester with
2 mol equiv CuCl/bpy in reflux DCE or benzene under inert N2 condition resulted to stereoselective
synthesis of Z-2,3-dichloro-but-2-enedioic acid diphenyl ester and/or formation of reductive de-
chlorination side product. The Z-stereochemistry of the product was confirmed by X-ray diffraction
spectroscopy of Z-2,3-dichloro-but-2-enedioic acid dinaphthalen-1-yl ester which crystalizes in
monoclinic system of P21/c symmetry elements. Hirshfeld surface analysis of experimentally
established structure of Z-2,3-dichloro-but-2-enedioic acid diphenyl ester unrevealed the
intermolecular interactions showing both H-bonding and short contacts. The stereochemistry was also
interpreted computationally using Density Functional Theory at B3LYP/6-311G(d,p) level of theory.
The calculated energies, energy gap and balance between the electrostatic potential on the molecular
surface were computed which revealed unusual formation of less stable Z-isomer.
Key Words: 1. CuCl/bpy, 2. Z-Stereoselective, 3. Z-Diphenyl 2,3-dichloromaleate, 4. DFT study, 5.
Hirshfeld Surface Analysis.
Introduction: The trichloromethyl group containing compounds are classically known for the
free radical reactions under transition metal salt and their salts complexed with nitrogen
containing ligands. The trichloromethyl group containing compounds easily form free radicals
by homolysis of an activated C-Cl bond with the help of UV-light, radical initiators, redox
active metal salts or its complexes.1 The study and behaviour of dichloromethyl radical
variously substituted at the β-position generated under redox condition of Cu(I) salts and its
complexes with nitrogen based ligands were extensively studied by Ram et al (Scheme 1).
The results revealed that generated radicals normally undergo intermolecular addition and/or
intramolecular cyclization (Path 1) on to suitably substituted carbon-carbon double bond,
which may be free or hindered by bulky groups.2-5 However, radicals substituted by weak or
strong nucleofugal groups at the β-position underwent predominantly rearrangement (Path
2) and/or fragmentation (Path 3) through the intermediate formation of contact ion pair (CIP)
which was extensively explored and established by inter or intramolecular tandem
nucleophilic attack followed by radical cyclization experiments.6-7 In two extreme cases the
β-position of radical substituted by hydrogen and oxy group underwent 1,2-H shift via
intermediate formation of a copper-carbenoid species and Z-stereoselective radical
dimerization to ene-1,4-diones (Path 4).8-9 Based on these observations it was thought to
check the behaviour of trichloromethyl esters under this reaction condition (Path 5).
Scheme 1 Study of dichloromethyl radical under redox condition of Cu(I)

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Stereoselective-alkenes are highly appraised functional groups used in chemical
transformations for the development of pharmaceutical products and essential building
blocks of biomolecules.10 The stereo-chemical outcome of any alkene reaction decide the
fate of the products. Both Z & E isomers of alkene possesses unique property, hence stereo
defined alkene are much demanding. The survey of the literature revealed that Z-alkenes
are important starting molecules for the synthesis of biologically active molecules and are
usually found in natural products, medicinal values such as (Z)-tamoxifen, (Z)-vinylic
sulfides, etc.11 However, the counter E-isomer has limited use in medicine. Also, the trans-
fat in food possesses E-alkenes which are considered harmful to the human health but not
the counter Z-isomer.12 As long as the stability of alkene is considered, the formation of E-
alkene is relatively easy, and thus in majority of cases, the synthesis of E-alkene
predominates over Z-alkene owe to its thermodynamic stability. Thus, the synthesis of
stereoselective and/or stereo defined alkenes remains a thrust area of research in organic
synthesis. Recently Ram and Tittal have reported synthesis of Z-stereoselective ene-1,4-
diones and the synthesis of Z & E-trisubstituted alkenes.13,3 The formation of counter E-
isomer was either present in very less amount which cannot be ascertained or might have
transformed to Z-form. This observation prompted the author to further investigate the
predominance of the stereochemistry of these alkene with the help of more reliable
computational method using DFT calculations. Herewith author is presenting the
stereochemistry of E- and Z-ene-1,4-diones.
Results and Discussion: The starting material trichloro-acetic acid naphthalen-1-yl ester 1a
and its positional isomer trichloro-acetic acid naphthalen-2-yl ester 1b were easily prepared
by the trichloro acetylation of commercially available 1- and 2-naphthol with trichloroacetyl
chloride using triethylamine as base in dry diethylether by following reported procedure with
little modofications.14 The reaction of starting material 1a and 1b was performed with
CuCl/bpy in 1:1 molar ratio in dry benzene under inert nitrogen atmosphere. The reaction of
1a completed in 2h with the consumption of 2 equivalents of copper complex CuCl/bpy. The
reaction mixture was worked up as per the usual procedures adopted until earlier.3,5,13 The
characterization of the final major product revealed it to be a Z-2,3-dichloro-but-2-enedioic
1
acid dinaphthyl ester 2a, which was successfully characterized by IR, H-NMR, 13C-NMR
and HRMS and finally supported by the X-ray crystallographic data. The formation of Z-2,3-
dichloro-but-2-enedioic acid dinaphthyl ester Z-2a was thought to be by the stereoselective
radical dimerization of initially formed dichloromethyl radical from trichloro-acetic acid
naphthalen-1-yl ester 1 with the help of CuCl/bpy along with the formation of small amount of
monochloro reduction product as shown in Scheme 2. The survey of literature revealed that
no other similar Z-2,3- dichloro-but-2-enedioic acid dinaphthyl ester Z-2a molecule and
dimerization reaction reported so far except our recent report on similar radical dimerization
of 2,2,2-trichloro-1-phenyl-ethanones to stereoselective Z-ene-1,4-diones.13
Scheme 2 Reaction of trichloro-acetic acid naphthalen-1-yl ester 1a and 1b with CuCl/bpy (1:1 mol ratio)

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The prolonged heating of 1b starting material underwent hydrolysis of trichloroester
completely along with the reduction of chlorine atom(s) to form corresponding 2-naphthol
along with the formation of chloro-acetic acid naphthalene-2-yl ester. The probable reason
for the selective radical dimerization with 1a may be due to the stabilization of dichloromethyl
radical of 1a. However, 2-naphthoxide is more stable than the dichloromethyl radical of 1b.
The unusual formation of Z-2,3-dichloro-but-2-enedioic acid dinaphthyl ester Z-2a selectively
from one of the two positional isomer of trichloro-acetic acid naphthalen-1-and 2-yl ester is
the unique feature of this study. The importance of this kind of alkene realizes in organic
synthesis like dimethyl maleate, maleic anhydride, and fumarates which may be used as the
electron-poor alkene ligands for the synthesis and characterization of zero valent platinum
complexes and also been used as catalysts in the hydrosilylation of styrene with
triethylsilane.15-16
The molecular structure of Z-2a crystalized in monoclinic system with (P21/c) symmetry
element and displayed a single molecule in the asymmetric unit. The ORTEP diagram of Z-
2a is given in Fig. 1. In an asymmetric unit the two aromatic rings are orthogonal to the
carbonyl groups displaying a torsion angle of 0.0(5)̊(O3-C14-O4-C15) & 9.0(5)̊ (O2-C11-O1-C1),
Fig. 1 The ORTEP diagram of Z-2.
where the carbonyl group was almost coplanar with the aromatic rings. This difference may
be attributed to the different type of carbonyl group present in the crystal structures.
However, the torsion angle displayed by the central atoms (Cl1-C12-C13-Cl4) was found to be
almost similar to the torsion angle observed in the reported crystal structures of (Z)-2,3-
dichloro-1,4-bis(4-chlorophenyl)but-2-ene-1,4-dione; Crystal structure of (Z)-2,3-dichloro-1,4-
bis(4-methoxyphenyl)but-2-ene-1,4-dione and other (Z)-ene-1,4-diones recently reported by
Ram and Tittal.13 The mean plane of the two naphthyl units displayed a torsion angle of
42.38(10)̊ and 14.16(8)̊ with respect to each other. Unlike the reported crystal structure of
the similar molecules by Tittal et al. and Ram & Tittal, no intermolecular hydrogen bonding of
C-H…O observed in the crystal packing diagram of the title compound. However, crystal
packing diagram reveals short contact (which is a sum of Van der Waals radii) at C-H…O
and Cl…Cl type intermolecular contacts and intramolecular hydrogen bonding between
C=O…O (Fig. 2 a) & b)). Also, short intermolecular π…π contacts were observed between

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Fig. 2 a) A crystal packing view of Z-2 along b axis showing short contacts C-H…O and Cl…Cl.; b) A crystal
packing view of Z-2 along b axis showing intramolecular H-bond at C=O…O.
the naphthyl units, Cg to Cg 3.702 Å (+x,-1 + y,+z) (Fig. 3) and 3.843 Å (+x,-1 + y,+z).
Similar π…π stacking of aromatic rings as well as C-H...O interactions were reported by
Subramanian et al wherein π-π stacking interactions between the aromatic rings and
imidazole rings and intermolecular π-π stacking interactions of aromatic groups observed.17
Fig. 3 A partial crystal packing view of the title compound showing π…π intermolecular contacts.
The data collection and structure refinement details are summarized in Table 1. All the
hydrogen atom connected to the aromatic carbon atom were placed at 0.93 Å and refined
using riding model with Uiso(H) = 1.2Ueq.
Table 1. Experimental and data collection details of crystal cell Z-2
Experimental details of Crystal data Z-2
Chemical formula
Mr
C24H14Cl2O4
437.25
Crystal system, space group
Temperature (K)
a, b, c (Å)
Monoclinic, P21/c
273
20.308 (8), 4.5337 (17), 22.738 (8)
β (°)
108.029 (6)
1990.8 (13)
4
V (Å3)
Z
Radiation type
Mo Kα
0.36
ꢀ (mm−1)
Crystal size (mm)
Data collection
Diffractometer
Absorption correction –
No. of measured, independent and
observed [I > 2σ(I)] reflections
Rint
0.32 × 0.12 × 0.11
CCD area detector
18321, 3709, 3489
0.037
0.606
0.071, 0.145, 1.25
(sin θ/λ)max (Å−1)
Refinement
R[F2 > 2σ(F2)], wR(F2), S
No. of reflections
No. of parameters
H-atom treatment
∆ρmax, ∆ρmin (e Å−3)
3709
271
H-atom parameters constrained
0.43, −0.18
Hirshfeld surface analysis: The intermolecular interactions in the title compounds were
analysed using Hirshfeld surface analysis.18 The asymmetric unit present in the crystal
structure of Z-2 was used to obtain the Hirshfeld surface. The Hirshfeld surface mapped

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using dnorm indicated the presence of a short contact by displaying intense red color spots as
shown in Fig. 4 a). Further analysis of the Hirshfeld surface indicated that the dark red stops
are due to the presence of C-H…O (Fig. 4 b)) and Cl…Cl (Fig. 4 c)) contacts. The 2-D
fingerprint plots of Z-2 were generated using this asymmetric unit. The fingerprint plot
produced a quantitative picture of intermolecular interactions of compound Z-2 are listed in
Table 2 and represented in Fig. 5. The 2-D fingerprint plots indicated that the C-H…π (H…H
Fig. 4 a) The Hirshfeld surface of the title compound mapped with dnorm.; b) The Hirshfeld surface showing C-
H…O interaction.; c) The Hirshfeld surface showing C-H…O and Cl…Cl interactions.
and C…H) interactions provide highest contribution (53.4%) to the total Hirshfeld surface.
The Cl…Cl interactions contribute 6.1% to the total Hirshfeld surface, while the Cl…H and
Cl…C interactions contribute 10.1% and 4.3%, respectively to the total Hirshfeld surface,
which indicated that the halogen interaction also plays an important role in stabilization of the
crystal packing. The intermolecular O…H interactions contribute 12.5% to the total Hirshfeld
surface. The C…C (π…π) interactions contribute 7.6% to the total Hirshfeld surface. The
other intermolecular interactions are listed in the Table 2.

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Fig. 5 (a-k) The two-dimensional fingerprint plots showing (a) complete Hirshfeld surface (b) H···H interactions
(c) C···H interactions (d) C···C interactions (e) O···O interactions (f) C···O interactions (g) O···H interactions (h)
Cl···H interactions (i) Cl···C interactions (j) Cl···O interactions (k) Cl···Cl interactions.
Table 2. The Percent contributions of the different interatomic contacts to the total Hirshfeld surfaces of Z-2.
Contact
H…H
C…H
C…C
O…O
C…O
O…H
Cl…H
Cl…C
Cl…O
Cl…Cl
(%) Contribution
35.2
18.2
7.6
1.6
2.5
12.5
10.1
4.3
2.0
6.1
To investigate the predominance of Z-ene-1,4-dione obtained in experimental results over E-
counterpart under our reaction conditions, we thought to calculate their energies, band gap,
and electrostatic potential balance on the molecular surface. The study was conducted on E
and Z-isomers of 2,3-dichloro-but-2-enedioic acid dinaphthalen-1-yl ester 1 and 2,3-Dichloro-
but-2-enedioic acid dinaphthalen-2-yl ester 2. With the help of density functional theory
(DFT) calculations using the Gaussian 09 program,19 the configurations of the structures Z-1,
Z-2, E-1 and E-2 were optimized by using B3LYP/6-311G(d,p) level of theory. The optimized
structures of these E-and Z- isomers are shown in Fig. 6.
Fig 6. (a-d) B3LYP/6-311G(d,p) optimized structures of Z-1, Z-2, E-1 and E-2.
The optimized DFT calculations on the configurations of these isomers showed surprising
result and reveal that the E-isomer is slightly stable than the Z-isomer with the relative
energy of 0.54 kcal/mol as shown in Table 3. We have used the energy gap between HOMO
and LUMO to assess the stability of the Z and E isomers.20 In general, the narrower energy
gap allows, the easier electron transition and reduces the molecular stability. The predicted
energy gap values for Z-1 and E-1 are 3.57 eV and 4.16 eV, respectively representing E
isomer more stable. The contour surfaces of the frontier molecular orbitals are shown in
Figures 2 and 3. In Z-1 isomer, the HOMO is delocalized over the whole (O)ClC=CCl(O)
group. By contrast, the LUMO is located over the aromatic ring only. A similar trend
observed in the case of E-1 isomer also.
Table 3. Calculated energies, HOMO and LUMO for Z-1 and E-1 isomers.
S. No.
Molecule
Z-1
Energy (a.u.)
HOMO (a.u.)
LUMO (a.u.)
1.
-0.09830
-2144.262897
-0.22969

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2.
E-1
-2144.263755
-0.23179
-0.07894
Fig 7. a: Optimized structure of Z-1, b: HOMO Z-1 and c: LUMO Z-1.
Fig 8. a: Optimized structure of E-1, b: HOMO E-1 and c: LUMO E-1.
Politzer et al. correlated the charge imbalance with a sensitivity of energetic materials.21 The
imbalance in the charge distribution on molecular surface can be linked with the molecular
stability. The v is the effective balance between positive and negative surface potentials
which may reach a maximum value of 0.250, when ꢀꢁꢂ=ꢀꢃꢂ. Where, ꢀꢁꢂ & ꢀꢃꢂ indicates the
variabilities of both positive and negative surface potential on molecular surface by following
equation,
ꢀꢁꢂꢀꢃꢂ
ꢄ =
(ꢀꢁꢂ + ꢀꢃꢂ)ꢂ
The imbalance in surface potential, v as shown in Table 4 has been correlated with the
stability of the molecule. The significant variation from 0.250 values represents the
decreased stability. The calculated values of v for Z- and E-1 isomers are 0.2020 and
0.2116, reveal that Z isomer has higher imbalance and thus lower molecular stability. The Z
and E-1 isomers contain the naphthalene rings, chlorine and ester groups, responsible for
electron withdrawing and create the imbalance in surface potentials. In both the isomers, we
observed that the negative regions are stronger than the positive regions (ꢀꢃꢂ > ꢀꢁꢂ). Figure 9
represents the calculated electrostatic potentials on the 0.001 electron/bohr3 molecular
surface of Z and E-1 isomers.
Table 4. Calculated energies, HOMO and LUMO for Z- and E-1 isomers.
ꢅꢁꢆ
ꢅꢆꢃ
v
S. No.
Molecule

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(kcal/mol)2
(kcal/mol)2
1
2
Z-1
E-1
25.66
65.68
78.51
0.2020
0.2116
34.30
The structural parameters of Z-1 isomer negative than 11. The structural parameters of Z-1
isomer obtained at the B3LYP/6-311G(d,p) level of theory gave good agreement with
experimental data of Z-1 as shown in Table 5. However, experimental results supported by
X-ray crystallography data of Z-1 and the same category of radical dimerization of
trichloromethyl ketones under similar condition to Z-ene-1,4-diones formation reported very
Experimental data of Z-1
B3LYP/6-311G(d,p) data of Z-1
S. No.
Description
C9-O11
Bond length (Å)
1.421
Description
C9-O11
Bond length (Å)
1
1.393 (2.0)
1.361 (1.6)
1.202 (1.6)
1.496 (0.4)
1.740 (1.2)
1.346 (1.7)
1.732 (0.9)
1.525 (1.3)
1.192 (1.1)
1.355 (1.4)
1.400 (1.5)
Bond angle (˚)
123.17 (4.7)
119.93 (4.7)
118.69 (6.3)
121.25 (1.6)
119.50 (1.7)
121.14 (2.9)
121.87 (0.8)
111.19 (1.3)
119.75 (3.6)
120.92 (4.0)
123.42 (5.0)
2
3
4
5
6
7
8
9
O11-C12
C12-O13
C12-C14
C14-Cl16
C14-C15
C15-Cl17
C15-C18
C18-O22
1.339
1.182
1.490
1.718
1.323
1.715
1.504
1.179
O11-C12
C12-O13
C12-C14
C14-Cl16
C14-C15
C15-Cl17
C15-C18
C18-O22
10
11
C18-O19
C23-O19
1.336
1.422
C18-O19
C23-O19
S. No.
Description
C9-O11-C12
O11-C12-O13
O11-C12-C14
O13-C12-C14
O12-C14-Cl16
Cl16-C14-C15
C14-C15-Cl17
Cl17-C15-C18
C15-C18-C20
O20-C18-O19
C18-O19-C23
Bond angle (˚)
117.29
125.64
111.16
123.20
117.40
124.74
122.94
112.66
124.10
125.84
117.13
Description
C9-O11-C12
O11-C12-O13
O11-C12-C14
O13-C12-C14
O12-C14-Cl16
Cl16-C14-C15
C14-C15-Cl17
Cl17-C15-C18
C15-C18-C20
O20-C18-O19
C18-O19-C23
1
2
3
4
5
6
7
8
9
10
11
Fig 9. Calculated electrostatic potentials on the 0.001 electron/bohr3 molecular surface of Z-2a and E-2a. White
circles denote the positions of atoms in a molecule. Color range (in kcal/mol): red greater than 13; yellow within
13 and 12; green within 12 and 11; blue more negative than 11.

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recently supported the Z-steroselective formation of ene-1,4-diones. The optimization of Z-
ene-1,4-diones by DFT calculations at the B3LYP/6-311G(d,p) level of theory is under
progress which would be communicated with full results.
Table 5. Comparison of experimental and theoretical structural parameters of Z-1.
Values in parenthesis shows % deviation relative to experimental results.
After ramrod study and the positive results for the stereoselective formation of 2,3-dichloro-
but-2-enedioic acid dinaphthyl ester Z-2a from trichloro-acetic acid naphthalen-1-yl ester 1a
under CuCl/bpy mediated reaction condition prompted the authors to explore their formation
from other derivatives to generalize the process and check the tolerance of other functional
groups under this reaction condition. Thus, other trichloroacetic acid aryl ester 1 (Scheme 3)
were prepared by the simple trichloroacetylation of the corresponding phenols by following
O
CCl3COCl (1.2 equiv)
Ar
Ar OH
O
CCl3
Et3N (1.5 equiv)
dry ether/ 0-5 C
20-60 min
°
1a-h
a: 1-C10H7; b: 2-C10H7; c: C6H5; d: O-CH3COC6H4; e: P-CH3COC6H4;
f: P-Cl-C6H4; g: P-Me-C6H4; h: P-MeO-C6H4
Scheme 3. Synthesis of aryl trichloroacetates (1a-h).
reported procedure as discussed earlier.14 In the same way, other trichloroesters 1b-h were
1
13
prepared, and all of them showed the satisfactory IR, H-NMR and C-NMR spectra. It has
been observed that these aryltrichloroesters were very unstable and were prone to
hydrolysis, especially O-acetyl and P-acetyl aryltrichloroesters 1d-e which readily hydrolysed
to corresponding phenols. The so formed aryl trichloroacetates 1a-h, when allowed to react
under the non-reducing condition of CuCl/bpy (1 mol equiv) in refluxing DCE under inert N2
atmosphere, the reaction did not complete even after prolonged reflux time of 12 hrs.
However, when one more equivalent of CuCl/bpy was added to the reaction mixture, it
completed in an hr. Thus, the completion of reaction required two equivalent of CuCl-
complex. The study of reaction products showed it did not give any decarboxylation,
rearrangement or fragmentation products rather the reaction of 1a gave Z-alkene 2a
(Scheme 4), in 50 and 60% isolated yield in dry DCE and benzene solvents along with the
formation of 35% and 20% of mono-reduction product 3a respectively. Other relevent
information are listed in Table 1. However, the reaction of 1b as discussed earlier did not
give the formation of any alkene dimer via radical dimerization. However, the reaction
product after purification showed the formation of di-reduction product 4 in 30 and 40%
isolated yield in DCE and benzene, respectively. The formation of Z-alkene 2c in 60 and
65% isolated yield was found in dry DCE and benzene, respectively in case of trichloroacetic
acid phenyl ester 1c but the corresponding reduction product was not isolated. These results
showed that the benzene is little better choice of solvent for supporting the formation of Z-
alkene and suppressing the formation of reduction products. The reaction of 1d-e did not
resulted to the radical dimerization to form corresponding Z-alkene, rather hydrolysed readily
to the corresponding phenol. The completion of reaction requires two equivalent of CuCl/bpy
1
13
(1:1 mol ratio each). The products were successfully characterized by IR, H-NMR and C-
NMR spectra and the stereochemistry was supported by X-ray crystallographic data.22
Ar
O O
Ar
O
CuCl/bpy (2 equiv)
(1:1 mol ratio)
O
O
O
O
Ar
+
+
O
CCl3
+
Ar
Ar
Ar OH
O
CHCl2
O
CH2Cl
Cl
Cl
1
3
2
4
a: 1-C10H7; b: 2-C10H7; c:C6H5; d: O-CH3CO-C6H4; e: P-CH3CO-C6H4; f: P-Cl-C6H4; g: P-Me-C6H4; h: P-MeO-C6H4

ACCEPTED MANUSCRIPT
S. No.
1a-h
a, 1-C10H7
a, 1-C10H7
b, 2-C10H7
b, 2-C10H7
c, C6H5
c, C6H5
d, 2-CH3CO-C6H4
d, 2-CH3CO-C6H4
e, 4-CH3CO-C6H4
e, 4-CH3CO-C6H4
f, 4-Cl-C6H4
Solvent
Time (h)
Yield(%)b
2
Phenol
3
35
20
-
-
-
4
-
-
30
40
-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
DCE
benzene
DCE
benzene
DCE
benzene
DCE
benzene
DCE
benzene
DCE
benzene
DCE
benzene
DCE
benzene
1.5
2
1
2
1.5
2
1
1
1
1
1.5
2
1
2
1.5
2
5
-
50
60
c
50
45
15
22
70
64
78
72
12
10
05
-
c
60
65
-
-
d
-
-
d
d
d
-
-
-
-
-
-
55
60
50
62
65
72
20
15
30
20
10
20
-
-
-
-
-
-
f, 4-Cl-C6H4
g, 4-Me-C6H4
g, 4-Me-C6H4
h, 4-MeO-C6H4
h, 4-MeO-C6H4
07
-
aThe reaction performed with 0.003 mol of 1 & 0.006 mol of CuCl/bpy in 20 mL DCE or benzene under inert N2 atm.
bYield(%) refer to purified products after column chromatography.
cThe dimer product was not obtained.
dStarting material is very unstable and hydrolysed readily to corresponding phenol.
Scheme 4 Reaction of aryl trichloroacetates 1a-h with 2 equiv. CuCl/bpy (1:1 mol ratio)
.
Table 6. Reaction of aryltrichloroacetates 1a-h with 2 equiv CuCl/bpy (1:1 mol ratio)a
In view of earlier observations attained while monitoring the progress of the reaction of 2,2,2-
trichloro-1-phenyl-ethanone with 2 mol equiv each of CuCl and bpy in refluxing DCE under a
N2 atm and progress of reaction of trichloro-acetic acid phenyl ester under similar reaction
condition, both underwent dimerization to Z-ene-1,4-dione and Z-2,3-dichloro-but-2- enedioic
acid diphenyl ester, respectively along with formation of small amount of reductive
dechlorination products.9 On the basis of these observations following mechanism (Scheme
5) may be proposed to support the stereoselective radical dimerization under the same
reaction condition.
Cl Cl
O
O
O
CCl3
X
Y
O
CCl2
O
O
R
R
R
R
O
O
R
O R
O
Cl Cl
O
CCl2
O
O
CCl2
X: CuCl/bpy; Y: -CuCl2/bpy
X Y
a: 1-C10H7; b: 2-C10H7; c:C6H5; d: O-CH3CO-C6H4;
e: P-CH3CO-C6H4; f: P-Cl-C6H4; g: P-Me-C6H4; h:
P-MeO-C6H4
Cl
R
O O
R
O
O
O
R
R
O
O
Cl
Cl
Cl
E
Z
Scheme 5. Proposed mechanism for stereoselective radical dimerization of aryl trichloroacetates.
In conclusions, the reaction of trichloro-acetic acid phenyl ester which is a one oxygen
homolog of trichloromethyl ketone acquired similar Z-stereoselective radical dimerization
with two mol equiv of CuCl/bpy in dry DCE or benzene to Z-2,3-dichloro-but-2-enedioic acid
diphenyl ester. However, the information for the formation of counter E-isomer was not
found, if formed may readily converted to Z-isomer. The observations revealed that the
thermodynamically stable E-isomer is predominated by kinetically controlled Z-isomer. The
products were successfully characterized by IR, 1H-NMR, 13C-NMR, and finally supported by
some HRMS data. However, the stereochemistry supported by the X-ray diffraction

ACCEPTED MANUSCRIPT
spectroscopy of Z-2,3-dichloro-but-2- enedioic acid diphenyl ester 2a. The unusual formation
of Z-2,3-dichloro-but-2-enedioic acid dinaphthyl ester Z-2a selectively from one of the two
positional isomer of trichloro-acetic acid naphthalen-1-and 2-yl ester is the unique feature of
this study. The electron-poor alkene like Z-2a may be used for the development of Pt(0)-
complexes of the type [Pt(η2-alkene)(η2-norbornene)2] by the reaction with [Pt(η2-
norbornene)3]. The complex reported for the hydrosilylation reaction of styrene by
triethylsilane.15 Due to extended electronic conjugation, Z-2a can find applications in electro-
optic devices, fluorescent probes, and dyes.
Experimental Section:
General Method: In a two-neck round bottom flask fitted with a rubber septum and
connected to a Schlenk tube through a condenser under inert N2 atm charged with CuCl
(0.60 g, 0.006 mol), 2,2'-bipyridine (0.94 g, 0.006 mol), and a magnetic bar under continuous
flow of nitrogen. Then, 8 mL benzene was injected into the flask and the heterogeneous
mixture was heated at 50 °C with stirring for 30 mi nutes to ensure the formation of the brown
Cu-bpy complex. Then the Cu-complex was cooled to room temperature and a solution of
the ester 1 (0.003 mol) in benzene (7 mL) was injected through the septum. The flask again
evacuated and filled with nitrogen after cooling it in an ice bath. The ice bath removed and
the reaction mixture heated at reflux with stirring under a slow and continuous flow of
nitrogen. The progress of the reaction monitored by TLC and the initial brown color of the
reaction mixture faded as the reaction progressed. After the completion of the reaction in 1-2
h as indicated by TLC, the greenish reaction mixture cooled and filtered through a celite pad.
The insoluble green solid on the celite pad was washed thoroughly with benzene and the
light brown filtrate then evaporated under reduced pressure on a rotary evaporator. The
residual mass subjected to column chromatography using silica gel as the solid support and
n-hexane and its mixtures with chloroform as the solvent system for elution. The Z-alkene
(Z-2a,c,f-h) eluted first followed by the reduced esters 3 and 5 were obtained afterwards as
a colourless thick liquid by elution with n-hexane-chloroform (95:5 v/v) mixture. In the last
hydrolysed phenolic starting materials or their polymeric material recovered.
Naphthalen-1-yl trichloroacetate (1a): Colorless solid, mp 74 °C. 75 % yield. Gives
1
positive Beilstein test for chlorine. H-NMR (300 MHz, CDCl3): δ 8.03-8.00 (m, 1H), 7.93-
7.90 (m, 1H), 7.83 (d, J = 8.2 Hz, 1H), 7.60-7.49 (m, 3H), 7.41 (d, J = 7.5 Hz, 1H) ppm; IR
(KBr): νmax 1772(s), 1632(m), 1596(m), 1504(m), 1462(m), 1389(s), 1222(s), 1069(s),
1027(s), 956(s), 872(s), 834(s), 756(s), 670(s) cm-1.
Z-2,3-dichloro-but-2-enedioic acid dinaphthyl ester (2a): White light weight foamy solid,
mp 112 °C. 60 % yield. Gives positive Beilstein tes t for chlorine and KMnO4 test for free
double bond. 1H-NMR (300 MHz, CDCl3) δ 7.98 (d, J = 7.9 Hz, 1H), 7.88 (d, J = 7.6 Hz, 1H),
7.78 (d, J = 7.9 Hz, 1H), 7.54-7.39 (m, 3H), 7.33 (d, J = 7.4 Hz, 1H) ppm; 13C-NMR (75.5
MHz, CDCl3): δ 159.8 (C), 145.8 (C), 134.7 (C), 131.4 (C), 128.0 (CH), 126.9 (CH), 126.7
(CH), 126.3 (C), 125.3 (CH), 120.8 (CH), 117.8 (CH) ppm; IR (KBr): νmax 1756(s), 1731(s),
1633(m), 1593(s), 1507(m), 1391(s), 1257(s), 1157(s), 1080(s), 1036(s), 1011(s), 971(s),
813(m), 780(s) cm-1. HRMS [M+K]+: m/z cal. for C24H14Cl2O4K 474.9906, found 474.9928.
Acknowledgements
We thank IIT Delhi and NIT KKR for extending facilities for synthesis and characterization.
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Highlights of the study:
1) The unusual stereoselective formation of Z-2,3-dichloro-but-2-enedioic acid
diphenyl ester.
2) Thermodynamically stable E-isomer is predominated by kinetically controlled Z-
isomer.
1
3) The products were successfully characterized by IR, H-NMR, 13C-NMR, and
HRMS data.
4) The stereochemistry of Z-2a was supported by the X-ray crystallographic data of
Z-2,3-dichloro-but-2-enedioic acid dinaphthyl ester.
5) The stability of Z-2a was interpreted by Hirshfeld surface analysis and DFT at
B3LYP/6-311G(d,p) level of theory.
Thank You,
Yours truly,
Ram Kumar Tittal