Practical and theoretical aspects of Wacker oxidation of tolanophanes: Synthesis and characterization of novel diketonic cyclophanes

Article abstract of DOI:10.1002/aoc.3812

Novel cyclophanes having 1,2-diketones 2a–c were synthesized by Wacker oxidation of the corresponding tolanophanes 1a–c having various alkyl chain lengths (n?=?2–4). Among them, tolanophane 1a with the shortest alkyl chain length (n?=?2) having more strained alkyne shows the lowest product selectivity even lower than that of acyclic analogues at various reaction temperatures. In contrast, the oxidation of tolanophane 1b (n?=?3) is clean with the highest activity and selectivity of 2b. Theoretical calculations confirm the experimental data. Based on ab initio calculations, the critical step of the reaction pathway is the interaction of oxygen atoms of 1 (in the absence of H₂O) with Pd ion of intermediate [PdCl₃(1)]^? to give [PdCl₂(1)] which keeps Pd ion close to the alkyne bond. This step is not observed computationally for 1a because the positions of the oxygen atoms are outside of its central part. This is in good agreement with almost no activity of 1a at room temperature to prove its rigid structure preventing the O…Pd interaction. Nevertheless, the alkyne…Pd interaction of 1b is not detected by NMR measurements which may be due to its too slow interaction at room temperature (35% after 24?h); however, this confirms ab initio calculation data that the preferred coordinating site is oxygen in the reaction pathway. In contrast, cyclic voltammetry measurements distinguish a different behaviour for the palladium complexation of 1. In general, it is found that the stereochemistry of tolanophanes rather than distortion of alkyne bond plays the critical role in the oxidation. The products are new and their structures were characterized.

Full text of DOI:10.1002/aoc.3812

Received: 9 December 2016
DOI: 10.1002/aoc.3812
Revised: 21 February 2017
Accepted: 24 February 2017
C O M M U N I C A T I O N
Practical and theoretical aspects of Wacker oxidation of
tolanophanes: Synthesis and characterization of novel diketonic
cyclophanes
Hossein Reza Darabi
Rohoullah Firouzi
| Saeed Rastgar | Ehsan Khatamifar | Kioumars Aghapoor | Hani Sayahi |
Nano & Organic Synthesis Laboratory,
Chemistry & Chemical Engineering
Novel cyclophanes having 1,2‐diketones 2a–c were synthesized by Wacker
Research Center of Iran (CCERCI), Pajohesh
Blvd, 17th Km of Tehran‐Karaj Highway,
Tehran 1496813151, Iran
oxidation of the corresponding tolanophanes 1a–c having various alkyl chain
lengths (n = 2–4). Among them, tolanophane 1a with the shortest alkyl chain length
(
n = 2) having more strained alkyne shows the lowest product selectivity even lower
Correspondence
than that of acyclic analogues at various reaction temperatures. In contrast, the oxi-
dation of tolanophane 1b (n = 3) is clean with the highest activity and selectivity of
2b. Theoretical calculations confirm the experimental data. Based on ab initio cal-
Hossein Reza Darabi, Nano & Organic
Synthesis Laboratory, Chemistry &
Chemical Engineering Research Center of
Iran (CCERCI), Pajohesh Blvd, 17th Km of
Tehran‐Karaj Highway, Tehran 1496813151,
Iran.
culations, the critical step of the reaction pathway is the interaction of oxygen atoms
−
of 1 (in the absence of H O) with Pd ion of intermediate [PdCl (1)] to give
2
3
Email: darabi@ccerci.ac.ir;
r_darabi@yahoo.com
[PdCl (1)] which keeps Pd ion close to the alkyne bond. This step is not observed
2
computationally for 1a because the positions of the oxygen atoms are outside of
its central part. This is in good agreement with almost no activity of 1a at room
temperature to prove its rigid structure preventing the O…Pd interaction.
Nevertheless, the alkyne…Pd interaction of 1b is not detected by NMR
measurements which may be due to its too slow interaction at room temperature
(35% after 24 h); however, this confirms ab initio calculation data that the preferred
coordinating site is oxygen in the reaction pathway. In contrast, cyclic voltammetry
measurements distinguish a different behaviour for the palladium complexation of 1.
In general, it is found that the stereochemistry of tolanophanes rather than distortion
of alkyne bond plays the critical role in the oxidation. The products are new and
their structures were characterized.
KEYWORDS
complexation, tolanophanes, Wacker oxidation
1
| INTRODUCTION
Diphenylacetylenes (or tolans) are usually applied for
larger systems which display interesting structural, electronic,
[
13,14]
1,2‐Diketones are fascinating building blocks in numerous
nonlinear optical and luminescent properties.
Oxidation
[1–5]
biologically interesting products.
utilized for the construction of elaborated compounds
and as photoinitiators for the free radical curing of polymer
They are broadly
of substituted tolans by Wacker reaction is one of the most use-
[
1,6–10]
[15,16]
ful methods for the synthesis of 1,2‐diketones.
Although
several methods have been reported for the oxidation of
alkynes, these reactions still suffer from drawbacks such as
[
11,12]
networks.
Appl Organometal Chem. 2017;e3812.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 6
https://doi.org/10.1002/aoc.3812

2
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DARABI ET AL.
harsh conditions, narrow substrate scope and low yield and/or
chemoselectivity.
tolan 1 (0.1 mmol), palladium chloride (2 mol%) and copper
[
17–35]
chloride (10 mol%) and dioxane–H O (4:1) as solvent. The
2
Concerning Wacker oxidation, we recently designed some
stilbenes (diphenylethylenes) to provide a better understand-
ing of the structure–function relationships of various
substituent groups within the molecule. It was found that ortho
arrangement of methoxy and alkene substitutions on linear
stilbenes plays a critical role in the complexation with
reaction was heated at 100 °C for 24 h under oxygen atmo-
sphere. After completion of the reaction (monitored by
TLC), CH Cl was added to the reaction mixture and the cat-
alyst was recovered by filtration. The organic medium was
removed with a rotary evaporator under reduced pressure to
give the corresponding product 2. The crude products 2b
and 2c were purified by column chromatography using ethyl
acetate–hexane (1:2) to afford pure products for analytical
measurements.
2
2
[
36]
PdCl2.
Furthermore, a careful study of Wacker oxidation
of stilbenophanes revealed the critical role of host–guest
[37]
chemistry in the oxidation rate of substrates.
In the work reported here, the Wacker oxidation of cyclic
and acyclic tolans 1 was investigated in detail. The objective
was to understand which factor, stereochemistry or distortion
of alkyne bonds of 1, plays a major role in their Wacker
oxidation. Therefore, we investigated the practical and
theoretical aspects of the reaction in detail. It is clear that
development of Wacker oxidation of tolans 1 would not only
expand the scope of the Wacker reaction but also pose an
interesting mechanistic question.
2
.3 | Selected spectroscopic data
1
2
b: H NMR (300 MHz, CDCl , δ, ppm): 7.88 (dd, J = 1.3,
3
7
.7 Hz, 2H), 7.55 (m, J = 1.4, 7.8 Hz, 2H), 7.13 (t,
J = 7.5 Hz, 2H), 7.05 (d, J = 8.3 Hz, 2H), 4.37 (t,
J = 4.88 Hz, 4H), 1.85 (m, 2H). C NMR (75 MHz, CDCl3,
δ, ppm): 193.3, 159.1, 135.5, 130.6, 127.4, 122.4, 116.9,
13
−
1
7
2.8, 27.7. IR (KBr, ν, cm ): 2956, 2926, 1729, 1670,
+
1
596, 1451, 1269, 766. MS‐ESI: m/z 282 (M , 100%), 197
(
45%), 121 (92%), 92 (73%).
1
2
c: H NMR (300 MHz, CDCl , δ, ppm): 7.99 (dd,
3
J = 1.5, 7.5 Hz, 2H), 7.57 (m, J = 1.8 Hz, 2H), 7.11 (t,
J = 7.2 Hz, 2H), 6.99 (d, J = 8.1 Hz, 2H), 4.18 (t,
1
3
J = 4.88 Hz, 4H), 1.82 (t, J = 4.88 Hz, 4H). C NMR
(
75 MHz, CDCl , δ, ppm): 193.1, 157.6, 135, 131.3, 125.5,
3
−1
1
1
2
21.4, 113.8, 68.7, 26.2. IR (KBr, ν, cm ): 2956, 2926,
2
| EXPERIMENTAL
+
729, 1658, 1594, 1480, 1281, 761. MS‐ESI: m/z 296 (M ,
2%), 197 (35%), 147 (31%), 121 (100%), 92 (50%).
2
.1 | Materials and methods
1
13
H NMR and C NMR spectra of 2b and 2c were recorded
in CDCl with a Bruker‐500. Mass spectra were obtained
3
3 | RESULTS AND DISCUSSION
with a Fisons instrument. Tolanophanes 1a–c were synthe-
[38–40]
13
sized from the corresponding stilbenophanes.
UV–vis-
Based on the reported C NMR data, the sp carbon atoms of
[
39]
ible spectra were recorded with an Agilent 8453. The cyclic
voltammetry system was a Metrohm Autolab BV trace
analyser. The auxiliary electrode was a Pt wire. The reference
electrode was Ag/AgCl. The measurements were performed
in dimethylformamide containing 0.3 M (butyl) N(BF )
1
a (95.4 ppm ) resonate strongly at lower field than those
[
38]
[39]
of 1b (92.2 ppm ), 1c (91.9 ppm ) and acyclic 1d
[
40]
(
89.7 ppm ), which is consistent with the increased ring
strain in 1a as a consequence of its decreased ring size.
The Wacker oxidation of 1a–c is shown in Scheme 1. To
gain more information on the reaction mechanism, two acy-
clic analogues 1d and 1e were also subjected to Wacker oxi-
dation. All the reactions of tolans 1a–e were carried out with
catalytic amount of PdCl /CuCl (5% PdCl and 10% CuCl )
4
4
under nitrogen atmosphere. Conductivity was measured
using a Philips PW 9526 digital conductivity meter. A Fisons
gas chromatograph 8000 equipped with a mass detector (Trio
1000) at 70 eV was used. A 60 m × 0.25 mm column packed
2
2
2
2
with WCOT fused silica CP‐sil 5 CB was employed. Carrier
gas was helium and inlet pressure was 14 psi.
in dioxane–water (8:2). All reactions were carried out for
4 h at various reaction temperatures under 1 atm of air and
2
the results are summarized in Table 1. Substrates 1b–e are
oxidized to the corresponding 1,2‐diketones in excellent
yields at 100 and 60 °C. In contrast, although 1a shows the
same activity as 1b and 1c, the reaction is not clean and
mostly leads to the formation of unknown by‐product
(Figure S1).
2
.2 | General procedure for Wacker reaction
of Tolanophanes 1
A 5 ml, two‐necked, round‐bottomed flask was fitted with a
magnetic stirrer. The flask was charged with a mixture of

DARABI ET AL.
3 of 6
of PdCl to substrate, and the reaction atmosphere to opti-
2
mize the reaction conditions for 1b (Table 2). Conducting
the reaction for 24 h stirring at 100 °C and 1 atm of oxygen
atmosphere gave 2b in 99% yield (Table 2, entry 1). It should
be noted that using air in place of oxygen could also give
good yield of product (Table 2, entry 2). Nevertheless, the
preference for oxygen over air in terms of the reaction rate
is evident when the reaction is conducted at lower tempera-
ture (Table 2, entries 3 and 4) or lower catalyst loading
SCHEME 1 Wacker oxidation of 1a–c with PdCl in dioxane–
H O (8:2)
2
2
(
(
Table 2, entries 5 and 6) or decreased reaction time
Table 2, entries 7 and 8). Decreasing the loading of the cat-
Therefore, to evaluate which structural elements of these
alyst from 5 to 2 mol% gave almost acceptable yield of prod-
uct (Table 2, entries 4 and 6).
Isotopic labelling experiments using H2 O revealed that
both oxygen atoms of the 1,2‐diketone 2b originated from
water rather than molecular oxygen as analysed using mass
spectrometry (Figure S12).
alkynes are more responsible for their activity and selectivity,
the reactions were carried out at room temperature. As evi-
dent from Table 1, although the reaction rate is very slow, a
clear difference in their activity is found in the order: 1b
18
>
> 1c > > 1d ≈ 1e > > 1a. Surprisingly, 1a having the most
strained alkyne bond shows almost no activity, even lower
than those of acyclic analogues.
To obtain more information on the oxidation rate of 1a–c,
we performed density functional theory (DFT) calculations
(Cam‐B3LYP functional coupled with the def2‐TZVP basis
set for Pd and 6–311++G(d,p) for H, C, O and Cl elements)
to investigate some important intermediate complexes of the
early steps of the mechanism of the Wacker process. In the
Wacker mechanism, two more possible active intermediates
The low by‐product formation, except for 1a, helped us to
separate products rapidly in useful yields by column chroma-
tography. Mass spectral analysis confirmed the molecular
mass of products (Figures S2–S4). The NMR spectra of
cyclophanes 2b and 2c are in accord with the symmetry of
these molecules (Figures S5 and S8). The aromatic protons
appear as two doublets and two triplets, while non‐aromatic
protons of 2b and 2c appear in different patterns, because
of the different chain length of the etheric oxygen.
TABLE 2 Optimization of reaction conditions for 1b
PdCl2
(%)
Reaction time
(h)/atmosphere
Reaction
temperature (°C)
Yield
(%)
13
In the C NMR spectra, nine signals are observed, which
is in agreement with a time‐averaged C2v symmetry in solu-
tion (Figures S6 and S9). The carbonyl signal is observed as a
singlet at about 195 ppm. The formation of carbonyl bond
was also confirmed using FT‐IR spectrophotometry (Figures
S7 and S10). The progress of the reactions determined using
UV–visible spectrophotometry is easy because products
show a blue shift of absorption maximum of materials
Entry
1
2
3
4
5
6
7
8
5
5
5
5
2
2
2
2
24/O
24/air
24/O
2
100
100
60
99
91
93
88
85
93
65
72
2
24/air
24/air
60
100
100
100
100
24/O
8/air
8/O
2
(Figure S11).
With the aforementioned reaction conditions in hand, we
examined the influence of the reaction time, the proportions
2
TABLE 1 Various reaction conditions for 1a–e in the presence of PdCl
2
(5%) and CuCl
2
(10%)
1
00 °C/24 h/air
60 °C/24 h/air
25 °C/24 h/air
Substrates
Conversion (%)
Product yield (%)
Conversion (%)
Product yield (%)
Conversion (%)
Product yield (%)
b
b
b
1
1
1
1
1
a
b
c
100
32
91
85
81
85
100
28
88
81
77
80
3
0.5
a
100
100
100
100
35
35
25
12
13
a
25
a
a
a
d
e
95
77
12
a
a
100
80
13
a
The remaining percentage is starting materials.
b
The starting material is mostly converted to unknown by‐products.

4
of 6
DARABI ET AL.
−
for the alkene bonds, [PdCl (C═C)] and [PdCl (C═C)
it seems reasonable to suppose that this hydrogen‐bonding
network is responsible for stabilizing the intermediate com-
plex structure. The existence of an additional short intramo-
3
[
2
41–45]
(
H O)], have been proposed.
We considered these
types of intermediates in the complexation of tolanophanes
a–c (Figures 1 and 2). Accordingly, the first chloride atom
from the coordination sphere of ([PdCl ] ) is removed to
form the first intermediate [PdCl (1)] as shown in Figure 1.
The optimized geometries of this intermediate complex indi-
cated that Pd ion is kept near the acetylene carbon atoms
through the interactions between chloride atoms and hydro-
gen atoms of methylene unit and aromatic rings. The topo-
logical analysis of the electron density within QTAIM
framework also demonstrate the existence of the weak
2
1
lecular hydrogen bond between O…CH in 1c (2.37 Å) is
2
2−
expected to weaken the interaction of O…Pd. This is the
4
−
reason why 1c shows lower activity than 1b.
3
−
A further chloride atom of intermediate [PdCl (1)] is
3
then removed by the oxygen atom, generating a complex with
the oxygen and acetylene carbon of 1 (Figure 2). The oxygen
source may be that of water or those of 1, giving rise to
[PdCl (1)(H O)] or [PdCl (1)] complex formation, respec-
2
2
2
tively. This critical step of the reaction mechanism, however,
was not observed computationally for 1a because the posi-
tions of the oxygen atoms are outside of the central part of
bonding interactions between the chlorine atoms of [PdCl ]
with hydrogen atoms of rings and CH fragments. Thus,
3
−
2
FIGURE 1 DFT‐optimized geometries of the first step of intermediate formation. Distances in angstroms and angles in degrees
FIGURE 2 DFT‐optimized geometries of the second step of intermediate formation. Distances in angstroms and angles in degrees

DARABI ET AL.
5 of 6
1a. Therefore, this process is not possible for 1a because the
oxygens of 1a cannot move easily to construct this intermedi-
ate complex. This is in good agreement with our previous ab
initio calculation for 1a which showed its rigid structure due
to a short distance (2.5 Å) between C─H (alkyl) and π‐system
[
46]
(alkyne).
On the basis of these results one concludes that
activity and product selectivity of 1a–c are strongly depen-
dent on structure and configuration.
As 1b showed the best activity and selectivity, we
1
13
measured its H NMR and C NMR complexation with
equivalent amount of PdCl in both DMSO‐d and CDCl .
2
6
3
However, the complexation was not detected using NMR
measurements which may be due to its too slow interaction
at room temperature (Figures S13–S16). These data
confirmed the ab initio calculation findings in which the
preferred coordinating site is oxygen in the reaction
pathway.
In contrast, cyclic voltammetry analysis of PdCl with 1b
2
and 1c showed that the current intensity of reduction and oxi-
dation peaks becomes higher than for 1a indicating better
electron transfer between PdCl and alkyne group (Table 3
and Figure 3). Moreover, new reduction and oxidation peaks
at −1.4 and −1.6 eV for 1b, unlike 1c, appeared showing
2
FIGURE 3 Top: cyclic voltammograms of 1b (dash‐dotted black
line); PdCl
Bottom: cyclic voltammograms of 1c (dash‐dotted black line); PdCl
dashed blue line); Pd complex of 1c (solid red line)
2
(dashed blue line); Pd complex of 1b (solid red line).
2
better interaction between PdCl and alkyne group.
2
(
In this study, we considered tolanophanes 1a–c with var-
ious alkyl chain lengths (n = 2–4) to investigate the progress
of Wacker oxidation at a variety of reaction temperatures. The
products are new and their structures were characterized. In
general, tolanophane 1a having the highest strain and rigidity
showed the lowest activity and selectivity compared to 1b
and 1c. Moreover, it showed lower activity than acyclic ana-
logues at room temperature. On the other hand, 1b showed a
clean reaction with the highest activity and selectivity. This
means that 1b has sufficient alkyl chain length to make the
best stereochemistry for hosting Pd ion. Ab initio calculations
showed that Pd ion is kept closer to alkyne bonds of 1b,
unlike 1a, through its interaction with oxygen atom of 1b to
confirm the difference in reaction rates of 1. However, this
complexation was not detected using NMR measurements
which may be due to its slow reaction at room temperature.
In contrast, cyclic voltammetry measurements showed a
different behaviour for the complexation of 1a and 1b with
palladium ion, confirming the experimental and theoretical
data. In general, it was found that the stereochemistry of 1b
is more suitable for enhancing the reaction rate.
Further investigation of various tolanophanes is currently
under way in our laboratory. The separation and characteriza-
tion of by‐products, especial those of 1a, are also being
investigated.
ACKNOWLEDGEMENT
The Ministry of Science, Research and Technology of Iran is
acknowledged for partial financial assistance of this work.
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Practical and theoretical aspects of Wacker oxidation
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