Palladium complexes of (benzoimidazol-2-ylmethyl)amine ligands as catalysts for methoxycarbonylation of olefins

Article abstract of DOI:10.1016/j.molcata.2015.05.012

Reactions of N-(1H-benzoimidazol-2-ylmethyl)-2-methoxy aniline (L1) and N-(1H-benzoimidazol-2-ylmethyl)-2-bromo aniline (L2) with either [PdCl₂(COD)] or [PdClMe(COD)] afforded the neutral palladium complexes [PdCl₂(L1)] (1), [PdClMe(L1)] (2) and [PdClMe(L2] (3), respectively. Treatment of 2 and 3 with one equivalent of PPh₃ in the presence of NaBAr₄ (Ar = 3,5-(CF₃)₂C₆H₃) produced the corresponding cationic species, [PdMe(L1)]BAr₄ (4) and [PdMe(L2)]BAr₄ (5). All the palladium complexes formed active catalysts in the methoxycarbonylation of alkenes to produce linear and branched esters. The catalytic behaviour was dependent on the catalyst structure, presence of PPh₃, acid promoter and alkene chain length.

Full text of DOI:10.1016/j.molcata.2015.05.012

Accepted Manuscript
Title: Palladium complexes of
(benzoimidazol-2-ylmethyl)amine ligands as catalysts for
methoxycarbonylation of olefins
Author: Thandeka A. Tshabalala Stephen O. Ojwach
Matthew A. Akerman
PII:
S1381-1169(15)00185-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.05.012
MOLCAA 9498
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
16-4-2015
15-5-2015
16-5-2015
Please cite this article as: Thandeka A.Tshabalala, Stephen O.Ojwach, Matthew
A.Akerman, Palladium complexes of (benzoimidazol-2-ylmethyl)amine ligands as
catalysts for methoxycarbonylation of olefins, Journal of Molecular Catalysis A:
Chemical http://dx.doi.org/10.1016/j.molcata.2015.05.012
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Palladium Complexes of (Benzoimidazol-2-ylmethyl)amine Ligands as
Catalysts for Methoxycarbonylation of Olefins
Thandeka A. Tshabalala, Stephen O. Ojwach*, Matthew A. Akerman
School of chemistry and Physics, Pietermaritzburg Campus, University of KwaZulu-Natal,
Private Bag X01, Scottsville 3209, South Africa
Graphical abstract
fx1
Highlights



Palladium complexes were used as catalysts in methoxycarbonylation of olefins
Linear and branched esters were afforded with 100% chemoselectivity
Activity was dependent on acid promoter, alkene chain length and complex
structure


Addition of PPh
3
to the reaction mixture favoured linear esters.
Higher alkenes gave more of the branched esters.
Abstract
Reactions of N-(1H-benzoimidazol-2-ylmethyl)-2-methoxy aniline (L1) and N-(1H-
benzoimidazol-2-ylmethyl)-2-bromo aniline (L2) with either [PdCl (COD)] or
(L1)] (1), [PdClMe(L1)]
2) and [PdClMe(L2] (3) respectively. Treatment of 2 and 3 with one equivalent of PPh in
(Ar = 3,5-(CF ) produced the corresponding cationic species,
(4) and [PdMe(L2)]BAr (5). All the palladium complexes formed active
catalysts in the methoxycarbonylation of alkenes to produce linear and branched esters. The
catalytic behaviour was dependent on the catalyst structure, presence of PPh , acid promoter
and alkene chain length.
2
[
(
PdClMe(COD)] afforded the neutral palladium complexes [PdCl
2
3
the presence of NaBAr
4
3
)
2
C
6
H
3
[
PdMe(L1)]BAr
4
4
3
Keywords: Methoxycarbonylation, alkenes, palladium catalysts, regioselectivity
1
| P a g e

*
Corresponding author: Tel.: +27 (33) 260 5239; Fax: +27 (33) 260 5009
E-mail: ojwach@ukzn.ac.za (S. O. Ojwach)
1
.
Introduction
The methoxycarbonylation reaction has become one of the most valuable organic
reactions in the functionalization of olefins to produce a wide range of industrial and
domestic products such as detergents, cosmetics and pharmaceuticals [1-3]. A number of
phosphine based transition metal complexes have been applied in methoxycarbonylation
reactions over decades [4-9]. To date, palladium-catalysts have attracted much interest in the
methoxycarbonylation of olefins due to their superior catalytic activities [10]. Depending on
the catalytic system used and reaction conditions employed, high regioselectivity towards
branched or linear products can be obtained using Pd(II) catalysts.
In recent years, a number of Pd complexes containing varied ligand architectures have
been reported to show high activity as well as selectivity for methoxycarbonylation of olefins
such as styrene1, 1- hexene [11] or ethene [12] under mild reaction conditions. For instance,
under low partial pressures of carbon monoxide and moderate temperatures, some of these
catalyst systems show high regioselectivity (>90%) towards branched esters [13-25].
Currently, one such catalyst design that is attracting significant attention are the mixed
nitrogen-phosphine donor Pd complexes. This is presumably due to their perceived stability
and tolerance to impurities. For example, Pd catalysts of the type [PdCl
and [PdCl(Ph PNHpy-k2 P,N)(PPh )]Cl were reported to be active and stable in the
methoxycarbonylation of styrene [11].
2
(Ph PNHpy-k2 P,N)]
2
2
3
In this paper, we report the syntheses of some neutral and cationic Pd(II) complexes
anchored on (benzoimidazol-2-ylmethyl)amine ligands and their applications as potential
catalysts in the methoxycarbonylation of alkenes. The effects of catalyst structure, identity of
2
| P a g e

alkene substrate, reaction conditions, and nature of acid promoter used have been
systematically studied and are herein reported.
2
. Experimental section 2.1 Material and methods
All moisture and air sensitive reactions were performed using standard Schlenk line
techniques. Methanol, acetone, diethyl ether, dichloromethane, absolute ethanol, ethyl
acetate, toluene, DMSO-d , potassium iodide (KI), sodium hydroxide (NaOH) and potassium
hydroxide (KOH) were purchased from Merck. Chloroform (CDCl ), styrene, σ-olefins, p-
TsOH, hydrochloric acid (HCl), oxalic acid, sulfuric acid (H SO ), HBr and PPh were
obtained from Sigma Aldrich. The chemicals, 2-chloromethylbenzimidazole, aniline
6
3
2
4
3
(
(
≥99.5%) 2-methoxyaniline (≥99.5%), 2-aminothiophenol (≥99%), and 2-bromoaniline
98%) were purchased from Sigma Aldrich and were used without further purification.
NaBAr
4
(Ar
4
= 3,5-(CF
3
)
2
C
6
H
3
) was obtained from Boulder Scientific and used as received.
Starting materials [PdCl
2
(COD)] [26] and [PdClMe(COD)] [27] were synthesized following
literature methods. Ligands L1-L2 were prepared following our recently published
1
procedures [28]. Nuclear magnetic resonance spectra were acquired at 400 MHz for H, 100
1
3
31
MHz for C and 162 MHz for P on a Bruker Avance spectrometer equipped with Bruker
magnet (9.395 T). All coupling constants are reported in Hertz, Hz. Elemental analyses were
carried out using CHNS-O Flash 2000 thermoscientific analyzer. Mass spectral analyses were
conducted using micromass LCT premier mass spectrometer.
2
2
.2 Syntheses of palladium complexes
.2.1 [{N-(1H-benzoimidazol-2-ylmethyl)-2-methoxyaniline}PdCl ] (1)
2
To a solution of L1 (0.20 g, 0.83 mmol) in CH
Pd(COD)Cl ] (0.22 g, 0.83 mmol) in CH Cl (10 mL) and the mixture was stirred for 24 h
to give a yellow precipitate. The crude product was filtered and recrystallized from CH Cl
| P a g e
2
Cl (10 mL) was added a solution of
2
[
2
2
2
2
2
-
3

1
hexane mixture to afford complex 1 as a light yellow solid. Yield = 0.27 g (77%). H NMR
3
(
DMSO): δ(ppm): 0.78 (s, 3H, OCH
3
); 4.90 (s, 2H, CH
2
); 6.48 – 6.57 (d, 1H, JHH = 7.60,
3
3
3
Ph); 6.59 (t, 1H, JHH = 5.36, Ph
,
); 6.61(d, 1H, JHH = 4.98, Ph
,
); 6.69 (d, 1H, JHH = 6.51,
3
3
Ph); 6.84 (dd, 2H, JHH = 4.80, Bzim); 6.86 – 7.27 (dd, 2H, JHH= 7.66, Bzim). MS (ESI)
+
m/z (%) 254 (M -Cl
2
– Pd, 100%). Anal. Calc. for C15
H
13
N
3
PdCl
2
.0.75CH
2
Cl : C, 38.97; H,
2
3
.06; N, 8.66. Found: C, 39.01; H, 3.66; N, 8.76.
2
.2.2 [{N-(1H-benzoimidazol-2-ylmethyl)-2methoxyaniline}PdClMe] (2)
Complex 2 was synthesized following the procedure described for 1 using L1 (0.10 g,
1
0
.41) and PdCl(COD)Me (0.10 g,0.35 mmol) in Et
2
O (10 mL). Yield = 0. 09 g (86%) H
NMR (DMSO-d
6
): δ(ppm): 0.78 (s, 3H, OCH
3
); 3.84 (s, 3H, CH
3
); 4.91 (s, 2H, CH
2
); 5.17
3
3
3
(
s, 2H, Ph); 6.50 (d, 1H, JHH = 3.74, Ph); 6.58 (t, 1H, JHH = 3.21, Ph); 6.69 (d, 1H, JHH
=
3
3
3
3
.34, Ph); 6.81 (t, 1H, JHH = 5.23, Ph); 7.35 (dd, 2H, JHH = 6.86, Bzim); 7.51 (dd, 2H, JHH
=
7.77, Bzim). Anal. Calc. for C16
H
18
3
N PdClO: C, 46.85; H, 4.42; N, 10.24. Found: C, 47.11;
H, 4.11; N, 8.58.
2
.2.3 [{N-(1H-benzoimidazol-2-ylmethyl)-2-bromoaniline} [PdClMe] (3)
Complex 3 was prepared according to the procedure for 1 using L2 (0.10 g, 0.34
1
mmol) and PdCl(COD)Me (0.10 g, 0.34 mmol). Yield = 0.09 (58%) H NMR (DMSO-d
6
):
3
�
H
(ppm): 0.80 (s, 3H, CH
3
); 4.61 (s, 2H, CH
2
) ;5.01 (s, 2H, CH
2
); 6.27 (d, 1H, JHH = 6.67,
3
3
3
Ph); 6.56 (t, 1H, JHH = 5.64, Ph); 7.06 (t, 1H, JHH = 6.77, Ph); 7.15 (d, 1H, JHH = 7.89, Ph);
3
3
7
.31 (dd, 2H, JHH = 6.78, Bzim); 7.48 (dd, 2H, JHH = 7.33, Bzim). MS (ESI) m/z (%) 482
4
| P a g e

+
(
M - Cl, 22%). Anal. Calc. for C15
H
15
3
N BrPdCl: C, 39.24; H, 3.29; N, 9.15. Found: C,
3
8.94; H, 3.87; N, 8.99.
2
.2.4 [{N-(1H-benzoimidazol-2-ylmethyl)-2-methoxyaniline} [PdMe(PPh
3
)] BAr (4)
4
To a suspension of 2 (0.02 g, 0.09 mmol), in CH
mmol) and NaBAr (Ar = 3,5-(CF ) (0.037 g, 0.09 mmol) were added and stirred for
4 h. The solution mixture was filtered and the filtrate concentrated to approximately 3 mL
and recrystallized from hexane/CH Cl solvent mixture to obtain compound 4 as a yellow
crystalline solid. Yield = 0.08 g (66%). H NMR (CDCl
H, OCH ), 5.52 (s, 2H, CH
2
Cl
2
(10 mL), PPh
3
(0.01 g, 0.09
4
4
3
)
2
C
6
H
3
2
2
2
1
3
): δ (ppm):0.95 (s, 3H, CH ), 3.43 (s,
3
3
3
3
2
), 6.47 (d, 1H, Ph-aniline), 6.69 (t, 1H, , JHH = 6.80, Ph), 6.75
3
3
3
(
d, 1H, , JHH = 7.20, Ph), 7.21 (m, 6H, JHH = 7.60, PPh
3
), 7.34 (d, 2H, JHH = 6.80, Bzim),
3
3
3
7
.36 (d, 8H, JHH = 8.00, BAr
4
), 7.50 (m, 9H, JHH = 8.00, PPh
3
), 7.71 (t, 4H, , JHH = 7.60,
3
31
BAr
4
), 7.72 (d, 2H, JHH = 7.88, Bzim). P NMR (CDCl
3
) δ(ppm): 37.85. Anal. Calc. for
C
66
H
45BF24ON PPd: C, 52.46; H,3.05; N, 2.82. Found: C, 52.94; H, 1.39; N, 3.09.
3
2
.2.5 [{N-(1H-benzoimidazol-2-ylmethyl)-2-bromoaniline} [PdMe(PPh
3
)] BAr (5)
4
Complex 5 was prepared according to the same procedure for 4 using 3 (0.04 g, 0.09
mmol), PPh
3
(0.02 g, 0.09 mmol) and NaBAr
4
(Ar
4
= 3,5-(CF
3
)
2
C
6
3
H ) (0.08 g, 0.09 mmol).
1
Yield = 0.06 (67%). H NMR (CDCl
3
): δ(ppm): 0.98 (s, 3H, CH
3
), 5.65 (s, 2H, CH ), 6.33 (d,
2
3
3
3
1
H, JHH = 6.40, Ph), 6.60 (t, 1H, JHH = 6.80, Ph), 7.10 (d, 1H, Ph), 7.20 (t, 1H, JHH = 6.40,
3
3
Ph), 7.25 (m, 6H, PPh
3
), 7.38 (d, 2H, JHH = 6.80, Bzim), 7.40 (8H, JHH = 8.40, BAr
4
), 7.59
3
3
3
(
m, 9H, JHH = 7.20, PPh
3
), 7.63 (d, 2H, JHH = 7.60, Bzim), 7.74 (4H, JHH = 7.60, BAr
4
)
5
| P a g e

3
1
P NMR (CDCl
3
): 30.63. Anal. Calc. for C65
H45BBrF24
3
N PPd:C, 50.30; H, 2.92; N, 2.70.
Found: C, 50.61; H, 2.72; N, 2.87.
2
.3 General procedure for the methoxycarbonylation reactions
The catalytic methoxycarbonylation reactions were performed in a stainless steel
autoclave equipped with temperature control unit and a sample valve. In a typical experiment,
complex 4 (48.78 mg, 0.08 mmol), PPh (0.04 g, 0.16 mmol), HCl (0.03 mL, 0.80 mmol) and
-hexene (2 mL, 15.93 mmol) were dissolved in a mixture of methanol (20 mL) and toluene
40 mL). The reactor was evacuated and the catalytic solution was introduced to the reactor
3
1
(
via a cannula. The reactor was purged three times with CO, set at the required pressure and
temperature and the reaction stirred at 500 rpm. At the end of the reaction time, the reaction
was cooled, excess CO was vented off and the samples drawn for GC analysis to determine
the percentage conversion of the alkene substrate to esters. GC-MS analyses were run under
the following standard chromatography conditions: -25 m CPSil 19 capillary column, 1.2 mm
film thickness, Helium carrier column gas 5 psi, injector temperature 250 ˚C, oven program
5
0 ˚C for 4 minutes rising to 200 ˚C at 20 ˚C/ min and holding at 200 ˚C. The branched and
linear esters were assigned using standard authentic samples.
3
. Results and discussion
1
.1. Syntheses of neutral and cationic Pd(II) complex synthesis
Compounds L1 and L2 were prepared by reaction of 2-(chloromethyl)benzoimidazole
with the appropriate aniline derivatives following our reported literature procedure [28].
| P a g e
6

Reactions of L1 and L2 with either [PdCl
corresponding neutral complexes 1-3 in moderate yields (58-86%), Scheme 1. Subsequent
treatment of complexes 1-3 with one equivalent of NaBAr (Ar = 3,5-(CF ) in the
2
(COD)] or [PdClMe(COD)] produced the
4
4
3
)
2
C
6
H
3
presence of PPh
3
afforded the respective cationic compounds 4–5 respectively in high yields
(
Scheme 2).
H
N
H
N
R
N
NH
PdCl (COD)
2
H
OR
PdClMe(COD)
N
N
Pd
R
Cl
X
R = OCH , X =Cl (1)
R= OCH (L1), Br (L2)
3
3
R = OCH , X = Me (2)
3
R = Br, X = Me (3)
Scheme 1
+
H
N
R
H
N
BAr4
R
H
H
PPh3
N
N
Pd
NaBAr4
N
N
Pd
Cl
X
Cl
X
R = OCH , X = Me (2)
R = Br, X = Me (3)
3
R = OCH , X = Me (4)
R = Br, X = Me (5)
3
Scheme 2
1
31
The new compounds were characterized by H, P NMR spectroscopies, mass
spectrometry, elemental analyses and single X-ray crystallography for complex 5. In a typical
1
H NMR spectrum, the CH
2
linker protons in the ligands were useful in determination of their
1
respective complex formation. For example, the H NMR spectra of L1 and its corresponding
complex 1 showed CH protons signals at 4.65 ppm and 4.90 ppm respectively. One
2
7
| P a g e

1
interesting observation in the H NMR data was the appearance of two broad signals for the
CH protons at 4.61 ppm and 5.01 ppm for complex 4 in addition to two sets of aromatic
2
protons. This was indicative of the presence of two isomers, which may arise from
unsymmetrical nature of complexes 3 and 4 due to Pd-Me and Pd-Cl bonds [29]. The cationic
3
1
complexes 4 and 5 were further characterized by P NMR in order to determine the presence
and nature of coordination of PPh ligand. Complexes 4 and 5 displayed one singlet in their
P NMR spectra between 37.85 ppm and 30.63 ppm respectively (Figures S1). These values
3
3
1
3
1
fall within typical P NMR signals between 30.70-35.70 ppm for related mono-coordinated
PPh compounds and agrees with the proposed structures in Scheme 2 [30-37]. Mass
3
spectrometry was also used to establish the formation and identity of these complexes. The
complexes showed typical fragmentation patterns consistent with a loss of a halide group. For
-
instance, complex 3 losses a Cl ion (m/z = 423.96) followed by a loss of PdMeCl fragment to
produce a peak at m/z = 304 corresponding to L4 (Figure S2).
In order to confirm the proposed coordination modes of ligands L1 and L2 to Pd,
attempts were made to grow single crystals suitable for X-ray analyses of the complexes. In
one such instance, single crystals of compound 5 suitable for X-ray analyses were isolated.
Interestingly, the structure obtained (5a) contained one monodentate L2 ligand, two PPh
3
ligands and a methyl group to complete a four coordinate geometry around the palladium
atom (Figure 1). Crystal data collection and structural refinement parameters are given in
Table S1. The emperical composition of 5a thus differs from 5 as was established in
elemental analyses. The monodentate coordination of L2 demonstrate the hemilabile nature
of these ligands as was recently reported by Attandoh et. al. [28]. Thus it is reasonable to
deduce that the change in coordination occurred during crystallization process. The bond
lengths of Pd(1)-N(3) of 2.1290 (19) Å is similar to those observed in related Pd(II)
complexes of 2.1300 (19) Å and 2.1289 (19) Å [38-39]. The average bond distance for Pd-P
8
| P a g e

of 2.3157(8) Å compares favourably to the average bond length of 2.3248 Å for other related
palladium phosphine complexes [40-42]. Significant distortions from the ideal square planar
geometry were evident in bond angles for N(2)-Pd(01)-P(003) of 94.19(5)° and C(1)-Pd(01)-
P(004)of 87.54(7)° presumably due to steric demands of the two bulkier PPh
solid state structure of 5a, there occurs hydrogen bonding between the benzimidazole NH and
fluorine atom of the adjacent BAr anion (Figure S3).
3
groups. In the
4
Figure 1: Thermal ellipsoid plot (50% probability) of 5a illustrating the square planar
coordination geometry of the Pd(II) ion. Hydrogen atoms and the BAr anion have been
4
omitted for clarity. Selected bond lenghths [Å] and angles [˚] for 5a: Pd(1)-C(1) 2.069 (2),
Pd(1)-N(3) 2.1290(19), Pd(1)-P(1) 2.3003(8), Pd(11)-P(2) 2.3311(8), C(1)-Pd(1)-N(3)
1
78.97(9), C(1)-Pd(1)-P(2) 87.54(7), N(3)-Pd(1)-P(1) 94.19(5).
3
3
.2. Methoxycarbonylation reactions of olefins catalyzed by complexes 1-5.
.2.1 Preliminary screening of palladium complexes 1-5 in methoxycarbonylation reactions
9
| P a g e

Preliminary evaluation of complexes 1–5 in the methoxycarbonylation of 1-hexene
were performed at 60 bar of CO, 90 ˚C and [1-hexene]:HCl:[Pd] = 200:10:1 (Scheme 3). The
major products were methyl heptanoate (linear product A) and methyl 2- methylhexanoate
(
branched product B). The identities and compositions of the products were determined by
GC-MS (Figures S4 and S5).
O
OCH3
A
CO/MeOH
+
CO Me
2
Catalysts 1-5
B
Scheme 3
3
.2.2. Effect of complex structure on methoxycarbonylation of 1-hexene
The catalytic activities of complexes 1–5 in the methoxycarbonylation of 1-hexene
were studied in order to investigate the effect of complex structure on catalytic performance.
The effect of ligand motif was observed to influence the catalytic activity of the resultant
catalysts. For example, complex 4 bearing electron donating OCH group on the phenyl ring
3
was found to be the most active (Table 1, entry 4), while the corresponding complex 5,
containing electron withdrawing bromide substituent was the least active (Table 1, entry 5).
This trend is consistent with OCH forming stable Pd(II) active species while the Br
3
substituent is likely to produce unstable Pd(II) catalysts. Less stable systems tend to promote
reduction of the catalytically active Pd(II) species to the less active Pd(I)/Pd(0) [43-46].
Another factor that affected the catalytic activity was the Pd-Cl/Me bond on the complex
structure. For instance, complexes 1 and 2 bearing Pd-Cl and Pd-Me bonds displayed
10 | P a g e

percentage conversions of 52% and 80% respectively (Table 1, entries 1 and 2). We
attributed this observation to the ease of CO insertion into Pd-Me bond, unlike in Pd-Cl
system which must activated by methanol solvent [45].
a
Table 1: Effect of catalyst structure in the methoxycarbonylation of 1-hexene.
b
c
-1
Entry
Catalyst
PPh
3
Conversion (%)
b/l (%)
43/57
TOF(h )
1
2
3
4
5
6
1
2
3
4
5
4
5
2
2
2
2
2
-
52
80
39
86
29
37
10
4.5
40/60
40/60
37/63
40/60
52/48
68/32
6.7
3.1
7.1
2.4
3.1
0.8
7
-
a
Reaction conditions: Pre-catalysts (0.07 mmol), solvent: toluene 40 mL and methanol 30 mL;
b
[
Pd]:[1-hexene] ratio 200:1, HCl:[Pd] ratio; 10:1; P (CO) = 60 bar; temperature: 90 °C; time, 24 h; %
c
of 1-hexene converted to esters; branched/linear ester ratio.
We also compared the behaviour of the neutral and the corresponding cationic Pd(II)
complexes. While the activity of 2 (80%) and its corresponding cationic species 4 (86%)
were comparable, the percentage conversion obtained for 3 (39%) was appreciably higher
than that observed for its cationic analogue complex 5 (29%) (Table 1, entries 2-5). We
postulate that the cancellation of enhanced solubility (expected to increase activity) and
increased steric bulk (expected to limit substrate coordination thus reduced activity) due to
11 | P a g e

the bulkier PPh
3
groups in the cationic systems might explain the insignificant differences
between these two systems.
The nature of the complex structure did not significantly influence the regioselectivity
of the esters produced. For example, the neutral and cationic complexes 2 and 4 afforded
0% and 37% of the branched esters respectively (Table 1, entries 2 and 4). It was expected
that the bulkier PPh groups in the cationic complexes may hinder isomerization of the
coordinated substrates leading to the formation of more linear products. Thus the
insignificant effect of the PPh group on product regioselectivity might point to a different
mechanism, possibly dissociation of the PPh group prior to alkene coordination.
4
3
3
3
3
.2.3. Effect of PPh on methoxycarbonylation reactions of 1-hexene
3
The catalytic performances of complexes 4 and 5 in the absence and presence of PPh
were performed in order to further probe the effect of triphenyl phosphine (PPh ) on the
catalytic activities and selectivity of these systems. From the results, it was apparent that
addition of PPh to the palladium complexes (Table 1, entries 6 and7) resulted in higher
catalytic activities. For instance, using complex 4, percentage conversions of 86% and 37%
were realized in the presence of 2 molar equivalent and absence of PPh respectively. From
literature reports [47-48], it is believed that the PPh group improves the stability of the active
species by limiting decomposition to Pd(0) [11]. Indeed, extensive decomposition of the
complexes to Pd(0) black was observed in reactions performed without PPh
Regioselectivity of the ester products was also influenced by addition of PPh to the catalytic
system. As an illustration, addition of PPh to the complex 5 resulted in a decrease of the
branched ester from 68% to 40% (Table 1, entries 3 and 7). This trend could largely be
attributed to steric hindrance from the bulkier PPh ligand thus limiting chain isomerization
2 | P a g e
3
3
3
3
3
3
.
3
3
3
1

to form the sterically demanding branched esters [47]. In related experiments, Zuniga [49]
and Seayad [48] and co-workers reported regioselectivities of 45–86% and 21-34% towards
branched esters in the absence and presence of PPh respectively.
3
3
.2.4. Effect of acid promoters on methoxycarbonylation reactions
The nature of the acid promoter has been established to play a major role in Pd(II)
catalyzed methoxycarbonylation of olefins [50-51]. We thus investigated the effect of various
acid promoters; HCl, H SO , and p-TSOH on the catalytic performance of complex 4 using
-hexene substrate. The results obtained clearly illustrate the importance of the acid
promoters; while no catalytic activity was reported using p-TsOH, conversions of 42% and
6% were achieved using HCl and H SO respectively (Table 2, entries 9-10). These results
2
4
1
8
2
4
are not surprising and have been associated with the coordinating abilities of the respective
acid promoters to stabilize the active Pd(II) species [52-54]. It is not clear how the nature of
the acid promoter changed the regioslectivity towards the branched esters from 37% to 47%
using HCl and H
2
SO respectively.
4
a
Table 2: Effect of reaction conditions in the methoxycarbonylation of 1-hexene using 4.
b
c
Entry
Acid
p-TsOH
HCl
P
CO/ bar T/ºC [1-hexene]:[Pd] Time Conv (%) b/l(%)
8
9
60
60
60
50
90
90
90
90
200:1
200:1
200:1
200:1
24
24
24
24
--
86
--
37/63
47/53
38/62
1
1
0
1
H
2
SO
4
42
50
HCl
13 | P a g e

1
1
1
1
1
2
3
4
5
6
7
HCl
HCl
HCl
HCl
HCl
HCl
60
60
60
60
60
60
60
90
90
90
90
90
200:1
400:1
100:1
200:1
200:1
200:1
24
24
24
12
32
36
85
55
89
49
92
99
34/64
31/69
50/50
28/72
47/53
55/45
1
a
Reaction conditions: Solvent: toluene 40 mL and methanol 30 mL; [Pd]:[acid] ratio; 1:10; time, 24
b
c
h; % of hexene converted to esters; Molar ratio between branched and linear ester.
3
.2.5 Effect of reaction conditions on methoxycarbonylation of 1-hexene
The effect of reactions conditions such as carbon monoxide pressure, temperature,
reaction time and substrate/catalyst ratio were also studied in order to optimize the catalytic
conditions using complex 4. Changing the CO pressure from 50 and 60 bar led to a
substantial increase in percentage conversion from 50% and 86% respectively (Table 2,
entries 11 and 12). This observation could be attributed to increased insertion of CO at high
concentrations. However, there was no appreciable change in product regioselectivity with
change in CO pressure. We did not observe any effect on the catalytic activity of 4 with
change in reaction temperature as was evident in comparable conversions of 85% and 86% at
o
o
6
0 C and 90 C respectively. The effect of catalyst concentration was also investigated by
varying substrate/catalyst ratio from 100 to 400 at fixed substrate concentration (Table 2,
entries 9, 13-14). Increasing the substrate/catalyst ratio from 100 to 400 led to a remarkable
decrease in activity from 86% to 55% accordance with the previous reports of Vavasori and
co-workers [55]. We noted that increasing substrate/catalyst ratio 100 to 400 (Table 2, entries
9
3
, 13-14) also resulted in decreased regioselectivity towards the branched esters from 50% to
1%. This hints that at higher catalyst loadings, isomerization reactions were favoured.
Lastly, the effect of time on catalytic activity and regioselectivity of the ester products was
4 | P a g e
1

probed using catalyst 4. Maximum conversion of 99% was obtained within 36 h, while only
9% was attained within 12 h (Table 2, entries, 15 and 17). It is noteworthy to mention that
4
increasing reaction time from 12 h to 36 h, resulted in significant shift to the branched esters
from 28% to 55% (Figure 2). This trend is consistent with increased chain isomerization of 1-
hexene prior to methoxycarbonylation with time.
Time (h)
Figure 2: Graph of percentage conversion and regioselectivity using complex 4 showing the
influence of time on the methoxycarbonylation reactions. Solvent: toluene 40 mL and
methanol 30 mL; [Pd]:[1-hexene] ratio 200:1, [Pd]:[HCl] ratio; 1:10;
temperature: 90 °C.
P
(CO) = 60 bar;
3
.2.6. Effect of olefin substrate on methoxycarbonylation reactions using 4
Upon establishing the optimum reaction conditions for the methoxycarbonylation
reactions, complex 4 was used in the methoxycarbonylation of a range of alkene substrates;
15 | P a g e

styrene, 1-octene, 1-nonene and 1-decene (Figure 3.) It was clear from the results that the
catalytic activity of 4 was significantly affected by the nature of the substrate. For example,
changing from 1-hexene to 1-decene drastically decreased the percentage conversion from
8
6% to 10%. It is therefore apparent that increasing the chain length lowers the catalytic
activity of 4 possibly due to increased steric hindrance or reduced reactivity of the higher
olefins. More importantly, regioselectivity towards the branched/linear esters was also
affected by the nature of the alkene substrate in which higher alkenes favoured the formation
of branched esters. For instance, compositions of 37% and 67% of the branched esters were
obtained for 1-hexene and 1-decene respectively. This occurrence could be partly assigned to
the higher number of possible isomers present in the higher alkenes compared to lower
alkenes. The lower selectivity towards the branched isomer using styrene substrate (18%)
compared to 1-hexene (37%) could be attributed to the bulkier phenyl groups in styrene
which might favour the less sterically demanding linear isomer and/or limit chain
isomerization to produce the branched isomers.
100
88
86
9
0
0
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
1
67
58
54
%
Conversion
37
36
%Branched
24
18
10
Styrene 1-hexene 1-octene 1-nonene 1-decene
16 | P a g e

Figure 3: The effect of substrate on percentage conversion and regioselectivity towards
branched product using catalyst 4, solvent: toluene (40 mL) and methanol (30 mL);
[
substrate]:[Pd] ratio 200:1, [Pd]:HCl ratio; 1:10; (CO) = 60 bar; temperature: 90 °C; time
P
2
4 h.
3
.2.7. Proposed mechanism for palladium catalyzed methoxycarbonylation reactions
Palladium catalysed methoxycarbonlylation of olefins have been proposed to occur
5
7
via two possible mechanisms: hydride and carbomethoxy routes (Scheme 4). For complexes
-5, both of these mechanisms are highly possible due to coordinative unsaturation of the
palladium metal center resulting from possible loss of PPh group. At this stage, we are
1
3
cannot unambiguously establish the preferred mechanistic pathway. In the carbomethoxy
route (A), the cycle is initiated by the formation of a Pd-OMe bond aided by HCl acid. This
is followed by migratory insertion of CO into the Pd-OMe to give an ester, prior to olefin
substrate coordination. In the hydride cycle (B), the formation of a Pd-H bond occurs in the
activation stage instead of a Pd-OMe bond. This is then followed by olefin coordination and
subsequently CO coordination and migratory insertion to form a coordinated ketone. The
product is ester product and generation of the Pd-H active species is achieved by
methanolysis reaction. In both cases, the formation of linear or branched isomers are
determined by the 1,2 or 2,1 olefin insertions. The role of the PPh
3
group is mainly to
stabilize the 14-electron species upon formation of the Pd-H or Pd-OMe active species. This
17 | P a g e

was supported by enhanced stability of the catalysts when PPh was added to the catalyst
3
system.
N
N
Pd
Cl
Cl
N
N
I
MeOH/HCl
PPh3
COOMe
Pd
Cl
Cl
R
N
N
II
PPh3 MeOH/HCl
Pd
R
COOMe
Ph3P
H
R
N
N
Pd
Ph3P
OMe
CO
N
N
N
N
N
Pd
Pd
R
H
OMe
Ph3P
R
N
Pd
A
O
Ph3P
B
N
N
O
R
Pd
OMe
Ph3P
PPh3
O
N
N
N
OMe
O
N
N
Pd
R
R
N
Pd
Pd
OC
Ph3P
R
PPh3
R
PPh3
CO
Scheme 4: Proposed hydride (A) and carbomethoxy (B) methoxycarbonylation mechanisms
using palladium complexes.
4
.
Conclusions
In summary, this work has demonstrated the potential of (benzoimidazol-2-
ylmethyl)amine palladium complexes to catalyze methoxycarbonylation of alkenes to afford
linear and branched esters with 100% chemoselectivity. Complex structure and alkene
substrate significantly affected both the catalytic activity and product regioselectivity of the
resultant catalysts. We are currently fine-tuning the catalyst structure in order to shift
regioselectivity towards the more desirable branched esters.
18 | P a g e

Acknowledgements
The Authors would like to acknowledge NRF-DST Center of Excellence in Catalysis,
c*change olefin program (OLE-UKZN 10.3) for financial support and bursary scholarship.
Supplementary information
Supplementary Table S1 contains the data collection and structural refinement
3
1
parameters of complex 5a. Figures S1-S2 represent P NMR and mass spectra of complex 5
respectively. Figure S3 represents the hydrogen bonding interactions in complex 5a. Figures
S4-S5 shows GC-chromatograms and GC-MS spectra of the ester products respectively.
CCDC number 1056837 contains the supplementary crystallographic data for complex 5a.
This data can be obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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