Palladium–Bis(carbene) Catalysts for the Bisalkoxycarbonylation of Olefins to Succinic Diesters

Article abstract of DOI:10.1002/ejoc.201601546

A series of Pd-bis(NHC) (NHC = N-heterocyclic carbene) complexes possessing various anions and substituents were used in the bisalkoxycarbonylation of olefins (α-olefins and ethylene) to afford industrially useful succinic diesters in modest to good yields. The influence of different ligands and counteranions of the Pd complexes on their catalytic activity was assessed, and it was found that dimeric Pd–bis(NHC)Br was the best catalyst for the bisalkoxycarbonylation reactions. The structure of dimeric Pd–bis(NHC)Br was confirmed by X-ray crystallographic analysis.

Full text of DOI:10.1002/ejoc.201601546

DOI: 10.1002/ejoc.201601546
Communication
Alkoxycarbonylation
Palladium–Bis(carbene) Catalysts for the Bisalkoxycarbonylation
of Olefins to Succinic Diesters
Yu Jin Cho,[a] Yu Na Lim,[a] Woojin Yoon,[a] Hoseop Yun,[a] and Hye-Young Jang*[a]
Abstract: A series of Pd-bis(NHC) (NHC = N-heterocyclic carb- counteranions of the Pd complexes on their catalytic activity
ene) complexes possessing various anions and substituents was assessed, and it was found that dimeric Pd–bis(NHC)Br was
were used in the bisalkoxycarbonylation of olefins (α-olefins the best catalyst for the bisalkoxycarbonylation reactions. The
and ethylene) to afford industrially useful succinic diesters in structure of dimeric Pd–bis(NHC)Br was confirmed by X-ray
modest to good yields. The influence of different ligands and crystallographic analysis.
Introduction
the alkyl–metal NHC complex intermediate formed during the
carbonylation of olefins, which is detrimental to catalytic activ-
ity.[5k,7] To avoid this undesirable alkylimidazolium reductive
elimination pathway, increasing steric hindrance of the carbene
ligand by incorporation of bidentate or tridentate NHC ligands
has been proposed.[1p,2d,7c] Accordingly, in this work, we used
palladium complexes with bidentate NHC ligands for the oxid-
ative carbonylation of olefins to afford succinic diesters, which
are important building blocks that are used in the synthesis of
biodegradable polymers.[8] The effects of the substituents,
tether, and counteranions of the bidentate carbene ligands on
the catalytic activity were investigated.
Palladium complexes with chelating N-heterocyclic carbene
(NHC) ligands are commonly used in Heck and Suzuki reactions.
Relative to other ligands, the coordination of chelating NHC
ligands to a palladium metal ion improves the stability and ac-
tivity of the resulting catalysts in such reactions.[1] Phosphane
ligands are commonly used in metal-catalyzed reactions; how-
ever, they are sensitive to oxygen, and their synthesis often
requires inert reaction conditions and multistep procedures. In
contrast, NHC ligands are much less sensitive to oxygen and
water, which provides stability to the catalyst, and the introduc-
tion of substituents on the nitrogen atom and backbone of bis-
NHC systems is relatively easy; this allows convenient modifica-
tion of the steric and electronic properties of these ligands.[2]
Pd complexes bearing various ligands have been employed
in the coupling of olefins to carbon monoxide (CO) to afford
polyketones and mono- and diesters.[3] Depending on the
structure of the ligands and the reaction conditions, two reac-
tion pathways are possible, that is, polymerization and mono-
mer formation (Scheme 1). Pd complexes containing bidentate
phosphane and nitrogen ligands favor the formation of poly-
ketones, as demonstrated by Drent and others.[4] The formation
of monomers (monoesters and diesters) occurs in the presence
of monodentate phosphane ligands instead.[5,6] In the case of
bidentate phosphane ligands with bulky substituents on the
phosphorus atom, monoester formation dominates over polym-
erization.[5d,5j] Other than phosphane ligands for Pd-catalyzed
coupling reactions of olefins and CO, monodentate NHC ligands
have also been used for the Pd-catalyzed alkoxycarbonylation
of olefins to afford monoesters.[5k] Except for Woo's work, carb-
ene ligands have rarely been used in the Pd-catalyzed oxidative
carbonylation of olefins owing to facile reductive elimination of
Scheme 1. Pd-catalyzed reactions between olefins and CO.
[a] Division of Energy Systems Research, Ajou University,
Suwon, Korea
Results and Discussion
E-mail: hyjang2@ajou.ac.kr
http://ajou.ac.kr/~hyjang2
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201601546.
We used six different Pd–bis(NHC) complexes for carbonylation,
and their structures are shown in Figure 1. Complexes A,[1o]
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B,[1b] D,[1b] E,[1b] and F[1o] are known compounds and were pre- temperature, as well as the pressure of CO were examined, and
pared according to literature procedures. The synthesis of di- the specified conditions are found to be optimal for this trans-
meric complex C is shown in Scheme 2. Although a similar pro- formation.[10] Under these oxidative carbonylation conditions,
cedure was reported to afford monocationic [Pd(NHC)(Br)- dialkyloxalates could be formed in principle, but our catalytic
(CH3CN)]PF6 starting from complex B,[1b] we isolated dimeric systems promoted the formation of diesters with 100 % select-
monocationic complex C instead. The dimeric structure of C ivity. Assuming that the catalytic activity was modulated by the
was confirmed by X-ray structure determination (Figure 2). A nature of the ligands, the anions, the tether length, and the
single crystal of C was obtained by slow evaporation of an acet- substituents on the nitrogen atom of the complexes were modi-
one solution of C at 4 °C to afford an acetone-incorporated fied (Scheme 3). The catalytic activities of catalysts A and B
dimeric complex. Palladium dimer C is assumed to undergo dis- were compared, and the effect of different halides on the yield
sociation to form the active catalyst during the catalytic reac- of 1b was not substantial. However, selective replacement of a
tion.
halide ion with a noncoordinating PF6– anion led to an improve-
ment in the yield of 1b (catalyst C). Further removal of the
halide ion was rather detrimental to the formation of 1b (cata-
lyst D). An increase in the steric bulk of the N-bound substitu-
ents by replacing the methyl groups with mesityl (2,4,6-trimeth-
ylphenyl) groups improved the yield of 1b (catalyst E). An in-
crease in the length of the tether did not lead to an improved
yield (catalyst F). Overall, dimeric Pd complex C possessing an
N-methyl-substituted bis(carbene) and a halide showed the
best catalytic performance.
Figure 1. Palladium complexes used in the bisalkoxycarbonylation of olefins.
Mes = mesityl = 2,4,6-trimethylphenyl.
Scheme 3. Pd-catalyzed bisalkoxycarbonylation of 1a.
Under the optimized conditions shown in Scheme 3, various
reactions between ethylene and CO were conducted
(Scheme 4). In the presence of methanol, ethanol, and benzyl
alcohol, desired succinic diesters were formed with the turnover
numbers (TONs) indicated in Scheme 4.
Scheme 2. Synthesis of complex C.
Scheme 4. Pd-catalyzed synthesis of succinic diesters by using ethylene and
CO.
In addition to 1b–4b, various succinic diesters were synthe-
sized from olefins (Table 1). Decene was exposed to the opti-
mized conditions to afford ethyl ester 5b and benzyl ester 6b.
Figure 2. X-ray structure of C.[9]
The Pd-catalyzed bisalkoxycarbonylation of 1-decene was Hexene participated in the reaction to form 7b in 78 % yield.
performed in the presence of catalyst A (0.02 mmol, 4 mol- The presence of a phenyl substituent distant from the alkene
%), para-toluenesulfonic acid (TsOH; 0.05 mmol, 10 mol-%), 1,4- functional group affected the yield, as shown for compounds
benzoquinone (BQ; 0.5 mmol, 1 equiv.), and methanol (MeOH; 8b–10b. Fluoro-, chloro-, and methyl-substituted styrenes were
5 mmol, 10 equiv.) in dichloromethane (CH2Cl2, 4 mL) at 80 °C converted into desired esters 11b, 12b, and 13b in yields of 61,
under 30 bar of CO. The amounts of additives and solvents, the 61, and 50 %, respectively.
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Table 1. Examples of succinic diesters.
gen anion is not clear, but it can be speculated that the decom-
position of the Pd catalyst to Pd0 might be retarded by reoxid-
ative addition of HX (X = Cl and Br).[6l] The addition of TsOH
also improves the yield of this transformation, as the oxidizing
power of BQ is known to increase in acidic media.[6g]
Conclusions
We synthesized various Pd–bis(NHC) complexes and employed
them in the synthesis of succinic diesters through the bisalkoxy-
carbonylation of olefins. A dimeric Pd–bis(NHC) complex was
synthesized and characterized by X-ray crystallography, and it
showed the best catalytic activity owing to the empty coordina-
tion site generated upon its dissociation in solution. A variety
of olefins participated in the alkoxycarbonylation reaction in
the presence of CO and alcohols to afford the corresponding
products in modest to good yields.
Experimental Section
Representative Procedure for the Reactions of Olefins: A 50 mL
autoclave was charged with a solution of BQ (54.05 mg, 0.5 mmol),
TsOH (9.5 mg, 0.05 mmol), catalyst C (10.2 mg, 0.02 mmol), 1-de-
cene (1a; 95 μL, 0.5 mmol), and MeOH (202.5 μL, 5 mmol) in CH2Cl2
(4 mL). The autoclave was pressurized with CO (30 bar), and the
mixture was stirred at 80 °C for 15 h. The vessel was cooled to room
temperature, and the unreacted gas was released. The solvent was
removed under vacuum, and the residue was purified by flash col-
umn chromatography (silica gel, 1 % EtOAc/hexane) to afford di-
methyl 2-octylsuccinate (1b; 101.2 mg, 78 %).
[a] BnOH (3 equiv.), BQ (1.5 equiv.). [b] BnOH (10 equiv.), BQ (1.0 equiv.).
A plausible catalytic cycle is proposed in Scheme 5. Dimeric
complex C dissociates in the presence of CO and an alcohol
to afford LnBrPd–CO2R. The empty coordination site of the Pd
monomer derived from C is readily occupied by incoming CO
and ROH. Migratory insertion of the olefin into the Pd–C bond
of LnBrPd–CO2R provides intermediate I. The addition of CO to
I followed by alcoholysis affords a succinic diester along with
the generation of LnBrPd–H. Intermediate I might undergo pro-
tonolysis to afford the monoester, but in the presence of the
bis(NHC)–Pd complex, no monoester formation was observed.
Representative Procedure for the Reactions of Ethylene: A
50 mL autoclave was charged with a solution of BQ (54.05 mg,
0.5 mmol), TsOH (9.5 mg, 0.05 mmol), catalyst C (10.2 mg,
0.01 mmol), and MeOH (202.5 μL, 5 mmol) in CH2Cl2 (4 mL). The
autoclave was pressurized with ethylene gas (5 bar) and subse-
Presumably, the formation of γ-chelate complex II is favored for quently charged with CO gas (10 bar) at room temperature. Then,
the contents of the autoclave were stirred at 80 °C for 15 h. The
vessel was cooled to room temperature, and the unreacted gas was
released. The solvent was removed under vacuum, and the residue
was purified by flash column chromatography (silica gel, 5 % Et2O/
hexane) to afford dimethyl succinate (2b; 61.6 mg, TON = 21.1).
the formation of the diester (succinic diester). In the presence
of BQ, Pd–H is converted into Pd–CO2R.[6l,6g] Accordingly, the
Pd–carboalkoxy complex (Pd–CO2R) re-enters the catalytic cycle
to promote the desired bisalkoxycarbonylation of the olefin.
The greater catalytic activity of dimeric complex C might be
explained by facile insertion of CO into the empty coordination
site to afford a Pd–carboalkoxy complex. The role of the halo-
Synthesis of Catalyst C: A solution of complex B (130 mg,
0.29 mmol) and K[PF6] (0.973 mg, 5.30 mmol) in acetonitrile (15 mL)
and H2O (10 mL) was stirred at 85 °C for 8 h. The solvent was
removed under vacuum and distilled water (10 mL) was added. The
resulting mixture was kept at 0 °C for 10 min. The white solid was
filtered off, washed with diethyl ether, and dried under vacuum
(111.8 mg, 75 %).
Acknowledgments
This study was supported by the National Research Foundation
of Korea (NRF) through the C1 gas refinery program (No.
2015M3D3A1A01065436) and the Korea Research Foundation
(No. 2016R1D1A1A09917100).
Keywords: Alkenes · Carbonylation · Carbenes · Palladium ·
Succinic esters
Scheme 5. A plausible catalytic cycle.
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Received: December 4, 2016
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