Olefins from biomass feedstocks: Catalytic ester decarbonylation and tandem Heck-type coupling

Article abstract of DOI:10.1039/c4cc09003a

With the goal of avoiding the need for anhydride additives, the catalytic decarbonylation of p-nitrophenylesters of aliphatic carboxylic acids to their corresponding olefins, including commodity monomers like styrene and acrylates, has been developed. The reaction is catalyzed by palladium complexes in the absence of added ligands and is promoted by alkali/alkaline-earth metal halides. Combination of catalytic decarbonylation and Heck-type coupling with aryl esters in a single pot process demonstrates the viability of employing a carboxylic acid as a "masked olefin" in synthetic processes. This journal is

Full text of DOI:10.1039/c4cc09003a

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Olefins from biomass feedstocks: catalytic ester
decarbonylation and tandem Heck-type coupling†
Cite this:DOI: 10.1039/c4cc09003a
Alex John, Levi T. Hogan, Marc A. Hillmyer* and William B. Tolman*
Received 11th November 2014,
Accepted 3rd January 2015
DOI: 10.1039/c4cc09003a
www.rsc.org/chemcomm
With the goal of avoiding the need for anhydride additives, the catalytic
decarbonylation of p-nitrophenylesters of aliphatic carboxylic acids
to their corresponding olefins, including commodity monomers like
styrene and acrylates, has been developed. The reaction is catalyzed by
palladium complexes in the absence of added ligands and is promoted
by alkali/alkaline-earth metal halides. Combination of catalytic decar-
bonylation and Heck-type coupling with aryl esters in a single pot
process demonstrates the viability of employing a carboxylic acid as
a ‘‘masked olefin’’ in synthetic processes.
Biomass, and chemicals derived therefrom, hold promise in transi-
tioning to a more sustainable bio-based economy.1 Carboxylic acids
are an important class of biomass-derived molecules that can be
used as starting materials for the synthesis of a variety of potentially
useful compounds. For example, polymerizable olefins may be
accessed through transition metal catalyzed dehydrative decarbony-
lation of aliphatic carboxylic acids (Fig. 1, top).2–4 Such reactions
Fig. 1 Comparison of previous work to decarbonylation and in situ Heck-
type coupling reactions reported herein.
Herein, we report (a) the development of a palladium-catalyzed
often employ Pd,3,4 Rh2d or Ir5 catalysts and phosphine based decarbonylation of p-nitrophenylesters of aliphatic carboxylic acids
ligands, with some recent attention to use of base metals like Fe6 to the corresponding olefins, and (b) demonstration of the viability
and Ni.7 An indispensable ingredient in all of the processes reported of a one-pot tandem Heck-type coupling reaction starting from the
thus far is a sacrificial stoichiometric anhydride, like acetic (Ac2O) or p-nitrophenylesters of hydrocinnamic acid (a styrene precursor) and
pivalic anhydride (Piv2O), which activates the carboxylic acid sub- the p-nitrophenylesters of various aromatic carboxylic acids (Fig. 1,
strate by forming a mixed anhydride that can readily undergo bottom). The catalytic decarbonylation of p-nitrophenylesters differs
oxidative addition to the metal center. A key goal of current research from the typical decarbonylation that involves intermediacy of a
is to increase the efficiency of the reaction and avoid the use of such mixed anhydride. The tandem Heck-type process is particularly
a stoichiometric (and wasteful) co-reagent. While some success notable, especially in light of recent efforts to incorporate biomass
in effecting decarbonylation of carboxylic acids directly at high derived chemicals in synthesis,9 and because Heck-coupling is a
temperatures (200–250 1C) using Ru catalysts has been reported,8 powerful synthetic tool for coupling alkenes with aromatics.10
attainment of high yields under mild conditions appears to
require activation of the acid.
An early precedent for the use of esters as substrates includes
the demonstration of a Ni(0)-mediated decarbonylation of phenyl
propionates to yield ethylene using simple phosphine ligands like
PPh3 and PCy3.11 It was also noted in this work that (i) more than
one turnover could be achieved with esters of electron-withdrawing
phenols like p-cyanophenol and (ii) that simple alkyl esters did not
participate in the reaction. We were also inspired by the report of
a palladium-catalyzed decarbonylation of p-nitrophenylesters of
aromatic carboxylic acids, which in the presence of alkenes resulted
in a Heck-type coupling via a putative aryl–Pd intermediate.12
Department of Chemistry and Center for Sustainable Polymers, University of
Minnesota, 207 Pleasant St SE, Minneapolis, MN, 55455-0431, USA.
E-mail: hillmyer@umn.edu, wtolman@umn.edu; Fax: +1-(612)-626-8659;
Tel: +1-(612)-625-4061
† Electronic supplementary information (ESI) available: Experimental proce-
dures, characterization data and NMR spectra for compounds synthesized and
isolated. See DOI: 10.1039/c4cc09003a
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Table 1 Effect of metal halides on the decarbonylation reactiona
product yield (entries 3 and 7). Addition of N-donor ligands (pyr-
idine, DMAP) had a detrimental effect (entry 6; styrene yield o 5%).
Through these combined studies, optimal conditions were identified
as PdCl2 (2.5 mol%), LiCl (100 mol%) in DMPU (ca. 0.5 mL) at
160 1C for 3 h (Table 2, entry 3).
Entry
MXn
Yieldb (%)
Using these optimized conditions, we explored the substrate
scope of the reaction (Table 3).‡ The p-nitrophenylester of tert-
butylsuccinate (which can be derived from bio-based succinic
anhydride3) was readily converted to tert-butylacrylate in 46%
yield (GC-MS; entry 2). Similarly, acrylonitrile was produced in
44% yield from 3-cyanopropanoic acid, which can be synthe-
sized via oxidative decarboxylation4b of glutamic acid (entry 5).
The substrates 4-phenylbutyric acid and 2-phenylbutyric acid
(entries 3 and 4) gave identical product distributions with trans-
b-methylstyrene as the major olefin product (trans : cis = 8 : 1);
allylbenzene was also detected as a minor coproduct (10%).
These results suggest that olefin isomerization occurs under the
reaction conditions, probably via re-coordination and chain-walking
of the p-allyl intermediate.14 Decarbonylation of the fatty acids
1
2
3
4
5
6
7
8
9
10
—
46
51
58
52
o5
62
71
78
53
66
NaCl
NaBr
KCl
CH3COONa
TBABc
CaCl2
LiCl
MgCl2
LiBr
a
Reaction conditions: p-nitrophenylhydrocinnamate (0.075 g, 0.28 mmol),
PdCl2 (0.004 g, 0.023 mmol, 10 mol%) and metal halide (0.26 mmol) in
DMPU (ca. 0.5 mL) at 160 1C for 16 hours. b Determined by GC using 1,3,5-
trimethoxybenzene as internal standard. c TBAB = tetra-n-butylammonium
bromide.
Initial test reactions focused on screening the decarbonylation of undecylenic acid and pelargonic acid (nonanoic acid; entries 7
the p-nitrophenylester of hydrocinnamic acid using various Ni and Pd and 8), also generated a mixture of the isomeric decadienes and
catalysts (e.g. NiI2, Ni(COD)2, PdCl2, PdI2) in the presence of phos- octenes in 31% and 35% yields, respectively. Attempts to synthesize
phine ligands (e.g. PPh3, PCy3, DPEPhos, dppe, dcype, dppb).13 1,3-butadiene by decarbonylation of 4-pentenoic acid resulted
Irrespective of the conditions employed, the reactions yielded only in o10% yield of the alkene, detected by trapping as tetrabromo-
B5–10% styrene and almost quantitative ester hydrolysis was noted. butane using in situ bromination at ꢀ10 1C (entry 6).
Reasoning that the ligands could be acting as nucleophiles and
Having identified a system that would catalytically generate olefins
promoting ester hydrolysis, we performed a reaction with PdCl2 from esters, we explored the possibility of coupling the decarbonylation
(10 mol%) in N,N0-dimethylpropyleneurea (DMPU) as solvent at
Table 3 Substrate scope for decarbonylation of p-nitrophenylestersa
160 1C in the absence of any added ligands, and found that styrene
was produced in 46% yield after 16 h. The reaction efficiency
Entry
Acid
Olefin
Yieldb (%)
improved upon addition of stoichiometric amounts of alkali and
alkaline-earth metal halides (Table 1). The nature of the halide as well
as the metal were found to affect the observed reactivity, as styrene
yield was 66% with LiBr while NaCl and KCl produced styrene in 51%
and 52% yield, respectively. LiCl and CaCl2 were found to increase
styrene yield to 78% and 71%, respectively.
1
53 (79)
2
(46)
The amount of the LiCl promoter was found to be important as
the yield dropped to 54% when its loading was reduced to 20 mol%
(Table 2, entry 2). The reaction temperature also was found to have
significant effect on the reaction efficiency (entries 4 and 5; 27%
yield at 120 1C over 16 h vs. 67% yield at 140 1C in 3 h). Loadings of
PdCl2 of 2.5 mol% and 1 mol% resulted in only modest decreases in
3
4
52
59
5
(44)
(o10)
35
Table 2 Decarbonylation of p-nitrophenylhydrocinnamatea
6
Entry
Conversion (%)
T (1C)
Time (h)
Yieldb (%)
1c
2d
3e
4
160
160
160
120
140
140
160
16
16
3
16
3
3
5
79
54
75
27
67
7c
Octene isomers
Decene isomers
65
8c
32
5
6f
7g
45
94
o5
47
a
Reaction conditions: p-nitrophenyl ester (0.26 mmol), PdCl2 (0.004 g,
a
Reaction conditions: p-nitrophenylhydrocinnamate (0.075 g, 0.28 mmol), 0.023 mmol, 5 mol%) and LiCl (0.010 g, 0.24 mmol, 100 mol%) in
b
PdCl2 (0.004 g, 0.023 mmol, 5 mol%) and LiCl (0.010 g, 0.24 mmol, DMPU (ca. 0.5 mL) for 3 h. Ar = p-NO2C6H4. Determined by GC using
100 mol%) in DMPU (ca. 0.5 mL). Determined by GC using 1,3,5- 1,3,5-trimethoxybenzene as internal standard (in parenthesis); isolated
trimethoxybenzene as internal standard. PdCl2 (5 mol%). LiCl yields were determined for reactions carried at 1 mmol scale and run
(20 mol%). PdCl2 (2.5 mol%). Pyridine (50%). PdCl2 (1 mol%).
b
c
d
e
f
g
c
for 5 h. Reaction carried out for 16 h.
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DMPU (ca. 0.5 mL) followed by a Teflon-coated stir bar to yield a light yellow
mixture. The Schlenk tube was sealed, brought outside the glove box and
set in an oil bath pre-heated to 155–160 1C. The reaction mixture was
allowed to stir at this temperature for the stipulated period of time during
which it darkened to a final red-brown color. At the end of the reaction, the
Schlenk tube was removed from the oil bath and allowed to cool to room
temperature. (a) For GC-MS analysis: the reaction mixture was diluted with
EtOAc (ca. 5 mL) and washed with 1 M HCl (ca. 5 mL ꢁ 2) and brine (ca.
5 mL). The organic layer was collected, dried over MgSO4, and analyzed by
GC-MS using 1,3,5-trimethoxybenzene as an internal standard. (b) For
olefin isolation (reaction carried out at 1 mmol scale): the reaction
mixture was diluted with 1 M HCl (ca. 10 mL) and extracted with pentane
(ca. 5 mL ꢁ 3). The combined organic layers were washed with 1 M HCl
(ca. 5 mL), the pentane extracts were dried with MgSO4, and then they were
concentrated under vacuum to remove solvent. Analysis of the residue by
1H NMR spectroscopy showed the olefin product in 490% purity.
Scheme 1 Tandem double decarbonylative Heck-type coupling.
reaction with the decarbonylative Heck-type coupling of aromatic esters
with olefins.12 Reports on Heck-coupling have focused on alternate
routes to incorporate the aromatic moiety in the cross-coupling reac-
tion; we are unaware of any efforts that explore novel methods of
introducing the olefin counterpart. Starting from equivalent amounts
of p-nitrophenylbenzoate and p-nitrophenolhydrocinnamate, trans-
stilbene was produced in 61% yield under our optimized reaction
conditions. This first demonstration of the use of a carboxylic acid ester
as a ‘‘masked olefin’’ in a Heck-coupling reaction has intriguing
potential as a tool in synthesis. Monitoring of the progress of the
tandem Heck-coupling reaction by GC-MS analysis showed initial build
up of styrene and loss of the hydrocinnamic ester in the mixture prior
to trans-stilbene generation accompanied by styrene and benzoate ester
consumption (Fig. S1, ESI†). Substituted benzoic acid esters partici-
pated in the reaction to yield the respective asymmetric stilbenes,
consistent with cross-coupling of components from the two different
ester starting materials (Scheme 1). The cross-coupling efficiency
decreased when activated aromatic esters (with electron withdrawing
substituents) were used, as their decarbonylation to yield the corres-
ponding parent arene via protonation of the Ar–Pd intermediate was
competitive. When p-nitrophenol-4-bromobenzoate was employed as
the coupling partner, a minor amount of trans-stilbene (o5%) was
observed in the reaction mixture along with 4-bromostilbene, presum-
ably arising from the coupling of styrene with bromobenzene.
In summary, we have identified a simple system for the catalytic
decarbonylation of p-nitrophenylesters of aliphatic carboxylic acids
that employs PdCl2 as catalyst and is promoted by alkali/alkaline-
earth metal halides like LiCl and CaCl2. The reaction generates
olefins, including commodity monomers like styrene, acrylates,
acrylonitrile and octene(s), in moderate to good yields. We have
also discovered that the olefins generated by the decarbonylative
pathway can participate in a tandem decarbonylative Heck-type
cross-coupling reaction. This reaction provides a new route to
stilbenes, notable synthetic targets.15 Ongoing efforts aim to
further improve upon these processes by generating activated
esters of bio-derived carboxylic acids in situ and, ultimately,
rendering the process catalytic in phenol additive.
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Funding for this project was provided by the Center for
Sustainable Polymers, a National Science Foundation supported
Center for Chemical Innovation (CHE-1413862).
13 DPEPhos = Bis[(2-diphenylphosphino)phenyl]ether, dppe = diphenyl-
phosphinoethane, dcype = dicyclohexylphosphinoethane, dppb =
diphenylphosphinobutane.
Notes and references
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