Highly regioselective anti-markovnikov palladium-borate-catalyzed methoxycarbonylation reactions: Unprecedented results for aryl olefins

Article abstract of DOI:10.1021/ol062646n

(Chemical Equation Presented) A general, highly efficient and regioselective methoxycarbonylation, by means of a palladium-salicylicborate- catalyzed protocol, of terminal alkyl and aryl olefins is described. The substrates include aliphatic alkenes, allylbenzenes, and styrene derivatives. The yields are very good (60-92%) and the regioselectivity, in favor of the linear ester, is up to quantitative - unprecedented in the case of styrenes.

Full text of DOI:10.1021/ol062646n

ORGANIC
LETTERS
2006
Vol. 8, No. 26
6143-6145
Highly Regioselective Anti-Markovnikov
Palladium-Borate-Catalyzed
Methoxycarbonylation Reactions:
Unprecedented Results for Aryl Olefins
Tiago O. Vieira,† Mike J. Green,‡ and Howard Alper*,†
Centre for Catalysis Research and InnoVation, Department of Chemistry,
UniVersity of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5, and
Sasol Technology, 1 Klasie HaVenga Road, Sasolburg 1947, South Africa
howard.alper@uottawa.ca
Received October 28, 2006
ABSTRACT
A general, highly efficient and regioselective methoxycarbonylation, by means of a palladium-salicylicborate-catalyzed protocol, of terminal
alkyl and aryl olefins is described. The substrates include aliphatic alkenes, allylbenzenes, and styrene derivatives. The yields are very good
(60−92%) and the regioselectivity, in favor of the linear ester, is up to quantitativesunprecedented in the case of styrenes.
Carbon-carbon bond formation is a long-standing challenge
to organic chemists and several catalytic approaches have
been developed during the past decades.1 The palladium-
catalyzed hydroesterification (Reppe carbonylation) is well-
known as a powerful strategysdue to its atom economys
for the preparation of esters from easily available unsaturated
moieties.2 There are numerous methods for the synthesis of
branched esters in racemic or asymmetric fashion.3 However,
only a few protocols lead to linear esters in reasonable
regioselectivity.4 As a matter of fact, all instances rely on
substrate specificity, and the ligand is tailored to the
substrate.2b
highly regioselective hydroesterification protocol still con-
stitutes a significant challenge for homogeneous and het-
erogeneous catalysis. Herein, we wish to report our results
concerning the first palladium-borosalicylic acid-catalyzed
methoxycarbonylation of alkenessincluding long-chain ole-
fins, allylbenzenes, and styrenessin high regioselectivity for
the linear product.
During studies on the Pd-catalyzed methoxycarbonylation
of ethylene, salicylborates were found to be quite effective
acid promoters.6 We found that, by using in situ generated
borosalicylic acid (BSA) (Scheme 1), 1-decene gave the
Given that long-chain esters play a crucial role in the
manufacture of lubricants and surfactants,5 together with the
fact that petroleum derivatives are eligible candidates as raw
materials, we believe that the establishment of a general and
Scheme 1. Formation of the Salicylborates
† University of Ottawa.
‡ Sasol Technology.
(1) (a) Tsuji, J. Palladium Reagents and Catalysts; Wiley-VCH: New
York, 1999. (b) Handbook of Organopalladium Chemistry for Organic
Synthesis; Neguishi, E.-I., Ed.; John Wiley and Sons: New York, 2002;
Vols. 1 and 2.
10.1021/ol062646n CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/30/2006

Table 1. Selected Screening for Methoxycarbonylation of 1-Decene with Pd(OAc)2, Using BSA and 5-Cl-BSAa
borate
(mol %)b
Pd(OAc)2
(mol %)
phosphine
(mol %)
CO
(psi)
time
(h)
conversion
(%)c
L/B
ratio
yield
(%)d
Pd
black
entry
1
2
3
4
5
6
7
8
9
BSA (10)
BSA (20)
BSA (20)
BSA (20)
5-Cl-BSA (10)
5-Cl-BSA (10)
5-Cl-BSA (10)e
5-Cl-BSA (10)
5-Cl-BSA (10)
1
1
1.5
1
1
1.5
1.5
1
PPh3 (10)
PPh3 (10)
PPh3 (15)
(m-tolyl)3P (10)
PPh3 (15)
PPh3 (15)
PPh3 (15)
(o-tolyl)3P (10)
(p-tolyl)3P (10)
300
300
300
300
300
300
400
300
400
15
12
20
15
15
15
18
20
18
100
100
100
85
100
100
100
5
97:3
92:8
87:13
95:5
95:5
87:13
96:4
55
74
71
64
69
78
64
yes
yes
yes
no
no
no
yes
no
no
1.5
100
97:3
92
a The reactions were performed with 1 mmol of 1-decene in 3 mL of MeOH at 100 °C. b Prepared in situ in a 2:1 ratio between the salicylic acid and
B(OH)3. c Determined by GC. d Isolated yields. e MeOH/Et2O (1:1) as solvent.
linear ester in up to 97% selectivity under 300 psi of CO.
Unfortunately, palladium black plating was observed, which
makes this approach of no utility.
and only ca. 5% of double bond isomerization was observed
by GC analysis.
We were gratified to observe that, when the protocol was
investigated for styrene derivatives (Table 3), an unprec-
edented regioselectivity was achieved (85 to 100%), in favor
of the linear ester. It is worth mentioning that the best
reported selectivity for such hydroesterifications of styrene
is 85%.9 Even the more hindered geminal disubstituted
styrenes (Table 3, entries 9-11) were susceptible to hy-
Preliminary experiments to determine the optimum phos-
phine ligands (see Table 1 for a selected screening) revealed
that the more electron-rich tris-p-tolylphosphine was a
suitable ligand, and 5-chloroborosalicylic acid (Table 1, entry
9) was a better promoter than BSA itself, leading to the
undecanoic methyl ester in comparable and excellent selec-
tivity, but in higher yield and without the undesirable
formation of Pd black. With preformed Cl-BSA,7 only 60%
conversion and 90% selectivity was achieved. In all attempts
to use bidentate phosphines, such as dppe, dppb, dppp, and
rac-BINAP, as well as more hindered aryl phosphines, only
low or no conversion was observed.
Table 2. Methoxycarbonylation of 1-Alkenes and
Allylbenzenes with Pd(OAc)2 in the Cl-BSA Systema
Having found the best catalytic system for the hydro-
esterification reaction, we applied the same conditions to
other aliphatic alkenes (Table 2, entries 2-4) and quite
similar selectivities were obtained. It is worthy of note that
such high regioselectivities are highly unusual for Pd systems
involving monodentate phosphine ligands, and until now only
achievable by using specific bulky bidentate phosphine
ligands.4a
When allylbenzenes (Table 2, entries 5-7) were used
instead, after a small increase in CO pressure, the selectivity
for our system proved to be higher than previously reported,8
(2) (a) El Ali, B.; Alper, H. In Transition Metals for Organic Synthesis:
Building Blocks and Fine Chemicals; Beller, M., Bolm, C., Eds.; Wiley-
VCH: New York, 1998; Vol. 1, p 49. (b) Kiss, G. Chem. ReV. 2001, 101,
3435 and references cited therein.
(3) El Ali, B.; Alper, H. in ref 2a, Vol. 2, p 2333 and references cited
therein.
(4) (a) Rodriguez, C. J.; Foster, D. F.; Eastham, G. R.; Cole-Hamilton,
D. J. Chem. Commun. 2004, 1720. (b) Ko, S.; Na, Y.; Chang, S. J. Am.
Chem. Soc. 2002, 124, 750. (c) El Ali, B.; Alper, H. J. Mol. Catal. 1992,
77, 7.
a The reactions were performed in a 2 mmol scale in 3 mL of MeOH,
under 400 psi of CO, at 100 (entries 1-4) and 110 °C (entries 5-7). The
L/B ratio was determined by 1H NMR. No Pd black plating was observed.
b Isolated yields.
(5) Boyde, S. Green Chem. 2002, 4, 293.
(6) Ferreira, A. C.; Bennie, L.; Meij, A. M. M.; Blann, K.; Green, M. J.;
Roodt, A. J. Angew. Chem. Accepted for publication.
(7) Cl-BSA was prepared by refluxing 5-Cl-SA and B(OH)3, in a 2:1
ratio, in toluene until 3 equiv of H2O was collected.
6144
Org. Lett., Vol. 8, No. 26, 2006

Although the exact role of the salicylborate remains unclear,
it is known that the counteranion is critical in the determi-
nation of the regioselectivity in hydroesterification reac-
tions.11
Table 3. Methoxycarbonylation of Styrenes with Pd(OAc)2 in
the Cl-BSA Systema
Another important feature of the borosalicylic counterion
is its bulkiness, which, by hindrance, would favor the
formation of the linear esters. Possible evidence for such an
effect is the failure, using this protocol, in the methoxycar-
bonylation of internal olefins, such as 2-octene and 1,2-
dihydronaphthalene. A postulated mechanism is depicted in
Scheme 2, based on a pathway proposed previously for the
time
(h)
L/B
ratio
yield
(%)b
entry
R1
R2
R3
R4
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
Me
Br
H
H
H
H
H
H
H
H
H
OMe
Cl
H
H
24
24
40
48
24
24
24
36
48
48
48
90:10
88:12
85:15
90:10
97:3
66
91
72
72
60
65
84
66
74
71
73
tBu
Cl
F
Scheme 2. Suggested Mechanism for the
Methoxycarbonylation of Styrene by Means of Palladium
Salicylborate Catalysis
H
H
H
H
H
Cl
H
98:2
85:15
89:11
97:3
95:5
100:0
Me
Me
Ph
10
11
H
H
H
H
a The reactions were performed with the same conditions reported in
Table 2, under 500 (600 for entries 9-11) psi of CO, at 110 °C. The L/B
ratio was determined by 1H NMR. No Pd black plating was observed.
b Isolated yields.
droesterification in up to quantitative regioselectivity. How-
ever, the protocol failed for substrates containing a nitrogen
atom, such as 1-allylimidazole and 4-vinylpyridine, and
palladium black deposition was observed.
hydroesterification of styrene with p-TSA as the acid
promoter.10a,b
In summary, this Letter describes the high catalytic activity
of Pd-borates that allows the preparation of one-carbon
elongated esters from olefins in very good isolated yields
and good to outstanding regioselectivity. This is a general
and highly effective entry for the synthesis of linear esterss
unprecedented for aryl olefinssand is of significant industrial
interest.
In mechanistic terms, a viable species that could be the
starting point for the catalytic cycle is a palladium(II) hydride,
as previously proposed in the literature.10 Assuming that the
borosalycilic counterion is weakly coordinated to the palla-
dium(II) species, it would favor the cationic reaction
pathway, which would support the observed selectivity.
Acknowledgment. We are indebted to SASOL Technol-
ogy for financial support of this work.
(8) Chenal, T.; Cipres, I.; Jenck, J.; Kalck, P. J. Mol. Catal. 1993, 78,
351.
(9) (a) Klingshirn, M. A.; Rogers, R. D.; Shaughnessy, K. H. J.
Organomet. Chem. 2005, 690, 3620. (b) Aghmiz, A.; Gime´nez-Pedro´s, M.;
Masdeu-Bulto´, A. M.; Schimidtchen, F. P. Catal. Lett. 2005, 103, 191. (c)
Bianchini, C.; Meli, A.; Oberhauser, W.; Parisel, S.; Gusev, O. V.; Kal’sin,
A. M.; Vologdin, N. V.; Dologushin, F. M. J. Mol. Catal. A: Chem. 2004,
224, 35.
(10) (a) de Pater, J. J. M.; Tromp, D. S.; Tooke, D. M.; Spek, A. L.;
Deelman, B.-J.; van Koten, G.; Elsevier, C. J. Organometallics 2005, 24,
6411. (b) Seayad, A.; Jayasree, S.; Damodoran, K.; Toniolo, L.; Chaudhari,
R. V. J. Organomet. Chem. 2000, 601, 100. (c) Grushin, V. V. Chem. ReV.
1996, 96, 2011.
Supporting Information Available: A general procedure
for the methoxycarbonylation reaction is described and full
characterization of new compounds is reported. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL062646N
(11) Bonnet, M. C.; Monteiro, A. L.; Tkatchenko, I. J. Mol. Catal. A:
Chem. 1999, 143, 131.
Org. Lett., Vol. 8, No. 26, 2006
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