Chelation-assisted hydroesterification of alkenes: New ruthenium catalyst systems and ligand effects

Article abstract of DOI:10.1021/ol500579n

New types of ruthenium catalysts were developed for the chelation-assisted intermolecular olefin hydroesterification that employs 2-pyridylmethyl formate as an ester source. Two classes of ligands, NHCs and phosphines, were found to facilitate the reactio

Full text of DOI:10.1021/ol500579n

Letter
pubs.acs.org/OrgLett
Chelation-Assisted Hydroesterification of Alkenes: New Ruthenium
Catalyst Systems and Ligand Effects
Bin Li,†,‡,§,∥ Seungeon Lee,‡,∥ Kwangmin Shin,‡,† and Sukbok Chang*,†,‡
†Center for Catalytic Hydrocarbon Functionalizations, Institute of Basic Science (IBS), Daejeon 305-701, Korea
‡Department of Chemistry, Korea Advanced Institute of Science & Technology (KAIST), Daejeon 305-701, Korea
§State Key Lab of Element-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China
S
* Supporting Information
ABSTRACT: New types of ruthenium catalysts were developed for
the chelation-assisted intermolecular olefin hydroesterification that
employs 2-pyridylmethyl formate as an ester source. Two classes of
ligands, NHCs and phosphines, were found to facilitate the reaction
delivering isomeric ester products (linear versus branched) with
different ratios, thus allowing access to ligand-guided selective
hydroesterification.
This issue becomes more critical especially when a selective
transformation is desired with regard to regio- or stereo-
selectivity. In addition, because the nature and composition of
catalytically active species was difficult to determine under the
conditions using the Ru3(CO)12 catalyst, a mechanistic
understanding of this reaction has been unclear. In this aspect,
a recent report of Manabe et al. was notable in that Ru3(CO)12-
catalyzed inter- and intramolecular olefin hydroesterification
was found to be tuned with the aid of imidazole ligands.7
We envisioned that a new catalyst system of hydroesterifi-
cation could be developed by a proper combination of metal
precursors and suitable ligands. Among various potential
benefits we can obtain from the well-defined catalytic systems,
control of the regioselectivity between two isomeric products
(linear versus branched) by tuning the electronic and/or steric
properties of ligands employed was especially desired.8
Described herein are our recent studies on the development
of two new catalyst systems for the intermolecular olefin
hydroesterification using 2-pyridylmethyl formate as an ester
source (Scheme 1, bottom).
ransition-metal-catalyzed hydroesterification of alkenes
1
Twith formates, an alternative route to olefin alkoxycarbo-
nylation using highly pressurized CO gas and alcohols,2
constitutes one of the most convenient processes for the
formation of one-carbon elongated esters. As a result, various
catalytic procedures have been developed aiming for high
efficiency and selectivity over the past few decades.3−5 One big
issue in the catalytic hydroesterification is in the destructive
decarbonylation of an alkoxycarbonyl metal hydride inter-
mediate, in situ generated from the reaction of formates with
metal species. This side pathway leads to the corresponding
alcohols and CO to finish a catalytic cycle.
In this regard, we demonstrated that a chelation-assisted
strategy could efficiently be utilized to inhibit the decarbon-
ylation path by using 2-pyridylmethyl formate as a carboxylate
reagent in the Ru3(CO)12-catalyzed intermolecular hydro-
esterification of alkenes and alkynes (Scheme 1, top).6 While
this procedure, as envisioned, does not require an external CO
atmosphere, the development of the so-called next generation
catalytic systems was not easy mainly due to poor ligand effects.
At the outset of optimization studies on the catalytic
hydroesterification, 3,3-dimethyl-1-butene was chosen as a
model substrate to react with 2-pyridylmethyl formate (1)
using various ruthenium species in combination with a wide
range of N-heterocyclic (NHC) ligands (Table 1).9
Scheme 1. Olefin Hydroesterification with 2-Pyridylmethyl
Formate
Whereas only a negligible amount of product was formed by
the action of Ru(nbd)Cl2 alone (entry 1), NHC ligands were
found to significantly affect the reaction efficiency. While
product yields were moderate to low with NHCs derived from
an imidazolium skeleton (entries 2−6), the steric bulkiness of
N-substituents induced a slightly higher efficiency although the
extent was varied depending on the reaction parameters.
Interestingly, a saturated NHC derivative having the same N-
Received: February 23, 2014
Published: March 27, 2014
© 2014 American Chemical Society
2010
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Letter
a
Table 1. Optimization of the Ru/NHC Catalyst System
Scheme 2. Hydroesterification of Various Alkenes Catalyzed
by Ru(II)/Ad2-bimy/KOtBu
a
b
entry
[Ru]
ligand
none
solvent
yield (%)
1
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
Ru(nbd)Cl2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
Ru(cod)Cl2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
MeCN
DMF
<5
14
7
2
ICy·HBF4
IiPr·HCl
ItBu·HBF4
IMes·HCl
IAd·HBF4
SIAd·HCl
Cy2-bimy·HCl
Cy2-bimy·HCl
Cy2-bimy·HCl
Cy2-bimy·HCl
Cy2-bimy·HCl
tBu2-bimy·HCl
Ad2-bimy·HCl
Ad2-bimy·HCl
Ad2-bimy·HCl
none
3
4
21
17
55
27
50
71
13
44
76
78
86
0
5
6
7
8
9
10
11
12
13
14
THF
THF
THF
c
15
THF
d
16
d
THF
82
0
17
THF
18
Ad2-bimy·HCl
Ad2-bimy·HCl
THF
66
d
19
(Cp*RuCl2)2
THF
42
a
b
1
a
b
c
1 (0.2 mmol), alkene (0.4 mmol). Yield determined by H NMR
Reactions in 0.2 mmol scale. Combined isolated yields. Ratio of
c
(internal standard: 1,1,2,2-tetrachloroethane). Benzyl formate was
used instead of 1. 2.5 Mol % of catalyst was used. Cp* =
pentamethylcyclopentadienyl.
linear and branched isomers in crude reaction mixture (measured by
NMR with 1,1,2,2-tetrachloroethane as an internal standard). d At 110
°C. e Ratio of 1-, 2-, and 3-isomeric esters. f Ratio of exo-/endo-isomer.
d
g
Ratio of two regioisomers (1-carboxylate/2-carboxylate).
Both acyclic (2a−2n) and cyclic (2o−2q) olefins were readily
reacted to afford the corresponding esters in moderate to good
yields. As anticipated, the ratio of regioisomers (linear versus
branched ester products) was sensitive to the steric environ-
ment around the olefinic double bonds. For instance, while
linear terminal alkenes (1-hexene, 2a; and 1-octene, 2b) were
converted to the corresponding esters with an ∼4:1 ratio
favoring linear isomeric products, hydresterification of 3,3-
dimethyl-1-butene (2c) and vinylcyclohexane (2d) took place
almost exclusively to favor linear isomers. On the other hand,
styrenes underwent the reaction with a Ru/NHC system to
afford branched isomers more favorably, and this preference
became more significant with substrates bearing electron-
withdrawing substituents. While selectivity in a reaction of 2-
vinylnaphthalene (2k) was moderate, 1,1,-disubstituted olefin
(2l) gave a linear product exclusively.
The reaction of linear internal alkenes (e.g., 2m and 2n) was
nonselective to give terminal esters as the major product in
addition to the expected two internal isomers, suggesting that
olefin isomerization occurrs during the course of hydroesteri-
fication probably initiated by a ruthenium hydride species.6a,d,10
Cyclic internal olefins (20−2q) were smoothly reacted to give
the corresponding ester products.
substituent was less effective when compared to unsaturated
NHCs (entries 6 and 7). We were pleased to see that NHC
ligands of a benzimidazolium skeleton were more effective than
the corresponding imidazolyl derivatives (e.g., entries 2 and 8).
Moreover, the reaction efficiency turned out to be improved
depending on solvents; THF was the most satisfactory (entries
8−12). By combining these variables together, the highest
product yield was obtained when an NHC derived from N-
adamantyl-substituted benzimidazole (Ad2-bimy) was em-
ployed in combination with Ru(nbd)Cl2 in THF at 90 °C
(entry 14).
The present hydroesterification was proved to proceed via a
chelation-assisted pathway by observing that a reaction with
benzyl formate instead of 1 was totally unsuccessful (entry 15).
Other ruthenium species were also examined; while a
combination of [Ru(p-cymene)Cl2]2 with Ad2-bimy gave a
similar outcome (entry 16), no conversion was observed in the
absence of the NHC ligand (entry 17). On the other hand,
Ru(cod)Cl2 and (Cp*RuCl2)2 displayed lower catalytic
activities under otherwise identical conditions (entries 18 and
19, respectively). To the best of our knowledge, this is the first
application of an NHC ligand in the olefin hydroesterification
reaction.
It needs to be noted that, in our previous olefin
hydroesterification procedure using Ru3(CO)12 as a catalyst,4
linear esters (anti-Markovnikov products) were obtained always
predominantly.11 In contrast, under the present conditions of
using Ru/NHC, not only linear but also branched adducts
(Markovnikov products) were formed as major products
depending on the substrates employed. As shown above,
whereas aliphatic terminal olefins were converted to linear
Under the optimized conditions, the reaction scope was next
investigated by using a range of alkene substrates (Scheme 2).
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Letter
adducts favorably (2a−2d), branched esters were formed
mainly in the reaction of styrenes (2e−2k).
active species. Again, as in the case of Ru/NHC, olefin
hydroesterification with benzyl formate did not occur with
Ru(II)/SPhos/HCO2Na, thus suggesting that the present
reaction involves a chelation-assisted pathway.
With a new catalyst system of Ru(nbd)Cl2−SPhos-HCO2Na,
the scope of alkene substrates was briefly examined (Scheme
4). In general, product yields of resulting esters were lower for
To see any dependency of the ratio of linear (L) versus
branched (B) isomers on the electronic nature of substituents, a
Hammett study was performed using various para-substituted
styrenes. Interestingly, when log(B/L) was plotted against σp, a
linear relationship was observed with a positive slope (ρ =
+0.52, R2 = 0.97, Figure 1). The positive ρ value suggests that
Scheme 4. Olefin Hydroesterification with a Catalyst System
a
of Ru(II)/SPhos/HCO2Na
Figure 1. Hammett plot.
negative charge builds up in the regioselectivity-determining
transition state. Thus, the preference for the branched isomers
in the hydroesterfication of styrenes with electron-withdrawing
substituents might be explained by the stabilization of negative
charge in transition states via delocalization onto an aryl ring
(compare A and B in Scheme 3). The regioselectivity should
Scheme 3. Proposed Intermediates in the Regioselectivity-
Determining Step with Ru(II)/NHC (L = NHC)
a
b
c
Reactions in 0.2 mmol scale. Combined isolated yields. Ratio of
linear and branched isomer of crude mixture determined by 1H NMR.
d
Ru(nbd)Cl2 (7.5 mol %), SPhos (22.5 mol %), HCO2Na (45 mol
e
f
%), and 3.0 equiv of alkene. Ratio of exo-/endo-isomer. Ratio of 1-
carboxylate/2-carboxylate. Ratio of 1-, 2-, and 3-isomeric esters.
g
the same alkenes when compared to those obtained from the
former Ru/NHC catalyst system. Aliphatic terminal alkenes
were reacted under the optimized conditions (in chlorobenzene
at 120 °C) to afford the corresponding esters although the
preference for the linear isomers was slightly lower than that
with Ru/NHC (in THF at 90 °C). It was noted that styrenes
underwent the hydroesterification to lead to linear products as
the major product irrespective of the electronic nature of the
substituents. This reversal in regioselectivity is highly
interesting in that branched isomers were obtained as major
products for all styrenes examined with the Ru/NHC system
(Scheme 2). Although it requires further investigation to
explain the observed dichotomy, this result may serve as a clue
for the future development of highly selective catalytic systems
which can control the formation of linear or branched isomers
just by tuning ligands. Cyclic olefins (2p and 2q) were also
reacted without difficulty although the yields were slightly lower
than in the case of the Ru/NHC system. As expected, the
hydroesterification of 1,1,-disubstituted olefins, being aliphatic
or styrenyl (2l or 2r), was highly selective to afford linear
products exclusively in both cases. Again, as in the case of Ru/
NHC, isomerization occurred in a reaction of 2n.
also be affected by the reversibility of the hydrometalation and
relative rate for reductive elimination between linear and
branched intermediates. It is assumed that the combination of
these effects seems to override the steric influence that favors
liner isomers.
Next, we tried to develop another type of well-defined
catalyst system that employs phosphines as a tunable ligand.
After extensive optimization studies (see the Supporting
Information for details), we found that Ru(II) precursors,
representatively Ru(nbd)Cl2, combined with monodentate
SPhos (2-dicyclohexylphosphino-2′,6′-dimethoxy-1,1′-biphen-
yl) most effectively facilitated the hydroesterification of alkenes
with 1 in the presence of HCO2Na. Chlorobenzene was the
solvent of choice regarding reactivity and selectivity. It needs to
be mentioned that an analogous catalyst system of [Ru(p-
cymene)Cl2]2-phosphines-HCO2Na was previously applied to
the hydroarylation of vinylsilanes and styrenes by Darses and
Genet.11c They showed that a catalytically active species was in
situ generated Ru(0) and that HCO2Na was essential for this
process. As anticipated, in our current hydroesterification
procedure, [Ru(p-cymene)Cl2]2 displayed similar catalytic
activity in the presence of SPhos when compared to the
Ru(nbd)Cl2/SPhos system,12 thus suggesting that the current
catalytic system might also be a similar type of catalytically
In summary, we have developed two new types of catalyst
systems for the intermolecular hydroesterification of alkenes
using 2-pyridylmethyl formate as an ester source. With the use
of a catalyst system Ru(nbd)Cl2/NHC, the reaction proceeds
more efficiently under milder conditions when compared to
2012
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Letter
(6) (a) Ko, S.; Na, Y.; Chang, S. J. Am. Chem. Soc. 2002, 124, 750.
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Ru(nbd)Cl2/phosphine/HCO2Na. Importantly, the regioselec-
tivity for the formation of linear versus branched isomeric
products was observed to be reversed in certain substrates by
the chosen ligands. Although the catalytic activity and
selectivity do not yet reach practical standards with these
present systems, they represent the first examples of employing
NHCs or phosphines for the chelation-assisted hydroesterifi-
cation reaction. On the basis of the present study, research
efforts to develop more efficient and selective catalyst systems
are now underway.
(7) Konishi, H.; Ueda, T.; Muto, T.; Manabe, K. Org. Lett. 2012, 14,
4722.
(8) For recent examples from this laboratory, see: (a) Kim, M.;
Chang, S. Org. Lett. 2010, 12, 1640. (b) Kwak, J.; Kim, M.; Chang, S. J.
Am. Chem. Soc. 2011, 133, 3780. (c) Kim, H. J.; Kim, M.; Chang, S.
Org. Lett. 2011, 13, 2368. (d) Kwak, J.; Ohk, Y.; Jung, Y.; Chang, S. J.
Am. Chem. Soc. 2012, 134, 17778. (e) Li, B.; Park, Y. S.; Chang, S. J.
Am. Chem. Soc. 2014, 136, 1125.
ASSOCIATED CONTENT
■
(9) For selected reviews on N-heterocyclic carbenes in transition
metal catalysis, see: (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002,
S
* Supporting Information
41, 1290. (b) Díez-Gonzal
2009, 109, 3612.
́
ez, S.; Marion, N.; Nolan, S. P. Chem. Rev.
Experimental procedure and characterization of new com-
pounds (1H and 13C NMR spectra). This material is available
free of charge via the Internet at http://pubs.acs.org.
(10) (a) Trost, B. M.; Toste, F. D.; Pinkerton, A. B. Chem. Rev. 2001,
101, 2067. (b) Ayllon, J. A.; Sayers, S. F.; Sabo-Etienne, S.; Donnadieu,
B.; Chaudret, B.; Clot, E. Organometallics 1999, 18, 3981. (c) Lee, M.;
Ko, S.; Chang, S. J. Am. Chem. Soc. 2000, 122, 12011.
(11) (a) Martinez, R.; Chevalier, R.; Darses, S.; Genet, J.-P. Angew.
Chem., Int. Ed. 2006, 45, 8232. (b) Martinez, R.; Genet, J.-P.; Darses,
S. Chem. Commun. 2008, 3855. (c) Martinez, R.; Simon, M.-O.;
Chevalier, R.; Pautigny, C.; Genet, J.-P.; Darses, S. J. Am. Chem. Soc.
2009, 131, 7887.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: sbchang@kaist.ac.kr.
Author Contributions
∥B.L. and S.L. contributed equally.
Notes
(12) See the Supporting Information for details of the optimization
study.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Institute of Basic Science
(IBS) in Korea. B.L. is also grateful to National Natural Science
Foundation of China (21372121) for financial support.
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