Palladium-Catalyzed Carbonylation of sec- and tert-Alcohols

Article abstract of DOI:10.1002/anie.201701950

A general palladium-catalyzed synthesis of linear esters directly from sec- and tert-alcohols is described. Compared to the classic Koch–Haaf reaction, which leads to branched products, this new transformation gives the corresponding linear esters in high yields and selectivity. Key for this protocol is the use of an advanced palladium catalyst system with L2 (py^tbpx) as the ligand. A variety of aliphatic and benzylic alcohols can be directly used and the catalyst efficiency for the benchmark reaction is outstanding (turnover number up to 89 000).

Full text of DOI:10.1002/anie.201701950

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201701950
German Edition: DOI: 10.1002/ange.201701950
Catalysis
Palladium-Catalyzed Carbonylation of sec- and tert-Alcohols
Kaiwu Dong+, Rui Sang+, Jie Liu, Rauf Razzaq, Robert Franke, Ralf Jackstell, and
Matthias Beller*
Dedicated to Professor Jan-Erling Bꢀckvall on the occasion of his 70th birthday
Abstract: A general palladium-catalyzed synthesis of linear
esters directly from sec- and tert-alcohols is described.
Compared to the classic Koch–Haaf reaction, which leads to
branched products, this new transformation gives the corre-
sponding linear esters in high yields and selectivity. Key for this
protocol is the use of an advanced palladium catalyst system
with L2 (pytbpx) as the ligand. A variety of aliphatic and
benzylic alcohols can be directly used and the catalyst
efficiency for the benchmark reaction is outstanding (turnover
number up to 89000).
been carbonylated in the past. For example, the palladium-
catalyzed carbonylation of allylic alcohols to give b,g-unsatu-
rated carbonyl compounds was explored by the groups of
Miura and Alper as well as us.[10] Obviously, in this case the
hydroxy group is activated and the stabilized (p-allyl)palla-
À
dium intermediate facilitates the cleavage of the C O bond.
In the context of the carbonylation of benzyl alcohol and its
analogs, a significant amount of halides is added to generate
the corresponding benzyl halide, which initiates the catalytic
cycle.[11] It is the same case for the industrial carbonylation of
methanol, which produces acetic acid with rhodium- or
iridium-based catalyst systems in the presence of iodide
(Monsanto[12] and Cativa[13] process).
V
arious catalytic carbonylation reactions of olefins, including
hydroformylations and alkoxycarbonylations, are powerful
tools in both industrial production and organic synthesis to
create all kinds of carbonyl compounds including acids, esters,
amides, alcohols, and aldehydes.[1] So far, most scientific
efforts in this area have been devoted to the development of
advanced catalyst systems to improve the activity and
selectivity as well as the extension of the range of nucleophiles
such as water,[2] alcohols,[3] amines,[4] and amides.[5] On the
other hand, the carbonylation of substrates other than
unsaturated carbon–carbon bonds[3a,6] and organic (pseudo)-
halides[7] has been much less explored. Despite significant
Interestingly, when using tert-alcohols as substrates, the
cobalt-catalyzed carbonylation (Koch–Haaf reaction) is
applied for the synthesis of branched esters/acids
(Scheme 1).[14] In this case, the cationic intermediates are
generated from the dehydration of tert-alcohols, followed by
carbon monoxide (CO) insertion to give the branched
compounds. In fact, this transformation is carried out using
over-stoichiometric amounts of strong mineral acids (H2SO4
or a mix of HF/BF3) under harsh reaction conditions.
À
À
improvements on the oxidative carbonylation of C H/Ar H
bonds in the past decades, obvious drawbacks (for example,
harsh reaction conditions, narrow substrate scope, and/or
poor selectivity) still exist due to the presence of stoichio-
metric amount of oxidants.[8] Advantageously, using alcohols
in such transformations avoids the necessity of an external
oxidant.[9] However, the poor leaving ability of the hydroxy
group and the possible side reactions caused by the released
water have hindered wider applications of these substrates.
As a result, only functionalized and/or special alcohols have
[*] Dr. K. Dong,[+] R. Sang,[+] J. Liu, Dr. R. Razzaq, Dr. R. Jackstell,
Prof. Dr. M. Beller
Scheme 1. Catalytic carbonylation reactions of alcohols: branched
versus linear selectivity.
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein Straße 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Prof. Dr. R. Franke
Compared to Koch–Haaf reaction, the carbonylation of
sec- and tert-alcohols to give linear compounds is not
developed. Early studies revealed that ruthenium/cobalt
binary catalysts were not effective for the conversion of
butanols into C5 homologous alcohols regarding activity and
selectivity.[15] Although palladium–phosphine complexes con-
stitute state-of-the-art catalysts for the alkoxycarbonylation
of olefins, their application in the carbonylation of alcohols is
still in its infancy. tert-Butanol has been carbonylated with low
Evonik Performance Materials GmbH
Paul-Baumann-Str. 1, 45772 Marl (Germany)
and
Lehrstuhl fꢀr Theoretische Chemie, Ruhr-Universitꢁt Bochum
44780 Bochum (Germany)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201701950.
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activity, poor chemo- and regioselectivity, and/or high catalyst
loading.[16] A general problem of such reactions is the
formation of the corresponding ether from the alcohol.
Hence, the methoxycarbonylation in methanol gave methyl
tert-butyl ether instead of the desired esters.[17] Moreover, the
application of sec-alcohols to give linear esters has not been
studied yet.[18]
Based on our continuous interest in transition-metal-
catalyzed reactions with CO, recently we became attracted to
investigate the carbonylation of alcohols in more detail.
Herein we report the first general and efficient protocol for
the carbonylation of various sec- and tert-alcohols to give
linear esters using a specific palladium–phosphine catalyst
system (Scheme 1).
At the beginning of our study, tert-butanol 1a was used as
the model substrate due to its availability and industrial
relevance. As shown in Table 1, the carbonylation was carried
out with the industrially applied palladium catalyst (Pd/L1/
H+, 0.05/0.2/1.5 mol%)[19] under 40 bar pressure of CO at
1208C in MeOH for 20 h. However, only 14% yield of the
desired ester 2a was obtained (Table 1, entry 1). Notably,
substantial formation of palladium black was observed after
the reaction which shows the detrimental effect of in situ
generated water by the dehydration of the alcohol.
Very recently, we introduced the new ligands L2 and L3
for the alkoxycarbonylation of various alkenes.[20] Here, the
additional basic pyridine substituent increases the rate of the
nucleophilic attack on the intermediate palladium acyl
complex. Notably, this last step in the alkoxycarbonylation
cycle is often rate-determining. When the benchmark reaction
was performed in the presence of these two ligands (both
contain a racemic mixture and meso compound), to our
delight the linear ester was obtained in 76% and 61% yields,
respectively (Table 1, entries 2 and 3). Interestingly, this
transformation which requires a strong acid to proceed at
all is significantly improved by the ligand with built-in basic
sites. Other well-known carbonylation ligands L4–7 were also
investigated and very low yield of 2a or no product was
observed under identical reaction conditions (Table 1,
entries 4–7).
To improve the catalyst system further on, several
palladium sources such as PdCl2, Pd(OAc)2, and Pd2(dba)3
were tested in the presence of L2 (Table 1, entries 8–10).
Using Pd0 as the catalyst precursor, the yield of 2a was
increased to 94% suggesting the facile generation of the
crucial palladium hydride species in the catalytic cycle
(Table 1, entry 10). In addition, amounts of PTSA as well as
other types of acids were investigated in the reaction, and 2a
was afforded in moderated to high yields (Table 1, entries 11–
15). Notably, the reaction worked well with only 11.2 ppm
palladium loading and gave the desired ester in high yield
with a turnover number (TON) of 89000 (Table 1, entry 16)!
The high productivity and selectivity indicated that the in situ
formed water was tolerated well with our catalyst systems of
L2.
Table 1: Pd-catalyzed carbonylation of tert-butanol 1a: Investigation of
ligands, Pd precursors, and acids.
Encouraged by these results, we investigated the Pd-
catalyzed carbonylation of sec-alcohols. With cyclohexanol 3a
as a model substrate, the reaction was carried out under
similar conditions to that for tert-alcohols. Unfortunately, less
than 10% yield of the desired product 4a was detected. To
improve the yield, the reaction conditions including acids,
catalyst loading as well as reaction temperature and time were
optimized again and 4a was attained in 82% yield finally (see
Table S1 in the Supporting Information for details).
Next, the carbonylation of both model systems was
compared under similar reaction conditions (Scheme 2). As
shown in Figure 1, almost all of tert-alcohol 1b was converted
within 30 minutes into the corresponding ether 1ba by
Entry
Ligand
Pd source
Acid (x)
Yield [%][b]
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L2
L2
L2
L2
L2
L2
L2
L2
L2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(OAc)2
PdCl2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (1.5)
PTSA (0.75)
PTSA (3)
14
76
61
14
0
trace
0
90
86
94
68
92
50
0
9
10
11
12
13
14
15
16[c]
CF3SO3H (1.5)
CH3CO2H (1.5)
H2SO4 (0.75)
PTSA (0.625)
62
75
[a] Reaction conditions: 1a (20 mmol), [Pd] (0.05 mol% Pd), L1–L3
(0.2 mol%) or L4–L7 (0.4 mol%), CO (40 bar), MeOH (3 mL), 1208C,
20 h. [b] The yield of 2a was determined by GC analysis using iso-octane
as internal standard. [c] TON=89000, 1a (120 mmol), [Pd2(dba)3]
(0.0005 mmol), L2 (0.1 mmol), PTSA (0.75 mmol), CO (40 bar), MeOH
(16 mL), 1008C, 100 h. PTSA, p-toluenesulfonic acid (monohydrate).
nBuPAd2, di(1-adamantyl)-n-butylphosphine.
Scheme 2. Reaction pathways of the carbonylation of tert- and sec-
alcohols.
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Table 2: The carbonylation of tert-alcohols 1b–l.
Figure 1. Compounds distribution in the carbonylation of a) tert-alco-
hol 1b and b) sec-alcohol 3a.
[a] Reaction conditions: 1 (2 mmol), Pd2(dba)3 (0.1 mol%), L2
(0.8 mol%), PTSA (6 mol%), CO (20 bar), MeOH (2 mL), 1008C, 20 h.
The yields were determined by isolated products. [b] GC yields using iso-
octane as internal standard. [c] 1208C.
nucleophilic attack and the internal alkene 1bb by dehydra-
tion (the ether was also converted into the alkene). Subse-
quent isomerization of the internal alkene to the desired but
thermodynamically unfavored terminal olefin 1bc, which was
observed in a very small amount, is the key and limiting
reaction step. Final alkoxycarbonylation of 1bc gives the
desired product 2b, which drives the equilibrium to the
expected direction. In the context of sec-alcohol 3a, the
corresponding ether 3aa and alkene 3ab were formed with
relatively lower activity owing to the thermodynamic insta-
bility of the sec-cationic intermediate. In this case, the slow
step is the conversion of the ether into the desired alkene.
With optimized reaction conditions in hand, the carbon-
ylation of various tert-alcohols was explored. Using [Pd2-
(dba)3]/L2/PTSA (0.1/0.8/6 mol%) as the catalyst, 12 differ-
ent alcohols were converted smoothly into the linear esters
under comparably mild conditions (Table 2, 20 bar pressure
of CO, 1008C, 20 h).
Specifically, other aliphatic alcohols (1b,c) were trans-
formed into the desired products (2b,c) in almost quantitative
yields with high selectivity. a,a-Dimethylbenzyl alcohol and
its analogs 1d–i with substituents on the aromatic ring
including fluoride, chloride, bromide, and methoxy groups
gave products 2d–i in very high yields and selectivity. The
same was true for a,a-diphenylethanol 1j. Interestingly,
dicarbonylated compounds 2k and 2l, which could be applied
in polymerizations as monomers, were afforded directly in
96% and 57% isolated yields by the carbonylation of di-tert-
alcohols 1k and 1l, respectively.
Due to the difference in reactivity various primary and
secondary alcohols can be used as nucleophiles in the
carbonylation of tertiary alcohols. Hence, carbonylation of
i
1a with EtOH, PrOH, cyclohexanol, and tetrahydrofurfuryl
alcohol afforded the corresponding esters 2m–p again in high
yields and selectivity.
Finally, the carbonylation of several sec-alcohols was
studied with [Pd2(dba)3]/L2/CF3SO3H (0.5/3/30 mol%) as the
catalyst under 40 bar pressure of CO at 1408C. As shown in
Table 3, cyclic alcohols 3a–d with five- to eight-membered
rings were converted into the corresponding esters 4a–d in
high yields. With 4-methylcyclohexanol 3e as the substrate,
the desired product 4e was attained in 80% yield with high
selectivity. Furthermore, the catalyst system allowed for the
carbonylation of a-arylethanols 3 f–i containing electron-
donating (methoxy) or withdrawing (fluoride and chloride)
group on the phenyl ring and the corresponding products 4 f–
i were obtained in high yields. Interestingly, both 1-indanol 3j
and 1-tetralol 3k worked well and the desired products 4j and
4k were afforded in high isolated yields and selectivity. When
employing 2- and 3-hexanol as the substrates, moderate yields
and selectivity were observed for the esters 4l and 4m. Other
aliphatic sec-alcohols including a,a-di-iso-propylethanol 3n
and a-tert-butylethanol 3o proved to be suitable substrates,
too. Notably, nature provides a variety of secondary terpenols,
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Table 3: The carbonylation of sec-alcohols 3a–p.
Acknowledgements
This work is supported by Evonik Performance Materials
GmbH and the State of Mecklenburg-Vorpommern. We
thank the analytical team of LIKAT for their kind support.
Conflict of interest
The authors declare no conflict of interest.
Keywords: alcohols · carbonylation · catalysis · esters ·
palladium
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4
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Manuscript received: February 22, 2017
Final Article published: && &&, &&&&
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Communications
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K. Dong, R. Sang, J. Liu, R. Razzaq,
R. Franke, R. Jackstell,
M. Beller*
&&&& — &&&&
Palladium-Catalyzed Carbonylation of sec-
and tert-Alcohols
Ligands with built-in function make pos-
sible the first general carbonylation of
tert- and sec-alcohols to give the linear
esters. The reactions complement the
classic Koch–Haaf carbonylation. A vari-
ety of aliphatic and benzylic alcohols can
be directly used.
6
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
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