Palladium-catalyzed carbonylation of aryl tosylates and mesylates

Article abstract of DOI:10.1021/ja711449e

A general protocol for the palladium-catalyzed carbonylation of aryl tosylates and mesylates to form esters has been developed using a catalyst system derived from Pd(OAc)₂ and the bulky, bidentate dcpp ligand. The system operates under mild conditions: atmospheric CO pressure and temperatures of 80-110 °C. A broad substrate scope has been demonstrated allowing carbonylation of electron-rich, electron-poor, and heterocyclic tosylates and mesylates, and the reaction shows wide functional group tolerance. Copyright

Full text of DOI:10.1021/ja711449e

Published on Web 02/08/2008
Palladium-Catalyzed Carbonylation of Aryl Tosylates and Mesylates
Rachel H. Munday, Joseph R. Martinelli, and Stephen L. Buchwald*
Department of Chemistry, Room 18-490, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received January 2, 2008; E-mail: sbuchwal@mit.edu
Table 1. Butoxycarbonylation of Aryl Sulfonates Using Ligand 1a
The Heck carbonylation1 is a powerful method for the synthesis
of aryl carboxylic acid derivatives. There are many published
protocols for the carbonylation of aryl iodides, bromides, and
triflates;2 however, cheaper, more readily available aryl chlorides
and tosylates have proved more recalcitrant. Methods have been
developed for the carbonylation of unactivated aryl chlorides3
using bulky, electron-rich, bidentate ligands which both facil-
itate oxidative addition and prevent catalyst decomposition by
carbon monoxide,4 including a recently published protocol from
our laboratories which addresses previous limitations with respect
to scope and the requirement to employ harsh reaction condi-
tions.5
entry
R
1 (mol %)
conversion (%)
yield (GC)
1
2
3
4
5
p-tol
Ph
Me
p-tol
p-tol
4
4
4
2.2
2.2
100
100
100
100
95
>99
>99
>99
>99
85b
a 1:
.
b No 4Å MS.
Aryl tosylates are easily prepared from cheap, readily available
starting materials and are convenient to handle, stable, crystalline
solids. Use of these compounds can have advantages over the
corresponding aryl halides in that the phenol is a useful directing
group for the introduction of other functional groups on the aromatic
ring and as such can allow access to a wider substrate scope. Aryl
mesylates have seen limited use in cross-coupling processes6 and
to the best of our knowledge have never been used in carbonylation
reactions. The relative acidity of the methyl group renders them
incompatible with procedures that require strong base. They,
however, are also easy to prepare and a procedure that could utilize
these substrates would have the advantage of being more atom
economical than one employing the corresponding aryl tosylates
due to their significantly lower molecular weight.7 This is of
increasing importance as greener processes are sought because of
environmental concerns.
the carbonylation of sulfonates was based on this system and sodium
phenoxide was used as the base. This resulted in a mixture of
carbonylated products due to a Williamson reaction between the
aryl sulfonate and sodium phenoxide (eq 1). To our delight, control
experiments revealed that the conditions which originally failed
for the carbonylation of aryl chlorides work well with aryl sulfonate
substrates. Indeed, the use of Pd(OAc)2 and 1 (a commercially
available,9 air stable HBF4 salt) with either K2CO3 or K3PO4 in
dioxane or toluene provided g95% yield (GC) for the butoxyes-
terification of 4-t-butylphenyltosylate.10 Further, it was found that
reaction carried out with K3PO4 or K2CO3 in DMSO also showed
clean formation of ester with electron-rich and electron-neutral
substrates.11
There is only limited precedent for the carbonylation of aryl
tosylates, with only two publications to date.8 The most general
procedure was recently published by Cai and co-workers for the
carbonylation of aryl arenesulfonates to form esters.8b A catalyst
system derived from Pd(OAc)2 and a Josiphos-type ligand was
employed at 90 psi pressure of carbon monoxide and elevated
temperatures which were required to allow oxidative addition of
the relatively inert C-O bond. The more reactive aryl p-fluoroben-
zene sulfonates gave good yields of esters (70-96%) for a wide
variety of aryl substituents at 135 °C; however, a single example
of an electron-rich aryl tosylate was included and its carbonylation
proceeded in lower yield. Therefore the need to develop a general
procedure for the carbonylation of unactivated aryl tosylates under
mild conditions still exists. Herein we report a general catalytic
process for the carbonylation of aryl tosylates that also allows
carbonylation of aryl mesylates for the first time. This process
proceeds at 80-110 °C and one atmosphere (balloon pressure) of
carbon monoxide.
We were curious as to whether these conditions could be applied
to other classes of aryl sulfonates and found that the analogous
benzenesulfonate and mesylate were also converted cleanly to ester
(Table 1, entries 2 and 3). Omission of molecular sieves from the
reaction mixture led to incomplete conversion of tosylate and
reduced yield with the remainder of the material presumably lost
as acid (entry 5).
The scope of these carbonylation processes with regard to the
aryl tosylate and mesylate and alcohol was next examined (Table
2). Electron-rich and electron-deficient aryl tosylates and mesylates
gave good yields of ester products; all substrates shown gave clean
conversion to ester with no competing formation of ether byprod-
ucts. Heteroaryl substrates were also efficiently transformed and
the reaction showed good functional group tolerance. Aryl tosylates
and mesylates with aldehyde, ketone, ester and cyano groups all
gave high yields of ester as did substrates with acidic hydrogens
(entries 3, 5). Esters with ortho substituents could also be obtained
in good yield (entries 5, 6, 7).
We recently reported an improved method for the carbonylation
of aryl chlorides that employed the electron-rich chelating ligand
1.5 Central to the success of this work was the use of sodium
phenoxide as the supporting base; it was demonstrated that phenyl
esters are key intermediates in this process. Our initial approach to
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10.1021/ja711449e CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Substrate Scope for the Palladium-Catalyzed
Carbonylation of Aryl Sulfonates at Atmospheric CO Pressure
With regard to the alcohol the reaction was general with a variety
of primary alcohols and a secondary alcohol. The synthesis of esters
derived from lighter alcohols was limited by the boiling point of
the alcohol. Ethyl esters could be synthesized by running the
reactions at 80 °C. Carbonylation of modestly activated tosylates
and mesylates occurred at this temperature, and their ethyl esters
were obtained in good yield (entries 9 and 14); for substrates where
the aryl group was unactivated the more reactive p-fluorobenzene
sulfonate was employed to allow complete conversion to product
(entries 15 and 16).
In summary we have developed a mild and general procedure
for the carbonylation of aryl tosylates and mesylates at atmospheric
pressure of carbon monoxide and relatively low temperatures using
commercially available, air stable dccp•2HBF4 as the supporting
ligand.
Acknowledgment. Generous financial support from the MIT-
Singapore Alliance is gratefully acknowledged. We are indebted
to Merck, Amgen, and Boehringer-Ingelheim for unrestricted
support. We are also grateful to Nippon Chemical Co. for providing
ligand 1 and to BASF for a donation of palladium acetate.
Supporting Information Available: Experimental procedures and
spectral data for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
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(10) See Supporting Information for details of butoxyesterification of 4-t-
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(11) Substrates with certain electron-withdrawing groups (e.g., 3-cyanophenyl
methanesulfonate) were transformed into mixtures of butyl ether and ester
when the reaction was carried out in DMSO. Ether formation occurred
without ligand and Pd in DMSO presumably via a base-mediated sulfonyl
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a Isolated yields (average of two runs). b Reaction was performed with
5% (Pd(OAc)2 and 6% 1 for 22h. c Reaction proceeded to 88% conversion
(GC analysis). d Reaction was performed with 10 equiv EtOH. e Reaction
was performed with 4% Pd(OAc)2 and 4.4% 1. f Reaction was performed
with 5 equiv of EtOH for 20 h. g Concentration: 0.67 M toluene.
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