Palladium-catalyzed conversion of aryl and vinyl triflates to bromides and chlorides

Article abstract of DOI:10.1021/ja107481a

The palladium-catalyzed conversion of aryl and vinyl triflates to aryl and vinyl halides (bromides and chlorides) has been developed using dialkylbiaryl phosphine ligands. A variety of aryl, heteroaryl, and vinyl halides can be prepared via this method in

Full text of DOI:10.1021/ja107481a

Published on Web 09/21/2010
Palladium-Catalyzed Conversion of Aryl and Vinyl Triflates to Bromides and
Chlorides
Xiaoqiang Shen, Alan M. Hyde, and Stephen L. Buchwald*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
Received August 18, 2010; E-mail: sbuchwal@mit.edu
potassium bromide has a low solubility in toluene, we reasoned
that the use of a phase-transfer catalyst (PTC) might be beneficial.
Adding polyethylene glycols improved the yield to 32% (entry 6).
All attempts to increase the yield of this reaction with more forcing
conditions or higher catalyst loading failed. We were initially
puzzled by this and wondered whether product inhibition was
Abstract: The palladium-catalyzed conversion of aryl and vinyl
triflates to aryl and vinyl halides (bromides and chlorides) has
been developed using dialkylbiaryl phosphine ligands. A variety
of aryl, heteroaryl, and vinyl halides can be prepared via this
method in good to excellent yields.
occurring. Therefore, we repeated the 4-n-butylphenyl triflate to
4-n-butylbromobenzene reaction (eq 1) in the presence of 1 equiv
Aryl halides are ubiquitous synthetic intermediates in organic
chemistry that are used for numerous transformations.1 They are
also present in a wide variety of natural products, pharmaceuticals,
and agrochemicals.2 Therefore, the development of general and
regioselective methods for the preparation of functionalized aryl
halides is of great importance. Sulfonate esters, often referred to
as pseudo-halides, can be employed as alternatives to aryl halides
in many cross-coupling reactions.3 Such pseudo-halides, however,
cannot be used as precursors for the generation of free radicals4 or
organometallic reagents.1 Traditional methods for the synthesis of
aryl halides (bromides or chlorides) from phenols require either
forcing conditions5 or multi-step procedures.6 Herein, we report
the first example of palladium-catalyzed direct conversion of readily
available aryl sulfonate esters to aryl bromides and chlorides.
Although a catalytic method has not yet been reported, several
mechanistically related metal-mediated carbon-halogen bond-
forming processes have been published.7 For example, the reductive
elimination of aryl halides from Pt(IV),8 Ni(III),9 Pd(IV),10 and
Pd(III)11 has been demonstrated. In addition, the reductive elimina-
tion of aryl halides from arylpalladium(II) halide complexes in the
of potassium triflate and found that the reaction was completely
inhibited (entry 7). This was surprising since the triflate anion is
known to be a poor nucleophile.19 In order to probe whether this
result was due to the existence of an unfavorable equilibrium, 5
was heated under similar catalytic conditions with KOTf as the
coupling partner. However, no conversion to aryl triflate was
observed, ruling out this possibility. Our next efforts focused on
the sequestration of KOTf as it was formed. After surveying a broad
Table 1. Evaluation of Reaction Conditions for Brominationa
presence of a large excess of P(t-Bu)3 has been reported.12,13
A
number of copper-14 and nickel-mediated15 halide exchange reac-
tions have also been developed in which carbon-halogen reductive
elimination processes are proposed to take place.
b
entry
MBr
ligand
PTC
additive
yield (%)
Despite these advances in aryl-halide bond formation, to the
best of our knowledge, there are few examples of the catalytic
conversion of an aryl sulfonate to an aryl halide,16 including
the ruthenium-catalyzed conversion of 2-naphthyl triflate to
2-bromonaphthalene16b and our recent report of the palladium-
catalyzed conversion of aryl triflates to aryl fluorides.17 This
prompted us to query whether we could uncover a catalytic process
to convert aryl triflates into corresponding aryl bromides and
chlorides; the results are disclosed herein.
Catalysts based on dialkylbiaryl monophosphines 1-4, which
have been successfully utilized as ligands in numerous transforma-
tions,17,18 were initially examined for the conversion of 4-n-
butylphenyl triflate into 4-n-butylbromobenzene (5). In initial
experiments, these reactions were carried out in the presence of
2.0 equiv of Bu4NBr, 1 mol % Pd2(dba)3, and 3 mol % ligand in
toluene at 100 °C. This set of conditions afforded only small
amounts of aryl bromide products (1-6%, entries 1-4). Other
bromide sources were tested, but the results were unimpressive,
with KBr providing the highest yield (11%, entry 5). Since
1
2
3
4
5
6
7
8
Bu4NBr
Bu4NBr
Bu4NBr
Bu4NBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
1
2
3
4
4
4
4
4
4
4
4
4
4
1c
2c
3c
6c
11c
32c
1c
PEG
PEG
PEG
PEG
PEG
PEG
PEG
PEG
KOTfd
Et3B
50c
54e
41e
1c
9
iBu2AlF
10
11
12
13
iBu3Al
(iPrO)3Al
iPr3Alf
25g
92
iBu3Al/2-butanoneh
a Reaction conditions: 0.25 mmol of aryl triflate, 2.0 equiv of MBr,
1.5 equiv of additive, 50 mg of PEG3400 [poly(ethylene glycol), Mn
≈
3400], 1.0 mL of toluene. b Yields determined by GC analysis with an
internal standard of dodecane. c The remaining material was the
4-n-butylphenyl triflate. d 1.0 equiv of KOTf. e The remaining material was
the 1-n-butyl-4-i-butylbenzene. f Triisopropylaluminum diethyl etherate as
additive. g The remaining material was the n-butylbenzene. h 1.5 equiv of
i-Bu3Al and 2-butanone as additives.
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14076 J. AM. CHEM. SOC. 2010, 132, 14076–14078
10.1021/ja107481a  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Palladium-Catalyzed Conversion of Triflates to Bromidesa
Table 3. Palladium-Catalyzed Conversion of Triflates to Chloridesa
a Reaction conditions: 1.0 mmol of triflate, 1.5-2.5 mol
% of
Pd2(dba)3, 3.75-6.25 mol % of 4 (4:Pd ) 1.25:1), 1.5 mmol of KCl,
1.5 mmol of 2-butanone, 1.5 mL of i-Bu3Al (1 M solution in toluene),
120 mg of PEG3400, 6.0-8.0 mL of toluene, 20-24 h. Isolated yields
(average of two runs). b 1.25 equiv of i-Bu3Al and EtOH as additives;
contains ∼5% of ethyl 4-i-butylbenzoate.
groups were well tolerated but can, in some cases, undergo
unwanted transesterification. With 4-trifluoromethylsulfonyl eth-
ylbenzoate as substrate, this undesired side reaction could be
avoided by using EtOH as an additive in place of 2-butanone (Table
2, compound 8). Aromatic amines are also tolerated in these
transformations, as exemplified by the formation of 7-bromo-1-
naphthylamine in moderate yield from the corresponding triflate
(Table 2, compound 11). In this case, the 2-butanone imine was
initially formed, which was then cleaved after workup and flash
chromatography. The bromination of 3-bromophenyl triflate and
4-chloro-1-naphthyl triflate demonstrates that polyhalogenated
products may be efficiently formed. Vinyl triflates are also effective
substrates for the bromination reaction. For example, a vinyl triflate
derived from estrone is easily converted to the corresponding vinyl
bromide in 92% yield (Table 2, compound 20).
a Reaction conditions: 1.0 mmol of triflate, 1.5-2.5 mol % of Pd2(dba)3,
3.75-6.25 mol % of 4 (4:Pd ) 1.25:1), 1.5 mmol of KBr, 1.5 mmol
of 2-butanone, 1.5 mL of i-Bu3Al (1 M solution in toluene), 120 mg of
PEG3400, 6.0-8.0 mL of toluene, 20-24 h. Isolated yields (average of
two runs). b 1.25 equiv of i-Bu3Al and EtOH as additives; contains
∼5% of ethyl 4-i-butylbenzoate. c Contains 1,4-dichloronaphthalene and
1,4-dibromonaphthalene (4-10% total).
range of Lewis acids, we found that the yield of the reaction could
be increased to 50% when Et3B was used as additive (entry 8).
Full conversions were observed when commercially available
i-Bu2AlF or i-Bu3Al was used; however, about 50-60% of the C-C
coupling product, 1-n-butyl-4-i-butylbenzene, was formed in both
cases (entries 9 and 10).20 The use of the bulkier i-Pr3Al additive
prevented the formation of the C-C coupling product, although
the desired aryl bromide was only formed in 25% yield, with the
reduction product, n-butylbenzene, now formed in 75% yield (entry
12). Fortunately, in situ formation of dialkylaluminum alkoxides
by the addition of ketones or alcohols suppressed both C-C
coupling and reduction byproducts; the yield was increased to 92%
when 2-butanone was used (entry 13).
As highlighted in Table 3, aryl and vinyl triflates can also be
converted to aryl and vinyl chlorides under similar reaction
conditions using KCl as the chloride source in good to excellent
yields.21
In conclusion, a new method for the direct conversion of aryl
and vinyl triflates to aryl and vinyl halides has been achieved using
palladium catalysis. The use of sterically hindered dialkylbiaryl
monophosphines is key for success in achieving this new
transformation.
With the optimal conditions for this new transformation in hand,
we next explored the reaction scope (Table 2). Both electron-poor
and electron-rich aryl triflates are suitable substrates and provide
aryl bromides in good yield. Significantly, heteroaryl triflates, such
as quinolines, indoles, carbazoles, and benzothiazoles, can be
efficiently converted to the corresponding aryl bromides. Ester
Acknowledgment. Generous financial support from the National
Institutes of Health (GM46059) is gratefully acknowledged.
Supporting Information Available: Procedural and spectral data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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C O M M U N I C A T I O N S
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