Palladium-catalyzed reductive carbonylation of aryl halides with N-formylsaccharin as a CO source

Article abstract of DOI:10.1002/anie.201303926

Easy peasy: The title reaction employs N-formylsaccharin, which is an easily accessible crystalline compound, as an effective CO source. The reactions proceed with a small excess of the CO source at moderate temperatures and were successfully applied to a wide range of aryl bromides. DMF=N,N- dimethylformamide, dppb=1,4-bis-(diphenylphosphino)butane. Copyright

Full text of DOI:10.1002/anie.201303926

Angewandte
Chemie
DOI: 10.1002/anie.201303926
Carbonylation
Palladium-Catalyzed Reductive Carbonylation of Aryl Halides with
N-Formylsaccharin as a CO Source**
Tsuyoshi Ueda, Hideyuki Konishi, and Kei Manabe*
Aromatic aldehydes are valuable synthetic intermediates in
knowledge, there is no precedent for the reductive carbon-
ylation of electrophiles other than aryl iodide without using
CO gas.[13]
À
C C bond-forming reactions. The reduction of carboxylic
acids or esters to aldehydes[1] is a commonly used synthetic
strategy in spite of its drawbacks, that is, the functional groups
that it can be applied to are limited, a relatively low
temperature is required, and a two-step procedure involving
initial activation of the carboxylic acid prior to the reduction
is sometimes necessary. Direct formylation of aryl halides is
Recently, we developed the alkoxycarbonylation of aryl,
alkenyl, and allyl halides, in addition to alkenyl tosylates, with
phenyl formates in the presence of a Pd/P(tBu)3 or xantphos
catalyst system.[10] In this approach, decarbonylation of
phenyl formate with a mild base (e.g., NEt3) generates
phenol and CO, which are subsequently used for the
palladium-catalyzed alkoxycarbonylation, thus affording the
corresponding phenyl esters which can be readily transformed
into various carboxylic acid derivatives. Our interest in
carbonylation reactions, especially in the conversion of aryl
halides into the corresponding aldehydes, prompted us to
explore the application of other CO sources to the synthesis
of aldehydes. It was hypothesized that s-acylpalladium
species, formed by the reaction of CO generated in situ with
s-arylpalladium species, could be trapped by a hydride donor
(e.g., Et3SiH[3,12]) to give the desired aldehydes.
Herein, we report a novel and practical method for
palladium-catalyzed reductive carbonylation of aryl bromides
with N-formylsaccharin, which works as an easily accessible
and highly reactive crystalline CO surrogate. The reported
reactions proceeded with a small excess of a CO source
(1.5 equiv) at moderate temperatures and were successfully
applied to a wide range of aryl bromides.
a
single-step alternative transformation which can be
employed to form aldehydes.[1] The conventional strategy
for this conversion involves halogen–metal exchange by
alkyllithium and subsequent addition of formylating agents
(e.g., DMF). In this procedure, the reaction requires a stoi-
chiometric amount of the metal reagent, the strong basicity of
which limits the scope of functional groups which can be
tolerated.
An efficient and complementary methodology is the
palladium-catalyzed reductive carbonylation of aryl halides,
which employs CO gas. Because of the pioneering work by
Heck and Schoenberg in 1974,[2] several groups have worked
on developing this conversion to enhance its utility as
a synthetic tool. However, there are still only a few general
protocols for reductive carbonylation,[3,4] especially in com-
parison to alkoxy- and aminocarbonylations.[4] Recently
Beller et al. reported the first industrially applied and
efficient palladium-catalyzed reductive carbonylation at
5 bar of synthesis gas (CO/H2 1:1).[5]
As an initial test, the reductive carbonylation of 4-
bromoanisole (1a) was carried out. Here, 1.5 equivalents of
formate (butyl, phenyl,[10a,b] or 2,4,6-trichlorophenyl forma-
te,[10c] and potassium formate with acetic anhydride[7]) were
used as a CO source, with 2 equivalents of Et3SiH as a hydride
donor under a Pd/xantphos[10,14] catalyst system at 858C
(Table 1, entries 1–4). Unfortunately, these experiments
resulted in extremely poor yields. The reaction using 2a
mainly afforded the dehalogenated compound, anisole,
because of the low reactivity of 2a as a CO source (entry 1).
By employing the more reactive formates 2b and 2c, the
corresponding phenyl esters, which are alkoxycarbonylation
products, were observed as the main products (entries 2 and
3). It was hypothesized that a fragment such as phenol
generated from a CO source has higher nucleophilicity to the
palladium center compared to silane, thus preventing the
conversion of acylpalladium into the desired aldehyde. In
agreement with this reasoning, the use of N-formylsaccharin
(2d)[15] as a CO source resulted in dramatic improvements,
thus providing complete conversion of 1a and a good yield
(80%) of 3a. Saccharin (pKa = 1.6),[16] which is generated
from 2d as a result of CO release, has much a lower
nucleophilicity than the phenols (pKa = 6–10). N-Formylsac-
charin (2d), developed by Cossy et al. as a new and
Over the last three decades, CO-free carbonylation
chemistry has been the focus of extensive research. Various
compounds such as formic acid derivatives and metal
carbonyl compounds have already been developed as alter-
natives to toxic CO gas.[6] Though some successful reports on
hydroxy-,[7,8] alkoxy-,[9,10] and aminocarbonylation[11] have
been published, there is still only one report on the reductive
carbonylation of aryl halides using an external CO source.
Cacchi et al. reported the palladium-catalyzed reductive
carbonylation of aryl iodides using thermally unstable acetic
formic anhydrides as a CO source.[12] To the best of our
[*] Dr. T. Ueda, Dr. H. Konishi, Prof. Dr. K. Manabe
School of Pharmaceutical Sciences, University of Shizuoka
52-1 Yada, Suruga-ku, Shizuoka 422-8526 (Japan)
E-mail: manabe@u-shizuoka-ken.ac.jp
Dr. T. Ueda
Process Technology Research Laboratories
Pharmaceutical Technology Division, Daiichi Sankyo Co., Ltd.
1-12-1 Shinomiya, Hiratsuka, Kanagawa 254-0014 (Japan)
[**] This research was supported by the Daiichi Sankyo Co., Ltd.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303926.
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Table 1: Reductive carbonylation of 1a with various sources of CO.[a]
Table 2: Ligand screening.[a]
Entry
Ligand[18]
Conversion [%][b]
Yield [%][b]
1
2
3
PPh3
PCy3
dppm
23
28
32
18
27
9
Entry
CO source
Conversion
[%][c]
Yield
[%][c]
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
dppe
dppp
dppb
dpppe
dppf
dppBz
P(tBu)3·HBF4
DCyPP·2HBF4
DCyPB
dpephos
xantphos
rac-binap
Dt-bpf
28
98
100
99
97
20
19
55
99
100
100
85
31
64
19
92
95
94
56
5
10
44
92
36
80
34
4
1
2
3
4
5
2a
2b
2c
100
97
100
76
0
5
1
2
HCOOK,[b] Ac2O
2d
100
80
[a] Reactions were conducted with 0.55 mmol of 1a and DMF (2 mL).
[b] 1.5 equiv of Ac2O and 2.5 equiv of HCOOK were used in place of
Na2CO3. [c] Determined using HPLC. DMF=N,N-dimethylformamide.
inexpensive formylating agent for amines, is a highly crystal-
line compound and can be easily synthesized from saccha-
rin.[15a]
BuPAd2·HI
P(o-Tol)3
62
0
11
[a] Reactions were conducted with 0.55 mmol of 1 and DMF (2 mL).
To confirm the reactivity of 2d as a CO source, the process
of decarbonylation to generate CO and saccharin (4) was
examined (Scheme 1). As expected, by using a mild base
(NEt3 or Na2CO3), rapid decarbonylation was observed at
room temperature, thus providing complete conversion of 2d
within 30 minutes.
[b] Determined using HPLC.
Table 3: Effect of base and solvent on the reaction of 1a and 2d.[a]
Entry
Solvent
Base
Conversion
[%][b]
Yield
[%][b]
1
2
3
4
5
6
7
8
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMI
NMP
CH3CN
DME
THF
CPME
toluene
EtOAc
DMF
DMF
DMF
Na2CO3
Li2CO3
NEt3
DIPEA
100
69
83
84
80
72
67
52
16
97
99
39
18
46
12
7
95
58
60
62
55
60
50
38
9
48
58
29
7
DABCO[c]
TMEDA[c]
N-methylmorpholine
N-methylimidazole
pyridine
Scheme 1. Decarbonylation of N-formylsaccharin (2d). [a] Reactions
were conducted with 0.1 mmol of 2d and [D7]DMF (1 mL). [b] Con-
version was determined using 1H NMR spectroscopy.
9
10
11
12
13
14
15
16
17
18[d]
19[d,e]
20[f]
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Several ligands were then tested in the reductive carbon-
ylation of 1a with 2d (Table 2). Monodentate ligands were
found to be ineffective in this reaction (entries 1, 2, 10, 17, and
18). However, dppp, dppb, DCyPB, and dpppe, which have
tethers greater than three carbon atoms long, showed
excellent catalytic activity, thus affording 3a in 92–95%
(entries 5–7, and 12). In contrast, DCyPP, which has a three-
carbon tether and was demonstrated by Buchwald et al. to be
a highly active ligand in alkoxy- and aminocarbonylation,[17]
was not particularly effective for this reductive carbonylation
(entry 11).
The effects of base and solvent were investigated using
dppb as the ligand (Table 3). Na2CO3 was found to be the only
effective base, thus giving the highest yield out of all the
conditions tested (entry 1). When an organic base such as
NEt3 was used, a significant quantity of anisole was produced
(entries 3–9). DMF was found to be the optimal solvent for
the reaction, with the others tested tending to give poor
30
0
4
14
100
100
100
2
Na2CO3
Na2CO3
Na2CO3
87
92
93
[c]
[a] Reactions were conducted on a 0.55 mmol scale in 1a and 2 mL of
solvent at 858C using 1.5 equiv of 2d, 2.0 equiv of Et3SiH, and 2.5 equiv
of base in the presence of 3 mol% of Pd(OAc)2 and dppb. Reaction time
was 16 h. [b] Determined using HPLC. [c] Used 1.5 equiv of base.
[d] Used 1.5 equiv of Et3SiH and 1.5 mol% of Pd(OAc)2 and dppb. [e] Pd/
P ratio was 1:3. [f] Used 1.3 equiv of Et3SiH. CPME=cyclopentyl methyl
ether, DABCO=1,4-diazabicyclo[2.2.2]octane, DIPEA=diisopropyl-
ethylamine, DME=1,2-dimethoxyethane, NMP=N-methylpyrrolidone,
THF=tetrahydrofuran, TMEDA=N,N,N’,N’-tetramethylethylene-
diamine.
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Table 4: Palladium-catalyzed reductive carbonylation of aryl bromides,
aldehyde yields while producing an increased amount of
anisole (entries 10–17). A further series of experiments
showed that even when reduced amounts of Na2CO3
(1.5 equiv), Et3SiH (1.3 equiv), and catalyst (1.5 mol%)
were used, the yields of the reaction remained high (87–
92%, entries 18–20). The P/Pd ratio was found to slightly
affect the catalytic activity, with the yield increasing to 92% at
P/Pd = 3 (entries 18 and 19).
iodide, and triflate with 2d.[a]
The identified optimal reaction conditions for 1a (1 equiv
of aryl halide, 1.5 equiv of 2d, 1.3 equiv of Et3SiH,[19] 1.5 equiv
of Na2CO3 in the presence of 3 mol% of Pd(OAc)2 and
4.5 mol% of dppb in DMF) were subsequently employed to
test the generality of the catalyst system for a variety of aryl
halides and triflates (Table 4). It was found that, in addition to
bromobenzene (1b), iodobenzene (1c) afforded 3b in high
yields (entries 1 and 2). Phenyl triflate (1d) also reacted,
although the yield was modest (entry 3). A variety of func-
tional groups (ether, ester, amide, aldehyde, amine, cyano,
and dioxolane) were tolerated (entries 4, 7–12). Although
electron-withdrawing groups at the 4-positions promoted
reductive debromination, slightly lowering the reaction
temperature resulted in a reduced amount of by-products,
thus giving the corresponding aldehydes in moderate to good
yields (entries 7–9, and 11). Though ortho-substituted aryl
bromides also gave good results (entries 13 and 14), the steric
hindrance of 2,6-disubstitution prevented the reaction from
taking place (entry 15). The protocol was found to work well
with bromonaphthalene (entries 16 and 17). Reaction of N-
and S-containing heteroaromatic bromides proceeded to
afford 3r–w in moderate to good yields (entries 18–23). In
addition, reductive carbonylation with Et3SiD was effective in
affording [D1]4-phenylbenzaldehyde 3x in good yield
(entry 24).
The initial hypothesis for the mechanism of these
palladium-catalyzed transformations was formed on the
basis that owing to the low nucleophilicity of saccharin (4)
generated from 2d, the s-acylpalladium species could be
trapped by silane without the participation of the saccharin
species (see Scheme 4, path 1). However, we cannot exclude
the possibility that acylsaccharin would be formed in situ in
the reductive carbonylation by reaction of the s-acylpalla-
dium species and saccharin. The palladium-catalyzed reduc-
tion of the in situ generated acylsaccharin with silane might
also give the desired aldehyde (see Scheme 4, path 2). To
further elucidate the mechanism, the palladium-catalyzed
reduction of N-benzoylsaccharin (6) was examined.[20] Sur-
prisingly, the reaction of 6 with Et3SiH in the presence of a Pd/
dppb catalyst system afforded 3b in 64% yield (Scheme 2). In
the absence of the palaldium catalyst, 3b was not obtained. In
[a] Reactions were conducted on a 0.5–1.5 mmol scale in substrate and
2–6 mL of DMF. [b] Yield of isolated product. [c] Determined using
HPLC. [d] 5 mol% of Pd(OAc)2. [e] 1.5 equiv of 2d, 1.5 equiv of Et3SiH,
and 2.5 equiv of base in the presence of 1.5 mol% of Pd(OAc)2 and
2.3 mol% of dppb. [f] Xantphos was used in place of dppb. [g] 1.3 equiv
of Et3SiD was used in place of Et3SiH.
addition, palladium-catalyzed carbonylation of 1q using 2d
without silane produced the acylsaccharin 7 (the correspond-
ing [M+H]+ was detected as a major peak on HRMS
analysis), which could be converted into the ester 8 in
a one-pot procedure (Scheme 3). These observations indicate
that the reaction intermediate is likely to be acylsaccharin, the
À
C N bond of which could oxidatively add to the palladium. A
plausible catalytic cycle is shown in Scheme 4. N-Formylsac-
charin (2d) is converted into sodium saccharin and CO by
Scheme 2. Palladium-catalyzed reduction of 6 with Et3SiH. [a] Deter-
mined using HPLC.
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Pd(OAc)2 (10.1 mg, 0.045 mmol, 3.0 mol%), dppb (28.8 mg,
0.068 mmol, 4.5 mol%), 2d (475 mg, 2.25 mmol, 1.5 equiv), and
Na2CO3 (238 mg, 2.25 mmol, 1.5 equiv) were added to a 30 mL test
tube, which was then evacuated and backfilled three times with N2. A
degassed solution of 1b (158 mL, 1.50 mmol) and Et3SiH (311 mL,
1.95 mmol, 1.3 equiv) in DMF (6 mL) was added to the test tube
under N2. The tube was immediately sealed by a plastic screw cap and
the mixture was stirred for 10 min at RT (this stirring is critical for the
selectivity of the reaction). The mixture was subsequently warmed to
758C and stirred for a further 16 h. The reaction mixture was cooled
to RT, then diluted with Et2O (15 mL), and washed with H2O
(15 mL). The aqueous layer was extracted two times with Et2O
(15 mL). The combined organic layer was dried over MgSO4, filtered,
and concentrated. The obtained residue was purified by flash column
chromatography on silica gel (3–5% Et2O in n-
Scheme 3. Palladium-catalyzed alkoxycarbonylation of 1q with 2d.
pentane) to provide 3b as a colorless oil (132 mg,
83%).
Received: May 8, 2013
Published online: July 3, 2013
Keywords: aldehydes · arenes · carbonylation ·
.
palladium · synthetic methods
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which results from the oxidative addition of 1b and insertion
of CO, thus leading to the formation of D. Palladium-
mediated reversible reductive elimination/oxidative addition
occurs between 6 and D, and then reacts with Et3SiH to form
3b and the active palladium species A (path 2). However, the
alternative mechanism which was part of the original
hypothesis (path 1) cannot be ruled out. In this case, C is
directly reduced by Et3SiH to afford 3b without the partic-
ipation of sodium saccharin.
In summary, we have reported a novel and practical
method for palladium-catalyzed reductive carbonylation of
aryl bromides with N-formylsaccharin (2d) without using CO
gas. The reported reactions proceeded with a small excess of
a CO source (1.5 equiv) at moderate temperatures and were
successfully applied to a wide range of aryl bromides.
Compared to other existing CO sources for palladium-
catalyzed reactions, 2d has a great number of advantages,
including low cost, good availability, ease of handling,
stability, and high reactivity as a CO source. Further inves-
tigations concerning the application of this methodology to
other CO-free, palladium-catalyzed reactions will be carried
out in due course.
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[18] Cy = cyclohexyl, o-Tol = 2-methylphenyl, dppm = bis(diphenyl-
phosphino)methane, dppe = 1,2-bis(diphenylphosphino)ethane,
dppp = 1,3-bis(diphenylphosphino)propane,
(diphenylphosphino)butane, dpppe = 1,5-bis(diphenylphos-
phino)-pentane, dppf = 1,1’-bis(diphenylphosphino)ferrocene,
dppBz = 1,2-bis(diphenylphosphino)benzene, DCyPP = 1,3-
dppb = 1,4-bis-
bis(dicyclohexylphosphino)propane, DCyPB = 1,4-bis(dicyclo-
hexylphosphino)butane, dpephos = bis(2-(2-diphenylphosphi-
no)phenyl) ether, xantphos = 4,5-bis(diphenylphosphino)-9,9-
dimethylxanthene,
binap = bis(diphenylphosphino)-1,1’-
binaphthyl, Dt-bpe = 1,1’-bis(di-tert-butylphosphino)ferrocene.
[19] Other silanes, including PhMe2SiH, Ph2MeSiH, Ph3SiH, poly-
methylhydrosiloxane, iPr3SiH, and (EtO)3SiH were tested in the
reaction of 1k, but only gave poor yields of 2k (3–50%).
[20] The preparation of 6 was performed according to: T. Imai, T.
Okunoyama, M. Okoshi, J. Chem. Soc. Jpn. 1975, 123. See the
Supporting Information for details.
[15] a) T. Cochet, V. Bellosta, A. Greiner, D. Roche, J. Cossy, Synlett
2011, 1920; b) N-Formylsaccharin is commercially available
from Tokyo Chemical Industry Co., Ltd. (TCI).
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