Palladium-Catalyzed Reductive Conversion of Acyl Fluorides via Ligand-Controlled Decarbonylation

Article abstract of DOI:10.1021/acs.orglett.8b01582

Ligand-controlled non-decarbonylative and decarbonylative conversions of acyl fluorides were developed using a Pd(OAc)₂/Et₃SiH combination. When tricyclohexylphosphine (PCy₃) was used as the ligand, aldehydes were obtained as simple reductive conversion products. The use of 1,2-bis(dicyclohexylphosphino)ethane (Cy₂P(CH₂)₂PCy₂, DCPE) as the ligand, however, favored the formation of hydrocarbons, which are decarbonylative reduction products.

Full text of DOI:10.1021/acs.orglett.8b01582

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed Reductive Conversion of Acyl Fluorides via
Ligand-Controlled Decarbonylation
Yohei Ogiwara,* Yuka Sakurai, Hiroyuki Hattori, and Norio Sakai*
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba
278-8510, Japan
S
* Supporting Information
ABSTRACT: Ligand-controlled non-decarbonylative and
decarbonylative conversions of acyl fluorides were developed
using a Pd(OAc)2/Et3SiH combination. When tricyclohex-
ylphosphine (PCy3) was used as the ligand, aldehydes were
obtained as simple reductive conversion products. The use of
1,2-bis(dicyclohexylphosphino)ethane (Cy2P(CH2)2PCy2,
DCPE) as the ligand, however, favored the formation of hydrocarbons, which are decarbonylative reduction products.
cyl carbon−heteroatom (acyl C−Z) bond cleavage of
For both types of transformations, acyl fluorides are
considered highly attractive and versatile carboxylic acid
derivatives. They are readily available from the corresponding
carboxylic acids or acyl chlorides,8 and are easy to handling due
to their chemical stability toward nucleophiles such as water. A
pioneering work by Rovis showed that acyl fluorides could be
applied as acylation reagents in a nickel-catalyzed coupling
with organozinc reagents.9 Our group also reported the A-type
coupling of acyl fluorides as carboxylic acid derivatives with
organometallic reagents such as organosilicon10a and organo-
boron10b compounds by palladium catalysis. In these reactions,
acyl fluorides acted as a novel acyl electrophile, and their acyl
C−F bonds were converted into the corresponding acyl C−C
bonds, which suppressed the loss of the carbonyl moiety and
yielded the desired ketones. Quite recently, Schoenebeck
reported a palladium-catalyzed decarbonylative coupling of
acyl fluorides and Et3SiCF3. This reaction is the first example
of the B-type transformation of acyl fluorides, which provided
trifluoromethyl arenes.11
A
carboxylic acid derivatives (RCOZ) by transition-metal
complexes to form acyl metal species has been widely
investigated and utilized as the initiation step in various
catalytic molecular transformations.1 In the presence of a
nucleophilic compound under the system, the acyl metal
species potentially displays the following two different reaction
modes. The first reaction mode is a simple acylation, when the
acyl fragment (RCO) of an acyl metal complex can be
transferred into nucleophiles from the transition-metal center
(Scheme 1A). Another mode includes a decarbonylation
Scheme 1. Two Reaction Modes of Carboxylic Acid
Derivatives via Acyl C−Z Bond Cleavage by a Transition-
Metal Catalyst
Since the reductive transformation of carboxylic acid
derivatives is an important synthetic strategy in organic
chemistry, various catalytic procedures have been developed
utilizing a hydrosilane,2−6 which is a mild reducing reagent
with highly functional group tolerance and chemoselectivity.
Selecting acyl fluorides as a novel electrophile in the reaction
with a hydrosilane, therefore, provides an alternative synthetic
route. Despite the synthetic potential, however, there has been
no report of the successful reductive conversion of acyl
fluorides.12
We describe, herein, a palladium-catalyzed reductive trans-
formation of acyl fluorides using a hydrosilane. In an attempt
to use a hydrosilane in the reductive transformation of acyl
fluorides, we found that C−H bond formation occurred via
acyl C−F bond cleavage, wherein the decarbonylation event of
process, wherein the carboxylic acid functions as a hydro-
carbon (R) source, and in many cases, these involve an
arylation (R = Ar) of nucleophiles (Scheme 1B).
Due to the potential utility of carboxylic acid derivatives as a
divergent building block in synthetic chemistry, extensive effort
has been devoted to developing a novel catalytic trans-
formation of carboxylic acids with or without the decarbon-
ylation process. To control the reaction mode using either a
non-decarbonylative (A-type) or a decarbonylative (B-type)
reaction, however, an individual fine-tuning of each reaction
system is essential.2−7
Received: May 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01582
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
acyl fluorides can be controlled by a simple switching of the
phosphorus ligand. By employing monodentate PCy3 as a
ligand, acyl fluorides 1 were converted to the corresponding
aldehydes 2 without decarbonylation. On the other hand, the
decarbonylation switch was turned “ON” by changing the
ligand from PCy3 to bidentate DCPE [Cy2P(CH2)2PCy2],
which formed hydrocarbons 3 (Scheme 2).13
ylphosphines, such as JohnPhos and CyJohnPhos, and a
trialkylphosphine, PtBu3, were less effective for the reaction in
terms of both reactivity and selectivity (entries 4−6). The best
result for the selective formation of aldehyde 2a was obtained
using PCy3 as a ligand (entry 7), and a decrease in the catalyst
loading (5 mol % to 2.5 mol % of [Pd]) slightly improved the
yield of 2a to 92% (with 99:1 selectivity of 2a/3a) (entry 8).
Next, we performed reactions using a series of bidentate
phosphine ligands: DPPBz, DPPE, DCPE, DPPP, DPPF, and
Xantphos. These ligands induced the decarbonylation step,
which provided naphthalene 3a (entries 9−15). In particular,
when DCPE was used as a ligand, a significant improvement
was observed in the formation of 3a (82% yield of 3a with 4:96
selectivity of 2a/3a) (entry 11). With a lower catalyst loading
(5 mol % to 2.5 mol % of [Pd]), the reaction also proceeded to
form 3a in an 84% yield (with 8:92 selectivity of 2a/3a) (entry
12). In the absence of a palladium catalyst, however, the
reductive conversions did not proceed (entries 16 and 17).
With employment of PCy3 as the ligand under the
conditions of entry 8 in Table 1, the scope and limitations
for the selective formation of aldehydes 2 from various acyl
fluorides 1 were examined (Scheme 3). A wide range of
benzoic acid fluorides was applicable to the selective
transformation that formed the corresponding benzaldehyde
derivatives. Benzoic acid fluoride 1b and its derivatives bearing
an alkyl or phenyl substituent at the benzene ring, 1c−g, were
successfully converted into aldehydes 2b−g with excellent
Scheme 2. This Work: Ligand-Controlled Divergent
Transformation of Acyl Fluorides in a Palladium-Catalyzed
Reduction Using a Hydrosilane
To achieve the selective conversion of acyl fluoride 1a to
either the corresponding aldehyde 2a or the decarbonylative
reduction product 3a, ligand screening was initially conducted
using a catalytic amount of Pd(OAc)2 and 1.4 equiv of Et3SiH
in toluene at 100 °C for 20 h (Table 1). When the reaction was
performed using PPh3 as a ligand, only trace amounts of the
desired products 2a and 3a were detected by GC analysis
(entry 1). Although P(4-CF3C6H4)3 gave similar results, the
use of P(4-MeOC6H4)3 increased the yield of 2a to 53% (with
85:15 selectivity of 2a/3a) (entries 2 and 3). Dialkylbiar-
Scheme 3. Substrate Scope for the Production of Aldehydes
2
Table 1. Ligand Screening for the Divergent Conversion of
Acyl Fluoride 1a
a
a
GC results (%)
selectivity
entry
ligand
PPh3
P(4-
conversion
2a
3a
(2a/3a)
1
2
21
12
2
<1
<1
<1
CF3C6H4)3
P(4-
MeOC6H4)3
3
83
53
9
85:15
4
5
6
7
8
9
10
11
12
13
14
15
JohnPhos
CyJohnPhos
PtBu3
PCy3
PCy3
DPPBz
DPPE
DCPE
DCPE
DPPP
DPPF
Xantphos
PCy3
77
63
31
17
10
<1
77
1
11
<1
2
<1
46
34
82
84 (84)
38
21
17
94:6
48:52
>99
>99
>99
>99
>99
>99
>99
>99
74
97:3
99:1
13:87
52:48
4:96
b
c
92 (90)
7
37
3
b
c
7
8:92
43
45
17
0
53:47
68:32
50:50
d
16
17
10
36
0
0
d
DCPE
4
a
Reaction conditions: 1a (0.2 mmol), Et3SiH (0.28 mmol),
a
Pd(OAc)2 (5 mol %), and ligand (15 mol % of P) in toluene (0.2
Reaction conditions: 1 (1.0 mmol), Et3SiH (1.4 mmol), Pd(OAc)2
b
mL) at 100 °C for 20 h. 1a (1 mmol), Pd(OAc)2 (2.5 mol %), and
(0.025 mmol), and PCy3 (0.075 mmol) in toluene (1 mL) at 100 °C
c
d
b
c
d
e
ligand (7.5 mol % of P) in toluene (1 mL). Isolated yield. Without a
palladium catalyst.
for 20 h. GC yield. Isolated yield. 1H NMR yield. Pd(OAc)2 (5
mol %, 0.05 mmol) and PCy3 (15 mol %, 0.15 mmol).
B
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selectivity. Reactions of methoxy-substituted versions at the 4-
and 3-positions, 1h and 1i, afforded the corresponding
aldehydes 2h and 2i in high yields with good selectivity.
When 2-methoxy 1j was used as a starting substrate, however,
the yield and selectivity of 2j were decreased. Functional
groups, including a sulfur and a halogen atom, 1k−n, were also
tolerant of the present non-decarbonylative conditions, giving
2k−n. Several acyl fluorides containing heterocycles, such as
an indole or a benzofuran moiety, and a 1-naphthyl one 1o−
1q, were available to provide 2o−q with high selectivity.
Vinylic carboxylic acid fluoride 1r, which is derived from
cinnamic acid, was also converted into aldehyde 2r. The use of
aliphatic acid derivative 1s bearing a fluorocarbonyl group at its
benzylic carbon failed to yield the desired selectivity of 2s/3s,
but the selective reaction of the starting acyl fluorides with
longer linear alkyl chains, such as 1t and 1u, proceeded to form
the corresponding aldehydes 2t and 2u, respectively.
Table 2. Substrate Scope for the Production of
Hydrocarbons 3
a
Next, we explored a decarbonylative reaction of acyl
fluorides 1 utilizing DCPE as the ligand, as shown by entry
12 in Table 1. In a manner similar to that for the reaction of
PCy3 in the preparation of aldehydes, the complementary
decarbonylative reaction was also applicable to various types of
acyl fluorides 1, leading to the corresponding hydrocarbons 3
with high yields and selectivities (Table 2). Benzoic acid
fluoride analogues 1b and 1d−n undertook a decarbonylative
reaction to produce arenes 3b and 3d−n in efficient yields and
selectivities with good functional group tolerance (entries 1−
12). Heteroaromatics and a fused aromatic substrate 1o, 1p,
1v, and 1q, were also available to form the corresponding
aromatic compounds 3o, 3p, 3v, and 3a, respectively (entries
13−16). Although starting substrates bearing a fluorocarbonyl
group at the vinylic carbon 1r or benzylic carbon 1s were
available for decarbonylative conversion to form 3r and 3s
(entries 17 and 18), aliphatic carboxylic acid fluorides with a β-
hydrogen, 1t and 1u, did not provide the expected decarbon-
ylative reduction products 3t and 3u. Then, the formation of
aldehydes 2t and 2u was observed along with olefins 3r and
3u′ via decarbonylation and β-hydrogen elimination (entries
19 and 20).
Plausible pathways of the divergent transformations of acyl
fluoride 1 to aldehyde 2 or hydrocarbon 3 are shown in Figure
1. In both cases, for the production of 2 using PCy3, or 3 using
DCPE, each mechanism is possible beginning with the
oxidative addition of an acyl C−F bond of 1 to palladium(0),
which would form an acylpalladium fluoride species (RCO-
[Pd]F). For the generation of 2 in the presence of PCy3,
transmetalation then occurs between the Pd−F and the Si−H
of a hydrosilane, and the subsequent reductive elimination of
the acyl C−H bond would provide aldehyde 2 (Figure 1A).
On the other hand, when DCPE is used as a ligand,
decarbonylation would proceed prior to the acyl C−H bond
formation of aldehyde 2, and then, the pathways involving
transmetalation and C−H reductive elimination would result
in the production of hydrocarbon 3 (Figure 1B).14
For both transformations of acyl fluoride 1l into aldehyde 2l
by PCy3, and into arene 3l by DCPE, the rates of the reactions
were affected by the initial ratios of the Pd/ligand. Monitoring
of the reaction of 1l and Et3SiH with three different ratios of
Pd/PCy3 (1:2, 1:3, and 1:4) by 19F NMR spectroscopy
revealed that 1:3 and 1:4 Pd/PCy3 ratios provided aldehyde 2l
at similar rates. However, the reaction rate was suppressed by a
Pd/PCy3 catalysis in a 1:2 ratio (Figure 2A). The effect of the
ratio of Pd/DCPE was also significant in the conversion of 1l
a
Reaction conditions: 1 (1.0 mmol), Et3SiH (1.4 mmol), Pd(OAc)2
(0.025 mmol), and DCPE (0.038 mmol) in toluene (1 mL) at 100 °C
b
c
for 20 h. GC yield. Isolated yield.
C
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Figure 3. Possible intermediates (X = H or F) for the production of
(A) aldehyde 2 and (B) hydrocarbon 3.
ylation is more favorable due to the presence of an open
coordination site at the palladium center (Figure 3B).
However, the detailed mechanism of the ligand effects for
the selectivity is unclear at this stage.15
Figure 1. Plausible pathways for the production of (A) aldehyde 2
and (B) hydrocarbon 3.
In summary, we have developed the first palladium-catalyzed
selective transformation of acyl fluorides using a hydrosilane.
The decarbonylation process can be completely controlled
with only a change of ligands, which affords different products
in high selectivity: aldehydes by PCy3 or hydrocarbons by
DCPE. These applications, the detailed mechanistic inves-
tigations, and computational studies on the effect of the ligands
are now underway by this research group.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01582.
Experimental procedures and characterization data for
all compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yoheiogiwara@rs.tus.ac.jp.
*E-mail: sakachem@rs.noda.tus.ac.jp.
ORCID
Yohei Ogiwara: 0000-0003-1438-0916
Norio Sakai: 0000-0003-0722-4800
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 2. Time−concentration plots of the conversion of 1l. Reaction
conditions: 1l (0.6 mmol), Et3SiH (0.84 mmol), and Pd(OAc)2
(0.015 mmol) in toluene-d8 (0.6 mL) at 100 °C. (A) Formation of 2l
using PCy3 (0.03−0.06 mmol). (B) Formation of 3l using DCPE
(0.0075−0.03 mmol). Concentrations of substrates were calculated
by 19F NMR analysis using CF3C6H5 as an internal standard.
This work was partially supported by JSPS KAKENHI Grant
No. JP16K21400, by a Research Grant from Shorai
Foundation for Science and Technology, by a grant from
Central Glass Co., Ltd. Award in Synthetic Organic Chemistry,
Japan, by a Research Grant Program for Younger Professors
from JGC-S Scholarship Foundation, and by a Research Grant
from the Japan Prize Foundation. We are grateful to Shin-Etsu
Chemical Co., Ltd., for the gift of silanes.
into arene 3l (Figure 2B). The reaction of 1l and Et3SiH with
̂
1:2 and 1:4 of Pd/P (1:1 and 1:2 of Pd/PP) proceeded to
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■
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E
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