Decarbonylative C-P bond formation using aromatic esters and organophosphorus compounds

Article abstract of DOI:10.1021/acs.orglett.8b00080

Ni-catalyzed C-P bond formation was achieved using aromatic esters as unconventional aryl sources. The key to success was the use of a thiophene-based diphosphine ligand (dcypt). Several aromatic esters including heteroaromatics can be coupled with phosphine oxides and phosphates, providing aryl phosphorus compounds. The synthetic utility of the method was demonstrated by application of the present protocol to the sequential coupling reactions.

Full text of DOI:10.1021/acs.orglett.8b00080

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Decarbonylative C−P Bond Formation Using Aromatic Esters and
Organophosphorus Compounds
Ryota Isshiki, Kei Muto, and Junichiro Yamaguchi*
Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan
S
* Supporting Information
ABSTRACT: Ni-catalyzed C−P bond formation was achieved
using aromatic esters as unconventional aryl sources. The key to
success was the use of a thiophene-based diphosphine ligand
(dcypt). Several aromatic esters including heteroaromatics can
be coupled with phosphine oxides and phosphates, providing
aryl phosphorus compounds. The synthetic utility of the method
was demonstrated by application of the present protocol to the
sequential coupling reactions.
hosphine-containing aromatic molecules have broad
Papplications from the field of medicinal chemistry to
materials chemistry, because of the strong influence of
phosphorus groups on the physical, chemical, and biological
properties of adjacent arenes. Furthermore, the high coordinat-
ing ability of the P atom to a metal center has been extensively
exploited for the development of various transition-metal
catalysts. Given these unique properties, it is not surprising that
many synthetic methods have been developed for the formation
of arene−phosphorus bonds.1 Classically, the nucleophilic
substitution of arylmetals with phosphine chlorides had been
used for the synthesis of aromatic phosphorus compounds. As a
catalytic method, the Hirao coupling of aryl halides and
organophosphorus compounds by palladium catalysis is
recognized (Figure 1A).2 Along with the development of
efficient metal catalysts, several synthetic methods for aryl
phosphorus molecules have been reported using phenol
derivatives,3 arene diazoniums,4 thiophenols,5 aryl nitriles,6
and arene itself7 through inert bond activation.8
Recently, our group has focused on the transformation of
aromatic esters through decarbonylation to construct C−C and
Figure 1. (A) Catalytic C−P forming cross-couplings. (B) C−P
C−O bonds by nickel and palladium catalysis.9,10 Because
coupling using arenecarboxylate derivatives.
aromatic esters are abundant, easy to prepare, and different as a
chemical feedstock from haloarenes, coupling reactions using
esters are attractive for the synthesis of substituted aromatic
molecules. Furthermore, since ester moieties are usually stable
under classic cross-coupling conditions such as the Suzuki−
Miyaura coupling, aromatic esters can be orthogonally utilized
as versatile building blocks for the synthesis of highly
functionalized arenes. Among several reports on the cross-
coupling of aromatic carboxylic acid derivatives,11 the Szostak
group recently developed a decarbonylative C−P bond-forming
reaction of aromatic amides with phosphites under the
influence of palladium and nickel catalysts (Figure 1B).12
Similarly, Wang and co-workers reported an aryl phosphine
synthesis through a catalytic decarbonylation of aroylphos-
phines.13 We herein report a Ni-catalyzed decarbonylative
cross-coupling of aromatic esters and organophosphorus
compounds such as phosphine oxides and phosphates.
Our study commenced with a survey of appropriate catalytic
conditions using phenyl isonicotinate (1A) and diphenyl
phosphine oxide (2a) (Table 1). In order to avoid catalyst
deactivation by coordination of 2a to the metal, we focused on
the use of robust ligands. To our delight, our in-house catalytic
system using Ni(OAc)2 /dcypt (dcypt: 3,4-bis-
(dicyclohexylphosphino) thiophene)14,15 showed high reac-
tivity toward decarbonylative C−P bond formation (Table 1,
Received: January 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00080
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Reaction Optimization
Scheme 1. Substrate Scope of Aromatic Esters 1
b
entry
Ni or Pd
ligand
dcypt
solvent
yield of 3Aa (%)
1
2
Ni(OAc)2
Ni(OAc)2
Ni(OAc)2
Ni(OAc)2
Ni(OAc)2
Ni(OAc)2
Ni(OAc)2
Ni(OAc)2
Ni(OAc)2
Pd(OAc)2
t-AmylOH
t-AmylOH
t-AmylOH
t-AmylOH
t-AmylOH
t-AmylOH
t-BuOH
74
6
dcype
3
dppe
9
c,d
4
PCy3·HBF4
0
c,d
5
IPr·HCl
0
c,d
6
ICy·HBF4
dcypt
0
7
50
47
65
0
8
dcypt
toluene
9
dcypt
1,4-dioxane
t-AmylOH
10
dcypt
a
Conditions: 1A (0.20 mmol), 2a (1.5 equiv), Ni or Pd (5 mol %),
b
ligand (10 mol %) in solvent (1.0 mL) at 150 °C, 12 h. Yield was
determined by GC analysis using n-decane as an internal standard after
c
the treatment of crude material with H2O2. 20 mol % of ligand was
d
used. NaOt-Bu (25 mol %) was added.
entry 1). Other diphosphine ligands, such as dcype and dppe,
significantly decreased the reaction yield (Table 1, entries 2 and
3). PCy3 and N-heterocyclic carbene ligands,17 which are
16
a
effective for inert bond activation using nickel catalysis,18 were
completely ineffective (Table 1, entries 4 and 5. See the
Supporting Information (SI) for details). Regarding the solvent
effect, it was found that tertiary alcohols give good results
(Table 1, entries 1 and 7). Nonpolar solvents such as toluene
and 1,4-dioxane were found to be applicable, albeit in slightly
lower yields (Table 1, entries 8 and 9). Palladium catalysis
showed no reactivity in the C−P bond formation (Table 1,
entry 10). It is noteworthy that this reaction proceeds without
base, in contrast to Hirao-type coupling, which often requires a
stoichiometric amount of base.
Conditions: Ni(OAc)2 (5 mol %), dcypt (10 mol %), 1 (0.40 mmol),
2a (1.5 equiv), t-AmylOH (2.0 mL), 150 °C, 12 h. b170 °C, 18 h. cNaF
(1.5 equiv) was added. HCO2Na (1.5 equiv) was added and toluene
(2.0 mL) was used instead of t-AmylOH.
d
and cyano groups were compatible under these catalytic
conditions.
Next, we investigated applicable phosphates (see Scheme 2).
At first, we subjected phosphate 2b to the reaction with 1A
under the optimized conditions; however, the reaction resulted
in a poor yield. After reinvestigating the reaction conditions, we
With optimized conditions (Table 1, entry 1) in hand, we
investigated the substrate scope of this catalytic reaction (see
Scheme 1). Various azinecarboxylates could be used in this
reaction, as 4-pyridyl (3Aa), 3-pyridyl (3Ba), and pyrazinyl
(3Ca) phosphine oxides were obtained in moderate to good
yields. Unfortunately, 2-pyridyl substrates diminished the
reaction yield (see the SI). Isoquinoline (3Ea) and quinoline
(3Fa and 3Ga) substrates also underwent the present reaction.
Flavone derivative 3Ha could be synthesized in a good yield.
Five-membered heteroaromatics, such as thiophenes, benzo-
thiophenes, and furans, were reactive, providing the corre-
sponding heteroaryl phosphine oxides (3Ia−3La). In these
cases, the addition of sodium formate and NaF improved the
reaction yield, although their effect is still unclear. This reaction
was also applicable to a range of benzoates (3Ma−3Sa).
Regarding the effect of substituents, it was revealed that this
reaction favors substrates bearing electron-withdrawing groups
(see the SI). Although ortho-substituents diminished the
reaction efficiency, meta- and para-substituted benzoates can
couple with 2a to give the corresponding aryl phosphines (3Ra
and 3Sa). Reactive functional groups such as acetal, alkyl esters,
a
Scheme 2. Substrate Scope of Phosphates 2
a
Conditions: Ni(OAc)2 (10 mol %), dcypt (15 mol %), 1 (0.40
mmol), 2 (1.5 equiv), NaF (1.5 equiv), toluene (2.0 mL), 150 °C, 12
h.
B
DOI: 10.1021/acs.orglett.8b00080
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
found that alternative conditions using NaF in toluene were
effective for the reaction of alkyl phosphates (see the SI for
details). Phosphinite 2e was also coupled to generate the
desired product 3Fe in an acceptable yield.
Since ester groups are generally intact under classic cross-
coupling conditions, we conducted a sequential cross-coupling
of 4 (Scheme 3). First, Suzuki−Miyaura coupling of 4 with 5
oxidative addition of the C−O bond to Ni(0) generates a
Ni(II) intermediate B. To this intermediate B, a phosphorus
nucleophile effectuates a ligand exchange, followed by
decarbonylation to form Ni(II) intermediate D. Finally,
reductive elimination produces product 3 with regeneration
of a Ni(0) species.21
In summary, we have developed a Ni-catalyzed C−P bond
formation of aromatic esters and organophosphorus com-
pounds such as phosphine oxides and phosphates. Intriguingly,
only the use of dcypt as a ligand enabled this reaction.
Sequential coupling reactions speak for the synthetic utility of
not only the present reaction, but also of aromatic esters as an
unconventional coupling partner. Further studies to expand the
scope of ester-based coupling are ongoing in our group and will
be reported in due course.
Scheme 3. Sequential Couplings
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.8b00080.
Detailed experimental procedures, spectral data for all
1
compounds, and H, 13C, and 31P NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: junyamaguchi@waseda.jp.
ORCID
Kei Muto: 0000-0001-8301-4384
Junichiro Yamaguchi: 0000-0002-3896-5882
Notes
smoothly produced biaryl 6 without any decomposition of the
phenyl ester moiety (Scheme 3A). The resulting aromatic ester
6 was subjected to the present C−P bond-forming reaction
with 2a, providing 7 in a good yield.19 In another example, a
sequential decarbonylative coupling was achieved in a site-
selective manner (Scheme 3B). A previously reported
decarbonylative etherification of 8,10g followed by the present
decarbonylative C−P bond formation, successfully generated
10 in a good yield.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Nos.
JP16H01011, JP16H04148 (to J.Y.), and JP17K14453, the
Early Bird Program of Waseda University, and JXTG Energy
for Young Researchers (to K.M.). We thank Mr. Ryosuke
Takise (Nagoya University) for the measurement of HRMS.
Scheme 4 shows a plausible mechanism for this reaction.20
We assume a Ni(0)/Ni(II) catalytic cycle as follows. First,
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