Ruthenium-Catalyzed Gram-Scale Preferential C-H Arylation of Tertiary Phosphine

Article abstract of DOI:10.1021/acs.orglett.9b00888

A general protocol for site-preferential mono-C-H arylation of tertiary phosphine ligands catalyzed by a ruthenium(II) complex was devised. This protocol gives access to a series of modified Buchwald-biaryl monophosphines on a gram scale in moderate to excellent yields. A catalytic cycle is proposed derived from knowledge of the intermediates observed by ESI-MS. Importantly, these monoarylated products could be further transformed into dibenzophosphole derivatives.

Full text of DOI:10.1021/acs.orglett.9b00888

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Ruthenium-Catalyzed Gram-Scale Preferential C−H Arylation of
Tertiary Phosphine
Jia-Wei Li,† Liang-Neng Wang,† Ming Li,† Pan-Ting Tang,† Xiao-Peng Luo,† Mohamedally Kurmoo,§
Yue-Jin Liu,*,† and Ming-Hua Zeng*,†,‡
†Hubei Collaborative Innovation Center for Advanced Chemical Materials, Ministry of Education Key Laboratory for the Synthesis
and Application of Organic Functional Molecules and College of Chemistry and Chemical Engineering, Hubei University, Wuhan
430062, China
‡Department Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry and
Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China
§
́
Institut de Chimie de Strasbourg, CNRS-UMR7177, Universite de Strasbourg, 4 rue Blaise Pascal, Strasbourg 67070, France
S
* Supporting Information
ABSTRACT: A general protocol for site-preferential mono-C−H
arylation of tertiary phosphine ligands catalyzed by a ruthenium(II)
complex was devised. This protocol gives access to a series of
modified Buchwald−biaryl monophosphines on a gram scale in
moderate to excellent yields. A catalytic cycle is proposed derived
from knowledge of the intermediates observed by ESI-MS.
Importantly, these monoarylated products could be further trans-
formed into dibenzophosphole derivatives.
C−H bond activation catalyzed by a coordination complex has
been an important approach in organic synthesis.1 The precise
and rational design of the catalyst is extremely important for
the sequence involving coordination, activation, and departure
of the catalyst with the reaction substrate, which define the
effectiveness of a catalytic reaction.2 Therefore, it is a great
challenge to choose a reasonable catalyst for a substrate with a
strong coordinating group. Tertiary phosphines (PR3) belong
to a critically important cornerstone where they are widely
used in transition-metal catalysis and functionalizing organic
materials for their unique optical properties.3 Moreover, their
steric and electronic properties can be altered by varying the R
groups.4 For instance, Buchwald−biaryl monophosphines with
a large steric hindrance have been widely used as supporting
ligands in metal-catalyzed C−C, C−O, and C−N bond
construction processes.5 Consequently, efficient and diverse
synthesis of PR3 has been arguably the most important
contributor to the advancement of the metal-catalysis field. In
recent decades, transition-metal-catalyzed C−H bond activa-
tion strategy has provided a powerful tool for synthesis of PR3,
which has enabled more efficient chemical synthesis by
avoiding the traditional requirement of preinstalled functional
handles.6,7 Among various modifications, arylation of PR3 via
C−H bond activation has received widespread interest. In this
respect, Lee,8 Kim,9 and other groups10 developed the first-
generation arylation of phosphine oxides via C−H activation
catalyzed by transition metals, such as Pd(II), Rh(III), and
Ir(III) (Scheme 1A). However, an inevitable limitation for the
first-generation method needs to utilize phosphine oxides as
substrates, which increases additional synthetic steps for
preoxidation of phosphines followed by reduction of the
Scheme 1. Modification of Tertiary Phosphines via Metal-
Catalyzed C−H Arylation
phosphine oxides. Therefore, a more efficient method for
direct C−H arylation of PR3 has been developed. Recently,
Shi11 and the group of Che and Yu12 realized the second-
generation P-atom-directed Rh(I)-catalyzed C−H arylation of
PR3 (Scheme 1B). Despite great success for atom- and step-
economical transformations, the development of a new
catalytic system for metal-catalyzed C−H arylation of PR3 is
Received: March 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00888
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
highly attractive and a great challenge due to their strong
coordination and instability.13
Scheme 2. Scope of Substituted Triarylphosphines
Prompted by the inspiring work of amino acid promoted C−
H activation14 and Ru(II)-catalyzed C−H functionalizations
by Ackermann15 and other groups16 and following a study on
the direct functionalization of inert C−H bonds,17 coordina-
tion-directed tandem reaction,18 and assembly process
mechanism,19 we herein describe a general and efficient
method for gram-scale synthesis of modified Buchwald−biaryl
phosphine ligands by Ru(II)-catalyzed preferential mono-C−H
arylation of PR3 assisted by monoprotected amino acid ligands
(Scheme 1C).
Initially, we selected readily available [1,1′-biphenyl]-2-
yldiphenylphosphane (1a) with 2-iodoanisole (2a) as a model
reaction (Table 1; for more details, see the Supporting
a
Table 1. Reaction Conditions Screening
entry
additive
base
yield of 3a (%)
1
2
3
4
5
6
7
8
1-AdCOOH
1-AdCOOH
nd
46
nd
47
52
13
64
60
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
PivOH
MesCO2H
N-Boc-lle-OH
N-Ac-lle-OH
N-Ac-L-Val-OH
N-Ac-L-Phe-OH
N-Ac-L-Ala-OH
N-Ac-β-Ala-OH
N-Ac-β-Ala-OH
9
10
11
61
60
69
a
Reaction conditions: 1 (3.0 mmol), 2 (4.5 mmol), [RuCl2(p-
cymene)]2 (5 mol %), N-Ac-β-Ala-OH (15 mol %) and CsOAc (6.0
mmol) in 2 mL of toluene at 120 °C under argon, 18 h, isolated
b
12
95 (1.69 g)
b
yields. 1 (4.0 mmol), 2 (6.0 mmol) in 2 mL of toluene, 120 °C, 18 h.
a
Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), [RuCl2(p-
cymene)]2 (5 mol %), additive (15 mol %), and CsOAc (0.2 mmol)
c
1 (2.0 mmol), 2 (3.0 mmol) in 1 mL of toluene, 120 °C, 16 h.
Reaction conducted at 140 °C.
d
b
in 1 mL of toluene at 120 °C under argon, 12 h, isolated yields.
(4.0 mmol), 2 (6.0 mmol) in 2 mL of toluene, 120 °C, 18 h.
1
moderate yields (24−74% yields, 0.36−1.49 g). Importantly,
even an active Cl− or Br− substituent could successfully
survive under these conditions, giving the desired products on
a large scale (3d, 51%, 0.97 g, and 3e, 61%, 0.95 g), which
could easily be further transformed. The ortho-substituent such
as Me (2i) gave the corresponding arylated product in 34%
yield (0.31 g), indicating a pronounced steric effect on the
reaction. Moreover, aryl groups attached to phosphine-bearing
electron-donating and electron-withdrawing substituents, such
as Me (1k, 1p), OMe (1o), F (1l, 1q), Cl (1m, 1r), and CF3
(1n), gave the corresponding products in moderate to
excellent yields on a large scale (49−88% yields, 0.73−1.30
g). Notably, trace biarylated product was detected in the
reaction system because the selectivity of this catalytic system
was excellent. The structures of 3d, 3f, 3i, 3k, and 3q were
determined from single-crystal X-ray diffraction.
We next explored the scope of aryl iodides (Scheme 3). To
our delight, substrates of aryl iodides bearing electron-donating
substituents on the aryl rings, such as OMe (2d, 2f), Me (2e,
2g), NMe2 (2i), and 3,4-methylenedioxyl (2p), gave the
corresponding products in reasonable yields (67−76% yields,
1.15−1.37 g). Aryl groups bearing electron-withdrawing
groups (CO2Me, CN, COMe) also led to the desired products
(4c, 4j, 4l, 4m) on a gram scale (30−65% yields, 0.53−1.22 g).
Information). The reaction gave the desired arylation product
in 46% yield in the presence of 5 mol % of [RuCl2(p-
cymene)]2, 15 mol % of 1-AdCOOH, and 2 equiv of CsOAc at
120 °C under argon in toluene (entry 2). No product was
detected in the absence of additive or base (entry 1, 3),
indicating that both play important roles in this transformation.
Then a series of ligands were screened (entry 4−11), and an
improved catalytic efficacy proved viable for N-Ac-β-Ala-OH
(entry 11). Finally, we tentatively increased the concentration
of the PR3, and as we expected, a higher yield of 3a up to 95%
was obtained (entry 12).
With the optimized reaction conditions in hand, we first
explored its applicability for a range of tertiary phosphines
(Scheme 2). Substrates with different substituents were tested,
considering the conversion of substrates and the oxidation of
the products during column chromatography separation
process, and the results were as follows. As we expected, the
reaction starting from diverse PPh2-based ligands led to a series
of ortho-arylated Buchwald−biaryl phosphines 3 on a gram
scale. A wide scope of functional groups, such as Me (1b), F
(1c), CF3 (1f), CO2Me (1g), PhO (1h), and naphthyl (1j),
were explored, affording the corresponding products in
B
DOI: 10.1021/acs.orglett.9b00888
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Scheme 3. Scope of Aryl Iodides
Scheme 4. Scope of Phosphole Derivatives
a
Reaction conditions: 4 (0.3 mmol), Pd(OAc)2 (5 mol %) in 1 mL of
toluene at 160 °C under argon, 12 h, isolated yields.
Scheme 5. Control Experiments
a
Reaction conditions: 1a (4.0 mmol), 2 (6.0 mmol), [RuCl2(p-
cymene)]2 (5 mol %), N-Ac-β-Ala-OH (15 mol %), and CsOAc (8.0
mmol) in 2 mL of toluene at 120 °C under argon, 18 h. Isolated
yields.
Excitingly, the Br− and OTf− substituents on aryl iodides gave
the corresponding arylated products in moderate yields (4b,
66%, 1.30 g, and 4k, 61%, 1.38 g). Dimethyl-substituted aryl
iodide was efficiently coupled and furnished the product 4n in
62% yield (1.10 g). Additionally, benzene- and naphthalene-
containing substrates were also found to be tolerable (4h, 58%,
1.14 g, and 4o, 66%, 1.23 g). It is noteworthy that heteroaryl
substrates with thiophene, indole, and quinoline efficiently
coupled and furnished a series of heteroaryl monophosphines
(4q−t) in moderate yields (50−66% yields, 0.84−1.23 g). In
addition, the structures of 4b, 4s, and 4t were confirmed by X-
ray crystallographic analyses. Unfortunately, no product was
observed when the P-atom was substituted with alkyls such as
Cy and t-Bu or simple tertiary phosphines, referred to PPh3.
Phospholes are widely used in organic materials for their
excellent characteristic optical and electronic properties.20 We
further transformed these arylated tertiary phosphines into
phosphole derivatives in a high-efficiency manner developed by
Chatani,21 and a series of phospholes derivatives bearing
electron-donating (OMe, Me, and 3,4-methylenedioxyl) and
electron-withdrawing groups (CN, COMe, and CO2Me) were
prepared in moderate yields (Scheme 4). Moreover, the
benzene- and naphthalene-containing substrates were also
found to be tolerable and gave the desired products in 30%
(5f) and 43% (5i) yields, respectively. Additionally, explora-
tion of their fluorescent properties is ongoing in our laboratory.
To gain insight into the reaction mechanism, an
intermediate Ru(II) Int A was prepared and the structure of
Ru(II) Int A was confirmed by X-ray crystallographic analysis
(Scheme 5a). The arylation of 1a under the optimal conditions
was suppressed in the presence of 5.0 mol % of Ru(II) Int A,
and a 20% yield of arylated product was obtained, indicating
that Ru(II) Int A may not be the active Ru(II) species
(Scheme 5b). Additionally, time-dependent control experi-
ments were conducted (Scheme 5c), and Ru(II) Int A showed
lower catalytic activity over time (Figure 1). These results
further confirmed that an active Ru(II) species, but not the
Ru(II) Int A, might be generated in situ initially with the aid of
MPAA and bases.
In order to provide information on the mechanism of this
transformation, the control reactions of [1,1′-biphenyl]-2-
Figure 1. Yield profile for the reaction of 1a with 2a in the presence of
[RuCl2(p-cymene)]2 (5 mol %) and Ru(II) Int A (10 mol %),
respectively. Each data point represents the average of three
independent trials.
C
DOI: 10.1021/acs.orglett.9b00888
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Organic Letters
Letter
yldiphenylphosphane (1a) with 2-iodoanisole (2a) were
monitored by ESI-MS.12,22 These results indicate the activated
intermediates I, II, and V involved in the catalytic process,
consistent with the C−H bond activation by Ru(II) species
(Figure S1; for details, see the Supporting Information).
On the basis of these experimental results and previous
reports,12,14e−g a plausible process is proposed (Scheme 6). In
monoarylation products were synthesized on gram scale, which
could be further transformed into phosphole derivatives.
Moreover, ESI-MS was used to elucidate the possible
mechanism of the reaction.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00888.
Scheme 6. Proposed Mechanism for the Catalytic Cycle
Detailed experimental procedures, compound character-
ization data (PDF)
Accession Codes
CCDC 1883107−1883108, 1883110−1883113, 1883116, and
1891452−1891453 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: liuyuejin@hubu.edu.cn.
*E-mail: zmh@mailbox.gxnu.edu.cn.
ORCID
Mohamedally Kurmoo: 0000-0002-5205-8410
Yue-Jin Liu: 0000-0003-4226-0890
Ming-Hua Zeng: 0000-0002-7227-7688
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
for Distinguished Young Scholars of China (No. 21525101),
the NSF of Hubei Province (Nos. 2017CFA006 and
2018CFB156), the Program of Ministry of Education Key
Laboratory for the Synthesis and Application of Organic
Functional Molecules (No. KLSAOFM1702), the NSF of
Guangxi Province (No. 2017GXNSFDA198040), and the
BAGUI talent program and scholar program (No. 2014A001).
M.K. is funded by the CNRS-France.
the absence of a Ru-N-Ac-β-Ala-O species and with the
knowledge that both additives are required, we speculate that
the CsOAc neutralizes the amino acid, which then coordinates
with [RuCl2(p-cymene)]2, thus generating the activated Ru(II)
species (Figure S2; for details see the Supporting Information).
Then the six-membered cyclometalated Ru(II) complex I
(detected by ESI-MS) is formed via the ortho-ruthenation of
1a with the activated Ru(II) species in the presence of base.
Oxidative addition of aryl iodide to the thus-formed Ru(II)
species II and formation of the complex III, followed by
reductive elimination, intramolecular C−H bond activation (V,
detected by ESI-MS), and coordination, releases the product 3
and regenerates the activated Ru(II) intermediate II.
In summary, we developed a general and efficient protocol
for C−H arylation of tertiary phosphine catalyzed by
ruthenium(II) complex. Its key rational design involves the
coordination of an amino acid to the [RuCl2(p-cymene)]2 that
enables site-preferential functionalization of tertiary phos-
phines via P-atom-directed mono-C−H arylation, affording a
series of Buchwald−biaryl phosphines with various functional
groups, such as CN, Br, and OTf. Importantly, all of the
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