Pd-Catalyzed Heck-Type Reaction: Synthesizing Highly Diastereoselective and Multiple Aryl-Substituted P-Ligands

Article abstract of DOI:10.1021/acs.orglett.9b02697

In this work, an efficient palladium-catalyzed Heck-type reaction was successfully used to synthesize a wide range of monoaryl-or diaryl-substituted P-ligands with excellent yields and diastereoselectivity (up to 98% yield, dr >20:1). The preliminary mechanistic studies demonstrated that it possibly underwent a cationic Heck reaction that was assisted by silver salt, and it revealed the significant role of P(O)Ar₂ for excellent yields and diastereoselectivity.

Full text of DOI:10.1021/acs.orglett.9b02697

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Pd-Catalyzed Heck-Type Reaction: Synthesizing Highly
Diastereoselective and Multiple Aryl-Substituted P‑Ligands
†
†
†
†
†
,†,‡
Chong Li, Xiao-Yue Qiang, Zhi-Chao Qi, Bin Cao, Jing-Yu Li, and Shang-Dong Yang*
†
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
‡
Key Lab of Functional Molecular Engineering of Guangdong Province, South China University of Technology, Guangzhou 510640,
P.R. China
*
S Supporting Information
ABSTRACT: In this work, an efficient palladium-catalyzed
Heck-type reaction was successfully used to synthesize a wide
range of monoaryl- or diaryl-substituted P-ligands with
excellent yields and diastereoselectivity (up to 98% yield, dr
>
20:1). The preliminary mechanistic studies demonstrated
that it possibly underwent a cationic Heck reaction that was
assisted by silver salt, and it revealed the significant role of
P(O)Ar for excellent yields and diastereoselectivity.
2
hosphine ligands bearing a biaryl scaffold have played an
Scheme 1. Synthesis of Aryl-Substituted Biarylphosphines
1
P
important role in transition metal catalysis. Consequently,
a series of efficient methods have been developed to modify
biarylphosphines. In the beginning, a coupling reaction acts as a
unique strategy for C−C bond formation, which was used to
synthesize different aryl-substituted phosphine ligands. In
general, phosphine-containing aryl halide substrates are coupled
with an aryl reagent to build aryl-substituted phosphine ligands
2
through oxidative addition (Scheme 1a). With environmental
concerns in mind, C−H bond activation has been an emerging
3
area in organic synthesis. As a significant advance, a process of
R (O)P-directed palladium-catalyzed C−H arylation via a rare
2
seven-membered cyclopalladium intermediate was reported by
4
our group in 2014 (Scheme 1b). Shi’s group also accomplished
the work of rhodium(I)-catalyzed PR -directed C−H arylation
2
5
in 2017. There is little doubt that the strategy has a high atomic
economy in the modification of phosphine ligands, which was
5
compared with other research results (Scheme 1c).
When it comes to C−C bond formation, the Mizoroki−Heck
reaction (Heck reaction) is the most momentous trans-
6
formation. During the past few decades, significant progress
7
8
6a,d,e
has been made by the groups of Hayashi, Jung, Sigman,
and Zhou,
6
b,c,f
etc. On the basis of a large amount of
experimental data, the cyclic olefins are common substrates for
the Heck reaction. These existing methods have advantages in
many aspects; however, obtaining a good aryl site selectivity is
always an ultimate challenge. Significantly, our group has been
committed to exploring new methodologies for the synthesis of
9
in the Supporting Information). One of the typical Heck
reaction conditions was investigated with iodobenzene (2a) in
the presence of Pd(OAc) and K CO in 1,2-dichloroethane.
novel phosphine ligands. The P(O)Ar group shows an excellent
2
2
3
2
10
Unfortunately, no desired product was detected. Surprisingly,
we could obtain the monoarylation (3aa) and diarylation (4aa)
performance in selectivity. Herein, we achieved a series of
highly diastereoselective and good aryl-site-selective phosphine-
containing products via the Heck reaction (Scheme 1d).
We selected (1-(cyclohex-1-en-1-yl)naphthalen-2-yl)-
diphenylphosphine oxide (1a), a modified cyclic olefin, as a
model substrate for reaction discovery (for details, see Table S1
products in 71% yield, with a ratio of 6:1 when Ag CO took the
2
3
Received: August 2, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02697
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
place of K CO (Table S1, entry 2). Subsequently, different
solvents, Ag salts, Pd species, and additives were screened, and
the optimal reaction condition of monosubstituted product
Moreover, the reactions with different substituted diary-
lphosphine oxides proceeded successfully with good yields and
acceptable dr. For example, diarylphosphine oxides bearing
para-Me, para-OMe, and meta-Me produced the desired
products with excellent yields and good dr (3ba, 82%, 10.1:1
dr; 3ea, 79%, 9.7:1 dr; 3ca, 67%, 10.8:1 dr). In addition,
diarylphosphine oxide bearing a bulky substitute (1d)
successfully afforded 3da (79%, 10.4:1 dr). Likewise, an
electron-withdrawing substituent at the para position on the
diarylphosphine gave moderate yield (3fa, 55%, 7.4:1 dr).
Based upon the importance of disubstituted phosphine
ligands that bear a biaryl scaffold, we explored this new strategy
with a variety of aryl iodide compounds. As expected, excellent
diastereoselectivity was obtained with dr up to >20:1. The para-
substituents bearing different substituents such as halogen, Me,
2
3
(
3aa) was obtained using Pd(OAc) (1.0 mol %) as catalyst,
2
AgTFA (1.5 equiv) in 2 mL of DCM for 0.3 mmol 1a, and 0.2
mmol 2a at 80 °C under an argon atmosphere (Table S1, entry
1
8). At the same time, the reaction could exclusively afford 4aa
with excellent yield and dr (Table S1, entry 26, 92% yield, dr
20:1) using Pd(OAc) (5.0 mol %) as catalyst, Ag CO (1.5
>
2
2
3
equiv), and TfOH (50 mol %) in 2 mL of DCM at 80 °C for 0.2
mmol 1a and 0.6 mmol 2a at 80 °C under an argon atmosphere.
With two optimal procedures in hand, the monosubstituted
products were first investigated using different substituted aryl
iodide compounds in Scheme 2 under conditions A. Good yields
Scheme 2. Scope of the Monoarylation Products ,
a b
t
OMe, Bu, and Ph on the aromatic rings could offer desired
products with good to excellent yields (4ab−4ah, 71−94%, dr
>
20:1). The structure of 4ae was confirmed by X-ray
crystallography. It is noteworthy that the desired products
could be obtained with aromatic rings bearing CN and NO in
2
3
9 and 32% yields, respectively (4ai, 4aj). Similarly, 2n and 2o
could give 4an and 4ao in good yields (68 and 79%) under
conditions B. However, when the aryl iodide compound was
used, an ortho-Me group monoarylation production 3al was
obtained exclusively (not shown in Scheme 3). 2m also could
not give the desired product under conditions B. 4ap was
obtained in 67% yield in the presence of 2-iodonaphthalene
(
2p). Furthermore, different substituted diarylphosphine oxides
were scoped. Diarylphosphine oxides containing electron-
donating groups, such as Me and OMe at meta or para positions,
provided desired products with decent yields and excellent dr
Scheme 3. Scope of the Diarylation Products ,
a b
a
Reaction conditions A: 1 (0.3 mmol), 2 (0.2 mmol), Pd(OAc) (1
2
mol %), and AgTFA (1.5 equiv) in 2 mL of DCM at 80 °C for 15 h.
b
c
31
Isolated yields based on 2. The dr value was determined using
P
d
NMR. n.r. = no reaction.
and moderate dr were obtained for substrates bearing halogen
groups (F, Cl, Br) (3ab−3ad, 76−81%, 6.2:1−7.9:1 dr). The
aryl iodide compounds that were substituted at the para position
with an electron-donating or electron-withdrawing group could
t
obviously impact the results. For example, Me, OMe, and Bu
were advantageous for the reaction, which gave the desired
product in excellent yields and moderate dr value (3ae−3ag,
7
7−81%, 8.6:1−10.7:1 dr). On the other hand, aryl iodide
derivatives bearing Ph also afforded the desired product in good
yield (3ah, 84%, 7.1:1 dr). However, the reaction would be
terminated by strong electron-withdrawing substituents, such as
CN or NO (3ai, 3aj). As we expected, the selectivity of the
2
product was somewhat affected by the steric effect. A higher dr
value was observed by stronger sterically hindered aryl iodide
compounds that carry a meta-Me or ortho-Me group (3ak,
a
Reaction conditions B: 1 (0.2 mmol), 2 (0.6 mmol), Pd(OAc) (5
2
1
2.5:1 dr; 3al, 16.8:1 dr). What disappointed us most was that 5-
mol %), Ag CO (1.5 equiv), and TfOH (50 mol %) in 2 mL of DCM
at 80 °C for 15 h. Isolated yields based on 1. The dr value was
determined using P NMR. n.r. = no reaction.
2
3
b
c
iodo-1-methylindole and other heterocyclic compounds could
31
d
not be reacted smoothly under these conditions (3am).
B
DOI: 10.1021/acs.orglett.9b02697
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
4ba−4ea, 68−89%, dr >20:1). Notably, 4f bearing CF could
conditions provided an alternative method for modifying such
complex natural products.
3
also give 46% yield with excellent dr (4fa, dr >20:1).
To our knowledge, there are few reports of different arylation
reactions under the same conditions. Most of them must have
other additives or increase the temperature because of the lower
reactivity or bulky steric hindrance of the monoarylation
To test the tolerance of the reaction further, some other types
of olefin derivatives were tested under conditions A/B in
Scheme 5. Moderate yields were obtained with cyclopentene
5
,11
a
products which were produced by the first step.
Taking the
Scheme 5. Scope of the Olefins
reaction applications into account, we further explored this
reaction with 3aa (Scheme 4). Interestingly, 3aa led to higher
Scheme 4. Scope of the Diarylation Products with 3aa ,
a b
a
Conditions A: 6 (0.2 mmol), PhI (0.3 mmol), Pd(OAc) (5 mol %),
2
and AgTFA (1.5 equiv) in 2 mL DCM at 80 °C for 15 h. Conditions
B: 6 (0.2 mmol), PhI (0.3 mmol), Pd(OAc) (5 mol %), Ag CO (1.5
2
2
3
equiv), and TfOH (50 mol %) in 2 mL DCM at 80 °C for 15 h.
b
c
Isolated yields. 6 (0.3 mmol), PhI (0.2 mmol). 6 (0.2 mmol), PhI
d
31
e
(
0.6 mmol). The dr value was determined using P NMR. 48 h.
n.r. = no reaction.
f
derivatives. However, 7aa′ was isolated with a low dr value
(
1.5:1 dr). In addition, 6b could produce the same product 7ba
in good yield and excellent dr value under conditions A/B (7ba,
A: 72%; B: 78%, dr >20:1). However, 7ca was only obtained in a
very low yield of 6% after 15 h under conditions A, whereas
prolonging the reaction time can increase the yield to 38%. It
may be due to the disadvantage of β-hydrogen elimination.
Incidentally, the 1,2-diphenylethylene derivative affords tetraar-
yl-substituted ethylene in good yields (7da, A: 54%; B: 79%).
The potential practicality of these methods was assessed in
Scheme 6. We explored the model substrate 1a. It can be scaled
a
Reaction conditions A: 1 (0.2 mmol), 2 (0.3 mmol), Pd(OAc) (5
2
mol %), AgTFA (1.5 equiv) in 2 mL of DCM at 80 °C for 15 h.
Reaction conditions B: 1 (0.2 mmol), 2 (0.3 mmol), Pd(OAc) (5
2
mol %), Ag CO (1.5 equiv), and TfOH (50 mol %) in 2 mL of DCM
2
3
Scheme 6. Gram-Scale Experiment and Derivatization
b
c
at 80 °C for 15 h. Isolated yields based on 3aa. The dr value was
determined using 3 P NMR. Twenty hours. n.r. = no reaction.
1
d
e
reactivity, and 4aa was obtained with excellent yields and
excellent dr (A: 95%; B: 98%, dr up to >20:1). To confirm the
position of the double bond and provide evidence for the
direction of β-hydrogen elimination, the structure of 5aae was
confirmed using X-ray crystallography. In addition, a range of
aryl iodide compounds with various electronic properties are
compatible with this method. Different groups at the para
position of aryl iodides, such as halogens, Me, OMe, Ph, and
CO Et, provided good yields (5aab−5aah, 67−92%, dr >20:1).
2
Satisfactory results were also obtained when substituents were at
different positions, for instance, para-Me (5aae, A: 89%; B: 82%,
dr >20:1), meta-Me (5aak, A: 76%; B: 71%, dr >20:1), and
ortho-Me (5aal, A: 83%; B: 75%, dr >20:1). Although strong
electron-withdrawing aryl compounds have been challenging
substrates for the above results, they delivered desired products
with excellent yield and dr under conditions A/B (5aai, A: 84%;
B: 75%, dr >20:1; 5aaj, A: 73%; B: 83%, dr >20:1). Inspired by
the high activity of 3aa, we obtained estrone-derived aryl iodide
compound 2q under this reaction in 47% yield. These
up to gram scale in good yields under both conditions A and B
(3aa, 1.05 g, 69%; 4af, 1.35 g, 77%). Furthermore, we
demonstrated the diverse and practical derivatization, which
could be readily converted to a series of P-ligands. Although P-
ligand 8a was obtained with a low dr value, a new type of diaryl-
substituted vinyl arene P-ligand 8b was obtained with excellent
C
DOI: 10.1021/acs.orglett.9b02697
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Organic Letters
Letter
12
dr (8b, 74%; dr >20:1) when product 4af was reduced. Finally,
in the presence of DDQ, monoarylation or diarylation of the P-
ligands’ precursors bearing a biaryl scaffold was obtained easily
elimination of intermediate C afforded the desired product I.
Finally, hydridopalladium reductive elimination releases acid
0
and regenerates the Pd species (cycle A). When AgTFA was
(
9a, 90%; 9b, 85%).
To clarify the P(O)Ar group’s participation, we switched 1a
replaced with Ag CO , product I also would be produced first. In
2
3
addition, the catalytic cycle still operates effectively, and product
II would be obtained after another catalytic cycle because TfOH
promoted hydridopalladium reductive elimination to release
2
to 1-(cyclohex-1-en-1-yl) naphthalene 10 under conditions A/B
and only observed a mixture of products. The desired products
were hard to separate (Scheme 7a, conditions A: totally 12%,
0
acid and regenerate the Pd species (cycle B).
In conclusion, we have developed a new strategy to synthesize
a wide range of monoaryl- or diaryl-substituted P-ligand
precursors with excellent yields and diastereoselectivity (up to
Scheme 7. Mechanistic Analysis
9
8% yield, dr >20:1). The monoarylation products have high
reactivity, and they could be obtained as asymmetrical diaryl-
substituted products under different conditions. It provides an
effective and valuable tool for P-ligand modification.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b02697.
Detailed experimental procedures, spectra data for all
1
13
31
19
compounds, copies of H, C, P, and F NMR spectra
PDF)
(
Accession Codes
CCDC 1889906 and 1889964 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
0a/10b = 2.4:1; conditions B: totally 10%, 10a/10b = 1:1.2).
To formulate a mechanistic model, we carried out a mass
spectrometry experiment to gain more insight into intermediates
1
3
(
9
Scheme 7b). First, a set of signal peaks in the ESI/MS at m/z
25.2327 and m/z 1077.2957 were detected, which correspond
AUTHOR INFORMATION
Corresponding Author
■
+
+
to [A + H] and [A′ + H] . It might show that both substrates
and products could be used as ligands that coordinate with the
*
E-mail: yangshd@lzu.edu.cn.
0
Pd species. Another set of signal peaks at m/z 1001.2643 and
ORCID
m/z 1153.3268 was detected, which probably were generated
from PhI via oxidative addition and then migratory insertion of B
or B′. Subsequently, we measured the changes in pH under
conditions A (before reaction, pH = 1.83, after reaction, pH =
Shang-Dong Yang: 0000-0002-4486-800X
Notes
The authors declare no competing financial interest.
1.32). This phenomenon indicated that there may be acid that
was produced during the reaction.
ACKNOWLEDGMENTS
■
Based on these results and those reported in the
6
h,13,14
We are grateful for financial support from the NSFC (No.
21532001) and the Open Fund of the Key Laboratory of
Functional Molecular Engineering of Guangdong Province
literature,
a possible mechanism was proposed (Scheme
0
8
). Take conditions A as an example: Pd was formed by an in
situ reduction of Pd(OAc) . Complex A was formed through
2
(
2016kf04).
coordination to both PO and the double bond. Then, PhI was
II
0
converted to Pd complex B through oxidative addition by Pd
1
5
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