New C2-symmetric six-membered carbene ligands incorporating two hydroxyl groups for palladium-catalyzed deprotonative-cross-coupling processes (DCCP) of sp3 C–H bonds in diarylmethanes

Article abstract of DOI:10.1016/j.tetlet.2017.09.022

A series of novel 3,4,5,6-tetrahydropyrimidinium salts, the precursor of N-heterocyclic carbene, were synthesized in high yields using a two-step procedure starting from commercially available amino alcohols. In situ prepared corresponding carbenes were tested in palladium-catalyzed C(sp³)?H arylation of 4-benzylpyridine with aryl bromides, affording triarylmethane derivatives in high yields.

Full text of DOI:10.1016/j.tetlet.2017.09.022

Accepted Manuscript
New C -symmetric six-membered carbene ligands incorporating two hydroxyl
2
groups for palladium-catalyzed deprotonative-cross-coupling processes
3
(
DCCP) of sp C−H bonds in diarylmethanes
Bi-Hui Zhou, Chen Wu, Xing-Xiu Chen, Hong-Xia Huang, Lin-Lin Li, Li-Mei
Fan, Jie Li
PII:
S0040-4039(17)31139-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.09.022
TETL 49292
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
19 June 2017
8 September 2017
12 September 2017
Please cite this article as: Zhou, B-H., Wu, C., Chen, X-X., Huang, H-X., Li, L-L., Fan, L-M., Li, J., New C 2-
symmetric six-membered carbene ligands incorporating two hydroxyl groups for palladium-catalyzed
3
deprotonative-cross-coupling processes (DCCP) of sp C−H bonds in diarylmethanes, Tetrahedron Letters (2017),
doi: http://dx.doi.org/10.1016/j.tetlet.2017.09.022
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New C -symmetric six-membered carbene
ligands incorporating two hydroxyl groups
for palladium-catalyzed deprotonative-cross-
2
Leave this area blank for abstract info.
3
coupling processes (DCCP) of sp C−H bonds
in diarylmethanes
a
b
b
a
a
a
a,
Bi-Hui Zhou, Chen Wu, Xing-Xiu Chen, Hong-Xia Huang, Lin-Lin Li, Li-Mei Fan, Jie Li ∗
a
Department of Pharmacy, School of Medicine, Zhejiang University City College, No. 48, Huzhou Road, Hangzhou 310015, PR China
b
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States

1
Tetrahedron Letters
journal homepage: www.els evier.com
New C -symmetric six-membered carbene ligands incorporating two hydroxyl groups
2
3
for palladium-catalyzed deprotonative-cross-coupling processes (DCCP) of sp C−H
bonds in diarylmethanes
a
b
b
a
a
a
a,
Bi-Hui Zhou, Chen Wu, Xing-Xiu Chen, Hong-Xia Huang, Lin-Lin Li, Li-Mei Fan, Jie Li ∗
a
Department of Pharmacy, School of Medicine, Zhejiang University City College, No. 48, Huzhou Road, Hangzhou 310015, PR China
b
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States
ARTICLE INFO
ABSTRACT
Article history:
Received
A series of novel 3,4,5,6-tetrahydropyrimidinium salts, the precursor of N-heterocyclic carbene,
were synthesized in high yields using a two-step procedure starting from commercially available
amino alcohols. In situ prepared corresponding carbenes were tested in palladium-catalyzed
Received in revised form
Accepted
3
C(sp )−H arylation of 4-benzylpyridine with aryl bromides, affording triarylmethane derivatives
in high yields.
Available online
2
009 Elsevier Ltd. All rights reserved.
Keywords:
N-Heterocyclic carbene
Tetrahydropyrimidinium
Pd-catalyzed
DCCP
Triarylmethanes
1
. Introduction
and have been widely used as ligands in organometallic
chemistry because of their attractive features such as stronger σ-
Triarylmethanes (TRAMs) have gained much attention
because of their widely application in the areas of organic
[12]
donor and weaker π-acceptor than phosphine ligands. While
most of the research focused on five-membered-ring carbenes,
[
1-2]
[
13]
synthesis and materials science.
In recent years, another
important application of TRAMs focused on the area of
current research efforts in this area indicated NHCs with six-,
[
14]
and seven-membered rings attracted increasing attention. In
comparison to their five-membered analogues, these ring-
expanded NHCs (RE-NHCs) feature much wider N−CNHC−N
bond angles, which result in an decrease in the corresponding
[
medicinal chemistry as antitubercular, anticancer, and
3]
[4]
[
antiproliferative agents. Furthermore, structurally diverse
5]
triaryl motifs are also important structural elements of some
[
6]
naturally occurring molecules. Consequently, considerable
efforts have been devoted to the synthesis of TRAMs, and the
Friedel-Crafts arylation of diarylmethanol or related derivatives
in the presence of an acid catalyst represent the most widely
employed method for the construction of triarylmethane
CNHC−N−R bond angles, forcing the N-substituents closer to the
[15]
metal centre. In addition, the wider N−CNHC−N angle also lead
to an increase in HOMO energy, resulting in higher basicity,
nucleophilicity, and σ-donor ability, which makes them
promising scaffolds for transition metal catalysis. Here, we
[
7-8]
present the synthesis of several new C -symmetric
2
structures.
Although this methodology is facile and
tetrahydropyrimidinium salts bearing two hydroxyl groups in N-
straightforward, the requirement of nucleophilic electron-rich
arenes and harsh reaction condition limited the application, and
mixtures of regioisomeric products are often formed. To address
these problems, several transition metal-catalyzed methods have
substituent. Along with their 5-membered counterparts, these
new ligands were tested in palladium-catalyzed DCCP, and the
latter show better activity than the traditional 5-membered
carbene, giving the DCCP products in higher yields.
[
9-10]
been developed.
developed by Walsh and co-workers, who established an
3
efficient palladium-catalyzed direct C(sp )–H arylation of
Among which an attractive method was
[
11]
2
. Results and Discussion
As shown in Scheme 1, the tetrahydropyridinium salts 3a−e
diarylmethanes with aryl bromide or aryl chloride through a
deprotonative cross-coupling process (DCCP).
In most cases, the ligands used in this process were
was prepared by reacting 1,3-dibromopropane with 2 equiv of
amino alcohols 1a−e to give bis(amino alcohol) 2, followed by
straightforward cyclization of bis(amino alcohol) to afford the
target product in good yields (75−86% for two steps). When 1,2-
phosphines, especially NiXantphos and Xantphos. N-heterocyclic
carbenes (NHCs) were first considered as mimics of phosphine
—
——
∗
Corresponding author. Tel.: +86 571 88016565; fax: +86 571 88018442; e-mail: lijie@zucc.edu.cn (J. Li)

2
Tetrahedron
dibromopropane was chosen as starting material, the five-
membered analogues 4a−c were synthesized smoothly by the
was used as solvent, giving the DCCP product in 95% yield.
Decreasing temperature was detrimental to the yield under these
1
conditions. 21% yield of 7aa was observed in crude H-NMR
same substitution/cyclization sequence.
o
spectra at 40 C (entry 6), while only trace of product was found
when it was lowered to room temperature (entry 7). It is
noteworthy that no reaction occured in the absence of ligand 3a
(entry 8), indicating the need for NHC supporting ligand in this
transformation.
Table 1. Optimization of Pd-catalyzed DCCP of 4-benzylpyridine (5a) with
a
3
a as ligand.
o
Temp [ C]
b
Entry
1
Solvent
dioxane
Yield [%]
Scheme 1. Synthesis of NHC precursors.
60
72
68
65
81
To search suitable conditions for NHC−Pd catalyzed DCCP in
the most efficient manner, we screened the synthesized ligands,
2
3
4
5
6
7
THF
60
60
60
60
40
rt
2-MeTHF
CPME
toluene
toluene
toluene
toluene
as well as three bases [LiN(SiMe
3 2 3 2
) , NaN(SiMe ) , and
KN(SiMe ] and four solvents [DME, THF, CPME, and
2
3
)
d
100 (95)
dioxane], for the cross-coupling of 4-benzylpyridine (5a) with 1-
o
bromo-4-tert-butylbenzene (6a) at 60 C using microscale high-
21
trace
−
[
17]
throughput experimentation (HTE, Scheme 2). As summarized
in Scheme 2, the six-membered ligands showed better activity
than their five-membered analogues and 3a emerged as the best
choice. Bases play an important role in this transformation,
c
8
60
a
Reactions conducted on a 0.1 mmol scale using 1 equiv of 6a, 1.2 equiv of
-benzylpyridine, and 3 equiv of NaN(SiMe , at 0.1 M.
1
Yield determined by H-NMR spectroscopy of the crude reaction mixture.
4
3 2
)
b
c
d
3 2 3 2
NaN(SiMe ) and LiN(SiMe ) afforded the DCCP product 7aa
Without 3a.
in good yield with 3a as ligand and the former was also
compatible with other ligands in this reaction. In sharp contrast,
Isolated yield.
With the optimal condition in hand, we next turned our
3 2
KN(SiMe ) showed no conversion at all. Among the solvents
attention to explore the scope of the DCCP with various aryl
bromides and 4-benzylpyridine. As presented in Table 2, aryl
bromides bearing various substituents exhibited good to excellent
screened, dioxane proved to be the best but the superiority was
not obvious when compared to CPME and THF.
a,b
Table 2. Scope of aryl bromides in Pd-catalyzed DCCP
R
Pd(OAc)2 5 mol%
R
3a 7.5 mol%
Ar
+
N
NaN(SiMe3)2 (3 equiv)
toluene, 60oC, 16 h
Ar
5
Br
N
6
7
OMe
N
N
N
N
7
aa 95%(87c
)
7ab 86%
7af 96%
7aj 73%d
7ac 93%
7ad 75%
Ph
OMe
N
N
N
N
Ph
7ae 86%
7ag 83%
7ah 71%
N
N
N
N
N
N
CN
O
7ai 83%
7ak 75%d
7al 78%d
Ph
N
N
N
Scheme 2. HTE results of NHC-Pd catalyzed DCCP of 4-benzylpyridine (5a).
7
ba 89%d
7ca 76%e
7da 0%e
7ea 0%e
The leading microscale result using 3a as ligand in dioxane
3 2
with NaN(SiMe ) was successfully translated to laboratory scale,
affording product 7aa in 72% analytical yield (Table 1 entry 1).
To further optimize the yield, we screened five solvents [dioxane,
THF, 2-MeTHF, CPME, toluene] for this reaction on laboratory
scale, and the lead result was obtained when toluene (entry 5)
N
7
fa 0%e

3
a
Reactions conducted on a 0.1 mmol scale using 1 equiv of aryl bromide, 3
equiv of NaN(SiMe3)2, and 1.2 equiv of diarylmethane at 0.1 M.
Isolated yield after chromatographic purification.
34, 438.
5
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Reaction conducted on 5 mmol scale.
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d
e
o
Reaction set up at 80 C.
o
Reaction set up at 100 C.
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ad, 7ah), electron-donating (7ad, 7ae), and electron-
7
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2
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o
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3
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coupling of 4-benzylpyridine with 1-bromo-4-tert-butylbenzene
using 5 mol % of catalyst. The coupling product 7aa was isolated
in 87% yield (1.3 g).
1
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3
In conclusion, the first NHC−Pd catalyzed C(sp )−H
arylation of diarylmethanes is reported, and the new synthesized
six-membered carbene showed better activity than their five-
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affords a variety of triarylmethane derivatives, a class of
1
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1
(
compounds with various applications and biological activity.
Further preparation of more chiral six-membered NHC ligands
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3
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1
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Acknowledgments
We are grateful to the National Natural Science Foundation of
China (31670357) and Hangzhou Science and Technology
Information Institute of China (20150633B45).
2
Huang, R.; Yang, X.; Walsh, P. J. Chem. Sci. 2016, 7, 611.
016, 138, 4260; (e) Cao, X.; Sha, S-C.; Li, M.; Kim, B-S.; Morgan, C.;
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5
Highlights
•
Several new 3,4,5,6-
tetrahydropyrimidinium salts were
synthesized.
•
•
These new ligands were effective catalysts
for DCCP.
Various triarylmethanes were obtained in
good to excellent yields.