Palladium-catalyzed benzylic C(sp3)–H arylation of o-alkylbenzaldehydes

Article abstract of DOI:10.1016/j.tetlet.2021.152865

The palladium-catalyzed benzylic C(sp³)–H arylation of o-alkylbenzaldehyde derivatives was achieved utilizing 2-dimethylaminoethylamine as a novel transient directing group. The γ-C(sp³)–H arylation reaction efficiently afforded a variety of arylated o-alkylbenzaldehydes and polycyclic aromatic hydrocarbons (PAHs) in one pot, exhibiting high functional group tolerance with broad substrate scope. The aliphatic diamine auxiliary represents a simple, inexpensive, readily available, and removable directing group for C–H activation. The resultant o-benzylbenzaldehyde products could be diversely transformed into potentially important synthetic intermediates under mild conditions.

Full text of DOI:10.1016/j.tetlet.2021.152865

Tetrahedron Letters 67 (2021) 152865
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Palladium-catalyzed benzylic C(sp3)–H arylation
of o-alkylbenzaldehydes
a,
Lan Lei a,1, Ping Wu b,1, Zhuqing Liu a,c, , Jiang Lou
⇑
⇑
a State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan 250353, China
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
c Advanced Research Institute for Multidisciplinary Science, Qilu University of Technology, Shandong Academy of Sciences, Jinan 250353, China
a r t i c l e i n f o
a b s t r a c t
The palladium-catalyzed benzylic C(sp3)–H arylation of o-alkylbenzaldehyde derivatives was achieved
Article history:
utilizing 2-dimethylaminoethylamine as a novel transient directing group. The c
-C(sp3)–H arylation reac-
Received 28 November 2020
Revised 13 January 2021
Accepted 20 January 2021
Available online 4 February 2021
tion efficiently afforded a variety of arylated o-alkylbenzaldehydes and polycyclic aromatic hydrocarbons
(PAHs) in one pot, exhibiting high functional group tolerance with broad substrate scope. The aliphatic
diamine auxiliary represents a simple, inexpensive, readily available, and removable directing group
for CAH activation. The resultant o-benzylbenzaldehyde products could be diversely transformed into
potentially important synthetic intermediates under mild conditions.
Keywords:
Diamine
Transient directing group
CAH Activation
Ó 2021 Elsevier Ltd. All rights reserved.
o-Alkylbenzaldehydes
o-Benzylbenzaldehydes
Introduction
used for the C(sp3)–H functionalization of a wide variety of aldehy-
des [11], ketones [12], and amines [13].
Transition-metal-catalyzed CAH activation has been employed
as a concise method for the construction of carbon–carbon and car-
bon-heteroatom bonds [1]. The directing group strategy of intro-
ducing coordinating moieties to the substrate has been well
documented and significant progress has been made to achieve
high reactivity and selectivity in CAH activation reactions [2].
However, the covalent installation and detachment of the external
directing groups diminishes the atom- and step-economy as well
as the practicability. In this aspect, the transient directing group
(TDG) strategy has been a useful breakthrough to overcome these
drawbacks, where the TDG could be reversibly linked to the sub-
strate before and after the reaction [3]. Since Jun’s work employing
2-amino-3-picoline as a TDG for rhodium-catalyzed C(sp2)–H acti-
vation [4], a variety of small molecules, such as amino acids and
their derivatives [5], aromatic and aliphatic amines [6], aldehydes
[7], orthanilic acid [8], anthranilic acid [9], and dioxazolone [10],
have been successfully applied as efficient TDGs to achieve C
(sp2)–H functionalization of aromatic aldehydes and ketones,
diphenyls, and ferrocenes. Recently, the TDG strategy has also been
2-Benzylbenzaldehydes are critical building blocks for the syn-
thesis of natural products, biologically active compounds, and spiro
and fused ring compounds [14]. Although numerous efforts have
been devoted to the synthesis of 2-benzylbenzaldehydes, employ-
ing prefunctionalized substrates, the use of toxic and unstable
reagents, and cost-efficiency limit the application of these syn-
thetic methods [15]. In this regard, efficient protocols for the syn-
thesis of 2-benzylbenzaldehydes are still highly desirable. Yu and
co-workers first disclosed the palladium-catalyzed c
-C(sp3)–H ary-
lation and fluorination of o-alkylbenzaldehydes using amino acids
and their derivatives as TDGs [16]. Subsequently, the TDG strategy
was applied for the construction of polycyclic aromatic hydrocar-
bons by Zhang’s and Kim’s groups [17]. Additionally, several
groups have described the c
-C(sp3)–H arylation and acetoxylation
of 2-methylbenzaldehydes using acetohydrazide, glycinamide
hydrochloride, and semicarbazide as effective TDGs [18]. Among
these reactions, TDGs played a critical role in site-selective CAH
activation (Scheme 1a). Thus, exploring novel, inexpensive, and
commercially available TDGs for the direct synthesis of 2-benzyl-
benzaldehydes via c
-C(sp3)–H activation of o-alkylbenzaldehydes
is still necessary.
⇑
Corresponding authors at: State Key Laboratory of Biobased Material and Green
Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan
250353, China (Z. Liu).
Taking into account the importance of 2-benzylbenzaldehydes
and the necessity for the development of new TDGs, we envisioned
that the aliphatic auxiliary 2-dimethylaminoethylamine, which
E-mail addresses: liuzhuqing@qlu.edu.cn (Z. Liu), loujyx@163.com (J. Lou).
These authors contributed equally.
1
https://doi.org/10.1016/j.tetlet.2021.152865
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

L. Lei, P. Wu, Z. Liu et al.
Tetrahedron Letters 67 (2021) 152865
Table 1
Optimization of the reaction conditions.a
Entry
Variation
Yield 3a (%)b
1
none
62 (61)c (60)d
2
3
4
5
6
7
Pd(TFA)2 instead of Pd(OAc)2
PdCl2 instead of Pd(OAc)2
AgOAc instead of AgTFA
Ag2CO3 instead of AgTFA
K3PO4 instead of AgTFA
TFE instead of HFIP
60
Trace
13
Trace
trace
21
8
AcOH instead of HFIP
trace
9
2-dimethylaminoethylamine (0.8 equiv.)
2-dimethylaminoethylamine (1.2 equiv.)
without Pd(OAc)2
49
52
n.d.
n.d.
Scheme 1. 2-Dimethylaminoethylamine as a simple directing group.
10
11
12
without 2-dimethylaminoethylamine
has been used as a bidentate directing group for C(sp2)–H and C
(sp3)–H functionalization (Scheme 1b) [19], might be employed
a
Reagents and conditions: 1a (0.3 mmol), 2a (0.45 mmol), Pd(OAc)2 (10 mol%), 2-
dimethylaminoethylamine (1 equiv.), AgTFA (2 equiv.), 4 Å MS (100 mg), HFIP (2
as a new TDG to achieve the direct c
-C(sp3)–H activation of o-alkyl
mL), 120 °C, air, 24 h.
b
Determined by 1H NMR analysis using 1,3,5-trimethoxylbenzene as the internal
benzaldehydes. Herein, we disclose the palladium-catalyzed aryla-
tion of o-alkylbenzaldehydes for the synthesis of 2-benzylben-
zaldehydes using inexpensive and commercially available 2-
dimethylaminoethylamine as a novel TDG (Scheme 1c).
standard.
c
Isolated yields.
d
1a (1 mmol), 2a (1.5 mmol), 4 Å MS (300 mg), HFIP (6 mL).
Results and discussion
aryl iodides gave products 3d and 3e in 60–62% yield. Halogen
atoms such as fluoro, chloro, and bromo did not affect the reaction
efficiency, affording products 3f-3h in 61–65% yield. 1,4-
Diiodobenzene (2i) gave a mixture of monobenzylation product
3i (42%) and dibenzylation product 3j (14%). Using an excess of
1a (2.5 equiv.) led to the dibenzylation product 3j in 51% yield.
4-Phenyl and 4-methoxycarbonyl substituted iodobenzenes also
reacted efficiently with 1a to give benzylation products 3k (65%)
and 3l (67%), respectively. The electron-withdrawing trifluo-
romethyl group resulted in diminished reaction efficiency, giving
the target product 3m in 46% yield. Interestingly, the polycyclic
aromatic hydrocarbon (PAH) products 3n0-3p0 could be directly
accessed as the major products in moderate yields (47–53%) using
m-methoxy and methyl iodobenzenes, and 2-iodothiophene as the
substrates. Similarly, Zhang and Kim achieved the synthesis of var-
ious PAHs, utilizing amino acids and their derivatives as TDGs [17].
In a similar manner, the reactions of 2-methylbenzaldehyde sub-
strates bearing various substituents such as 4-Me, 4-OMe, 4-F, 4-
Cl, 4-Br, and 5-F with various aryl iodides also proceeded efficiently
to give the target products 3q-3y in 50–60% yield. 2,3-Dimethyl-
benzaldehyde 1i did not show an obvious steric effect, producing
the corresponding product 3z in 56% yield. In addition, 2-ethylben-
zaldehyde 1j reacted with 4-iodobiphenyl to afford the benzylation
product 3z1, albeit in 30% yield, indicating that the methylene C
(sp3)–H could also be activated using this synthetic protocol. How-
ever, aldehydes containing heteroatoms such as 3-methylthio-
phene-2-carboxaldehyde did not undergo the reaction. Further
derivatization of the 2-benzylbenzaldehyde products has also been
demonstrated (see the ESI for details).
Initially, the reaction of 2-methylbenzaldehyde (1a) and
iodobenzene (2a) was conducted to screen the reaction conditions
(Table 1). After systematic experimentation, the optimal reaction
conditions were identified (see the ESI for details). In the presence
of 10 mol% Pd(OAc)2, 2-dimethylaminoethylamine (1 equiv.),
AgTFA (2 equiv.) and 4 Å MS, in hexafluoroisopropanol (HFIP) at
120 °C, the target product 2-benzylbenzaldehyde (3a) was formed
in 61% isolated yield. 2-Benzylbenzaldehyde 3a could also be
obtained in 60% yield on a 1 mmol scale (Table 1, entry 1). Other
TDGs such as 2-diethylaminoethylamine (TDG2), 2-diisopropy-
laminoethylamine (TDG3), 2-methoxyethylamine (TDG4), acethy-
drazide (TDG5), glycine (TDG6) and L-tert-leucine (TDG7) were
much less effective (see the ESI for details), which might be due
to the stronger or weaker coordination of these TDGs to the cata-
lyst. Pd(TFA)2 could also promote the reaction, but PdCl2 was not
effective (Table 1, entries 2 and 3). The use of other bases such
as AgOAc, Ag2CO3, and K3PO4, or other solvents such as 2,2,2-triflu-
oroethanol (TFE) and AcOH, dramatically diminished the reaction
efficiency (Table 1, entries 4–8). Decreasing or increasing the
amount of 2-dimethylaminoethylamine gave reduced yields
(Table 1, entries 9 and 10). Excess TDG might directly coordinate
with the catalyst and resulted in its deactivation. Control experi-
ments revealed that the palladium catalyst and diamine were
essential for the reaction (Table 1, entries 11 and 12).
Under the optimal conditions, the scope of the c
-C(sp3)–H ary-
lation reaction was investigated by reacting various aryl iodides
with o-alkylbenzaldehydes (Scheme 2). A negative steric effect
was observed using ortho-methoxy substituted iodobenzene,
which only afforded trace amounts of the target product 3b. The
aryl iodide (2c) bearing a strong electron-withdrawing nitro group
reacted smoothly with 2-methylbenzaldehyde, leading to the cor-
responding product 3c in 44% yield. Introducing an electron-donat-
ing group such as methyl or methoxy to the para-position of the
To gain insights into the reaction mechanism, 2,2,4,4-tetram-
ethyl-1-piperidinyloxy (TEMPO, 1 equiv.) was added under the
standard reaction conditions, and the target product 3a was iso-
lated in 58% yield, which indicated that the reaction might not pro-
ceed via a radical mechanism (see the ESI for details). On the basis
2

L. Lei, P. Wu, Z. Liu et al.
Tetrahedron Letters 67 (2021) 152865
Scheme 3. Proposed reaction mechanism.
Conclusion
In summary, we have demonstrated a palladium-catalyzed ben-
zylic C(sp3)–H arylation of o-alkylbenzaldehydes with inexpensive
and commercially available 2-dimethylaminoethylamine as an
effective TDG to synthesize diverse 2-benzylbenzaldehydes and
PAHs. The protocol exhibits good efficiency, broad substrate scope,
and high compatibility with functional groups.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This project was supported by the Foundation (No. 23190411)
of State Key Laboratory of Biobased Material and Green Papermak-
ing, Qilu University of Technology, Shandong Academy of Sciences.
Appendix A. Supplementary data
Scheme 2. Scope of the o-alkylbenzaldehydes and (hetero)aryl iodides. Reagents
and conditions: a1a (0.75 mmol), 2i (0.3 mmol), AgTFA (0.75 mmol).
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.152865.
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