Palladium-Catalyzed Alkylation with Alkyl Halides by C(sp3)?H Activation

Article abstract of DOI:10.1002/anie.201706418

Utilizing halogens as traceless directing goups represents an attractive strategy for C?H functionalization. A two C?H alkylation system, initiated by the oxidative addition of organohalides to Pd⁰, has been developed. The first reaction involves an intermolecular alkylation of palladacycles to form C(sp³)?C(sp²) bonds followed by C(sp²)?H activation/cyclization to deliver alkylated benzocyclobutenes as the final products. In the second reaction, two C?C bonds are formed by the reaction of palladacycles with CH₂Br₂, and provides a facile and efficient method for the synthesis of indanes. The alkylated benzocyclobutene products can be transformed into tricyclic hyrocarbons, and the indane derivatives are essential structural motifs in bioactive and odorant molecules.

Full text of DOI:10.1002/anie.201706418

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Accepted Article
Title: Pd-Catalyzed Alkylation with Alkyl Halides via C(sp3)-H
Activation of Aryl Halides
Authors: Zhuo Wu, Ding Ma, Bo Zhou, Xiaoming Ji, Xiaotian Ma,
Xialing Wang, and Yanghui Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201706418
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10.1002/anie.201706418
Angewandte Chemie International Edition
COMMUNICATION
Pd-Catalyzed Alkylation with Alkyl Halides via C(sp3)-H Activation
of Aryl Halides
Zhuo Wu, Ding Ma, Bo Zhou, Xiaoming Ji, Xiaotian Ma, Xiaoling Wang, Yanghui Zhang*
Abstract: Utilizing halogens as traceless directing goups represents
an attractive strategy for C-H functionalization. We have developed
two C-H alkylation systems initiated by the oxidative addition of
organohalides to Pd0. The first reaction involved an intermolecular
alkylation of palladacycles to form C(sp3)-C(sp2) bonds followed by
C(sp2)-H
activation/cyclization,
delivering
alkylated
benzocyclobutenes as the final products. In the second reaction, two
C-C bonds were formed via the reaction of palladacycles with
CH2Br2, which provides a facile and efficient method for the
synthesis of indanes. The alkylated benzocyclobutene products can
be transformed into tricyclic hyrocarbons, and the indane derivatives
are essential structural motifs in bioactive and odorant molecules.
Figure 1. C−H functionalization using halogens as traceless directing groups .
C(sp3)−H activation reactions. Likewise, the most of the
reactions involve intramolecular cyclization[6] and very rare
intermolecular reactions were reported.[7]
Recently, we found that dibenzometallacyclopentadiene
prepared via C-H activaiton of 2-iodobiphenyl exhibited novel
reactivity.[8] This palladacycle can selectively react with alkyl
halides, whose reactions are usually challenging in transition-
metal-catalyzed reactions. Actually, the palladacycles formed via
C(sp2)−H activation from aryl iodides and norbornene in
Catellani reaction can react with alkyl halides efficiently.[9]
Inspired by these reactions, we envisioned that palladacycles
might be desirable models for the development of Pd-catalyzed
alkylation with alkyl halides. In Catellani and our reactions, the
palladacycles consist of two carbon-metal bonds, and the
advantage of this type of palladacycles is that these two carbons
can be functionalized. However, norbornene just functions as
catalyst in Catellani reaction, and the C(sp3)-Pd bond can
usually not be manipulated. We were interested in the
difunctionalization of the two carbon-metal bonds in the
palladacycles, which may offer opportunities to develop novel
organic reactions. Herein, we report the alkylation reaction of the
palladacycle derived from 2-tert-butylaryl halides with alkyl
chlorides and dibromomethane. The reaction with alkyl chlorides
In the past few decades, transition metal-catalyzed C−H
functionalization has made noticeable progress and is emerging
as a novel and valuable strategy in organic synthesis.[1] Most of
the current C−H functionalization reactions rely on the use of
directing groups, which can lead to great regioselectivity and
accelerate C−H cleavage process.(Figure 1, A) [2] However, this
strategy restricts the scope of accessible products. Although
some directing groups can be manipulated after C−H
functionalization, additional synthetic steps are often required.[3]
Moreover, some directing groups have to be installed via
complex synthetic steps.
An alternative method of activating C−H bonds is to utilize
halogens as traceless directing goups. For Pd-catalyzed
reactions of this type, the catalytic cycles are usually initiated by
the oxidative addition of organohalides to Pd0 precatalysts. The
resulted PdII species then cleave proximal intramolecular C−H
bonds and form palladacycles, which then undergo further
transformations (Figure 1, B). The major advantage of this
method is that the halo groups are removed and the resulted Pd-
carbon bonds can be manipulated readily. Furthermore,
halogens are ubiquitous functionalities in organic molecules and
can be introduced comparatively readily. While PdII-initiated C−H
functionalization reactions require the use of a stoichiometric
amount of external oxidants, organohaldies act as oxidants
themselves. Although some reactions of this type have been
developed,[4] the majority of them are intramolecular cyclization
reactions.[5] Notably, this strategy has also been applied to
was
a
tandem
process
and
provided
alkylated
benzocyclobutenes as the final products. A catalytic protocol for
the reaction of the palladacycle with dibromomethane has also
been developed.
Table 1. Survey of the reaction conditions for Pd-catalyzed alkylation of 1-
bromo-2-tert-butylbenzene with 4-chlorobutyl acetate.
[*]
Z. Wu, D. Ma, B. Zhou, X. Ji, X. Ma, X. Wang, Prof. Dr. Y. Zhang
School of Chemical Science and Engineering, Shanghai Key
Laboratory of Chemical Assessment and Sustainability, Tongji
University
3aa
4a
5a
entry
ligand
(%)[a]
(%)[a]
(%)[a]
1
2
3
---
0
16
0
0
2
0
8
3
1239 Siping Road, Shanghai 200092, China
E-mail: zhangyanghui@tongji.edu.cn
The work was supported by the National Natural Science
Foundation of China (No. 21372176) and Shanghai Science and
Technology Commission (14DZ2261100). We thank Prof. Jin-Quan
Yu (The Scripps Research Insitute) for helpful discussions.
PPh3
[**]
P(o-tol)3
85 (82[b]
)
[a] The yields were determined by 1H NMR analysis of the crude reaction
mixture using CHCl2CHCl2 as the internal standard. [b] Isolated yield.
Supporting information for this article is given via a link at the end
of the document.
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10.1002/anie.201706418
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We commenced our study by investigating the reaction of 1-
bromo-2-tert-butylbenzene (1a) with 4-chlorobutyl acetate (2a).
Unexpectedly, the reaction formed alkylated benzocyclobutenes
3aa in the presence of PPh3 (Table 1, entry 2). Inspired by this
exciting result, we sought to improve the yield of 3aa by
screening phosphine ligands, and found that the yield increased
dramatically to 85% when P(o-tol)3 was used (entry 3). (For
detailed condition screening, see SI).
The substrate scope was next examined. We first
investigated the performance of a range of 1-bromo-2-tert-
butylbenzene derivatives. As shown in Figure 2, the substrates
bearing a methyl, tert-butyl, methoxyl, or acetamido group were
suitable, and the desired products were formed in good yields
(3ba, 3ca, 3da, and 3ea). The derivatives bearing electron-
withdrawing groups such as ester, carbonyl, and aldehyde also
underwent the domino reaction, giving the alkylated products in
moderate or good yields (3fa, 3ga, and 3ha). Fluoro and chloro
were also tolerated (3ia and 3ja). The substrate bearing a
methyl group at the position meta to the tert-butyl group was
also transformed into desired product 3ka in 72% yield. Notably,
bromobenzenes bearing derivatized tert-butyl groups were also
reactive, albeit in lower yields (3la, 3ma, and 3na).
Figure 3. Alkyl chloride scope. [a] Isolated yields. [b] 4 equiv 2. [c] 90 ℃.
palladacycle could be functionalized simultaneously, which
offers opportunities to develop new synthetic methods. We
envisioned that if the alkyl chlorides were replaced with
dihaloalkanes, the palladacycles could react with the
dihaloalkanes to form benzocycloalkanes. Notably, the same
palladacycles, which were obtained from Grignard reagent
Mg(CH2CMe2C6H5)Cl, could react with CH2Br2 or CH2I2 to form α,
α-dimethylindane.[10] Indanes are very important carbocyclic
derivatives. They are ubiquitous in various drugs and natural
products and find applications in material science and
asymmetric catalysis.[11] Therefore, we sought to develop a
catalytic process for the synthesis of indanes starting from 1-
bromo-2-tert-butylbenzenes and dihalomethanes. Gratefully, by
subjecting 1a and CH2Br2 to the above alkylation reaction
conditions, we obtained desired indane prodcut 7a in 13% yield.
The yield was improved to 29% when aryl iodide 6a was
employed. The optimal yield (70%) was achieved under reaction
conditions as shown in Scheme 1. (For detailed condition
screening, see SI).
Figure 2. Aryl bromide scope. [a] Isolated yields. [b] 90 ℃. [c] 15 mol%
Pd(OAc)2, 30 mol% P(o-tol)3, 36 h. [d] 4 equiv 2a, 100 ℃, 12 h.
Scheme 1. Synthesis of indane from 1-iodo-2-tert-butylbenzene and CH2Br2.
[a] Isolated yield.
Subsequently, we investigated the substrate scope with
respect to alkyl chlorides. As shown in Figure 3, n-butyl and n-
hexyl chlorides were reacitve, and a variety of functionalities
including phenyl, ester, carbonyl, cyano, and acetal on n-propyl
chloride were tolerated in the reaction (3ad, 3ae, 3af, 3ag, and
3ah). The performance of n-butyl chloride derivatives was also
examined, and a range of butyl chlorides proved to be effective
alkylating reagents, generating the desired products in good
yields (3ai, 3aj, 3ak, and 3al). Notably, 1,4-dichlorobutane was
also compatible with the second chloro group intact during the
reaction (3am), and the reaction of 1-chloro-2-methoxyethane
was high-yielding (3an). Finally, sterically hindered 1-chloro-2-
methylpropane and 2-(chloromethyl)oxirane could alkylate 1a,
albeit in lower yields (3ao and 3ap).
We then explored the substrate scope of the catalytic
protocol (Figure 4). The tolerance of functional groups was
examined by investigating the reactivity of substrates bearing a
substituent at the position para to the tert-butyl group. Gratefully,
a range of functional groups, including alkyl, methoxyl, ester,
ketone, aldehyde, and chloro, were compatible (7b-7g). Next,
we examined the performance of substrates bearing a meta
substituent. Whereas 7h was formed as a single isomer, two
regiomers 7i-1 and 7i-2 were obtained in a ratio of 1 : 1 in the
reaction of 6i. The formation of isomers 7i-2 implied that the
palladacycle tended to decompose to form C(sp3)-Pd species,
which could activate the other C-H bond ortho to the tert-butyl
group due to the small size of fluoride and formed a second
palladacycle. Interestingly, whereas the substrate bearing two
methoxyl groups (6j) yielded a single product 7j, 6k formed two
The palladacycles in the above alkylation reaction consist of
two carbon-metal bonds. The two carbons forming the
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isomers 7k-1 and 7k-2. The substrates bearing derivatized tert-
butyl groups could be converted into corresponding indane
derivatives, the yields were lower, though (7l-7o).
It should be noted that α,α-disubstituted indanes are
essential structural motifs in many bioactive and odorant
molecules and also find applications in material science.[12,13]
(Scheme 3, A). On the other hand, the reactions with alkyl
chlorides provide a simple method for the synthesis of alkylated
cyclobutarenes. The products can undergo cyclization to form
tricyclic hydrocarbons with a central benzene ring fused to two
saturated carbocyclic rings, which are not only important
intermediates in organic synthesis but also interesting molecules
for bonding studies (Scheme 3, B).[14]
Figure 4. Aryl Idoide Scope. [a] Isolated yields. [b] 24 h. [c] 15 mol% Pd(OAc)2,
30 mol% P(o-tol)3.
Scheme 3. Applications of the synthetic products.
On the basis of the products fromed in the reactions and the
previous reports,[6e, 7b, 8a, 10] tentative mechanisms for these two
alkylation reactions are proposed. The key palladacycle B is
formed via intramolecular C(sp3)-H activation. For the reaction
In conclusion, we have developed tandem Pd-catalyzed
C(sp3)-H activation/alkylation reactions. The iodo and bromo
groups functioned as the traceless direcitng groups, which
represents an advantageous strategy for C-H functionalization.
In the reaction of 1-bromo-2-tert-butylbenzenes with alkyl
chlorides, two C(sp2)-C(sp3) bonds were formed via a tandem
process, yielding alkylated benzocyclobutenes as the final
products. A catalytic protocol for the reaction of 1-iodo-2-tert-
butylbenzenes with CH2Br2 was also developed.
with simple alkyl chlorides,
B
undergoes oxidative
addition/reductive elimination or metathesis with the alkyl
chlorides to generate E. Next, intramolecular C(sp2)-H activation
forms a second palladacycle F, and final reductive elimination
yields product 3aa. For the reaction with CH2Br2, the oxidative
addition of CH2Br2 to B forms G. G is transformed into
palladacycle J via intermediate H or carbene complex I. The
reductive elimination of J generates final product 7a and
releases PdII, which is reduced to Pd0. Furthermore, isotope
effect experiments were conducted using deuterated 1n and 6l
(with one of the methyl groups fully deuterated). The kinetic
isotope effects were 5.9 and 6.1 for the reaction with 2a and
CH2Br2 respectively, which implies that C-H bond cleavage was
the rate determining step in both of the alkylation reactions.
Keywords: C–H activation • alkylation • alkyl halides • palladium
catalysis
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COMMUNICATION
COMMUNICATION
Zhuo Wu, Ding Ma, Bo Zhou, Xiaoming
Ji, Xiaotian Ma, Xiaoling Wang, Yanghui
Zhang*
Page No. – Page No.
Pd-Catalyzed Alkylation with Alkyl
Halides via C(sp3)-H Activation of Aryl
Halides
Pd-catalyzed intermolecular alkylation reactions with alkyl halides via C(sp3)-H
activation of aryl haldies have been developed. The reactions involved a tandem
process and two C(sp2)-C(sp3) bonds were formed, yielding alkylated
benzocyclobutenes as the final products. A catalytic protocol for the reaction with
CH2Br2 was also developed, which provides a facile and efficient method for the
synthesis of indanes.
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