Angewandte
Chemie
À
C H Activation
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Palladium(II)-Catalyzed meta-C H Olefination: Constructing
Multisubstituted Arenes through Homo-Diolefination and Sequential
Hetero-Diolefination**
Milan Bera, Arun Maji, Santosh K. Sahoo, and Debabrata Maiti*
Abstract: Divinylbenzene derivatives represent an important
class of molecular building blocks in organic chemistry and
materials science. Reported herein is the palladium-catalyzed
substrates, and that the flexible nature of the sulfonyl (SO2),
compared to carbonyl (CO) moiety, in the scaffold would
enhance the directing power of the nitrile (Scheme 1).[7] We
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synthesis of divinylbenzenes by meta-C H olefination of
sulfone-based arenes. Successful sequential olefinations in
a position-selective manner provided a novel route for the
synthesis of hetero-dialkenylated products, which are difficult
to access using conventional methods. Additionally, 1,3,5-
trialkenylated compounds can be generated upon successful
removal of the directing group.
Scheme 1. Development of a new scaffold.
À
T
ransition-metal-catalyzed functionalization of arene C H
bonds has been extensively investigated over the last two
decades by using a variety of directing groups (DGs).[1] In the
C H activation realm, directing groups are almost universally
were particularly interested in the synthesis of di-alkenylated
benzylsulfonyl scaffolds because of the importance of dio-
lefinated compounds in organic synthesis and materials
chemistry.[8] However the second olefination reaction is
expected to be challenging since the electronic and steric
properties of mono-olefinated species is completely different
from those of the starting materials.[9] Such challenges were
overcome and we herein disclose an approach for meta-
selective homo-diolefination and sequential hetero-diolefina-
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ortho-selective.[2] Compared to ortho-functionalization, the
related meta-selective reactions pose a significant challenge.
In this regard, examples of meta-selective functionalization
have only recently been reported and are based on steric and
electronic bias.[3] An alternate template-based approach,
initiated by the group of Yu[4] and subsequently extended by
Tan and co-workers[5] as well as our group,[6] demonstrated
meta-selective functionalization in the presence of a suitable
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tion of C H bonds of sulfone-based arenes (Scheme 2).
[10]
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directing group. These meta-selective C H functionalizations
are usually independent of the substitution pattern and
electronic nature of the substrate. Importantly, activation of
meta C H bonds hinges upon the combined use of a weakly
coordinating nitrile-based template with a protected amino-
acid ligand.
Although ortho-C H olefination of benzyl sulfonamides
and ortho-selective hetero-diolefinations have been docu-
mented,[9] meta-selective hetero-diolefination is yet to be
reported.
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Initially, benzylsulfonyl ester derivatives were tested with
various of nitrile-based directing groups (Scheme 3; for
structures see Figure 1).[6,11] Upon optimization, 2-hydroxy-
benzonitrile successfully delivered the diolefination product
During our previous work on palladium-catalyzed meta-
C H olefination with arylacetic acid derived scaffolds,
[6]
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a number of limitations were encountered: 1) unsuccessful
diolefination, 2) transesterification of the directing group
with the solvent hexafluoroisopropanol (HFIP), and 3) mod-
erate to good meta-selectivity. Some of these drawbacks
probably originate from the conformational and electronic
bias of arylacetic acid based substrates. We reasoned that
a more electron-withdrawing SO2 group may reduce electron
density at the ortho- and para-position of the sulfone-based
[*] Dr. M. Bera, A. Maji, Dr. S. K. Sahoo, Prof. D. Maiti
Department of Chemistry, Indian Institute of Technology Bombay
Powai, Mumbai-400 076 (India)
E-mail: dmaiti@chem.iitb.ac.in
[**] This activity is supported by SERB, India (SB/S5/GC-05/2013).
Financial support was received from DST under the Fast Track
Scheme (M.B.), CSIR (A.M.), and IIT-B (S.S.).
Supporting information for this article is available on the WWW
Scheme 2. Outline of this work.
Angew. Chem. Int. Ed. 2015, 54, 8515 –8519
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8515