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10.1002/hlca.201900229
Helvetica Chimica Acta
HELVETICA
Unprecedented Nucleophilic Attack of Piperidine on the Electron
Acceptor during the Synthesis of Push-Pull Dyes by a Knoevenagel
Reaction
Corentin Pigot,*,a Guillaume Noirbent,a Sébastien Peraltab, Sylvain Duval,c Malek Nechab,a Didier Gigmesa
and Frédéric Dumur*,a
b Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI), Université de Cergy Pontoise, 5 mail Gay Lussac, F-95000 Neuville-sur-
Oise, France
c Université de Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France
An unprecedented nucleophilic addition of piperidine on an electron acceptor, namely, 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-
ylidene)malononitrile is reported. This unexpected behavior was observed during the synthesis of push-pull dyes using the classical
Knoevenagel reaction. To overcome this drawback, use of diisopropylethylamine (DIPEA) enabled to produce the expected dyes PP1 and PP2.
The optical and electrochemical properties of the different dyes were examined. Theoretical calculations were also carried out to support the
experimental results. To evidence the higher electron-withdrawing ability of this electron acceptor, a comparison was established with two dyes
(PP3 and PP4) comprising its shorter analogue.
Keywords: Knoevenagel reaction • piperidine • push-pull • electron acceptor • electron donor
Among amines, piperidine is the most widely used.[18,19] In this
Introduction
field, the first condensation of benzaldehyde with ethyl acetoacetate
During the past decades, push-pull dyes have been extensively
in the presence of piperidine was reported as soon as 1896 by
studied with regards to their numerous applications ranging from
Knoevenagel providing the product in 95% yield.[20]
sensors,[1,2] energy conversion,[3,4] field effects transistors,[5,6]
In this work, an unprecedented nucleophilic attack of piperidine
organic light-emitting diodes,[7,8] nonlinear optics[9,10] or
during a Knoevenagel reaction with 2-(3-oxo-2,3-dihydro-1H-
photoinitiators of polymerization.[11-15] To access to these
cyclopenta[b] naphthalen-1-ylidene)malononitrile Napht-EA was
structures consisting in an electron donor connected to an electron
evidenced (See Figure 1). It has to be noticed that only few materials
acceptor by mean of a conjugated or none-conjugated spacer, the
have been designed with Napht-EA, despites its extended core that
Knoevenagel reaction is undoubtedly the most popular reaction,
makes it an ideal electron-withdrawing candidate for the design of
opposing an aldehyde with an electron acceptor bearing an activated
materials for photovoltaic applications. In this field, the preliminary
methylene group, in the presence of a catalytic amount of a base.
results revealed Napht-EA to be promising as a non-fullerene
Popularity of this reaction also arises from the facile availability of
electron acceptor for the design of push-pull materials for energy
electron acceptors possessing activated methylene groups and
conversion, justifying the development of new structures.[21-23] The
malononitrile, indane-1,3-dione derivatives, (thio)barbituric
low availability of Napht-EA-based push-pull dyes also originates
derivatives, Meldrum derivatives, 1,1,3-tricyano-2-substituted
from the difficult access to Napht-EA so that a careful optimization
propenes, isoxazolones, hydantions and rhodanines, and
of the reaction conditions was required to prepare the different dyes.
dicyanovinyl-thiophen-5-ylidenes can be cited as the most widely
After optimization, Napht-EA could be prepared with reaction yield
studied.[16,17] Interest for this reaction also originates from the easy
as high as 91% yield. To avoid the nucleophilic attack on Naphth-EA,
work-up following the synthesis of the dyes. Indeed, the reaction is
classically done by refluxing the reagents in ethanol and use of
alcohols as solvents favors the precipitation of the dyes upon
cooling. As a result, the expected products can be generally
recovered in pure form by a simple filtration on a glass filtrate.
a non-nucleophilic base i.e. diisopropylethylamine (DIPEA) was used
and the two targeted push-pull dyes PP1 and PP2 could be finally
obtained in high yields. To evidence the benefits of the polyaromatic
nature of Napht-EA, push-pull analogues PP3 and PP4 based on
indane-1,2-dione derivatives were designed and synthesized for
1
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