Synthesis of stilbene, 1,4-distyrylbenzene and 4,4′-distyrylbiphenyl via Horner–Wadsworth–Emmons reaction in phase-transfer catalysis system
Graphical abstract
Introduction
Stilbene, 1,4-distyrylbenzene and 4,4′-distyrylbiphenyl bear remarkable optical, photochemical and photophysical properties [1]. They have widespread applications in many fields ranging from fine chemistry and materials science to biomedicine. They can be used as fluorescent whitening agents in textile, paper manufacturing and household detergents [2], two-photon absorbing materials [3], [4], [5], [6] and blue electroluminescent materials [7], [8], [9], [10], [11] in the optoelectronic device, drugs and antitumor agents in pharmacology [12], [13], molecular probes and labels in bioassay [14] and etc. With the discovery of new applications, such as molecular conformational switches [15], integral part of phenylene vinylene-based oligomers and polymers [16], energy transporter in one-dimensional channels [17], [18] and bridge spanning redox-active moieties [19], [20], they have drawn great interests of more and more researchers.
Many synthetic methods for stilbenes, 1,4-distyrylbenzenes and 4,4′-distyrylbiphenyls have been developed. The condensation reactions of arylmethyl compound are the most common methods, such as Grignard reaction, Knoevenagel reaction, Wittig reaction and Horner–Wadsworth–Emmons (HWE) reaction. Particularly, HWE reaction is the main pathway employed in industry. Some other condensation routes for new stabilized carbanion have also been reported [21]. Nonetheless, the low atom economy, the high (Z)-factor and the high cost of solvents are the main disadvantages of the condensation reaction. Hence, the new synthetic method of stilbenes, 1,4-distyrylbenzenes and 4,4′-distyrylbiphenyls has been a hot area of research. The dimerization reactions had been used been used for many years, such as Meerwein arylation [22], Stille coupling reaction [23] and Heck reaction [24]. As one of basic organic synthetic methods, Heck reactions has been extensively investigated in the synthesis of stilbene, 1,4-distyrylbenzene and 4,4′-distyrylbiphenyl [25], [26], [27], [28], [29], [30]. However, the high cost of catalyst, ligand and dipolar aprotic solvent limits their application in industry. As an alternative to these methods, we proposed here an easy and convenient phase transfer catalysis (PTC) routes via HWE reaction (Scheme 1).
PTC is one of the most widely used synthetic techniques and has had more than 700 industrial applications [31], [32]. The major advantages of PTC include its simplicity, use of inexpensive reagents under mild conditions, high reaction rate and high selectivity to the desired product [33]. HWE reaction under PTC conditions was firstly reported in 1974 [34]. The synthesis of α,β-unsaturated compounds via HWE reaction in solid–liquid PTC (SL-PTC) system was then researched [35], [36]. At present, HWE reaction in PTC system have been used in the synthesis of polymer [37] and dipolar organometallic complexes [38], [39]. Nevertheless, those reactions suffer from one or more problems, such as long reaction time, low yield and expensive solvent. Moreover, there are few studies on the mechanism of HWE reaction under PTC conditions and the parameters influenced the activity of HWE reaction in PTC system have rarely been investigated.
In this work, HWE reactions for monophosphonate or bisphosphonate with aldehyde were preceded in liquid–liquid PTC (LL-PTC) and SL-PTC system (Scheme 1). The yield and geometric selectivity for HWE reaction in these two PTC systems were compared. The effects of the side reaction, reactants and the third phase on the activity of HWE reaction in both PTC systems were studied. Furthermore, the reusability of the aqueous phase in LL-PTC system was discussed. These results will provide a potential application for HWE reaction in industry and show new insights into the mechanisms of PTC reaction.
Section snippets
Chemical and instruments
Diethyl benzylphosphonate (DEBP), diethyl 2-cyanobenzylphosphonate (DCBP), tetraethyl (1,4-phenylenebis(methylene))bis(phosphonate) (TEPP) and tetraethyl ([1,1′-biphenyl]-4,4′-diylbis(methylene))bis(phosphonate) (TEBP) were prepared according to the literature [40], [41], [42]. Other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. and used directly as received without further purification. Dry toluene was obtained by distillation under nitrogen in the presence of sodium and
Synthesis of stilbene, 1,4-distyrylbenzene and 4,4′-distyrylbiphenyl in PTC system
A series of stilbenes, 1,4-distyrylbenzenes and 4,4′-distyrylbiphenyls were prepared according to the synthetic route 1 and 2 shown in Scheme 1. The yield and geometric selectivity for each reaction were listed in Table 1. The structure of the product was fully characterized by IR, 1H NMR and 13C NMR spectrometry and element analysis. The datum were given in the Supplementary Information.
It can be observed from Table 1 that the activity of HWE reaction is varied with PTC system. The yield of
Conclusions
The synthesis of stilbene, 1,4-distyrylbenzene and 4,4′-distyrylbiphenyl via HWE reaction were developed in LL-PTC and SL-PTC systems. These synthetic routes showed a high yield and geometric selectivity. For the aldehyde with electron-donating substitute, the yield of the product in both PTC systems was more than 90%. The (E)-isomer was the predominate product in both PTC systems, especially in the synthesis of 1,4-distyrylbenzenes or 4,4′-distyrylbiphenyls.
The activity of HWE reaction in PTC
Acknowledgements
One of the authors is thankful to Dr. Guimin Zhang, Shanghai Heliya Fine Chemicals Co., Ltd., for his constant encouragement. Financial assistance from Donghua University is acknowledged gratefully.
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