Sonogashira Coupling Applied in the Synthesis of 1,2,4-Oxadiazole-Based Nonsymmetrical Liquid Crystals

Abstract

A series of highly p-conjugated nonsymmetrical liquid crystals, based on the core 3,5-(disubstituted)-1,2,4-oxadiazole with a shape similar to a hockey stick, were successfully synthesized by convergent Sonogashira coupling reaction between the building blocks of aryl iodides and the corresponding terminal arylacetylenes, using 3-[4-(decyloxy)phenyl]-5-(haloaryl)-1,2,4-oxadiazoles or decyl 4-iodobenzoate (10) and terminal arylacetylenes. This versatile synthetic route yielded luminescent mesogens with smectic and nematic phases, typical of calamitic compounds.In an attempt to optimize molecular properties for display and optical applications, considerable attention has been given to the design and synthesis of liquid crystalline compounds with suitable selection of the core fragment, linking group, and terminal functionality. 1 Highly p-conjugated liquid crystal molecules containing heterocycles have attracted increasing interest in the field of organic light emitting diodes (OLEDs), where electron-deficient heteroaromatic rings can lead to a good charge-transporting materials allied with its inherent self-organization ability, and strong fluorescence. 2 1,3,4-Oxadiazole-based liquid crystals with high thermal stability and very efficient blue luminescence have been synthesized. 3 In addition, the carbon-carbon triple bond is a good spacer group in liquid crystals, which produces a highly p-polarizable and conjugated moiety. Liquid crystals containing a carbon-carbon triple bond are very interesting materials due to their large birefringence, 4 large index of refraction parallel to the director, large electric polarizability and the ability to form optical films transparent in the visible. The Sonogashira coupling 5 between aryl halides and terminal acetylenes, the most common employed synthetic technique to build carbon-carbon triple bonds, has been widely used in the synthesis of functional materials. 6Herein, we report an efficient synthesis of a series of liquid crystalline compounds 1a-f, which possess a 1,2,4-oxadiazole heterocyclic nonsymmetrical core and similar hockey stick shape (Figure 1). 7 In our attempts to gain a better understanding of the chemical structure versus thermal/optical property relationships, different aromatic moieties were linked to the central core by a carbon-carbon triple bond. These aromatic groups [phenyl, 2-naphthyl, 4-(piperazin-1-yl)phenyl, or 4-(alkoxycarbonyl)phenyl], were chosen by considering an elongation of the liquid crystal molecule rod, thus increasing the length-to-breadth ratio, and also an extension in the pconjugated system, with strong electron-donating (e.g., OR, NR 2 ) and electron-withdrawing (e.g., NO 2 , C=O) substituents.The initial synthesis of the intermediates containing the 1,2,4-oxadiazole ring (compounds 6a,b) was carried out as shown in Scheme 1. First, N-hydroxyimidamide 3 was prepared from 4-(decyloxy)benzonitrile (2) 8 by the Tiemann reaction 9 using hydroxylamine hydrochloride, and potassium hydroxide in a methanol-water mixture.The formation of...

Oxadiazoles

“…Thus, compounds 366 were prepared in 74-82% yield, by reaction of an aryl iodide containing the 1,2,4-oxadiazole ring (364) with the terminal arylacetylenes 365, using 10 mol.% of dichlorobis(triphenylphosphine)palladium, 5 mol.% of the co-catalyst copper(I) iodide, in a triethylamine-tetrahydrofuran mixture (7 : 3). The obtained compounds showed liquid crystal phases, in particular smectic and nematic typical of calamitic structures, and moreover exhibit strong blue fluorescence in solution (Scheme 13.132) [282]. 13.3.6 1,2,4-Oxadiazoles in Medicine 1,2,4-Oxadiazole ring occurs widely in biologically active synthetic compounds, and is often used in drug discovery as a hydrolysis-resisting bioisosteric replacement for amide or ester functionalities [283] because of its electronic properties.…”

Rapid synthesis of di-1,2,4-oxadiazoles pyridyl compounds

“…These compounds have also been applied as ligands on metal complexes 2 and liquid crystal materials. 3 In this work, we have investigated the rapid synthesis of some new di-1,2,4-oxadiazoles pyridyl compounds by two different methods between as shower the scheme 1. …”

Lanthanide nitrates as Lewis acids in the one-pot synthesis of 1,2,4-oxadiazole derivatives

“…13 Also, 1,2,4-oxadiazole compounds have been applied as ligands of metal complexes 14 and liquid crystalline materials. 15,16 Synthesis of 1,2,4-oxadiazole derivatives by using ketones, nitriles and nitric acid as starting materials and Fe(NO 3 ) 3 as catalyst has been reported by Itoh et al 17 However, this method has the disadvantage of low reaction rates and drastic conditions, resulting in low reaction yields. Recently, yttrium triflate [Y(OTf) 3 ] 18 was applied instead of Fe(NO 3 ) 3 by using the same reaction condition in the synthesis of various 1,2,4-oxadiazole with low reaction times and good yields.…”