A series of monosubstituted piperazine derivatives were obtained by the reaction of piperazine with trimethylacetic arylcarboxylic anhydrides in good yields, which were prepared in situ from arylcarboxylic acid with trimethylacetyl chloride in the presence of triethylamine.Recently, we have prepared a series of azo-dye liquid crystalline molecules containing piperazine moiety 1 for the lower power consumption reflective liquid crystal devices ( Figure), as the three-layered guest-host (G-H) system with subtractive color mixing of yellow, magenta and cyan are expected to be used for developing the full-color reflective displays. 2 Investigation of their mesogenic behaviors shows that the O=C-N< moiety of the piperazine does help the formation of the SmC phase, which are potentially useful materials in the application of anti-ferroelectric or ferroelectric liquid crystal display. In the G-H system, the dye molecules are mixed with liquid crystals to increase the desired mesogenic phase or other physical properties. 3 However, the phase separation between the guest and host materials are serious issues for application.
FigureTo broaden our research scope and avoid the phase separation problem when mixed with the piperazine-containing azo-dye liquid crystals as shown in Figure, we are interested in synthesizing liquid crystals containing O=C-N< moiety together with piperazine unit. Intermediates containing the O=C-piperazine unit are not only versatile for synthesizing various kinds of liquid crystals, 4a,b but are also useful in drug chemistry. 4c,d Moreover, the amide functionality is highly thermostable, which is one of the most important factors to be considered in designing photoelectric materials. However, direct monoacylation of symmetrical diamines is frequently problematic due to competitive bisacylation. 5 For example, under normal basic conditions, the bisadduct 4 is isolated as the major product. A possible explanation of the uncontrollable bisacylation of the symmetrical secondary diamines under these conditions is that the monobenzoylated intermediate 3 is more soluble in the solvent than piperazine and it reacts preferentially with arylcarboxyl derivative to provide the observed disubstituted product 4. Therefore, a number of indirect, multi-step processes have been developed, 6a-d which involve protecting one of the nitrogen atoms, acylating the other, and deprotecting to give the desired products. The same purpose was also achieved by a rather laborious procedure involving the addition of starting materials in several batches with very cautious control of the pH value (range from 3.8 to 5.4) of the reaction system. 6eHowever, Wang et al. overcame this problem by reacting the dilithium salt of the piperazine with benzoyl chloride. 7a In his method, butyl lithium in THF was used for generating the dilithium piperazine amide, which was then allowed to react with benzoyl chloride. In such a case, solvents used for the reaction have to be dried. Usually, the carboxylic acid was treated with excess of the ...