New Conjugated Systems Derived from Piperazine-2,5-dione

The preparation of monoarylidene and both symmetrical and unsymmetrical bisarylidene derivatives of piperazine-2,5-dione is described. The use of 1,4-diacetylpiperazine-2,5-dione make it possible to prepare unsymmetrical bisarylidenes. The introduction of a dicyanomethylene moiety into the para position of one of the arylidene groups gave a remarkable deepening in the colour of the resulting compounds 11 and 16.

The structural similarity of derivative 3 to the chromophore of indigo 4, led us to assume that if the arylidene (piperazine-2,5-dione) could be obtained with donor-acceptor substituents, then merostabilization of the excited state [10] should occur to give deeply coloured compounds that might be novel dyestuffs.This paper deals with the synthesis of mono and bis-arylidene derivatives possessing donor and acceptors substituents.
Compound 5 showed an NH absorption at 3266 cm -1 , a band at 1683 cm -1 for the carbonyl group and 1625 cm -1 for C = C (See Table 2).

Mono Arylidene and Unsymmetrical Bis-arylidene Derivatives
Condensation of piperazine-2,5-dione 1 with aromatic aldehyde always afforded symmetrical bis-arylidene derivative.However condensation using 1,4diacetylpiperazine-2,5-dione 5 with aldehydes could be controlled to occur stepwise.Two novel monoarylidene 7 and 8 were synthesized from the reaction of equal molar quantity of 2 and the appropriate aldehyde.
The H 1 -NMR spectrum of compound 7 showed a singlet at δ 4.5 ppm attributed to the methylene signal indicative of mono arylidene derivative.Its ir spectrum showed an NH absorption band at 3255 cm -1 and broad bands at 1693 cm -1 and 1661 cm -1 for the two C = O groups.
The unsymmetrical diaryldene derivatives were prepared from the mono arylidene derivatives 7 and 8 as shown in Scheme 1.

SCHEME 1
Compound 10 was synthesized by condensing one equivalent of compound 7 with the equivalent amount of terphthaldehyde in dimethylformamide at room temperature.Its ir spectrum showed an absorption band at 1705 cm -1 for the aldehydic C = O group of the free aldehyde.Compound 10 was readily undergo knovenagel condensation with active methylene compounds readily.Thus the treatment of the aldehyde derivative 10 with malononitrile using piperidene as a base to afford the red dicyanomethylene adduct 11 in good yield (Scheme 1).The ir spectrum of the later showed a CN absorption band at 2197 cm -1 .The introduction of the powerful electron withdrawing group CH = C(CN) 2 into the para position of the phenyl group in compound 11 gave a remarkable deepening in colour when compared with the yellow compound 10.This deepening in colour is believed to be due to the stabilization of half of the molecule brought about by the hybrid resonance 12 (Scheme 2).

SCHEME 2
To get a clear insight of this colour change we synthesized two other bis arylidene derivatives 13 and 14 which both contains a donor group on one arylidene and an acceptor on the other one.Compound 14 was prepared from the mono arylidene 7 and phthalic anhydride in good yield.The formation of the 1,3-dione derivative 14 is believed to be via the intermediate 15.The structure of compound 14 was followed from its ir spectrum which showed an absorption for the carbonyl groups of the 1,3-diketone moiety at 1670 cm -1 which expected to appear at wavenumber greater than 1750 cm -1 for compound 15.

Experimental
Melting points were recorded on a Thomas-Hoover capillary melting apparatus without correction.IR spectra were taken as KBr disk on a Nicolet Magna 520 FT IR spectrometer, 1 H NMR were recorded solution in CDCl 3 on a Brucker DPX 400 MHz spectrometer using TMS as internal standard.Microanlyses were carried out using a Perkin Elmer 240B Analyzer.

TABLE 1 .
Physical and analytical data of synthesized compounds.

TABLE 2 .
IR and 1 H-NMR data of synthesized compounds.