Reactivity of 2-substituted hydrazinecarbothioamides towards tetracyanoethylene and convenient synthesis of (5-amino-2-diazenylthiazolylmethylene)malononitrile derivatives

2-{Amino-[5-amino-2-(substituted diazenyl)thiazol-4-yl]methylene}malononitriles were synthesized from the reaction of 2-substituted hydrazinecarbothioamides with tetracyanoethylene (TCNE) to give tetracyanoethane adduct, followed by heterocyclization afforded the target compounds. The structure of ( E )-2-{amino-[5-amino-2-(phenyldiazenyl)thiazol-4-yl]methylene}malononitrile was supported by single crystal X-ray crystallography.


Results and Discussion
Treatment of the hydrazinecarbothioamides 1a-e with TCNE 2 (1.1 equiv) in dry ethyl acetate at room temperature resulted in a pink coloration of the reaction solution which quickly turned reddish orange.Tentatively, the color change observed may be owed to the formation of unstable charge-transfer (CT) complexes.The mixture was stirred and then left to stand for 24 hours at room temperature, resulting in the formation of single products 3a-e in 81-88% yields (Scheme 1).The gross formula C 13 H 9 N 7 S represents a product from one molecule of 1a and one molecule of TCNE 2 without elimination of any atoms.Two NH2 groups are present in 1 H NMR (exchangeable with D2O) as broad signals, the downfield at 10.22 ppm due to NH2 attached to vinyl group in 3a, the other NH2 resonate at 7.94 because of NH2 attached to thiazole C5.The aromatic protons observed at 7.83-7.10ppm.In its 13 C NMR spectrum, thiazole C2, C4 and C5 resonate at 161.2, 150.4 and 152.6, respectively.In the (aminomethylene)malononitrile fragment, of 3a, the dicyanovinyl carbons C2 and C1 resonated at 164.4 and 61.6 ppm, respectively, and were in accord with the observed trends in the δ values for C-atoms in push-pull alkenes. 22,23urther peaks at 115.6 (CN), besides the aromatic carbons support the assigned structure.
Absorption bands around 3373-3320 cm -1 relating to NH2 groups appeared in IR spectra of 3ad.The IR spectra of 3a-e showed two sharp absorption bands at 2220-2210 cm -1 (CN) and 1623-1612 cm -1 that assigned to C=N vibration.The absence of C=S signal in IR and 13 C NMR, also support the structures 3a-e.Moreover, the structure of (E)-2-{amino-[5-amino-2-(phenyldiazenyl)thiazol-4-yl]methylene}malononitrile 3a has been strongly supported by a single crystal X-ray structure analysis (Figure 1 and Tables S1-S7 in the supplementary data).The asymmetric unit of (E)-2-{amino-[5-amino-2-(phenyldiazenyl)thiazol-4-yl]methylene}malononitrile 3a (C13H9N7S), confirms two independent molecules whose conformations differ primarily in the orientations of phenyl and substituted vinyl groups with respect to the thiazole ring.The X-ray structure analysis confirms a transoid geometry of thiazole and substituted groups with respect to the N=N double bond.All bond lengths and angles in compound 3a are normal.The thiazole ring is planar; mean deviation from the S1/C2/N3/C4/C5 plane 0.019 Å.The amino group and the planar phenyldiazenyl substituent are coplanar to the thiazole moiety, while the planar aminomethylenemalononitrile is twisted by 43° to the thiazole plane.
A rationale for the formation of compounds 3a-d given in Scheme 2. Nucleophilic attack from the terminal NH2 of 1a-d on the C=C double bond of 2 to give the tetracyanoethane derivatives 4; charge-transfer complexes may, but do not necessarily have to, 24

Conclusions
In conclusion, novel (diazenylthiazolyl)methylene]malononitriles have been synthesized from the nucleophilic addition reactions of 2-substituted hydrazinecarbothioamides on TCNE.The products were synthesized from readily accessible starting materials using a simple experimental procedure.

Experimental Section
General.Gallenkamp melting point apparatus was used for determining the melting points; the results are uncorrected.The IR spectra (KBr discs technique) were recorded on Alpha, Bruker FT-IR and Shimadzu 408 instruments.The 1 H-NMR (400.13MHz) and 13 C-NMR (100.6 MHz) spectra were determined on a Bruker AM 400 spectrometer; s = singlet, m = multiplet, b = broad.The 13 C NMR signals were assigned based on DEPT 135/90 spectra.Chemical shifts are expressed as δ in parts per million (ppm).The mass spectra (70 eV, electron impact mode) were recorded on a Finnigan MAT 312 instrument.The elemental analyses for C, H, N and S were carried out at the Microanalytical Centre, Cairo University, Egypt using an Elmyer 306.Preparative layer chromatography (PLC) used air-dried 1.0 mm thick layers of slurry-applied silica gel (Merck Pf 254 ) on glass plates 48 × 20 cm using the solvents listed.

Reaction of 2-substituted hydrazinecarbothioamides 1a-e with tetracyanoethylene (2).
General procedure.A solution of TCNE 2 (141 mg, 1.1 mmol) in dry EtOAc (10 mL), was added dropwise to a solution of 1a-e (1.0 mmol) in dry EtOAc (15 mL), which causes a spontaneous change of color from yellow to pink and finally to reddish orange.The mixture was stirred for 2 h, then left to stand for 24 h at room temperature.A red-orange precipitate was formed, filtered and recrystallized from acetonitrile to give pure crystals 3a-e.In case of the reaction between 1d and TCNE 2, the mixture of the reaction was subjected to PLC and using toluene/ethyl acetate (5:1) as eluent to give an intense red-orange zone from 3d.The zone was separated by using acetone and recrystallized from acetonitrile.

Single crystal X-ray structure determination of 3a
Single crystals were obtained by recrystallization from acetonitrile.The single crystal X-ray diffraction study was carried out on a Bruker D8 Venture diffractometer with Photon100 detector at 123 K using CuKα radiation (λ = 1.54178Å) 3a.Direct Methods (SHELXS-97) 32 were used for structure solution and refinement was carried out using SHELXL-2014 33 (full-matrix least-squares on F 2 ).Hydrogen atoms were localized by difference electron density determination and refined using a riding model (H (N) free).A semi-empirical absorption correction was applied.Compound 3a: C13H9N7S, M = 295.32g mol -1 , red plates, crystal size 0.12 × 0.08 × 0.02 mm, triclinic space group, P-1 (no.  2 (for all data) = 0.068, S = 1.06, largest diff.peak and hole = 0.266 eA -3 /-0.245 eA -3 .Crystallographic data (excluding structure factors) for structure reported in this work have been deposited with Cambridge Crystallographic Data center as supplementary publication no 1048447 (3a) Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44(1223) 336 033: e-mail: deposit@ccdc.cam.ac.uk.

Figure 1 .
Figure 1.Molecular structure of 3a in the crystal (TIF file).The crystallographic numbering does not reflect the systematic IUPAC numbering.

Scheme 2 :
Scheme 2: Mechanistic rationale for the formation of compounds 3a-e.