Formation of thiadiazole, thiadiazine, thiadiazepine and pyrazole derivatives in the reaction of 2,4-disubstituted thiosemicarbazides with tetracyanoethylene

The reaction of 2,4-disubstituted thiosemicarbazides with tetracyanoethylene gave mixtures of 1,3,4-thiadiazole, 1,3,4-thiadiazine, 1,3,4-thiadiazepine and pyrazolidine derivatives. Rationale mechanisms for these transformations are presented

We investigated the behavior of other analogous 2,4-disubstituted thiosemicarbazides 7a-e towards TCNE 2 (Scheme 2).Treatment of 7a-e with two molar equivalents of TCNE 2 in ethyl acetate at room temperature resulted in a green coloration of the solution which later turned to dark brown.This behavior may be due to initial formation of unstable charge-transfer (CT) complexes followed by a chemical reaction.Monitoring of the reaction by visible spectroscopy failed since the reaction was fast and also at lower concentration no significant color changes were observed.Concentration of the preparative runs gave a precipitate which by washing with ethyl acetate yielded a yellowish brown solid of compounds 8.The remaining soluble materials were subjected to preparative layer chromatography giving numerous colored zones, from which products 9-12 were isolated (Scheme 2).Scheme 2. The products formed during the reaction of 7a-e with TCNE 2.
From their elemental composition and spectroscopic characteristics, these products can be regarded as resulting from 1:1 combinations of the starting materials 7a-e and TCNE 2 with loss of one or two fragments giving rise to the gross compositions listed in Table 1 together with the eliminated fragments and the structural requirements as delineated from spectroscopic evidence.
Thiadiazepines 8a,b,d,e: The molecular ions in their EI-mass spectra support the molecular masses and the gross compositions.Further, the following common features of fragmentation patterns supported the assigned structures: Loss of C5H2N4 giving intense (M + -118) ions, and loss of RN=C=S giving rise to the ion m/z 209 common in the spectra of compounds 8a,d,e.The IR spectra show characteristic absorption for the NH2 group in the ranges 3333 to 3170, and 2210-2220 for cyano groups and 1617-1625 cm -1 for C=N.The 1 H NMR spectra show the presence of a NH2 group by broad signals in the range 7.01-7.09ppm for two protons, and for 8e, the additionally expected signals for the allyl group at H: 4.21-4.26,5.07-5.16and 5.86-5.93ppm. 13C NMR spectra clearly support the absence of a C=S group in 8a,b,d,e and the presence of an isothiourea carbon in the range from 165.81 to 166.78 ppm.In the 13 C NMR spectrum of 8a, thiadiazepine C-6 and C-7 resonate at C = 108.12and 158.24 ppm, respectively, in accordance with the observed trends in the  values for C-atoms in push-pull alkenes. 23,24 showed the presence of thiadiazolidine NH (H 10.81 ppm) being due to conjugation with a π system or adjacent to a C=N double bond, whereas an isolated NH in a five membered ring like 18 has been reported to resonate at 4.5 pm. 30n the 13 C NMR spectrum of 10b the thiadiazolidine C-2 bearing two carbonitrile groups was observed at C 66.17This can be explained in the following way: 2,4-diarylthiosemicarbazides are known for their thermal instability relative to their constitutionally isomeric 1,4-diarylthiosemicarbazides 31 and therefore it is likely that products 10 result from the latter rearranged starting materials.In addition, the rearrangement of 7 may be facilitated in the radical cationic stage of 7 inside the initial charge transfer complexes formed upon mixing the reactants 7 and TCNE 2 (see Scheme 3).In principle, one can discuss initial tricyanovinylation products 20 and 22 originating from the primary adducts 19 and 21 from thiosemicarbazide 7 and TCNE 2 (Scheme 4) since products 11 may also be formed from 20 with loss of malononitrile, and adducts of 2 to N-4 of 1,4-disubstituted thiosemicarbazides have been discussed before. 21heme 4. Conceivable alternative adducts from the reaction of thiosemicarbazides 7a-e with TCNE 2. For meanings of R and R' see Scheme 2.
It has also been reported earlier in an investigation of the interaction of TCNE 2 with two thiosemicarbazides that the -SH group is the reactive center, 35 but the formation of the products 8,9 and 11 supported our rationalization via the involvement of the tricyanovinylation product 14 for at least these three compounds (Scheme 3).
As outlined there, the NH2 group first attacks the double bond of 2 forming the products.Since, the reaction required multiple steps and by necessity moderate yields (see Table 2) have to be accepted and it will not be possible to clarify every detail.This needs to be taken into account when determining and evaluating yields (see Table 2).

Conclusions
The reactions and products presented provide insight into the spontaneous reactions between the electrondonating 2,4-disubstituted thiosemicarbazides 7a-e and TCNE 2. In fairly complex and multistep processes five types of heterocyclic products 8-12 are formed from 7a-e and TCNE 2. Consequently, TCNE 2 acts as a building block in heterocyclization of 2,4-disubstituted thiosemicarbazides.

Experimental Section
General.Melting points were determined using open glass capillaries on a Gallenkamp melting point apparatus and are uncorrected .The IR spectra were recorded with a Shimadzu 408 or a Bruker Vector 22 spectrometer using KBr pellets. 1 H 300 MHz and 13 C NMR 75 MHz spectra, all in DMSO-d6, have been registered using a Bruker WM 300 instrument, 500 MHz 1 H and 125 MHz 13 C NMR spectra on a Bruker DRX 500 spectrometer.Chemical shifts are expressed as δ [ppm] with reference to tetramethylsilane as an internal standard, s = singlet, d = doublet, m = multiplet, br = broad, 13 C assignments were made with the aid of DEPT 135/90 spectra.Mass spectra were obtained with an AMD 604 doubly focusing instrument using electron-impact ionization (70 eV).Elemental analyses were run by the microanalytical center, Cairo university.Preparative layer chromatography (plc) was made on 1.0 mm thick air-dried layers of slurry applied silica gel Merck PF254 on 48 cm wide and 20 cm high glass plates, zones were detected by their color and indicator fluorescence quenching upon exposure to 254 nm light and extracted with acetone.Starting materials: 2,4-disubstituted thiosemicarbazides 7a-e were prepared according to the literature 36 adapting Noto's pocedure. 31eaction of 2,4-disubstituted thiosemicarbazides 7a-e with 2. General procedure.A solution of 2,4disubstituted thiosemicarbazide 7a-e (1.0 mmol) in dry ethyl acetate (30 mL) added dropwise with stirring at room temperature to a solution of 2 (2.0 mmol) in ethyl acetate (20 mL).The color changed gradually from green to brown.Stirring was continued for 48 h with admission of air to complete the reaction.The reaction mixture was filtered and the precipitate was washed several times with cold ethyl acetate until the washing remained colorless.The precipitate was dried and recrystallized from ethanol to give thiadiazepine derivatives 8a,b,d,e.The filtrate was concentrated to dryness and the residue was kept at 80 °C under vacuum to sublime off all unreacted TCNE 2. The residue was then separated by plc (100 mg per plate) using a suitable solvent mixture as eluent (c-hexane/ethyl acetate 5:1 for the reactions of 2 with 7a, 7c and 7e, c-hexane/ethyl acetate 3:1 for the reactions of 2 with 7b and 7d) to give numerous colored zones, the intense of which were removed and extracted.The fastest migrating zone contained the thiadiazoles 11b,c,d, the second zone contained the thiadiazolidines 10b,c,d, the third characteristically yellow zone contained the thiadiazine derivatives 9a,b,d, and finally the slowest migrating zone contained the pyrazolidine derivatives 12a,c,e.Extraction of zones with gave residues, which were rechromatographed to separate the pure compounds.7-Amino-2-(benzylimino)-3-phenyl-2,3-dihydro-1,3,4-thiadiazepine-5,6-dicarbonitrile (8d).Dark yellow crystals (294 mg, 41%), mp 227-229 °C (MeCN).IR (ṽ/cm -1 ): 3325-3210 (NH2), 2210 (C≡N), 1625 (C=N), 1590 (phenyl ring stretching). 1 H NMR (300 MHz): H 4.92 (2H, s, CH2), 7.09 (2H, br, NH2), 7.17-7.58(5H, m, Ar H). 13

29 1, 3 , 4 -
support the assigned structure.The presence of a Ph-CH2 group is also evident from the13  C DEPT-NMR spectrum exhibiting a negative signal at C 53.66 ppm.The molecular formulas of compounds 9a,b,d were supported by elemental analyses and mass spectra which showed the expected molecular ion peaks.For 9a the EI-mass spectrum needs a brief comment: m/z 91 represent a C4HN3 fragment formed by release of RN=C=S + RN from the molecular ion.Alternative structures could be ruled out according to the13  C NMR spectra of 9a,b,d since the thiadiazine C-5, C-6 are regularly downfield shifted [9a (C-5: 140.22,C-6: 101.16), 9b (C-5: 140.56,C-6: 100.83), 9d (C-5: 140.65,C-6: 100.76)] 25 compared to =C(CN)2 (which would resonate in the range 61-74 ppm.26-Thiadiazolidines 10b,c,d: These compounds show a characteristically yellow color.The molecular structure of 10b was supported by the following findings: The gross formula C17H13N5S represented a product from one molecule of thiosemicarbazide and one molecule of TCNE 2 with loss one molecule of malononitrile CH2(CN)2.The presence of two cyano groups at C 117.92 and 118.48 ppm is documented, and a signal for thiadiazolidine C-5 at C 162.92 as well as the absence of C=S favor structure 10b.The 1 H NMR spectrum of 10b ppm.The mass spectra of 10b,d show a fragment at m/z 292 (representing the release of HCN from the molecular ions) whereas loss of RN=C=S gives rise to fragments with m/z 157.Furthermore, treatment of 10b,c,d with diluted alcoholic KOH followed by neutralization did not change the compounds to 11b,c,d (see below) as expected.Therefore compounds 10 carry the R'-group at N-3 and not at N-4.

Table 1 .
Composition and essential features of products obtained by 1:1 combination of starting materials 7 (RR'CH3N3S) and TCNE 2 (C6N4) with elimination of low molecular weight fragments.For meanings of R, R' see Scheme 2 spectrum (DMSO-d6) of 9d clearly shows the presence of thiadiazine NH at H 6.99 ppm and benzyl CH2 at 4.91 ppm in addition to the phenyl protons (H 7.15-7.68ppm).In its13C NMR spectrum, thiadiazine C-2, C-5 and C-6 resonate at C 164.91, 140.65 and 101.76 ppm, respectively.Further, peaks at 118.06-118.27ppm for CN [32][33][34]showed also the presence of an allyl group which gave rise to three multiplets centered at 4.25, 5.22-5.35and5.92-5.97ppmdue to allyl CH2N, allyl CH2= and allyl -CH=, respectively.The presence of allyl group was also evident from the13C DEPT NMR spectrum exhibiting a positive signal at C 135.13 (allyl CH=) and negative signals at 43.08 and 115.37 ppm due to allyl CH2N and allyl CH2=, respectively, further peaks at 116.85, 116.96, 117.08, 117.33 [C(CN)2].[32][33][34]Fordata of the analogous compounds 12a,c see the Experimental part.Overview of product formation:A rationale for the formation of products 8-12 is presented in Scheme 3. Disubstituted thiosemicarbazides 7a-e and TCNE 2 give the neutral adduct 13.Elimination of a molecule of HCN from the adduct 13 generates the tricyanovinylation intermediate 14 which cyclizes to thiadiazepine derivatives 8. Also the products 9 and 11 require the intermediate formation of 14. Elimination of another molecule of HCN from the latter affords the thiadiazine derivatives 9, whereas elimination of malononitrile from 14 gives rise to thiadiazole derivatives 11.
1,3,4-Thiadiazoles 11b,c,d: The 1 H NMR spectrum of 11b clearly does not show any thiadiazole NH (neither isolated nor in conjugation with a C=N bond).The 13 C NMR gave signals at 153.38 and 165.11 assigned to thiadiazole C-2 and C-5, respectively.The mass spectrum of compound 11b exhibits a molecular ion peak at m/z 292 (28%).Analogous data was found for 11c,d.There was no13C signal to indicate a C=S bond.Pyrazolidines 12a,c,e: These compounds do not contain sulfur but each one contains seven N atoms, thus the molecular mass of 12e represents a gross composition (C16H11N7) reached from the rearranged 1,4-disubstituted isomer 17e (see Scheme 3) and 2 after loss of H2S.This was confirmed by the mass spectrometry (m/z 301, 100%, M + ).The IR spectrum showed bands at 2212 and 2215 cm -1 due to cyano groups, 3336 (NH) as well as 1627 (C=N).The 1 H NMR spectrum indicated the presence of broad band at (H 11.32 ppm) due to the pyrazolidine NH conjugated to a C=N bond.Scheme 3. Rationale for formation of products 8-12.For meanings of R and R' see Scheme 2.

Table 2 .
Yields of products formed according to Scheme 3