Reaction of N , N ’ -disubstituted hydrazinecarbothioamides with 2-bromo-2-substituted acetophenones

Reaction of hydrazinecarbothioamides with 2-bromoacetophenones furnished the formation of thiazole-, bis- thiazole-, pyrazole-and 1,3,4-thiadiazole-derivatives in good yields. The mechanism was discussed. The structures of products were proved by MS, IR, NMR, elemental analyses and X-ray structure analyses


Introduction
Substituted hydrazinecarbothioamides were found to exhibit antifungal, 1-3 antiviral 4 and antioxidant activities 5 .Free radicals and reactive sulfur species such as thiol radicals are frequently synthesized through many biological processes and may be considered as indicators of biological inadequacy.In recent times, the applications of thiazoles have found in drug development for the treatment of allergies 6 , hypertension, 7 inflammation, 8 schizophrenia, 9 bacterial, 10 HIV infections, 11 and hypnotics 12 .Moreover, thiazoles have been used for the treatment of pain, 13 as fibrinogen receptor antagonists with antithrombotic activity 14 and as new inhibitors of bacterial DNA gyrase B. 15 Mukhija it was reported that thiourea reacted quantitatively with various 2-halocarbonyl compounds to form 2-amino salts of thiazoles. 16Many classes of thiadiazole have been known to possess interesting widespread biological properties such as antimicrobial, 17 and anticancer. 18,19reviously, we utilized by N,N'-disubstituted-hydrazinecarbothioamides in heterocyclic synthesis, such as 1,3thiazin-2-ylidene-substituted hydrazides and 1,2,4-triazolo [3,4-b]-1,3-thiazine-5-carboxylates 20 .Continuation of our research program included the synthesis of various thiazoles, [21][22][23][24][25][26] we herein report the results of our investigation on the reactions of symmetrical hydrazinecarbothioamides 1a-d with 2-bromoacetophenones 2a,b.
The structures of thiazoles 3cb, 4bb, 5ba, and 6db were elucidated by IR, NMR, mass spectra and elemental analyses.For 3cb, its molecular formula was proved from elemental analysis and mass spectrometry as C28H22N4S2 (Experimental section).The IR spectrum of 3cb showed absorption band at = 3330-3320 cm -1 for the NH group.No absorption was noted for carbonyl or hydroxyl groups in the IR spectrum.The 1 H NMR spectrum of 3cb showed two broad singlets; each for one proton at  = 11.45 and 11.30 assigned to the two protons of thiourea.In 13 C NMR spectrum of 3cb, it was observed carbon signals at C = 181.0for C=S, 154.0 (thiazole-C-2), and 118.4 ppm for C-5 of thiazole moiety (Experimental Section).
For compound 4bb, its molecular formula was proved by mass spectrometry and elemental analysis as C30H26N4S2 (Experimental Section).Mass spectrum showed also the molecular ion peak at m/z = 506 (55%), whereas the basic ion peak at m/z = 91, which related to the prescence of benzylic fragment pattern.The IR spectra of 4bb showed the NH groups at = 3220-3215 cm -1 .In the 1 H NMR spectral data of 4bb (Experimental Section), it can be seen two broad singlets appeared at H = 10.25 and 10.08 ppm, which assigned to the two NH thiourea protons.Moreover, the two-dissimilar benzylic-CH2 protons resonated at H = 4.86 and 5.02 ppm, respectively.The 13 C NMR spectra was also in accordance to the proposed structure and showed the functional groups of carbon signals C = 181.0for the thioamide carbon and the two benzylic carbons at C = 48.9 and 46.9 (Experimental section).The thiazole carbon signals were also resonated at C = 156.8(C-2), 145.0 (C-4) and 118.3 (thiazole-C-5).When, the derivative 1b reacted with 2-bromo-1-phenylpropan-1-one (2a), the reaction gave thiazole 5ba (Scheme 1).Mass spectrum of 5ba showed the molecular peak at m/z = 444.The 1 H NMR spectrum of compound 5ba gave two broad singlets at H = 10.20 and 8.20 for the two NH thiourea protons (Experimental Section).Moreover, the asymmetrical structure was corroborated via appearance of three singlets at H = 4.8, 5.0, 2.3 and 2.1 of the two benzylic-CH2 protons, CH3 and thiol-H, respectively.The 13 C NMR spectrum supported the 1 H NMR spectroscopic data due to the prescence of the thione-C, thiazole-C-2 and the two benzylic carbons at C =179.7,156.6, 48.9, and 46.4,respectively.(Experimental Section).Again, and in case of 6db, the two NH protons absorbed as two broad singlets at H =11.00 and 10.20.Most indicative that 13 C NMR spectrum of 6db elucidated the asymmetric structure of 6db via the appearance the two carbon signals of the p-toyl-methyl groups at C = 21.2 and 21.4 (see Experimental Section).
Under the condition mentioned above, 1d reacted with 2a to give the bis-thiazole 7da in 74% yield (Scheme 1).The IR spectrum of 7da didn't reveal any absorption corresponding to NH, OH and C=S groups (Experimental Section).Mass and elemental analysis revealed that the molecular weight of 7da equals to the sum of 1d with two moles of 2a accompanied with elimination of two molecules of hydrogen bromide.In the meanwhile, 13 C NMR spectrum reveal carbon signals of an asymmetric molecule.Disappearance of the thione carbon in 13 C NMR spectrum, indicated that it was involved in cyclization process.The structure of Econfiguration in 7da was ultimately proved by X-ray structural analysis (Figure 1).
The suggested mechanism described the formation of thiazoles 3cb, 4bb, 5ba and 6db was based upon attacking of thione-lone pair to the -bromo-C in 2a,b (Scheme 2).That was followed by salt formation as in intermediate 10.Subsequently, salt 10 would be neutralized and nitrogen lone-pair would attack to the carbonyl-C to form intermediate 11.Elimination of water and HBr from 11 would give the thiazoles 3cb, 4bb, 5ba and 6db (Scheme 2).Ultimately, addition of a second molecule of 2a to 11 along with extrusion of two molecules of water and HBr, would produce 7da (Scheme 2).Scheme 2. Suggested mechanism describing the formation of thiazoles 3cb, 4bb, 5ba, 6bb and free bisthiazole 7da.
In different manner, reaction of equal equivalents of both 1a with 2-bromo-1,2-diphenylethan-1-one (2b) gave pyrazole 8ab (Scheme 1).The IR spectrum of 8ab showed the NH and aliphatic groups at  = 3425-3400 and 2933-2974 cm -1 , respectively.The mass and elemental analysis elucidated the gross molecular formula of 8ab as C18H17N3.Mass spectrum showed the molecular ion peak at m/z = 275 (51%), whereas the base peak at m/z = 102.The 1 H NMR spectrum proved the prescence of allyl group and its protons.The NH-pyrazole was absorbed in 1 H NMR at H =12.00 (Experimental Section).The 13 C NMR spectrum supported the 1 H NMR © ARKAT USA, Inc spectroscopic data via the appearance of the allylic-carbons at C = 45.7,114.8 and 133.3.The structure of 8ab was corroborated by X-ray structure analysis (Figure 2).Scheme 3. Suggested mechanism describing the formation of compound 8ab.
The suggested mechanism for the formation of 8ab starts also from the salt 10 (Scheme 3).Neutralization accompanied by elimination of HBr molecule, then addition of nitrogen-lone pair of the N-2 of the hydrazine group to the carbonyl-C would give intermediate 13 (Scheme 3).Elimination of water molecule from 13 would, thus, give 14.Rearrangement was then occurred via amidine-like reaction to C-6 of the formed thiadiazine would led to salt 15.Elimination of allyl isothiocyanate and extrusion of sulfur from 15 would, finally give 8ab (Scheme 3).
Finally, on reacting hydrazinecarbothioamide derivative 1c with 2a, the reaction yielded the known product 9c 27 , Scheme 1).Heating the compound 1c alone under the same condition didn't proceed to give 9c, therefore we concluded that the presence of 2a would enhance the formation of thiadiazole 9c.Starting from the previous formed intermediate 10, the other thione-lone pair would attack to the positively charged thiamido-C to form salt 16 (Scheme 4).Rearrangement and neutralization process in 16 and recombination of the eliminated Br anion would form 10 and recycled compound 2a together with extrusion of H2S (Scheme 4).Based upon TLC analysis with authentic sample of known N,N'-diphenyl-1,3,4-thiadiazole-2,5-diamine 27 and its IR and 1 H NMR, the structure compound 9c was proved.

Conclusions
Our paper describes the synthesis of different types of heterocycles during the reactions of hydrazinecarbothioamides with electrophilic reagents.Therefore, much work will be done in our lab to investigate further reactions of hydrazinecarbothioamides.

Experimental Section
General.Melting points were determined on Stuart electrothermal melting point apparatus and were uncorrected.TLC analysis was performed on analytical Merck 9385 silica aluminum sheets (Kiselgel 60) with PF254 indicator.The IR spectra were recorded as KBr disks on Shimadzu-408 infrared spectrophotometer, Faculty of Science, Minia University.The NMR spectra were measured using a Bruker AV-400 spectrometer at the Karlsruhe Institut für Technologie (KIT), Institute of Organic Chemistry, Karlsruhe, Germany.Chemical shifts were expressed as δ (ppm) with tetramethylsilane as internal reference.The samples were dissolved in DMSO-d 6 , s = singlet, d = doublet, dd = doublet of doublet and t = triplet.Mass spectrometry were recorded on a Varian MAT 312 instrument in EI mode (70 eV), at the Karlsruhe Institut für Technologie (KIT), Institute of Organic Chemistry, Karlsruhe, Germany.Elemental analyses were carried out using Varian Elementary device in National Research Center, Giza, Egypt.

Scheme 4 .
Scheme 4. Suggested mechanism describing formation of compound 9c during reaction of 1c with 2a.