The guanidine group has attracted considerable attention since it is found in a wide array of natural and synthetic biologically active compounds [1,2,3]. The guanidine groups are categorized as organosuperbases whose basicity is magnified because of the resonance stabilization of the corresponding conjugated acids. These molecules are basic enough (pKa of their conjugated acids is around 12.5) to form intermolecular contacts mediated by H-bonding interactions [4]. Its positive charge, resulting from protonation in a wide range of pH values, plays an important role in forming specific intermolecular interactions, comprising key-steps of many biological reactions including enzyme-mediated processes and interaction of hormones with their receptors [5]. The guanidinium moiety interacts with functional groups present in enzymes or receptors on the basis of hydrogen bonds and electrostatic interactions to form intermolecular associations. Thus, they are useful pharmacophores in medicinal chemistry [6]. Moreover, synthetic guanidines have found wide applications in the engineering of advanced synthetic molecular recognition devices, sensors, organic materials and phase-transfer catalysts [7,8]. Recently, it has been demonstrated that by introduction of chirality in one of the guanidinyl nitrogen atoms [9,10,11], the resulting chiral guanidines were effective in catalytic [12,13,14,15] and stoichiometric asymmetric synthesis [16,17]. Due to this, continued interest has been shown in the transformation of amines into the corresponding guanidines, because the guanidine group, instead of an existing amino group, can significantly increase the potency and/or selectivity of biologically active compounds [18,19,20].

Typically, the synthesis of guanidine-containing compounds involves the treatment of an amine with an electrophilic amidine species. The most commonly used reagents include cyanamide (1) [21], O-methylisourea hydrogen sulfate (2a) [22], S-methyl isothiouronium salts (2b) [23,24,25], pyrazole-1-carboxamidine (2c) [26], N-protected thiourea (3a) [27,28,29], (S-methyl or -aryl)isothiourea (3b) [30,31,32,33] or pyrazole-1-carboxamidine (3c) derivatives [34,35] (Figure 1). To increase yields, new reagents with electron-withdrawing substituents have been developed in the recent years [36,37].

The synthesis of guanidines from thioureas and isothioureas are the most commons strategies used for the construction of the guanidine functionality. In the case of thioureas, they are activated through the reaction with DIC, EDCI, Hg²⁺ (most popular but toxic), 2-chloro-1-methylpyridinium iodide, 2,4-dinitrofluorobenzene, etc. Recently, a bismuth catalyst has been found to afford high yields (70%–97%) [38]. The attachment of thiourea groups to a solid phase has been used as precursor of guanidinium groups [39]. Recently, it has been demonstrated that guanylilation of an amine from a thiourea, involves the attack of the amine on what is generally accepted to be a carbodiimide intermediate [40,41,42,43,44,45,46].

Diphenylcarbodiimide has been used for the synthesis of diphenylguanidinobenzothiazole [47] and dicyandiamide for the synthesis of 2-guanidinobenzothiazole [48]. However, to the best of our knowledge, there is no reports about non-symmetrical guanidines derived from 2-aminobenzothiazole.

We have reported [49] a detailed study and characterization of the intermediates involved in the synthesis of dimethyl benzo[d]thiazol-2-ylcarbonodithioimidate (5), from the reaction of 2-aminobenzothiazole (4) with carbon disulfide in basic media, using DMF as solvent [50]. Two molecules of HSMe are displaced when 5 reacts with o-XH substituted anilines in refluxing DMF to get NH-bisbenzazoles 7 [51,52,53] (Scheme 1). We found that the reaction proceeds through the intermediacy of isothiourea derivatives 6a–c when o-XH anilines are refluxed 16 h in ethanol. Under these conditions, isothiourea 6c could be isolated and the reaction was extended to m- and p-phenylenediamines [54]. In the same work, we reported the synthesis of S-methyl-N-alkylbenzothiazolyl-isothioureas 8a–d when ammonia, methylamine, pyrrolidine, aniline and 1,4-piperazine were used (Scheme 2).

In continuation of this work, we were interested in extending the routes of synthesis of guanidine compounds, in this sense, we generalized this reaction to prepare symmetrical 10g,h,j and nonsymmetrical 10b–f,i guanidine derivatives obtained from 2-aminobenzothiazole (4, Scheme 2). Herein we report the preparation and ¹H and ¹³C-NMR structural study of a series of guanidines 10b–j. The isolation of S-methylisothiourea intermediates 8 is important since their remaining reactive S-methyl group was subsequently substituted by amines to form the guanidine group.

When dithiocarboimidate 5 reacts with one molar equivalent of ammonia, alkylamine or aniline in ethanol, one molar equivalent of thiomethanol was evolved to afford the corresponding S-methylbenzothiazolyl-isothiourea 8a–d as isolable intermediates (Scheme 2). Ammonia required 72 h and alkylamine 48 h on stirring at room temperature, to be completed, whereas aniline required 24 h in refluxing ethanol.

The reaction of isothiourea intermediates 8a–d with a second equivalent of several amines (ammonia, methylamine, aniline and pyrrolidine) was then carried out. When the reaction of isothiourea intermediate 8a was performed with excess ammonia in refluxing ethanol, 2-cyanamido-benzothiazole 9a crystallized as the only product in a 62% yield. The X-ray diffraction structure of 9a is shown in Figure 2, along with a summary of representative distances and angles. The N12-C11 bond length is the shortest N–C bond, its value of 1.155(3) Å is in the characteristic range of a triple bond [55], confirming the presence of the cyano group. In addition, N10-C11 [1.337(3) Å] and N3-C2 [1.338(3) Å] are in agreement with a single bond whereas the shorter N10-C2 [1.311(3) Å] bond length is more appropriate for a double bond character. On the other hand, the X-ray diffraction structure of 9a shows that the mobile hydrogen atom prefers to stay on the benzothiazole nitrogen because of two intermolecular interactions, N3-H3∙∙∙N10, stabilize the molecule as a dimer [H3∙∙∙N10 = 2.01 Å, N3∙∙∙N10 = 2.866(2) Å, N3-H3∙∙∙N10 = 174°; symmetry code: –x, −y, 1−z]. In this arrangement, the nitrile group is cis positioned to the sulfur atom, thus the electronic conjugation is extended from the benzothiazole system to the nitrile group. The torsion angles S1-C2-N10-C11 [−1.0°(3)] and C11-N10-C2-N3 [178.3°(2)] are in agreement with a planar molecule. The nitrile is a polarized functional group whose interaction with sulfur atom, as Lewis acid, is favored by cis configuration [N12∙∙∙S1 = 3.160(3) Å] and C7-H7∙∙∙N12 interaction [H7∙∙∙N12 = 2.677 Å, C7-N12 = 3.343(4) Å, C7-H7∙∙∙N12 = 138°; symmetry code: 1−x, 1−y, 1−z].

Compound 9a is formed when S-methylisothiourea 8a suffers HSMe group elimination promoted by the basic media (Scheme 3). The proposed mechanistic pathway involves the participation of NH₄OH to remove one NH hydrogen atom from 8a to in situ form the ammonium intermediate I, which is stabilized as the tautomer intermediate II. A second molecule of NH₄OH promotes the elimination of the HSMe group to generate the nitrile 9a. The intermediate II was transformed to 8-NMe, (δ = 3.80 NMe, 2.66 SMe) by reaction with one molar equivalent of NaOH and CH₃I, which is readily transformed into 2-cyanamide-N-methylbenzothiazole compound 9b by a second molar equivalent of NaOH (Scheme 3).

Compound 9a was characterized in solution by NMR. The nitrile carbon atom appears as a small signal at 117.7 ppm, very similar to the observed value of 118.7 ppm in benzonitrile [56]. The mass spectrometry [M⁺ = 175 m/z (100%)] and elemental analysis, are in agreement with the proposed structure. Under the same conditions already described for 8a, isothioureas 8b and 8c failed to react with an excess of ammonia to give nonsymmetrical guanidines 10b and 10c respectively, and the starting materials were recovered, however, the reaction of isothiourea 8d with an excess of ammonia, affords the nonsymmetrical guanidine 10d as the only product. In this case, the acidic aniline hydrogen is intramolecularly engaged with the benzothiazole nitrogen atom. This hydrogen bonding interaction is strong enough to polarize the imine carbon and favors the substitution of the SMe group by ammonia.

On the basis of this result, the reaction of isothiourea 8a with methylamine and pyrrolidine were carried out to get nonsymmetrical guanidines 10b and 10c, after refluxing in ethanol for 4 and 2 days, respectively. The reaction of 8a with aniline failed to give the corresponding guanidine compound 10d, even after 4 days in refluxing ethanol. Under these conditions, aniline is not nucleophilic enough to add to the carbodiimide intermediate II. The reaction of isothiourea 8b or 8c with one molar equivalent of methylamine or pyrrolidine in refluxing ethanol for 8 h afforded the corresponding guanidines 10h,i or guanidines 10i,j, respectively. Symmetric guanidines 10h,j can also be obtained when dithiocarboimidate 5 is reacted with two molar equivalents of the corresponding amine in refluxing ethanol for 8 h.

The reaction of isothiourea 8d with one molar equivalent of methylamine, pyrrolidine and aniline was tested. After 3 days in refluxing ethanol, the corresponding nonsymmetrical guanidines 10e and 10f, were obtained. The reaction with aniline required harsh conditions: refluxing DMF or solventless heating. In the ¹H-NMR spectrum of 10g, only aromatic protons and a broad signal at 12.4 ppm, assigned to the N-H protons, were observed. Moreover, sixteen signals in the ¹³C-NMR spectrum are indicative of the presence of the 2-N,N-diphenyl guanidinebenzothiazole 10g (Table 1 and Table 2). The mass spectrometry data [M⁺ = 344 m/z (19%)] is in agreement with the proposed structure.

Comparison of ¹³C-NMR chemical shifts of guanidine compounds 10 to those of dimethyl benzo[d]thiazol-2-ylcarbonodithioimidate 5, shows that C2 and C11 are shifted to low frequencies by approximately 10 and 2 ppm, respectively (Table 2). This effect is explained by the extended conjugation of N12 and N13 electron pairs to the benzothiazole ring, increasing the electronic protection of C2 atom. This effect also shifts C7 and C8 to low frequencies by 6 and 3 ppm, respectively. In contrast, C2 is shifted by 6.5 ppm to higher frequencies in 2-aminonitrilebenzothiazole 9a.

Nonsymmetrical guanidines 10f and 10i were crystallized from ethanol; their molecular structures are shown in Figure 3 and Figure 4, respectively. The aniline N-H proton in 10f is intramolecularly bridged with benzothiazole nitrogen [H17∙∙∙N3 = 2.13 Å, N17∙∙∙N3 = 2.697(3) Å, N17-H17∙∙∙N3 = 123°], forcing the guanidine group to be in the same plane of the benzothiazole ring [N17-C11-N10-C2 = −1.7(3)°] and [N12-C11-N10-C2 = −179.62(17°)]. In addition, two phenyl hydrogen atoms make intermolecular CH∙∙∙π contacts: H19 with guanidine carbon atom [H19∙∙∙C11 = 2.792 Å, C19∙∙∙C11 = 3.684(3) Å, C19-H19∙∙∙C11 = 160.9°; symmetry code: 1−x, −y, 1−z], and H20 with the π electrons of the benzothiazole aromatic ring [H20∙∙∙π = 3.130 Å, C20∙∙∙π = 3.898(3) Å, C20-H20∙∙∙π = 141.2°, symmetry code: 1−x, −1/2+y, 1/2−z].

Moreover, in the case of guanidine 10i, derived from two alkylamines, the conformation is not fixed by intramolecular hydrogen bonding as in 10f, the methylamine N-H proton is intermolecularly hydrogen bonded with benzothiazole nitrogen of a second molecule and so on to get an helix-like polymer [H17∙∙∙N23 = 2.16Å, N17∙∙∙N23 = 2.973(6), N17-H17∙∙∙N23 =158°; symmetry code: x, y ,z and H37∙∙∙O3 = 2.19 Å, N37∙∙∙O3 = 2.997(7) Å, N37-H37∙∙∙O3 =157°; symmetry code: x, 1−y, z], this forces the plane of the guanidine system approximately 35° out of the benzothiazole mean plane [N17-C11-N10-C2 = −144.6(6)°, N12-C11-N10-C2 = 39.2 (9)°] in agreement with the steric demand of the pyrrolidine moiety.

The proton NMR chemical shift of the NH protons of compounds 10f and 10i are in 11.5 and 8.9 ppm respectively. These results show that aniline N-H hydrogen atom of guanidine 10f is involved in hydrogen bonding in solution whereas the NH of 10i is not. This result is explained because of, in the case of compound 10f, the electron pairs of the aniline nitrogen atom are conjugated with the aromatic ring making the NH more acid and thus able for hydrogen bonding. The same effect is observed in the ¹H-NMR spectra of the series of non-symmetric guanidines based on aniline derivatives. The NH protons are in 11.2 for 10e, 11.6 for 10f and 12.4 for 10g. This intramolecular interaction, make the benzothiazole ring and the guanidine system to be in the same plane, increasing the electronic protection on C2 atom and, shifting the corresponding ¹³C-NMR signals by approximately 3 ppm to lower frequencies, in comparison with guanidines without this group, Table 2.

We have demonstrated the preparation of symmetric and non-symmetric guanidines from the reaction of dimethyl benzo[d]thiazol-2-ylcarbonodithioimidate (5) and primary or secondary amines in refluxing ethanol, through the displacement of two molecules of HSMe. The reaction proceeds through isothiourea intermediates which, in strongly basic media, are transformed in (Z)-2-cyanamidabenzothiazoles. Alkylamines are nucleophilic enough to easily perform both substitutions leading to guanidines, whereas the second substitution with aniline requires harsh conditions. Intramolecular hydrogen bonding between the NH of the aniline group and benzothiazole nitrogen in S-methylisothiourea 8d, leads to a cis-disposition between them and thus controlling the stereochemistry of the second substitution. The NH of alkylamines is not acidic enough to form intramolecular hydrogen bonding and intermolecular interactions are found instead. In any case the preferred rotamer observed in the solid state is trans to the sulfur atom of benzothiazole ring.