Anhydrous Aluminum Chloride Catalyzed Methylene Group Inclusion: Mechanistic, Spectral and Single Crystal X-Ray Structural Study on Methanediyl Bis(Cyclohexylmethylcarbamodithioate)

. In this study anhydrous AlCl 3 is used as a catalyst for the inclusion of a methylene group in to cyclohexylmethyldithiocarbamic acid to form methanediyl bis(cyclohexylmethylcarbamodithioate). Dichloromethane is used as a methylene group bearer in the reaction. A suitable mechanistic pathway involving + CH 2 Cl is discussed. FTIR, NMR and Mass spectral techniques have been used in the analysis. Single crystal X-ray structure of the compound was determined. FTIR spectrum of the compound showed υ c-s band at 1073 cm -1 and υ C-H vibrations appeared at 2853 and 2928 cm -1 . Thioureide stretching band was observed at 1473 cm -1 . The molecular ion peak in the Mass spectroscopy confirmed the proposed formula. H 1 NMR spectrum of the compound showed a signal at 4.33(s) ppm for α-CH of the cyclohexyl ring and - CH 3 protons attached to nitrogen appeared at 3.40 ppm. Methylene proton (S-CH 2 -S) signal appeared at 3.16 ppm which is largely deshielded by the presence of two electronegative sulphur atoms on either side. The characteristic methylene carbon (S-CH 2 -S) signal appeared at 45.46 ppm in the 13 C NMR spectrum. Single crystal X-ray structural analysis of the compound showed it to be monomeric. Methylene carbon in S-CH 2 -S, C(9) is tetrahedrally bonded to two hydrogen atoms and two sulphur atoms S(2), S(3). The molecule stacks its cyclohexyl rings along ‘c’ axis of the unit cell. Short contacts in the form of supramolecular interactions such as C---S and S---S exist in the solid state at 3.49 and 3.50 Å respectively.


Introduction
Organosulphur compounds are important intermediates in organic synthesis. Recently, a new reaction protocol was developed that consists of a reductive lithiation of alkyl or arylthioesters [1][2][3][4][5]. Lewis acids serve as a catalyst for the methylene group inclusion reaction of thiols. Reactions of thiols with dimethoxymethane in the presence of four equivalents of Lewis acids, such as AlCl 3 , TiCl 4 and SnCl 4 were investigated [6][7][8][9][10][11][12]. Aluminum hydrogen sulfate acts as a catalyst for the efficient synthesis of 3,4-dihydropyrimidin-2(1H)-ones via the condensation of aromatic aldehydes, β-keto esters and urea (or thiourea) in methanol under solvent-free conditions [13]. Though there is a growing importance of dithioacetals or sulfur-stabilized carbanions in organic synthetic chemistry. Generally bis(alkyl or arylthio)methanes are prepared by the reaction of diiodomethane with sodium thioalkoxide or thiophenoxide [14,15] and in this case diiodomethane is used as a methyl inclusion agent as it can easily give the methylene group by the abstraction of iodine atoms by Lewis acid. The rate order to synthesize dithioacetals in the presence of Lewis acids was decreasing in the order: TiCl 4 / Zn [Ti] > AlCl 3 > TiCl 4 > SnCl 4 [16]. The formation of S-CH 2 -S bond between two N-CBZ-protected cysteins, is having great significance as there is an immense interest due to their biological activity [17]. In the present investigation, methanediyl bis(cyclohexylmethylcarbamodithioate) was prepared by using anhydrous AlCl 3 as a catalyst and dichloromethane as methylene bearing source for inserting methylene group in cyclohexylmethyldithiocarbamic acid and the product is characterized by IR, mass, nmr and single crystal XRD techniques.

Experimental
Cyclohexylmethyl amine (Sigma-Aldrich, 99.7%), carbon disulfide and dichloromethane (Sd fine Chemicals, India) were used in the preparation. All the reagents and solvents employed were commercially available analytical grade materials and were used as supplied without further purification. IR spectra were recorded as KBr pellets on ABB Bomen MB 104 spectrometer (range: 4000-400 cm -1 ). NMR spectra were recorded on a Bruker 400 MHz spectrometer at room temperature using CDCl 3 as solvent. Mass spectra were recorded on a VG Autospec GEIFAB and a Hewlett Packard MS-Engine thermospray and ionization by electron impact at 70 eV.
Single crystal X-ray crystallography Intensity data were collected at ambient temperature (295 K) on Bruker SMART 1000 CCD diffractometer using graphite monochromated MoKα radiation (λ = 0.71073Å). Data were corrected for absorption using the SADABS program and ω-scan technique was used for data collection [8,9]. The structures were solved by SIR97 [10] and were refined by full matrix least squares with SHELXL-97 [11]. All the non-hydrogen atoms were refined anisotropically and the hydrogen atoms were fixed geometrically. ORTEP-3 program was used for drawing the molecular plots [12].

Synthesis of methanediyl bis(cyclohexylmethylcarbamodithioate)
Cyclohexylmethyl amine (0.27 mL, 2 m mol) in dichloromethane (25 mL) and carbon disulfide (0.16 mL, 2 m mol) in dichloromethane were mixed under ice cold condition (0-5 ˚ C) to obtain yellow cyclohexylmethyldithiocarbamic acid solution. To this solution, anhydrous aluminum chloride (0.2 g, 2 m mol) was added slowly with vigorous stirring using magnetic stirrer, for about one and half an hour. Colourless solid formed was removed by filtration and the filtrate was kept for slow evaporation. On evaporation of the solvent a pale yellow solid appeared (Scheme 1) which was than recrystallized for three times with acetonitrile. Yield: 56 %, m. p.: 81 °C. Elemental analysis: % Anal. Calcd., for C 17  dichloromethane and forms carbocation (electrophile) due to its Lewis acidity. AlCl 4 has the tendency to produce dithiocarbamate anion from corresponding dithiocarbamic acid as in step (2). In step (3), chloromethylcyclohexylmethylcarbamodithioate is formed. In step (4), AlCl 3 abstracts a hydrogen from chloromethylcyclohexylmethylcarbamodithioate and the corresponding carbocation is formed. The carbocation interacts with another dithiocarbamate anion (step (5)) to form methanediyl bis(cyclohexyl methylcarbamodithioate). The present method is a relatively simple process in which a methylene group is inserted to form an -S-CH 2 -S-moiety.

Spectral studies FTIR and Mass spectrum
In the IR spectrum, characteristic thioureide stretching bands are observed at 1473 cm -1 for the alkylated compound, which is a clear evidence for the partial double bonded nature of C-N bond. The υ c-s band appears at 1073 cm -1 and υ C-H vibrations appear at 2853 and 2928 cm -1 respectively (Figure 1). In the mass spectrum (Figure 2), the molecular ion peak is observed at (M + = 391.83) which clearly confirmed the formation of the methylene inserted compound.

Scheme 2. Mechanism of formation of methanediyl bis(cyclohexylmethylcarbamodithioate)
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Single crystal X-ray structural analysis Details of data collection and refinement parameters of the compound are given in Table 1. Selected bond distances and angles are given in Table 2. ORTEP diagram of the compound is given in Figure 6. Packing diagram of the compound is shown in Figure 7.

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The compound is monomeric in nature. Four formula units are present in one unit cell. Methylene carbon in S-CH 2 -S, C(9) is tetrahedrally bonded to two hydrogen atoms, H(9A), H(9B) and two sulphur atoms, S(2), S(3). The C(9)-S(2) and C(9)-S(3) bond distances around the methylene carbon are 1.8072(16) and 1.7934(16) Å respectively. The two hydrogen atoms subtend 108. 19(1)˚ at C(9). However, S(2)-C(9)-S(3) angle is 109.53(1)˚. The partial double bonded distances do not show large changes compared to the previous reports. [27][28][29][30]. The molecule stacks its cyclohexyl rings along 'c' axis of the unit cell as shown in Figure 8. Short contacts in the form of supramolecular interactions such as C---S and S---S exist in solid state at 3.49 and 3.50 Å respectively, which are marginally less than the sum of their van der Waals radii. Figure 8. Stacking of cyclohexyl rings and the short contacts in methanediyl bis(cyclohexylmethylcarbamodithioate)

Conclusions
In this report anhydrous AlCl 3 is used as a catalyst for the inclusion of a methylene group in to cyclohexylmethyldithiocarbamic acid to form methanediyl bis (cyclohexylmethylcarbamodithioate). Dichloromethane is used as a methylene group bearer in the reaction. In the FTIR spectrum of the compound, the υ c-s band appears at 1073 cm -1 and υ C-H vibrations appear at 2853 and 2928 cm -1 . Thioureide stretching band is observed at 1473 cm -1 . The molecular ion peak in Mass spectroscopy confirmed the proposed formula. H 1 nmr spectrum of the compound showed a signal at 4.33(s) ppm for α-CH of the cyclohexyl ring and -CH 3 protons attached to nitrogen appeared at 3.40 ppm. Methylene proton (S-CH 2 -S) signal appeared at 3.16 ppm which is largely deshielded by the presence of two electronegative sulphur atoms on either side. The characteristic methylene carbon (S-CH 2 -S) signal appeared at 45.46 ppm in the 13 C NMR spectrum. Single crystal X-ray structural analysis of the compound showed it to be monomeric. Methylene carbon in S-CH 2 -S, C(9) is tetrahedrally bonded to two hydrogen atoms and two sulphur atoms S(2), S(3). The molecule stacks its cyclohexyl rings along 'c' axis of the unit cell. Short contacts in the form of supramolecular interactions such as C---S and S---S exist in solid state at 3.49 and 3.50 Å respectively.  122.53 (9) 68 ILCPA Volume 68