(+)-(S,S)-1,3-Bis[(tetrahydrofuran-2-yl)methyl]thiourea

The title compound, C11H20N2O2S, is an enantiomerically pure heterocycle-substituted thiourea synthesized under solvent-free conditions. The thiourea unit adopts a ZZ conformation, with the HN—(C=S)—NH core almost planar and the tetrahydrofurfuryl groups placed below and above this plane. The whole molecule thus approximates to noncrystallographic C 2 symmetry. Unexpectedly, the C=S group is not involved in intermolecular hydrogen bonding, as generally observed in homodisubstituted thioureas. Instead, molecules form a one-dimensional network based on weak N—H⋯O(heterocycle) hydrogen bonding, resulting in a zigzag ribbon-like structure around the crystallographic 21 screw axis along [100].

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: CI2734).

Comment
The development of straightforward and eco-friendly synthetic procedures remains an important aim in organic synthesis. Many organic solvents, particularly chlorinated hydrocarbons, that are used in large quantities in organic reactions are potential threat to human health and environment. Thus, the design of chemical reactions under solvent-free conditions is getting a renewed interest. In this regard, solvent-free organic syntheses have great applied value and expansive prospects considering their advantages such as high efficiency and selectivity, easy separation and purification and environmental acceptability. All these merits are in accord with the green chemistry's requests of energy-saving, high efficiency and environmentally benign features (Tanaka & Toda, 2000;Jeon et al., 2005). On the other hand, N,N'-disubstituted thioureas have recently received much interest due to their diverse applications, such as, inter alia, antiviral, antituberculous, fungicidal, herbicidal activities, as well as tranquilizing and antidiabetic drugs, agrochemical properties, antioxidants in gasoline, corrosion inhibitors, etc. In view of these and in continuation of our earlier work on the synthesis of thioureas (Vázquez et al.,

2004), we synthesized the title compound under solvent-free conditions (see experimental).
The asymmetric unit contains one molecule in general position (Fig. 1). As the amine used as starting material was enantiopure, the thiourea is found to be a pure (S,S) isomer. The central core HN-(C═S)-NH unit is close to be planar, the r.m.s. deviation from the mean plane S1/C1/N2/H2/N12/H12 being 0.039 Å. This core adopts a ZZ conformation (i.e. amine H atoms are arranged syn) and tetrahydrofurfuryl groups are placed below and above the central HN-(C═S)-NH plane. The whole molecule thus approximates a local C 2 point symmetry. The observed conformation is identical to that found in other related homosubstituted thioureas (Lai & Tiekink, 2002;Bailey et al., 1997).
The ZZ conformation avoids the formation of intramolecular hydrogen bonds (Saxena & Pike, 2007). Regarding the packing structure, it is clear that the thioketone functionality does not participate in intermolecular contacts. Such a situation is unexpected, since for previously X-ray characterized chiral and non-chiral homosubstituted thioureas, one-dimensional supramolecular structures based on C═S···H-N hydrogen bonds are predominant, providing that the thiourea is in a ZZ conformation (e.g. Vázquez et al., 2004;Custelcean et al., 2005;Shashidhar et al., 2006;Sadiq-ur-Rehman et al., 2007).
Instead, the crystal structure of the title compound is determined by weak N-H···O(heterocycle) hydrogen bonds, aggregating molecules in a backbone arrangement (Fig. 2) Methylene and methine H atoms were placed in idealized positions and refined as riding to their carrier C atoms. Amine H atoms, H2 and H12, were found in a difference map and refined with N-H bond lengths restrained to 0.86 (1) Å. For all H atoms, isotropic displacement parameters were calculated as U iso (H) = 1.2U eq (carrier atom). Fig. 1. Molecular structure of the title compound, with 30% probability level displacement ellipsoids for non-H atoms. Fig. 2. Part of the crystal structure of the title compound, showing the network of N-H···O hydrogen bonds (dashed lines). The 2 1 screw axis forming the backbone supramolecular structure is shown with a standard symbol, and each molecule has a color corresponding to symmetry related positions in the crystal: grey: asymmetric unit; red: -1/2 + x, 3/2 -y, 1 -z; green: 1/2 + x, 3/2 -y, 1 -z; blue: 1 + x, y, z; purple: 3/2 + x, 3/2 -y, 1 -z.