Pyridoxal Hydrochloride Synthesis Essay

Synthesis of fused pyridines in the presence of thiamine hydrochloride as an efficient and reusable catalyst in aqueous conditions

I. R. Siddiqui,*a  Pragati Rai,a  Rahila,a  Anushree Srivastava,a  Arjita Srivastavaa  and  Anjali Srivastavaa 

Abstract

Efficient and straightforward synthesis of fused pyridine derivatives was achieved from electron-rich amino heterocycles and Knoevenagel products derived from aldehyde and malononitrile under aqueous media at 90 °C in the presence of thiamine hydrochloride as a reusable, green catalyst. The strategy in this protocol involves addition on an activated olefinic bond formed in situ by Knoevenagel condensation between an aromatic aldehyde and an active methylene compound. The Michael product on subsequent cyclo condensation yielded fused pyridine in high yield. It offers several advantages such as inexpensive, easily available and recyclable catalyst, simple operational procedure, excellent yield and use of aqueous medium that is considered to be relatively eco-friendly. Vitamin B1 was recovered and reused thrice.

The XRD pattern of the obtained product is shown in Figure 1. Five diffraction peaks are indexed to the digenite Cu1.8S phase (JCPDS card, File No. 47-1748). The absence of peaks corresponding to other phases of copper sulfide, such as CuS, Cu1.75S, Cu1.95S, Cu2S, and materials related to the precursors and copper oxides indicates the purity of the product. The product is crystalline, as reflected by the strong and sharp diffraction peaks. These results implied that the digenite Cu1.8S phase was obtained from thiamine hydrochloride and the copper precursor under hydrothermal conditions.

The SEM images of the sample synthesized from thiamine hydrochloride and the copper precursor under hydrothermal conditions exhibit short rod-like structures as shown in Figure 2. The EDX analysis confirms that the atomic ratio of Cu:S in the sample is about 1.8:1. This is well-consistent with the result of the XRD analysis, and indicates a pure phase of Cu1.8S. Cu1.8S with dendritic structures can be clearly seen in the TEM images (Figure 2d). The size and diameter of the trunk of the dendritic structure are 100–300 nm and 30–50 nm, respectively. An inset of Figure 2d displays the high-resolution TEM image of the tip position of dendrites (main trunk and secondary trunk), and the observed lattice spacing of 0.196 and 0.278 nm match with the (0 1 20) and (1 0 10) planes of Cu1.8S, respectively. It can be concluded from the analysis that the main trunk of a Cu1.8S dendrite grows along the (0 1 20) direction.

To understand the formation mechanism of the Cu1.8S dendrite, we investigated the morphology evolution of Cu1.8S as a function of the hydrothermal process time. Burford et al. reported that the functional groups in biomolecules, e.g., –NH2, –COOH, and –S–, are strongly inclined to interact with inorganic cations based on a mass spectrometry study [13]. This indicates that metal ions could interact with biomolecules to form stable complexes. In this experiment, copper nitrate and thiamine hydrochloride is dissolved in water to form a mixture in which Cu2+ ions coordinate with thiamine hydrochloride to form a complex. When the mixture was sealed and kept at 180 °C under high pressure, the complexes decompose and Cu1.8S nuclei are produced, as described by Equation 1:

To give a detailed description of the complex, we performed density functional theory (DFT) calculations with a cluster model. In this cluster model, two Cu atoms were added to C12H17ClN4OS·HCl to represent possible interactions. The geometry optimization of the cluster was carried out by using the DMol3 package [14]. The Perdew–Burke–Ernzerhof (PBE) functional and double numerical basis set with polarization functions (DNP) were employed [15]. As can be seen in Figure 3a, the two Cu atoms could form two chemical bonds with S, exhibiting a distorted local tetrahedron configuration. The bond lengths of Cu–S are 2.496 and 3.198 Å, respectively, which indicates that the interaction between Cu and S is significant. In particular, the Mayer bond orders of Cu–S bonds are 0.402 and 0.138, which means that the Cu–S bonds exhibit a covalent component. In fact, such an interaction between Cu and S can also be understood from the deformation density, as shown in Figure 3b. The DFT results show that an interaction between Cu and S indeed exists.

Figure 4 shows the morphological changes of the Cu1.8S dendritic structure in dependence on different treatment times. The Cu1.8S nuclei grew into nanoparticles after a reaction time of 1 h under hydrothermal conditions, as shown in Figure 4a. With the reaction time increasing to 2 h and further to 4 h, the nanoparticles self-assembled into rod-like structure (Figure 4b,c). A large number of petiole-like structures were formed and surrounded by small nanoparticles after 8 h of reaction time (Figure 4d). When the reaction time prolonged to 12 h, leaflet morphology was observed (Figure 4e). Longer reaction time (16 h) resulted in Cu1.8S with a dendritic structure, as shown in Figure 3f. After a reaction time of 24 h under hydrothermal conditions, the perfect dendrite was obtained through Ostwald ripening. The secondary and third level dendrite appears and leads to the formation of a dendritic net structure. Most of the product evolved into fully 2D dendritic structure, as shown in Figure 2c.

Li et al. and Liu et al. have discussed the growth process and revealed the mechanism of metal sulfide synthesis by using L-cysteine and L-methionine, respectively [12,17]. They suggested that the growth process of metal sulfide crystals exhibit two stages: an initial nucleating stage and a subsequent growth stage. Metal cations reacted with biomolecules to form a complex, then the coordinate bonds ruptured because of the high reaction temperature. In the present system, thiamine hydrochloride plays a significant role in the synthesis of Cu1.8S dendrite. Firstly, it is an environmental-friendly and cheap sulfur source. Secondly, the functional group (–C–S–C–) in the Cu (thiamine hydrochloride) complexes breaks at 180 °C and releases free S2− ions in water. The Cu2+ ions interact with free S2− ions and produce Cu1.8S nuclei. Then, due to the larger amount of thiamine hydrochloride in comparison with that of copper nitrate, the excessive thiamine hydrochloride in the system probably acts as a structure-directing agent for the self-assembly of the nuclei into dendritic structures. This is consistent with the result that the presence of L-cysteine was in favor of the formation of Cu3BiS3 dendrites [16].

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