1,4-dihydropyridine is achieved in the presence of organometallic complexes of magnesium and zinc, and (Δ3,4)-tetrahydropyridine is obtained by electrochemical reduction of pyridine. Birch reduction converts pyridine to dihydropyridines. Lewis basicity and coordination compounds Pyridine is a Lewis base, donating its pair of electrons to a Lewis acid. Its Lewis base properties are discussed in the ECW model. Its relative donor strength toward a series of acids, versus other Lewis bases, can be illustrated by C-B plots. The plots shown in this paper used older parameters. Improved E&C parameters are listed in ECW model. One example is the sulfur trioxide pyridine complex (melting point 175 °C), which is a sulfation agent used to convert alcohols to sulfate esters. Pyridine-borane (, melting point 10–11 °C) is a mild reducing agent. Transition metal pyridine complexes are numerous. Typical octahedral complexes have the stoichiometry and . Octahedral homoleptic complexes of the type are rare or tend to dissociate pyridine. Numerous square planar complexes are known, such as Crabtree's catalyst. The pyridine ligand replaced during the reaction is restored after its completion. The η6 coordination mode, as occurs in η6 benzene complexes, is observed only in sterically encumbered derivatives that block the nitrogen center. Applications Pesticides and pharmaceuticals The main use of pyridine is as a precursor to the herbicides paraquat and diquat. The first synthesis step of insecticide chlorpyrifos consists of the chlorination of pyridine. Pyridine is also the starting compound for the preparation of pyrithione-based fungicides. Cetylpyridinium and laurylpyridinium, which can be produced from pyridine with a Zincke reaction, are used as antiseptic in oral and dental care products. Pyridine is easily attacked by alkylating agents to give N-alkylpyridinium salts. One example is cetylpyridinium chloride. It is also used in the textile industry to improve network capacity of cotton. Laboratory use Pyridine is used as a polar, basic, low-reactive solvent, for example in Knoevenagel condensations. It is especially suitable for the dehalogenation, where it acts as the base for the elimination reaction. In esterifications and acylations, pyridine activates the carboxylic acid chlorides and anhydrides. Even more active in these reactions are the derivatives 4-dimethylaminopyridine (DMAP) and 4-(1-pyrrolidinyl) pyridine. Pyridine is also used as a base in some condensation reactions. Reagents As a base, pyridine can be used as the Karl Fischer reagent, but it is usually replaced by alternatives with a more pleasant odor, such as imidazole. Pyridinium chlorochromate, pyridinium dichromate, and the