Pyridines s. a. Piperidines

N-sulfonylimines and 1,3-dienes 31, 623 Pyridines (s. a. Piperidines) dimerization, reductive 31, 295... 

Pyridine, like benzene, is an aromatic system with six jt electrons (see Section 11.3). The ring is planar, and the lone pair is held in an sp orbital. The increased s character of this orbital, compared with the sp orbital in piperidine, means that the lone pair electrons are held closer to the nitrogen and, consequently, are less available for protonation. This hybridization effect explains the lower basicity of pyridine compared with piperidine. Pyrrole is also aromatic, but there is a significant difference, in that both of the lone pair electrons are contributing to the six-jr-electron system. As part of the delocalized Jt electron system, the lone pairs are consequently not available for bonding to... 

Physiology biosynthesis Like the tropane alkaloids, T. a. are formed in the roots and transported to the aboveground parts for storage by the plant s phloem system. In some sorts of tobacco plants a part of the nicotine is demethylated to nomicotine during transport to the shoot. Nomicotine and anabasine are often the main alkaloids in the so-called nicotine-poor tobacco plant types. The T. a. are formed biogenetically from nicotinic acid, made available via the pyridine nucleotide cycle (see nicotinamide), and a pyrrolidine or piperidine building block (figure). In the case of nicotine, like for the tropane alkaloids, Al-methylpyr-roline is an intermediate, in the biosynthesis of anabasine the intermediate is a piperidine derived from the amino acid lysine (see piperidine alkaloids). 

W(CO)5S] (S = a variety of solvents) is formed more rapidly from [W(CO)sL] (L = pyridine or piperidine) than from and this is associated with... 

N-Subst. pyrrolidines. A 10 M soln. of BH3-dimethyl sulfide in THF added dropwise via syringe to a soln. of startg. succinamic ester in the same solvent under N2, the mixture refluxed 7-9 h, and quenched with methanol and 2 N NaOH N-phenylpyr-rolidine. Y 82%. The method failed for highly hindered amines. F.e. incl. N-subst. piperidines s. M.C. Venuti, O. Ort, Synthesis 1988, 985-8 review of pyrrole, pyrrolidine, piperidine, pyridine and azepine alkaloids (1986-7 literature) s. A.R. Pinder, Nat. Prod. Reports 6, 67-78 (1989). 

The relative production volumes of pyridine compounds can be ranked in the following order pyridine (1) > P-picoline (3) > a-picoline (2)> niacin (27) or niacinamide (26)> 2-vinylpyridine (23)> piperidine (18). U.S. and Japanese production was consumed internally as well as being exported, mainly to Europe. European production is mosdy consumed internally. Growth in production of total pyridine bases is expected to be small through the year 2000. 

Blanco et al. (1994) reported measurements of the vapor pressure (P,at) for p-xylene, y-picoline, piperidine, pyridine and tetralin. The data for piperidine and pyridine are given in Table 3.4. A suitable equation to correlate these data is Antoine s relationship given next... 

Fig. 4.1. Potential ranges of solvents, (a) h.n.p.s of acids. I, Acetic acid II, benzoic acid III, formic acid IV, salicylic acid V, sulphuric acid VI, p-toluenesulphonic acid, (b) h.n.p.s of conjugate acids of I, n-butylamine II, piperidine III, ethylenediamine (1) IV, ammonia V, ethylenediamine (2) VI, pyridine.

Pyridine (and quinoline) which in so many respects are aromatic and comparable to benzene, lose this character completely on hydrogenation to piperidine (and hydroquinoline), which are entirely of the same nature as secondary aliphatic amines. The completely hydrogenated heterocyclic bases undergo degradation reactions which have become important particularly in the investigation of the constitution of alkaloids. A. W. Hofmann s method of opening rings by means of exhaustive methylation may be illustrated with piperidine. By thermal decomposition of the quaternary ammonium base a C—N-link-age is broken and at the same time water is eliminated. 

The term Knoevenagel reaction however is used also for analogous reactions of aldehydes and ketones with various types of CH-acidic methylene compounds. The reaction belongs to a class of carbonyl reactions, that are related to the aldol reaction. The mechanism is formulated by analogy to the latter. The initial step is the deprotonation of the CH-acidic methylene compound 2. Organic bases like amines can be used for this purpose a catalytic amount of amine usually suffices. A common procedure, that uses pyridine as base as well as solvent, together with a catalytic amount of piperidine, is called the Doebner modification s of the Knoevenagel reaction. 

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