Piperidines from pyridine

Pyridine.—Ahrens1 accomplished the electrolytic reduction of pyridine and the derivatives of pyridine, and obtained piperidine from pyridine, and a-pipecoline from a-picoline. In these electrolyses lead cathodes and 10% solutions of sulphuric acid were employed. 

Heterocyclic Compounds. - The largest electrochemical reduction in this category is the production of piperidine from pyridine by Robinson Brothers. The 120 tonne p.a. produced by this method is supplementary to the bulk of the production which is produced by catalytic hydrogenation of pyridine. The electrochemically produced piperidine is believed to contain fewer by-products and can be produced from pyridine which contains sulphur. 

Piperidines from pyridines s. 2, 71 1,2,3,4-tetrahydroquinolines from quinolines or 1,2-dihydroquinolines s. S.W. Goldstein, PJ. Dambek, Synthesis 1989, 221-2. 

The preparations of over two hundred tetrahydro- and octahydro-pyrido[4,3-d]pyrimidines from piperidines or from purely aliphatic starting materials are described in the patent literature. Fully aromatic examples of the system have been prepared from pyridines and pyrimidines. 

The position of aniline in the above reactivity order deserves special comment. Aniline is less basic than pyridine by a relatively small factor, 0.65 pA units, but is appreciably more polarizable it then seems likely that the inverted order of reactivity is caused by the polarizability term in accordance with Edwards equation. If this is correct, in the reactivity order piperidine > aniline > pyridine, inversion with respect to basicity appears to result from an abnormally high reactivity of aniline rather than from a particularly low reactivity of pyridine. This view differs from that based on relative steric requirements of the reagents, but other factors besides basicity and polarizability may well contribute to the quantitative experimental picture. 

From the pATa values shown, there is relatively little difference in basicities for diethylamine, pyrrolidine, or piperidine. Note, however, that morpholine and piperazine are weaker bases than piperidine. This is the result of an electron-withdrawing inductive effect from the second heteroatom, making the nitrogen atom both less basic and also less nucleophilic. This makes morpholine a useful base with basicity between that of piperidine and pyridine pK 5.2) (see Section 4.6). The second pK value for the diamine piperazine is substantially lower than the first, since the inductive effect from the protonated amine will withdraw electrons away from the unprotonated amine (see Section 4.7). 

Tables I through VIII summarize the occurrences of alkaloids from ants and other insects. Each table presents chemical structures as well as specific sources of particular types of alkaloids e.g.. Table I covers piperidines and pyridines. Table II, pyrrolidines, pyrroles, and indolizidines.

Piperidine derivatives and acyclic substances such as trihalogeno-alkanes and dihalogenoalkylamines may be used as starting materials. The piperidine derivatives are obtained mainly from pyridine compounds. Dihalogenoalkylamines and trihalogenoalkanes are prepared from tetrahydropyran derivatives, dialkoxy-substituted malonic esters, or alkane-tetracarboxylic esters. 

The study of pyridine HDN indicates that the hydrogenation of pyridine to piperidine is of first order with respect to H2 at 250 °C and 1.5 at 300-375 °C and of first order with respect to the pyridine partial pressure. The strong adsorption of NH3 proposed by Mcllvried220 was not observed and the deviation from the first-order rate is explained because the reverse reaction of piperidine to pyridine, thermodynamically favoured at 315°C, was neglected. The order in H2 of the ring opening was found to be near zero. 

The formation of piperidines from the borohydride reduction of pyridine and picoline methiodides has been reported by Ferles.17 He assumed that this complete reduction arose from initial formation of the 1,4-dihydropyridine, and that the ratio of tetrahydropyridine to piperidine represented the ratio of attack of hydride at the 2-position to that at the 4-position. 

The pyridine ring is easily reducible. Hydropyridines are formed from pyridine and its derivatives, and piperidines by complete hydration. Quinoline and acridine are also easily converted into hydro-compounds. 

Preparation of secondary (or tertiary) carbinols from pyridines and an aldehyde (or a ketone) in the presence of magnesium or aluminum and mercuric chloride is known in pyridine chemistry as the Emmert reaction. 7 70 For example, dimethyl-2-pyridylcarbinol is obtained in this way from pyridine and acetone. When a mixture of pyridine and acetone is subjected to an electrolytic reduction in dilute sulfuric acid at lead electrodes, a mixture of two main products results, namely, 2-(2-hydroxy-2-propyl)-3-piperideine and 4-(2-hydroxy-2-propyl)piperidine. Analogous compounds are obtained with the use of methyl ethyl ketone as the reactant. The mixed electrolytic reduction of 2-methylpyridine and acetone affords 2-(2-hydroxy-2-propyl)-6-methyl-3-piper ideine (74) and 2-methyl-4-(2-hydroxy-2-propyl)-piperidine.71... 

Not only has binding of imidazoles and pyridines to Fe protoporphyrin IX been studied, as discussed in Section 4.1.2, but also photodissociation of axial ligands such as pyridines, imidazoles, or piperidines from six-coordinate, low-spin Fe porphyrins, in which the porphyrin is derived from protoporphyrin IX, or proto- or deuteroporphyrin IX dimethyl ester, has been investigated in nonaqueous solvents using picosecond transient absorption spectroscopy (see Photochemistry of Transition Metal Complexes). It has been shown that photodissociation leads to the formation of five-coordinate complexes, that is, only one ligand appears to be released upon excitation of the six-coordinate complex. ... 

Miscellaneous piperidines and pyridines from plant sources... 

Ladenburg s preparation of piperidine from pyridine14 will be described as an example Pyridine (20 g) in anhydrous ethanol (150 g) is placed in a round-bottomed flask under a reflux condenser and warmed on the water-bath. Then sodium pieces (75 g) are added not too slowly. As soon as the reaction slackens or sodium ethoxide separates, more ethanol is added and the reaction is brought to conclusion as fast as possible. When all the sodium has been consumed, the mixture is allowed to cool, treated with an equal volume of water, and distilled in steam. The distillate is neutralized with hydrochloric acid and evaporated to dryness. Piperidine hydrochloride is obtained as residue in almost quantitative yield and can be recrystallized from ethanol. 

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