Reduction of nitriles to aldehydes
Synthesis of aldehydes from nitriles is a process of great practical importance. To implement it, you can use different methods. However, to the strength of its simplicity and economy, the most promising is a method based on the reaction of catalytic conversion of nitriles.
If the process is carried out in water under mild conditions, i.e. at low temperatures and pressures, in the presence of low-activity catalysts based on base metals, it is possible to hydrolyze the aldimines formed in the first stage of the reaction to aldehydes [5, 6, 30, 32]:
H2 H2О
R-C≡N → RCH=NH →RCHО+NH3
However, undesirable side reactions are inherent in this process: part of the aldimine is reduced to amines that react with the aldehyde to form schiff bases and secondary amines. To avoid this, different authors suggest slightly different methods of catalytic conversion of nitriles to aldehydes, but their essence boils down to one - to strengthen the hydrolysis of the aldimine formed during hydrogenation and to create conditions that prevent its further hydrogenation. In a number of studies this problem is solved by introducing into the reaction zone various kinds of compounds capable of interacting with aldimines or aldehydes.
Italian researchers Pietra and Trinchera developed a method for obtaining aldehydes in the presence of hydrazine:
H2,Ni
2RC≡N+3NH2- NH2 → RCH=N- NH2+ N2+2 NH3
RCH=N- NH2+H20 → RCHO+ NH2- NH2
In particular, from benzonitrile, benzaldehyde was obtained in a yield of 87%. However, this method was not widely used for the reason that in the presence of skeletal nickel, especially its large amounts, hydrazine is destroyed.
Another method is described in the works of Plininger [5, 6, 30, 34] and Felder. The authors have shown that aromatic, substituted heterocyclic nitriles at a temperature of 60-100 °, a hydrogen pressure of 1-100 kgf / cm2 in the presence of Group VIII and semicarbazide catalysts during the reaction smoothly turn into semicarbazones, which are converted to aldehydes by hydrolysis.
|
|
|
The question of the possibility of obtaining pyridinaldehydes and aromatic aldehydes from the corresponding nitriles is considered in detail in Geyf's studies [5, 6, 30]. Believing that the compound of the first mole of hydrogen with the formation of aldimine occurs much faster than the further reduction of the imine to the amine, the author believes that the yield of the aldehyde should depend on the amount of phenylhydrazine introduced. Indeed, with an increase in the amount of phenylhydrazine from 1 mole to 4 moles per one mole of nitrile input, the aldehyde yield increases from 50 to 90%. The following reaction mechanism is proposed:
RC≡N + H2 → RCH = NH
RCH = NH+ H2N – NH-Ph ↔RCH=NNHPh + NH3
RCH=NNHPh → RCHO + PhNH - NH2
RCH= NH + H2 → RCH2 NH2
The acceptor of imine (phenylhydrazine) - prevents its further recovery. Direct hydrolysis of imine to aldehyde, according to the author, is fraught with the danger of possible amide formation. Geyf showed that the reduction of nitriles to aldehydes with the participation of phenylhydrazine is most easily carried out in the presence of a skeleton nickel catalyst [5]. Palladium, platinum and rhodium are not capable of quantitatively carrying out this reaction.
The author pays much attention to the separation of pyridinealdehydes from catalysts, since these compounds are less stable due to their high reactivity in comparison with aldehydes of other classes. It is concluded that to bind aldimines, a more suitable reagent is semicarbazide, since semicarbazones, in comparison with phenylhydrazones, more easily form crystalline precipitates and are more hydrolyzed more smoothly.
To create a soft condition for the reduction and prevention of the hydrogenation of aldimine to the amine, several other catalytic systems have been proposed in the works. Their essence lies in the fact that the reduction of nitriles is performed on nickel with a skeleton in the presence of compounds capable of acting as hydrogen donors. One such compound is sodium hypophosphite, in the decomposition of which water liberates hydrogen, using this compound it was possible to obtain from benzonitrile benzaldehyde in up to 85% yield, and 2-cyanopyridine the corresponding aldehyde in 70% yield. Aldehydes were isolated in the form of hydrazones. In the case where the starting nitriles had substituents, the yield of aldehydes dropped sharply. Thus, toluunitrile was converted to o-toluic aldehyde by no more than 10%, which is related, in the opinion of the authors, to the steric effect.
|
|
|
Very interesting was a system of equal weight amounts of nitrile, wet skeletal nickel and formic acid [6]. The latter serves as a source of hydrogen. The reaction is carried out at a temperature of 70-80 °. Under these conditions, most of the aromatic nitriles studied were converted to aldehydes in 75% yield. From 3-cyanopyridine, 3-pyridinealdehyde was obtained in an amount of 50% based on the reacted nitrile.
As follows from the patent [34], the reduction of cyanopyridines with formic acid can also be carried out in the gas phase. For this, a mixture of nitrile, formic acid and water vapor is passed at a temperature of 250-650 ° C over a catalyst consisting of various oxides, in particular Th2 / Al2O3 (10% Th).
As a hydrogen donor, formaldehyde is sometimes used. For example, Hungarian researchers [5] obtained aromatic aldehydes with a good yield when hydrogenating nitriles on a Nickel-Rhenium catalyst in a mixture of formaldehyde and formic acid.
The hydrolysis of aldimine to the aldehyde, as mentioned above, can be enhanced by adjusting the pH medium, namely by acidifying the solution. In this case, obviously, the rate of hydrolysis of the imine into the aldehyde becomes higher than the rate of its hydrogenation into the amine and, consequently, the reaction must shift towards formation of the aldehyde. It is this property of the nitrile group, especially for the nitriles of the pyridine series, used to obtain aldehydes in the next series of works [6].
According to the US patent [30], 3-pyridinealdehyde is prepared from 3-cyano pyridine by reduction of the latter in an acidic medium at 45 ° and a hydrogen pressure of more than 1 atm on a skeletal nickel catalyst. The aldehyde yield reached 70%. The disadvantage of the process is the high consumption of the skeleton nickel catalyst.
|
|
|
The nickel catalyst was also used in the work of Tinap [5, 6], who succeeded in obtaining in quantitative yield in the solution of strong sulfuric acid a large number of aromatic aldehydes.
Shigei and co-workers [26] also used skeleton nickel and cobalt catalysts. But to increase the yield of aldehydes, they recommend adding an organic base (pyridine) or tin chloride to the acidic solvent (phosphoric or glacial acetic acid) without explaining the nature of the effect of these additives.
Very interesting data on the preparation of pyridinaldehydes of different structures from the corresponding cyanopyridines are given by American researchers. The method proposed by the authors is industrial. In the work, it was possible to achieve high yields of pyridinealdehydes from 2-, 3- and 4-monocyanopyridines by hydrogenating them in the presence of platinum and palladium catalysts in the form of blacks or on carriers in an acidic medium at reduced hydrogen pressures. 2-Pyridinaldehyde was obtained in a yield of 58-70%, 4-pyridinealdehyde-61% and 3-pyridinealdehyde - in a yield of up to 80%.
It is necessary to dwell on studies on the production of pyridinealdehydes, in particular, 3-pyridinealdehyde, which were carried out by German researchers. The method originally proposed for the conversion of 3-cyanopyridine to skeletal nickel or nickel on aluminum oxide catalysts at a pressure of 100-150 atm of hydrogen in a solution of ≈5N. sulfuric acid was further improved. Instead of sulfuric acid, which causes the dissolution of nickel and the corrosion of autoclaves, solid carbon dioxide is used. Under these conditions, nitriles of nicotinic and isonicotinic acids at room temperature for 6 to 8 hours are converted to the corresponding pyridinealdehydes by 50-70%.
In some cases, the reaction does not stop at the aldehyde production stage and the process continues until the aldehyde is reduced to alcohol. According to the data of the patent, when 3-cyanopyridine is hydrogenated in a hydrochloric acid aqueous solution (pH = 2), 3-pyridinecarbinol is formed on 5% palladium-carbon at atmospheric pressure H2 and temperature 30 ° instead of 3-pyridinealdehyde; the aldehyde has time to peroxide to alcohol. Modernization of this method made it possible to use it for the synthesis of 3-pyridinecarbinol, which is used to produce medical preparations. The yield of the desired product reaches 90%.
|
|
|
Pyridinecarbinols can also be obtained by hydrogenating the nitriles of pyridinecarboxylic acids in the presence of ion exchange resins of an acidic nature. It is believed that in this case the ammonia released during the formation of the aldehyde group from nitrile is absorbed by the resin, and the aldehyde is reduced to alcohol. Thus, for example, in the hydrogenation of 3-cyanopyridine on Raney nickel in the presence of an ion exchange resin, 3-pyridinecarbinol was obtained with a yield of about 26%. At the same time, 44% of the initial nitrile was converted to 3-aminomethylpyridine [5, 6, 30].
Ketimine is another product that can be obtained by reducing nitriles to aldehydes. With a strict amount of water, the aldehyde is able to react with amines, forming ketimines.
And, finally, it should be noted that under severe conditions the possibility of restoring the formed aldehyde group to the alkyl one is not included. Thus, for the vapor-phase hydrogenation of aromatic nitriles at 250-300 ° C, the formation of aromatic hydrocarbons takes place over catalysts from copper and nickel oxides on kieselguhr. Similar results were obtained for liquid-phase processes. Conversion of aromatic and heterocyclic nitriles to the corresponding alkyl derivatives was carried out in high yield in boiling n-cymene in the presence of a palladium-on-charcoal catalyst.
Experimental part
2.1.Solvents and reagents:
| 1. | Ethanol, 'х.ч.' |
| 2. | Acetonitrile, 'х.ч. |
| 3. | Sodium hydroxide, 'х.ч.' |
| 4. | Hydrogen from a cylinder; |
| 5. | Ammonia NH3 from a cylinder; |
| 6. | Nickel - niobium - aluminum alloy. |
Catalyst Preparation
The initial nickel-aluminum alloy is crushed, sieved and a fraction with a particle diameter of 0.2-0.5 mm is selected. A skeleton nickel catalyst is obtained by leaching aluminum from a nickel-aluminum alloy with an alkali solution. The catalyst leaching reaction is expressed by equation:
2Al + 2NaOH + 2H2O → 2NaAlO2 + 3H2↑ (13)
The catalyst is weighed into a Kyeldal's flask, treated with a 20% aqueous solution of sodium hydroxide (based on 1 g of the alloy, 80 ml of 20% NaOH). After slowing down the violent reaction, the flask with the catalyst is placed in a boiling water bath and held for two hours, after which it is left for one day at room temperature. The leached catalyst is washed with distilled water until neutral (phenolphthalein) and stored under conditions that exclude contact with air. Before the experiment, the catalyst is washed twice with a solvent. Weigh it on a hydrostatic analytical balance under a solvent. It must be remembered that the surface of the catalyst should not come in contact with air to avoid oxidation [5, 6, 17, 26]. Skeletal nickel catalyst - pyrophoric !
Дата добавления: 2018-06-27; просмотров: 376; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!