Hi Typpihappo, that's a really interesting find and sort of confirms the way I was thinking. I started writing done my thoughts and experiments so I
have added what I have done to date. I will dig out the paper on the asymmetric 3-acetyl-collidine and see what they did differently. I am not at home
at present so I can't do any further experiments for a while. I wonder what colour those rare earth amino-pentanone complexes are and do they
fluoresce?
A preliminary investigation into the condensation of acetylacetone with itself and
other carbonyls in the presence of ammonia
Boffis February 2022
Following the appearance of Typpihappo’s thread on his attempts to prepare lanthanum acetylacetonate and his rather unusual yellow-green fluorescent
product I became intrigued by the reaction and the more I read into it the more interesting it looks. As discussed above I originally thought that the
fluorescent compound indicates that a Hantzsch type 1,4-dihydropyridine derivative has formed but although a little bit of googling soon found many
references to this type of reaction as a preparative method for substituted 1,4-dihydropyridines, a method for measuring ammonia or primary amines5)
or a method for detecting carbonyl compounds one reference made it clear that this is not strictly a Hantzsch type react1). In a Hantzsch type
pyridine reaction three molecules provide carbon atoms to the pyridine ring in the ratio 2-1-2. I found that this reaction has been studied before2)
and that the product of this reaction is 3-acetyl-collidine (3-acetyl-2,4,6-trimethylpyridine) and the reaction is 3-2, that is only two molecules
provide carbons to the pyridine ring both molecules are acetylacetone but they react asymmetrically. Further more when the third molecule is involved
and is a ketone, the reaction is of “Hantzsch type” and the resulting molecule is doubly substitute in the 4 position. The products of this type
of reaction tend to become aromatic by loosing one of the alkyl moieties in the 4 position in an alkaline environment3). A methyl group is never lost
so in our case the part that would be lost is the other group in the 4 position ie. Acetone. Theoretically, the product that results would be
identical to the condensation product or 2 moles of acetylacetone and one mole each of acetaldehyde and ammonia.
There are also numerous papers describing the preparation of acetylacetonate complexes; of particular interest here are the uranyl acetylacetonate
ammonia complexes in which it has been shown that the ammonia is not coordinate with the uranyl centre but becomes combined with a molecule of the
acetylacetone to form beta-ketoamine. This is part way to the Hantzsch type condensation above and the paper describes the uranyl complex as
undergoing hydrolysis when a solution is stored at 25°C precipitating UO3.2H2O. Presumably the ketoamine and the additional acetylacetone ligand are
liberated into the aqueous phase at this point. It is tempting to speculate that lanthanum salts can perhaps catalyse the formation of a
dihydropyridine derivative rather than the collidine derivative. Dornow & Schacht2) used 4 to 6 days for their preparation 3-acetyl-collidine this
reaction appears slow under room temperature conditions so perhaps it doesn’t take much to accelerate a competing reaction.
The experiments described below were undertaken well before I had done any reading up on these reactions and with the benefit of hindsight I could
have done a better job.
Experimental
A series of four small-scale experiments were carried out to investigate the conditions under which reactions occur and the formation of products that
might be useful to me.
Experiment 1
5.009g of GPR acetylacetone was diluted with 10ml of 95% ethanol and 1.2ml of 28% ammonia solution added, a faint yellow colour soon appeared. The pH
was adjusted to about 5 with 5M acetic acid and the mixture warmed gently to about 40-45°C. A pale yellow ppt began to form almost immediately the
ammonia was added but on acidification and warm it disappears again giving a pale orange solution. On cooling to room temperature (~8°C) and standing
overnight no ppt formed so the solution was partly evaporated to about 10ml and 10ml of water added. No crystals formed only a pale orange brown oil
that smelled in part of acetylacetone. The reaction failed to give anything resembling the desired product though the liquid 3-acetyl-collidine is a
possibility.
Thing that the issue was the acidification too early I repeated the reaction using water in place of 95% ethanol and waited 36 hours before lowering
the pH with acetic acid.
Experiment 2
5.012g of acetylacetone were mixed with 15ml of water and 2.5ml of 28% ammonia solution. The mixture was warmed gently and stirred vigorously until
all had dissolved and then left to stand for 36 hours at room temperature (~10°C). No ppt formed some the mixture was warmed to 35°C and acidified
to pH 5 with 28% hydrochloric acid. A ppt appeared to form but it was soon apparent that it was a liquid separating that again smelled mainly of
acetylacetone. No solid could be isolated.
Experiment 3 Reaction of acetylacetone with formaldehyde and ammonia
5.015g of acetylacetone was diluted with 10ml of 95% ethanol, 2ml of 37-38% formaldehyde solution were added followed by 1.2ml of 28% ammonia solution
and the solution warm gently to 30-35°C. The addition of the ammonia caused a yellow colour to develop which darkened slightly on warming but no ppt
formed even after several hours so the pH was lower to about 6 with 80% acetic acid and the mixture warmed again to about 30°C for 30 minutes during
which time a deep yellow precipitate formed. After cooling to room temperature, the mixture was vacuum filtered and the deep yellow cake sucked dry
but not washed. After air drying at 35-40°C for 24 hours the product weighed 2.703g of crude or a yield of 56% of theory of
2,6-dimethyl-3,5-diacetyl-1,4-dihydropyridine.
Experiments revealed that methanol was the best solvent for recrystallisation (16ml per gram) as it gave the coarser, less fibrous crystals.
Isopropanol may also be used (24ml per gram) but the crystals are rather wool-like. The recrystallised product is brilliantly fluorescent yellow green
like Typpihappo’s but should be a quite different compound.
Following the success of experiment 3, I decided to see if an aromatic aldehyde would react in a similar way to formaldehyde. It is reported that
acetaldehyde and benzaldehyde react similarly to formaldehyde in this classic Hantzsch synthesis. I am interested in compound with hydroxy group in
proximity to acetyl groups so I thought I would try the reaction with salicylaldehyde.
Experiment 4
To 5.007g of acetylacetone diluted with 6ml of 95% ethanol were added 3.054g of salicylaldehyde. With vigorous stirring and gentle warming 1.5ml of
33% ammonia solution were added. An immediate ppt forms that quickly dissolves again to give an orange solution. The solution was kept at 40-45°C for
20 minutes and then allowed to stand overnight. No precipitate formed after 36 hours so the mixture was warmed to 30-35°C and acidified with 28%
hydrochloric acid to pH 3-4. After about 5 minutes a ppt started to form but by the following morning had disappeared and nothing crystalline could be
recovered only a dark brown oil that consisted, at least partly, of unreacted salicylaldehyde.
Discussion
Much of the reading I have done into this area of chemistry was done after I had carried out my initial experiments and, as a result, I can now see
where I went wrong in many cases. It is quite likely that experiments 1 and 2 would eventually have yielded 3-acetyl-collidine of Dornow & Schacht
but I simply didn’t let the reaction run for long enough before buffering them. Likewise experiment 4 might also work if left for long enough.
I could not prepare the yellow compound of Typpihappo but there again I didn’t use exactly his method or the addition of a lanthanum salt. I intend
to repeat his experiment exactly as he reported it when time permits, essentially, he used a solvent free reaction of AcAc and concentrated ammonia
solution to give an initial white solid product and then added the buffered lanthanum salt solution. This route raises some interesting issues:
1. Why did Dornow & Schacht2) not obtain the yellow solid that Typpihappo obtained; I am sure if it had been present they would have reported it.
They used ammonium acetate as the source of ammonia and water bath temperatures for 3 to 5 days to achieve their 75% yield, so hot, dry, prolonged
neutral reaction conditions.
2. Is the initial solid formed by adding ammonia to acetylacetone the ammonium acetylacetonate salt or a reaction product such as a β-ketoimine?
3. Is lanthanum essential for the reaction or just a spectator? In the latter case the important function of the salt solution is its buffering
action, my initial experiments would seem to suggest that this latter conjecture is not the case and therefore, that the lanthanum salt performs some
catalytic function.
4. If lanthanum is essential for the reaction does it function as a catalyst or does it, say, precipitate the reaction product as a different type of
complex (ie not an La(AcAc)3 complex) and this drives an otherwise unfavourably slow reaction forward in preference to the 3-2 type reaction. This
then raises the question:
5. Is lanthanum a component of the yellow compound? With 3 acetyl groups in close proximity it is possible that the compound would be a reasonably
good ligand for a RE metal known for preferring oxygen containing ligands.
6. According to Guareschi the 4,4-disubstituted lose one of the alkyl groups as a hydrocarbon when boiled with ammonia or magnesium hydroxide (weakly
alkaline conditions). However, if Typpihappo’s compound had lost acetone to yield a 2,4,6-trimethyl-3,5-diacetylpyridine would this compound still
be fluorescent? The 1,4-dihydro-compound is fluorescent but less intensely and in a darker shade of green than the equivalent compound lacking the
4-methyl group5) but I have not found a reference to the fluorescence of the aromatic pyridine derivatives.
1) Chemistry of Heterocyclic compounds v14 Pyridine and its derivatives Ed. Klingman
2) Dornow & Schacht, Chem. Berichte., is6 p502 (1947)
3) I. Guareschi & C. Mensio, Mem. Real. Acad. Sci. Torino; ii v50, p235-288 (1901) see Chem. Abstracts, 1901, I, p341
4) J. M. Haigh and D. A. Thornton, Inorg.Nucl.Chem. v33, p1787 (1971)
5) A. E. Comyns, B. M. Gatehouse and E. Wait, J.Chem.Soc. p4655 (1958).
