..but, I'm sorry to say, these experiments won't tell us the whole story, because it's difficult to compare them. The reason is that these experiments must all be conducted under the same reaction conditions like time, temperature, order and time of addition, equimolarity of catalyst and reactants etc.
But it is good to see you are trying to work on a scientific base, trying to find the optimal catalyst for this kind of reaction. Please keep on with the good work, Hest!
But I've been digging in my literature this weekend and
I've stumbled upon one thing in your tests, you will see later what I mean.
It's primarily the very question of discussing the pathways of the reaction, Rhodium is right when he says both ways of reaction, the one via imine formation and the one via an aldol-like condensation will happen at the same time, both leading to the same product. As already said, I've done some reading the last days because I was curious about this subject. One book of mine states in a single sentence
(which I have overlooked to this very day):
"The simoultaneousness of acid- and base-catalysed reaction steps explains why ammonium salts are so particulary effective as catalysts
(in the Knoevenagel condensation), because these are able to accomplish both functions".
[Translated from:
Uhl/Kyriatsoulis: "Namen - und Schlagwortreaktionen in der Organischen Chemie, Vieweg, 1984; italics mine]
In other words, ammonium salts are more effective than other catalysts because these are able to catalyse the imine pathway as well as the aldol pathway of reaction!
So it appears to me that the stronger bases like 1,3-diaminopropane (pKa
1=10.94, pKa
2=9.03) and cyclohexylamine (pKa=10.66), both are primary amines, give less yields than the not so strong ammonium acetate (pKa=4.75 for acetic acid, ammonia pKa=9.25) in your experiments because the ammonium acetate catalysed both pathways.
(Note, I'm not talking about hardness of bases, just about base strength!Isn't this a nice and simple explanation for your results
?
The question is now, is it more preferable to work with ammonium salts as catalysts because these will assist both reaction pathways? I think the answer is yes, when I look at your yields.
But here comes the next question then, is it more preferable to work with the salt of a primary amine, or a secondary or tertiary amine?
I remember the famous EDDA is very often used in Knoevenagel condensations because it's weakly basic, showing an almost neutral pH in water, but if I remember right EDDA wasn't much superior to plain ammonium acetate in our special kind of Knoevenagel condensations.
Another question, can all reactants, including catalyst, simply be dumped together in a reaction flask or will some order of addition or pre-reaction of catalyst with one reactant (aldehyde or nitrocompound) have a great impact on the yield?
Questions, always questions...