Chemists from Russia studied the hydrogen borrowing reaction necessary for the formation of carbon-nitrogen bonds. They found that it often does not require a catalyst at all, and also developed recommendations for successfully carrying out the reaction in the laboratory. The study was published in the journal Journal of Catalysis.
Organic compounds containing an NH amino group in the molecular structure2 can react with alcohols to form a carbon-nitrogen bond and eliminate water. This process is called the hydrogen borrowing reaction because it occurs through the oxidation and reduction of reactants with the formal transfer of a hydrogen molecule. One of the possible mechanisms of this reaction is as follows: first, the alcohol gives up a hydrogen molecule to a carrier – a catalyst or, for example, an oxygen molecule, and itself turns into an aldehyde. The amine then reacts with the aldehyde with the assistance of a base to form a carbon-nitrogen double bond, and then the double bond is reduced to a single bond by the hydrogen bonded to the carrier. The result is a substituted amine.
Most often, to carry out the hydrogen borrowing reaction, organometallic catalysts based on expensive metals are used, which act as hydrogen carriers. But there are cases when this reaction occurs on its own in the presence of oxygen and a base. Moreover, chemists cannot predict whether an organometallic catalyst is needed to carry out this reaction between specific substances, and they have to conduct experiments at random.
Chemists led by Denis Chusov (D. A. Chusov) from the effective catalysis group at the Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences decided to tackle this problem. They suggested that in many cases an organometallic catalyst is not needed for this reaction. And to test their hypothesis, the scientists optimized the conditions for the hydrogen-borrowing reaction between aniline, the simplest aromatic amine, and benzyl alcohol. It turned out that the reaction between them easily occurs without an organometallic catalyst at a temperature of 90 degrees Celsius, and the product is obtained with a yield of 89 percent. At the same time, the choice of solvent for the reaction turned out to be important – in non-polar solvents, for example, chlorobenzene, the reaction proceeded much better than in polar ones, for example, methanol.
Next, the scientists examined how the choice of amine affected the reaction. It turned out that the higher the acidity of the amine, the worse it reacts, and the more heat is required to complete the process. For example, the reaction of tosylamide (it is much “more acidic” than aniline) with benzyl alcohol had to be carried out at 160 degrees Celsius, and its yield was 88 percent.
Chemists then tried hydrogen-borrowing reactions with aliphatic amines, in which the nitrogen atoms are linked not to an aromatic benzene ring, but to a chain of carbon atoms. As a result, even with high heating, these reactions did not take place, and scientists concluded that they require catalysts. The same turned out to be true for secondary amines, which have not one carbon substituent at the nitrogen atom, but two.
As a result, chemists were able to figure out when a hydrogen-borrowing reaction should be carried out simply in the presence of a base, and when it was necessary to do so without an organometallic catalyst. “A clear understanding of this border is very important,” says one of the authors of the article, Oleg Afanasyev. “Many scientific groups around the world are actively developing hundreds of new organometallic catalysts and using them, including in this reaction. Our work shows in which cases such developments are justified and in which cases they are a waste of resources. For example, to carry out a hydrogen borrowing reaction at room temperature, a catalyst is always needed. And at 100 degrees, aniline will react with benzyl alcohol without a catalyst. And you don’t need to add anything there.”
Another process that, as it turns out, does not always require a catalyst is the hydrogen transfer reaction. You can read about how chemists figured this out in our material “The Darkness of Catalysis.”