Borrowing hydrogen catalysis, also called hydrogen autotransfer, is an important catalytic concept. "Borrowing hydrogen" can be seen as an example of Green chemistry. It is based on the intermediate oxidation of an alcohol substrate to the corresponding aldehyde by the catalyst (which "borrows" hydrogen from the substrate); the intermediate aldehyde then reacts with e.g. a secondary amine in a condensation reaction to produce e.g. an imine, that is then reduced by the catalyst in the final step to yield e.g. a tertiary amine. The method is highly atom economic, because is circumvents the activation of the alcohol (which is a poor electrophile itself, see tosylation or Swern oxidation).
The method is not limited to the preparation of amines, it can also be used to form Carbon–carbon bonds (C-C bonds).
Alcohols can be temporarily converted into carbonyl compounds by the metal-catalysed removal of hydrogen. The carbonyl compounds are reactive in a wider range of transformations than the precursor alcohols and can react in situ to give imines, alkenes, and α-functionalised carbonyl compounds. The metal catalyst, which had borrowed the hydrogen, then returns it to the transformed carbonyl compound, leading to an overall process in which alcohols can be converted into amines, compounds containing C-C bonds and β-functionalised alcohols.
Microwave heating enables a Borrowing Hydrogen strategy to form C-N bonds from alcohols and amines, removes the need for solvent and reduces the reaction times, while the results are comparable with those using thermal heating.
Nitroaromatics can also give this reaction, reducing nitro to amine and imine giving secondary amines.