Borrowing hydrogen catalysis is an important catalytic concept that 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

Imine

An imine is a functional group or chemical compound containing a carbon–nitrogen double bond, with the nitrogen attached to a hydrogen atom or an organic group. If this group is not a hydrogen atom, then the compound is known as a Schiff base...

, that is then reduced by the catalyst in the final step to yield e.g. a tertiary amine^,,. The method is highly atom economic

Atom economy

Atom economy describes the conversion efficiency of a chemical process in terms of all atoms involved . In an ideal chemical process, the amount of starting materials or reactants equals the amount of all products generated and no atom is wasted...

, because is circumvents the activation of the alcohol (which is a poor electrophile

Electrophile

In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...

 itself, see tosyl

Tosyl

A tosyl group is CH3C6H4SO2. This group is usually derived from the compound 4-toluenesulfonyl chloride, CH3C6H4SO2Cl, which forms esters and amides of toluenesulfonic or tosylic acid...

ation or Swern oxidation

Swern oxidation

The Swern oxidation, named after Daniel Swern, is a chemical reaction whereby a primary or secondary alcohol is oxidized to an aldehyde or ketone using oxalyl chloride, dimethyl sulfoxide and an organic base, such as triethylamine...

). "Borrowing hydrogen" can be seen as an example of sustainable (thus "green") chemistry.
The method is not limited to the preparation of amines, it can also be used to form C–C-bonds.

Further reading

• (Cymene)ruthenium dichloride dimer
  (Cymene)ruthenium dichloride dimer
  ruthenium dichloride dimer is the organometallic compound with the formula [RuCl2]2. This red-coloured, diamagnetic solid is a reagent in organometallic chemistry and homogeneous catalysis.-Preparation and reactions:...
  
  
• Dichlorotris(triphenylphosphine)ruthenium(II)
  Dichlorotris(triphenylphosphine)ruthenium(II)
  Dichlorotrisruthenium is a coordination complex of ruthenium. This chocolate brown solid is a precursor to other complexes including those used as homogeneous catalysis.-Synthesis and basic properties:...
  
  
• dppf
• Eschweiler-Clarke reaction
  Eschweiler-Clarke reaction
  The Eschweiler–Clarke reaction is a chemical reaction whereby a primary amine is methylated using excess formic acid and formaldehyde. Reductive amination reactions such as this one will not produce quaternary ammonium salts, but instead will stop at the tertiary amine stage...