A metal-free catalytic method for the direct hydrogenation of imines and reductive ring-opening of aziridines has been developed by scientists in Canada.

Douglas Stephan and co-workers from the University of Windsor, Canada, have discovered that a simple commercially available Lewis acid (B(C₆F₅)₃) can be combined with a bulky imine to act as a so-called 'frustrated Lewis pair' (FLP). FLPs are capable of acting as both Lewis donors and acceptors since their steric demands are such that Lewis acid-base adducts are not formed. The team propose that these FLPs react with hydrogen gas, forming an iminium-hydridoborate ion pair. This iminium can then undergo nucleophilic attack by the borohydride, producing the desired amine product after dissociation from the borane.

Traditionally this transformation uses either a transition metal catalyst (which can be expensive and potentially toxic) or a stoichiometric amount of a main group hydride such as LiAlH₄ (which requires waste remediation)_.  Some organocatalytic methods do exist though these generally do not use hydrogen directly but instead rely on other molecules as the hydrogen source.

Stephan has shown that the new technique can hydrogenate several sterically hindered imines, and also carry out the reductive ring-opening of an aziridine, with very good yields. Adding a bulky phosphine Lewis base to the reaction also allowed the reduction of electron-poor imines and protected nitriles, which shows the potential generality of the procedure.

'We hope to expand the scope of substrates for which metal-free hydrogenation catalysts are applicable as well as extend this concept to asymmetric synthesis,' says Stephan.

The hydrogenation reaction is used extensively by chemists, so this new method could have a wide range of future applications, as well as being cheaper and more environmentally friendly. 

Frances Galvin