Metal-free catalytic hydrogenation
The ability of ‘frustrated Lewis pairs’ to activate dihydrogen has been known for some time and in around 2006 the mixture of (mesityl)3P/B(C6F5)3 was shown to form the phosphonium/hydridoborate salt (ie (mesityl)3PH+/B(C6F5)3H-) in the presence of dihydrogen at ambient temperature. Furthermore, the ‘frustrated Lewis pair’ catalysed the hydrogenation of an imine at elevated temperature in the presence of dihydrogen. In later advance, the reagent (1) shown below was found to catalyse the hydrogenation of a variety of enamines and some bulky imines under very mild conditions at room temperature.
The paper by Erker et al (Chem Commun, 2008, 5966) provides an interesting addition to this chemistry wherein they describe the reduction of silyl enol ethers as test substrates using a mixture of the diphosphine (2) and B(C6F5)3 in the presence of hydrogen. The hydrogen activation equilibrium is shown in the scheme below. They have isolated and characterised the salt (3), using single crystal X-ray analysis in addition to extensive NMR charcaterisation.
The hydrogenation reactions were carried out at room temperature under 2 bar pressure of hydrogen using 20mol% of the diphosphine (2) and B(C6F5)3 to give >99% conversion, with just one example of 5 shown below.
In summary, this paper potentially offers a very interesting alternative to catalytic hydrogenation over precious metals, thereby avoiding issues of metals residues. Although not mentioned in the paper, subject to the precise mechanism one wonders if it offers a new avenue for asymmetric hydrogenation?