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Frustrated Lewis pair
In chemistry, a frustrated Lewis pair is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form an adduct.〔Stephan, Douglas W. "Frustrated Lewis pairs": a concept for new reactivity and catalysis. ''Org. Biomol. Chem.'' 2008, ''6'', 1535-1539. 〕 Because of their "unquenched" reactivity, such systems are very reactive and are able to split the hydrogen molecule heterolytically, which makes them potentially useful as metal-free catalysts for hydrogenation reactions.
In the scheme below, compound 1 has a frustrated Lewis pair: the lone pair on the phosphorus atoms cannot be donated to the boron atoms because of the large substituents on both atoms (Mes = mesityl). However, when exposed to hydrogen at 1 atm, the zwitterionic compound 2 is formed. This hydrogen addition is reversible, and it is possible to transfer the activated hydrogen to a sterically hindered imine, which results in a catalytic cycle that produces the hydrogenated product, using only 0.05 equivalents of 1 as a catalyst.
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