Activation of H2 is typically a domain of transition metal catalysis -even nature uses metal-centred reactions for this process. However, very recently Stephan and Erker have described metal free systems able to activate dihydrogen by the use of Frustrated Lewis Pairs (FLP), a combination of sterically hindered Lewis acids and Lewis bases that for this geometric reason are not able to quench each other. Moreover they were able to apply these systems for the reduction of some imines using H2 directly instead of any organic surrogate.
Although this process is quite interesting, its applicability is nevertheless quite limited. Only silylenol ethers and very bulky imines can be satisfactorily reduced, non-encumbered ones simply react directly with B(C6F5)3, the ubiquitous Lewis acid used to form FLPs. To circumvent this obvious drawback I propose a new approach consisting in the use of completely organic molecules such as electron-poor allenes, alkenes, iminium salts and ketones as attractive alternatives to B(C6F5)3. The development of a totally organic FLP able to activate H2 reversibly goes far beyond the state-of-the-art as it is expected to allow the reduction of carbon-carbon and carbon-heteroatom double bonds without the use of any metal and employing directly H2. This wide-ranging extension of the FLP reactivity will clearly surpass classical metal catalysis in environmental issues as no noble metal will be necessary. Also, organocatalytic reductions will be outstripped in atom economy criteria as H2 and no heavier surrogate will be used as reducing agent.
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Funding SchemeERC-SG - ERC Starting Grant