Formyl cation, protosolvated

Whereas the formyl cation could not be directly observed by NMR spectroscopy its intermediacy has been well established in aromatic formylation reactions In order to account for the failure to observe the formyl cation, it has been suggested that CO is protonated in acid media to generate protosolvated formyl cation a very reactive electrophile. Protosolvation of the carbonyl oxygen allows facile deprotonation of the methine proton, thus resulting in rapid exchange via involvement of the isoformyl cation. 

Insertion of protosolvated formyl cation into the C—H a-bond has further been substantiated by carrying out the reaction of 1,3,5,7-tetradeuteroadamantane with CO in superacid media. 3,5,7-Trideutero-l-adamantanecarboxaldehyde-H and 3,5,7-trideutero-l-adamantanecarboxaldehyde-D were obtained in the ratio 94 6. This work provides the first direct evidence for the electrophilic formylation of a saturated hydrocarbon by CO under superacid catalysis. 

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