Carbon Atom as an Extremely Strong Nucleophilic and Electrophilic Center: Dendritic Allenes Are Powerful Organic Proton and Hydride Sponges.

Gas-phase proton affinities (PAs) and hydride affinities (HAs) of organic bases possessing an allene moiety and substituted with methyl, dimethylamino, cyano, and vinyl substituents were examined with the B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) model. It was shown that a number of superbases and hyperbases can be obtained, as well as the potent hydride sponges. Methyl or dimethylamino substituents increased the proton affinity of the parent molecule, and the cyano substituents increased its hydride affinity. When the vinyl substituents are placed on allene, both the hydride and the proton affinities increased. A disubstituted allene with two dimethylamino groups is the smallest studied superbase, whereas the allene tetrasubstituted with four vinyl groups gives the smallest superbase possessing only alkene substituents. By introducing the vinyl group as a repeating subunit, one can obtain dendritic structures with the investigated substituents determining its properties. By changing the dimethylamino with the cyano group, a dendrimeric molecule can change from a hyperbase with a proton affinity of 324.6 kcal mol-1 to a very strong hydride ion acceptor with a hydride affinity of 205.4 kcal mol-1, while possessing the same proton or hydride ion attachment site.