Phenanthrenes, proton sponges

In 1964, Paquette reported that although the 9,10-double bond of simple phenanthrenes can be formed using the Ramberg-Backlund reaction (NaOH in aqueous dioxane), the sterically crowded 4,5-dimethyl-phenanthrene cannot be made by this method. Staab has now shown that the highly strained proton sponge 4,5-bis(dimethylamino)phenanthrene (15) can be prepared using Meyers modification of the re-... 

On the basis of ab initio calculations using HF/6-31G and BLYP/G-31G levels, Howard has concluded that for naphthalene 1 and phenanthrene 12 proton sponges, the IHB contribution in the overall increase of basicity is not less than 65%, whereas the other 35% are equally divided between the relief of steric strain and the electronic repulsion on... 

Platts, J.A. and Howard, S.T. (1996) Ab initio studies of proton sponges 3. 4,5-Bis(dimethy-lamino)fluorene and 4,5-bis(dimethylamino)phenanthrene. The Journal of Organic Chemistry, 61, 4480-4482. 

From these data, further basicity increase should be expected for the phenanthrene sponge 12. This expectation, however, is not justified and the pKa value of 12 is only 11.5, despite its being rather short (2.54 A) and linearity of the IHB (167°) in the cation 12411 (Table 5). It is assumed that the principal reason for the reduced basicity of diamine 12 lies in its helicene-like structure, which is retained in the corresponding cation, i.e. the protonation in this case does not result in steric relief1. Even more pronounced helicene strain is manifested in the cation of benzophenanthrene sponge 13411, having a basicity (pKa = 5.8) only a little higher than that of dimethylaniline. 

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