The Next Generation of Phosphorus Bisylide Superbases – Synthesis, Structures, Basicity and Proton Self-Exchange
On the search for the most basic proton sponge ever described the effects of stronger donating pyrrolidino vs. dimethylamino P-substituents as well as the homologisation of previously described bis phosphorus ylide base MHPN is investigated by synthesis, spectroscopy, crystallography, pKaH measurements and DFT calculations.
Herein we present two phosphorus ylide superbases to enhance the basicity of 1,8-bis (methylylidene (hexamethyltriamino) phosphorane) naphthalene (MHPN) – the first superbase with interacting carbon atoms as basicity centers. Its 1-pyrrolidinyl substituted analog 1,8-bis (methylylidene (tris (1-pyrrolidinyl)) phosphorane) naphthalene (MTPN) and MHPN's theoretically predicted higher homologue P2-MHPN exhibit extreme pKaH values of 26.0 and 29.5 (experimental) in THF solution and 33.6 and 37.4 (estimated) in acetonitrile solution. The corresponding calculated gas phase basicity values are 281.4 and 284.6 kcal mol−1, respectively. We prepared the neutral free bases together with the corresponding mono- and bisprotonated species which were characterized by NMR spectroscopy, ESI mass spectrometry, IR spectroscopy, elemental analysis and partly XRD analysis. The monoprotonated forms exhibit a rapid proton self-exchange between the two carbon atoms in peri-position and dynamic NMR spectroscopic methods revealed self-exchange rates of 2298 s−1 and 300 s−1 at 300 K for MTPN and P2-MHPN, respectively. However, computational studies reveal that the proton chelating effect, which typically considerably contributes to the basicity of proton sponges with basicity centers on nitrogen, is negligible in bisylides, as the 1,8-substitution pattern yields almost the same basicity as the corresponding 2,7- or 1,5-substituted analogues.