Intrinsic basicities of phosphorus imines and ylides: A theoretical study

A density functional theory (B3LYP/6-311+G**), ab initio (HF/3-21G*), and semiempirical (PM3) study of intrinsic basicities, protonation energies, or protonation enthalpies of organic phosphorus imine (iminophosphorane) including phosphazene, phosphorus ylide (phosphorane), and phosphine superbases has been performed. The study shows that representatives of the first two classes of the above-mentioned organic superbases can reach the basicity level of the strongest inorganic superbases such as alkali-metal hydroxides, hydrides, and oxides. The strongest organic phosphazene imine superbases are predicted to reach the gas- phase basicity level of ca. 300 kcal/mol (number of phosphorus atoms in, the system n greater than or equal to 7), whereas the strongest organic phosphazene ylide superbases are estimated to have (at n greater than or equal to 5) gas-phase basicities around, or beyond 310-320 kcal/mol. The phosphine superbases, including the Verkade's bicyclic phosphines (proazaphosphatranes) are predicted to have a basicity comparable to P-2 phosphazenes or P-1 phosphorus ylides, whereas the respective proazaphosphatrane imines and ylides are expected to be the strongest organic superbases which contain only a single phosphorus atom. Extremely high expected basicity values and handling preferences over inorganic superbases. make representatives of novel organic superbases possible partners for observing the spontaneous gas-phase proton transfer between neutral Bronsted superacids and -bases. For the comparison, the basicities of some alkali-metal substituted ammonia, phosphine, phosphorus, and nitrogen ylides and imines have been also calculated.