Intrinsic Basicities of Phosphorus Imines and Ylides:  A Theoretical Study

Abstract

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 gasphase basicity level of ca. 300 kcal/mol (number of phosphorus atoms in the system n g 7), whereas the strongest organic phosphazene ylide superbases are estimated to have (at n g 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 Brønsted 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.

In Search of Ultrastrong Brønsted Neutral Organic Superacids: A DFT Study on Some Cyclopentadiene Derivatives

“…Therefore, one can safely conclude that aromatization, as well as cationic and anionic resonance in the corresponding conjugate acids and bases, are the key concepts in designing neutral organic superbases and superacids. These ideas, combined with other intrinsically highly basic functional groups such as S=N [42] and iminophosphorane [43] and new general notions such as the use of zwitterionic systems [44] or cooperative multiple intermolecular hydrogen bonding, [45] will undoubtedly close the gap which now separates the ladders of superbases and superacids.…”

Design of Brønsted Neutral Organic Bases and Superbases by Computational DFT Methods: Cyclic and Polycyclic Quinones and [3]Carbonylradialenes

“…Examples include amides and imidates, [19] cyclic and acyclic guanidines, [20][21][22][23] phosphazenes, [24][25][26] quinoimines and related systems, [27] quinolylboranes, [28] extended 2,5-dihydropyrrolimines [29] and C 2 diamines. [30] The extensive theoretical and experimental work carried out by Koppel, Leito and co-workers should be singled out as it includes both the theoretical design and the synthesis of organic bases and superbases as well as measurements of their pK a values in various solvents.…”

Structural investigations of phosphorus–nitrogen compounds. 6. Relationships between molecular parameters in per-X-substituted bridged spermine derivatives and basicity constants ΣαR of substituents

“…In general it is found that, with the increasing value of AE R , the bond angles and increase, whilst those for , and decrease; concomitantly, the bond length a decreases, whilst b, c, d and e increase. Although the present structural data refer to molecules in their unperturbed ground state in the crystalline solid and the basicity measurements were made in nitrobenzene solution (Feakins et al, 1965(Feakins et al, , 1968Feakins, Last et al, 1969;, where the molecule is perturbed by the approach of a proton (Koppel et al, 2001), there is a de®nite relationship between the molecular parameters of (2a)±(2g) and the substituent basicity constant, analogous to the effects observed previously (Bes Ëli et al, 2002). Other molecular parameters, such as Á(PÐN) values and the sum of the bond angles AENHsp and AENCsp in the series of molecules (2a)± (2g), also decrease as AE R increases, indicating a similar trend in increasing electron density provided by the X substituents, from X = Cl through to X = NHBu t .…”