Cyclopropenimine as pincer ligand and strong electron donor in proton sponges

Abstract

The basicity of cyclopropenimines (CPIs) and cyclopropenimino-based proton sponges is investigated by means of density functional theory (DFT) calculations and for the first time directly compared with the basicity of phosphazenes. It is found that CPIs are more basic than the corresponding phosphazenes in the gas phase. Proton sponges based on CPI as pincer ligands also possess higher or at least the same gas-phase proton affinity as proton sponges based on phosphazenes. However, in comparison with phosphazenes, CPI-based pincer ligands possess greater conformational flexibility, which enables almost complete avoidance of nitrogen lone pairs. This leads to the substantially smaller destabilization of neutral base in CPI proton sponges. Utilizing homodesmotic reactions, we have shown that significant contribution to the proton affinity of CPI proton sponges is an electron-donating effect of the second CPI substituent, whereas only a smaller portion of the stabilization energy should be attributed to intramolecular hydrogen bond. Further, it was shown that tetrasubstituted CPI naphthalenes possess very low ionization potential, which qualifies them as very strong electron donors. Finally, utilizing cyclopropenimino substituents, it was found that theoretical gas basicity limit of 370 kcal mol À1 recently calculated by Leito and coworkers could be extended beyond 370 kcal mol À1 in a case of cyclopropenimino phosphorus carbenes and cylopropenimino phosphorus ylides. Figure 3. Ball and stick representation of conformers (top view) of proton sponges 22-24 together with phosphazene sponge HMPN Scheme 4. Homodesmotic reactions referring to molecules 25-27 Scheme 3. Homodesmotic reactions referring to proton sponges 22-24

Stable Singlet Carbenes as Organic Superbases

“…Compared to traditional alkali metal bases, [6] neutral “superbases” have been shown to offer milder reaction conditions and cleaner deprotonations, [7] allowing them to find numerous applications in synthetic chemistry and catalysis [8] . Today, with promising designs informed by computations, [9] neutral superbases extend well beyond phosphazenes and phosphatranes to encompass phosphines, [10, 11] amidines, [12] guanidines, [13] and other cyclic compounds, [14] or hybrid superbases (combination of above‐mentioned subgroups) [15] . While truly remarkable, these developments still largely rely on iterative improvements around a heteroatom core (i.e.…”

Push–Pull Effect on the Gas-Phase Basicity of Nitriles: Transmission of the Resonance Effects by Methylenecyclopropene and Cyclopropenimine π-Systems Substituted by Two Identical Strong Electron Donors

“…This choice does not imply a priori that the The cyclopropenimine (or cyclopropeneimine, less common name) skeleton (Z: N) has a special status among the heteroatomic analogs of methylenecyclopropene. It is present in many systems, which have been largely studied experimentally and theoretically, in particular in the last ten years, as platforms for developing superbases and organocatalysts [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. The imino bond carrying a nitrile group on the nitrogen, >C=N−CN, is present in molecules of biological interest, in particular the neonicotinoids [35][36][37], and in materials developed for their electronic applications [38][39][40].…”

Step Forward to Stronger Neutral Organic Superbases: Fused Troponimines

“…The deprotonation of the reactant is usually the most crucial step in many organic reactions, which sometimes require very strong superbases. Neutral organic superbases (NOS) are organic molecules that have higher basicity than that of 1,8-bis­(dimethylamino)­naphthalene (DMAN) with gas basicity (GB) of 239 kcal mol –1 and proton affinity (PA) of 245.3 kcal mol –1 . , The NOS are commonly categorized into amidines, guanidines, − phosphazenes, , phosphines, , aromatization-assisted cyclo-organic compounds like cyclopropeneimines and troponimines, − and compounds where above-mentioned subgroups are combined into one molecule, which are usually called hybrid superbases. Superbases, where basic substituents are brought into close proximity by substitution of aromatic backbone, are termed proton sponges.…”