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The spatial and electronic structure of highly basic organic molecules: cyclopropeneimines and some related systems

It is shown, by utilizing a reliable model at the MP2 level of theory, that the imino group attached to the cyclopropene moiety exhibits a high proton affinity (PA). The reason behind the appreciable PA is identified as significant aromatization of the three-membered ring spurred by the protonation. Further amplification of the PA can be achieved by NH2 substitutions at the CC endo double bond, since amino groups stimulate aromatization of the cyclopropene fragment. Additionally, they release some of their lone pair electron density thus contributing to a uniform distribution of the positive charge over the entire molecular system. This effect is even more pronounced if the diaminoiminocyclopropene system is substituted by sizeable alkyl groups. The aromatic stabilization in the protonated 2, 3-diaminocyclopropeneimine, estimated by the corresponding homodesmic reaction, seems to be as high as 59.5 kcal/mol. Very potent organic bases can be obtained by designing polycyclic systems possessing two or more cyclopropene fragments, where the aromatization occurs by the conjugation interaction transmitted through the p-electron network in a typical domino fashion. The highest PA value is found, however, in a calicene-like structure embracing a quinoid six-membered ring and a cyclopropene moiety. Its proton affinity is well above the PA value of the Schwesinger's proton sponge, which is known as the stongest organic base so far. Importance of the iminocyclopropene motif in tayloring of the strong organic superbases is stressed. The role of the intramolecular hydrogen bonding in some specific systems is briefly discussed.

Cyclopropeneimines; Highly basic organic molecules; Aromatic domino effect; Ab initio calculations

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