Molecular superbase

This reduces the energy of low-lying vacant molecular orbitals of free acetylene in this complex, as compared with analogous orbitals of free acetylene, and consequently the triple bond becomes more accessible to nucleophilic attack. As for nucleophiles, they become supernucleophiles in superbase media because of a sharp increase in their energy (76G817 77AP0133). 

Lochmann et al. were the first to attempt postmodification of the internal molecular framework of Frechet-type dendrons [46]. Multifunctionalisation of the interior of the dendron via metallation (superbase followed by reaction with various electrophiles) proved to be of a somewhat random nature owing to a lack of regioselectivity. It is admittedly suitable for engineering the solubility properties of the dendrons, but not for producing dendritic structures having functional groups in defined positions of the internal scaffold of the molecule. 

In comparison with molecular catalysts, solid catalysts can be isolated from the reaction mixtures by filtration or used in continuous processes this is both environmentally friendly and useful in laboratory-scale experiments. The most important reactions catalyzed by solid superbases are isomerization reactions and the alkylation of substituted arenes in the side chain (Scheme 2). They proceed at room temperature or below with high yield (typically >99%). The surperbase-cata-lyzed alkylation of aromatic compounds complements the acid-type Friedel-Crafts alkylation and acylation, because the latter results in ring alkylation, whereas the former results in side-chain alkylation. 

Gor/awski. H Hocldcrich, WF. Preparation of superbases and their u,sc as catalysts for double-bond isomerization. Journal of Molecular Catalysis A Chemical. 1999 144, I8I-187. 

Good choice of molecular backbone for attachment of bis(tetramethylguanidine) fragments leads to a strong superbase. The substitution of the aromatic backbone could also affect the basicity, as shown for l,2-bis(dimethylethyleneguanidino)benzene (86) by... 

There have been extensive efforts put into the accurate calculation of PAs, gas phase basicities and solution p fa values in recent years. Results vary depending on the level of sophistication of the applied calculations, and the fit to experimental values varies with the theoretical model employed. Although the calculated PA, gas phase basicity and p fa values differ from the experimentally determined, these calculations appear reliable enough that the major effects found in superbases can be clearly demonstrated. To generalize, more high level the theory method the better the results, but it is not always, dependent on the system under study. Large molecular systems need some trade-off between accuracy and computational effort (CPU time). 

As a first approximation, all this leads to the decrease in energy of low-lying molecular orbitals of acetylene in such a complex as compared to similar orbitals of free acetylene, and, consequently, the triple bond becomes more available for the nucleophilic attack. The nucleophiles themselves in the superbase systems turn to supernucleophiles due to the dramatic increase of their free energy [157,158]. 

Fig. 5 Molecular structures of central cores of superbase-like complexes [(tBuNH)4(OtBu)4Li4K4(C6H6)4l top) and [(PhK)4(PhLi)(tBuOLi)(THF)6(C6H6)2] bottom). Neutral coordinating ligands have been omitted for clarity...

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