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Containers and Vessels for Supramolecular Catalysis
Published in Jubaraj Bikash Baruah, Principles and Advances in Supramolecular Catalysis, 2019
Cyclophane terms refer to compounds that have one or more aromatic groups linked together to form a cyclic basal unit and such basal units connected to another unit bridging at two nonadjacent positions. Cyclophane molecules provide confinement to a substrate or ions. Hence, they are able to cause many catalytic reactions. Thiamine diphosphate is a coenzyme that assists keto-transfer reactions, nonoxidative decarboxylations of β-keto acids and oxidative decarboxylations of α-keto acids. The active site of the coenzyme 3.6b has a six-member heterocyclic ring with an amine as well as imine unit; the amine group is stabilised by hydrogen bonding with a glutamate residue of the protein. This heterocyclic ring has the propensity to aromatise by losing a proton (Figure 3.6).
Structure and properties of some chiralanes and chirolanes
Published in Molecular Physics, 2018
13C-NMR spectrum shows four groups of peaks with combined intensity ratio 1:4:4:12 and chemical shifts of 108.4, 54.6, 53.6 and 46.5 ppm (vs. TMS), respectively. The 108.4 ppm peak corresponds to endohedral carbon which is heavily deshielded which is consistent with its positive partial charge of +0.199. The use of NBO partial charges in the interpretation of 13C-NMR chemical shifts has been reported previously [12]. The smallest shift of 46.5 ppm is experienced by tertiary carbons which are CC bonded to the quaternary ones. This is again consistent with their carrying of largest negative partial charge of −0.205. It is interesting to note that chemical shifts of tertiary carbons which are CC bonded to quaternary carbons appear upfield (i.e. are more shielded) compared to the tertiary carbons whose CC bonds do not involve quaternary carbons. The presence of intra-molecular strain is known to affect 13C chemical shifts as has been demonstrated by experiment and theory in the case of cyclophane derivatives [16,17]. However, in cyclophane derivatives, the aliphatic carbons are influenced not only by molecular distortion, but also by the proximity of vicinal, distorted aromatic rings. The comparison with cyclophanes therefore does not provide a clear rationalisation of the anomalous chemical shifts in some chiralanesFigure 2.