Phenyl group stacking

There are two kinds of Pl SnCLt2- anions in 476, one placed between two cations with its phenyl groups stacking over their pyrimidine rings, and the other without a close relationship to any cation. The two Sn atoms lie on a crystallographic inversion center930. 

A decrease in the growth rate along a may conceivably be achieved either by disruption of the hydrogen bonds within the cyclic dimer at the site of the additive, or by further weakening of the interactions between the stacked phenyl groups. 

Murray and colleagues199 developed some 2,5-diketopiperazines as new chiral auxiliaries and examined their asymmetric induction in the Diels-Alder reactions of their A-acryloyl derivatives with several dienes. Some of their results with dienophile 320 have been summarized in Table 19 (equation 89). When the benzyl group on 320 was substituted by an isopropyl or /-butyl group, the diastereofacial selectivity dropped dramatically. It was proposed that tv-tt stacking between the phenyl group and the electron-poor double bond provided a more selective shielding of one face of the double bond in this special case. 

The Ag cations are coordinated to two sulfur atoms of different cavitands with Ag-S distances in the range 2.47-2.50 A. In the solid, efficient r-stack-ing of the P-phenyl groups with the picrate anions stabilizes the supramolecular complex (Fig. 10). The two cavitands are aligned along their common C4 axis and offset by about 45°, leading to a helical structure. The inner space is reduced by the occupancy of the sulfur atoms, and there is probably not enough room to accommodate small guests inside the cavity. 

Pentafluorination diminishes n electron density of phenyl, making it more suitable for participation in stacking or charge transfer with the phenyl groups of other aromatic amino acids of the active site. As an example, the inhibitory power of the carbonic anhydric hydrolase by the pentafluorophenyl analogue of methazolamide is tenfold increased (Fig. 5) [31]. 

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