Geometries and interaction energies of benzene complexes with pyridine, pyridinium, N-methylpyridinium were studied by ab initio molecular orbital calculations. Estimated CCSD(T) interaction energies of the complexes at the basis set limit were -3.04, -14.77, and -9.36 kcal/mol, respectively. The interactions in the pyridinium and N-methylpyridinium complexes should be categorized into a cation/pi interaction, because the electrostatic and induction interactions greatly contribute to the attraction. On the other hand, the interaction in the pyridine complex is a pi/pi interaction. The dispersion interaction is mainly responsible for the attraction in the benzene-pyridine complex. Short-range interactions including charge-transfer interactions are not important for the attraction in the three complexes. The most stable pyridinium complex has a T-shaped structure, in which the N-H bond points toward the benzene, while the N-methylpyridinium complex prefers a slipped-parallel structure. The benzene-pyridine complex has two nearly isoenergetic (Slipped-parallel and T-shaped) structures.

The cation−π interaction is an attractive noncovalent interaction between a cation and a π system. Due to the stronger interaction energy than those of the other π interactions, such as π–π and CH−π interactions, the cation−π interaction has recently been recognized as a new tool for controlling the regio- and stereoselectivities in various types of organic reactions. This review attempts to cover a variety of organic reactions controlled by cation−π interactions, which includes not only recent examples but also those reported before the term “cation−π interaction” was defined in 1990. This review will provide comprehensive knowledge on the role of cation−π interactions in organic synthesis.

In order to explore the contribution of the cation−π interaction in the photocyclodimerization of styrylpyridines, the effects of the acid concentration and the substituent on the aryl ring on the product distribution were investigated. The structures of the product dimers were clarified by X-ray crystallographic analysis. In addition, the X-ray packing structures of trans-4-styrylpyridine (1) and its HCl salt were compared. On increasing the acid amount, the yield of the syn-HT dimer significantly increased, whereas that of the cis-isomer decreased. The substituent on the aromatic ring had a significant effect on the product distribution. Irradiation of the substrate styrylpyridine bearing a CF3 group resulted in much lower selectivity, whereas the photolysis of the substrate having a MeO group resulted in a syn-HT dimer in 95% selectivity. Comparison of the X-ray packing structures of 1a and 1a·HCl clarified the significant differences between them. The molecules of 1a·HCl are packed alternately in antiparallel alignment in a face-to-face manner, the distance of which is 3.295 Å, strongly suggesting the existence of an intermolecular cation−π interaction. On the other hand, no such interaction was observed in 1a. These results lead to a conclusion that pyridinium−π interactions govern the orientation of the trans-styrylpyridinium in solution, the irradiation of which would result in the selective formation of the syn-HT dimer.

These findings show that milnacipran acts as a SNRI in vitro and in vivo and may be useful for the treatment of anxiety as well as depression.

Selective shielding of one side of the pyridinium face by way of intramolecular cation-pi complex formation enabled nucleophiles to attack only from the non-shielded side, and consequently, chiral 1,4-dihydropyridines were produced stereoselectively with up to 99% de. The structure of the cation-pi complex was elucidated by 1H NMR studies and X-ray analysis.

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H 7 H 8 R 9 MCPBA, 25'C, 10 CH,CI, min * (H2@1"~SiPh, 10 1 . 0,, CH,C12,-78'C 8b 2. Me,S a Scheme 3. a: I? = 1. b: n = 2.of propellane ring systems has been demonstrated by the stereospecific epoxidations to oxa[3.3.l]propellane 1Oa and 0~4 4 . 3 . llpropellane lob, which proceed quantitatively. Moreover, ozonolysis of bicycloolefin 8 b to afford cyclononadione 11 (46% yield) emphasizes the potential of these intermediates as precursors for compounds with medium-sized rings.In conclusion we have found a versatile, stereospecific method for the annulation of silylcyclopentanes by cycloaddition of an allylsilane 1 as the synthetic equivalent of the 2-silyl-substituted 1,3-dipole 12. The stereoselective synthe-1 12 ses and further synthetic transformations of the products emphasize the potential of this method for the synthesis of cyclopentanoid natural products. The oxidative cleavage of the carbon-silicon bond should occur with retention of configuration['61 and is currently under investigation. Preliminary results indicate that asymmetric syntheses of cyclopentanes are feasible by using chiral allylsilanes, and that cyclopentanes with five stereogenic centers can be generated stereospecifically by employing 1,3-disubstituted allylsilanes. The present method thus opens the way for the enan- [I] H.-J.[12] For more recent discussions on the intermediacy of pentavalent silicon cations (siliranium ions or siloniacyclopropanes; these species have been mistakenly referred to as "siliconium ions") see: T. [15] X-ray analysis of5c: C,,H,,OSi, triclinic, space group, Pi, a = 1044.1(5), b =1195.6(6),c =1206.9(6)pm,a = 88.27(3),1(= 65.14(2),y= 69.55(3)", V =1269 .lo6 pm3, Z = 2, pcalcd =1.15 g~m -~. p = 0.77 cm-'; Mo,, radiation (graphite monochromator); 28 range: 3-56"; 5069 independent reflections, 3186observed ( l > 2u(/)), R = 0.053, R , = 0.050. Furtherdetails of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, D-76344 Eggenstein-Leopoldshafen. on quoting the depository number CSD-57205, the names of the authors. and the journal citation. [16] I. Fleming, R. Henning. H. Plaut.

A series of the C(O)−N twisted amides, 3-acyl-4-alkyl-1,3-thiazolidine-2-thiones 1a − e, was synthesized, and the structures were elucidated by X-ray crystallographic analysis. The relationship between the C(O)−N twist angles τ, the 13C, 15N, and 17O NMR chemical shifts, and the infrared absorption of carbonyl groups were investigated in order to provide insight into the changes in charge distribution dependence on the C(O)−N twist angle. Furthermore, the relationship of the νC  O and the 15N chemical shift was also investigated. Because the spectral data reflect considerable substituent effects, the 13C and 17O chemical shifts and νC  O were compared with those of corresponding N,N-dimethylamides 2a − c, and the 15N chemical shifts were compared with those of corresponding N-methyl-1,3-thiazolidine-2-thiones 3a − c. As the twist angle increased, the Δδ13C and Δδ17O increased, whereas, the Δδ15N decreased. Furthermore, the ΔνC  O increased with increasing τ and decreased with increasing Δδ15N. The relationship of the results to the classical amide resonance model and recently proposed model is also discussed.