Reactivity In Nucleophilic Reactions Research Articles

Pseudo-pentacoordination in silylcarbamates: structure-based prediction of silylating power

N-substituted trimethylsilyl carbamates (NTSCs) are powerful silylating agents. A survey of eight available X-ray molecular structures reveals an unusually short Si⋯O C distance that seems to convey a pseudo-pentacoordinated character of the silicon atom. The molecular structure of some representatives of NTSCs and their carbon analogues have been optimized at the MP2/DZ(2)P level. The theoretical structures confirm the existence of a Si⋯O through-space interaction and the pseudo-pentacoordination at the silicon atom. A structure-based predictive model is developed in which the high reactivity of NTSCs in displacing mobile hydrogens (i.e. the silylating power) is explained by both the pseudo-pentacoordination at the silicon and the unusually long Si–O ester bond, that is, by the use of ground-state structural parameters. On this basis, O-silylcarbamimidates are predicted to show higher reactivity in nucleophilic reactions, as compared to NTSCs, due to the stronger Si⋯N C interaction. Beyond NTSCs, the current predictive model can be applied to a wider range of compounds. O-alkylcarbamimidates are predicted to be good alkyl donors in nucleophilic substitutions. This conclusion is in accordance with the fact that O-alkyl-isoureas are general alkylating agents. Preliminary experimental results on the rate constants of hydrolysis of N-silylated cyclic ureas show strong correlation with the shift toward pseudo-pentacoordination around silicon, proving the validity of the current predictive model.

Reactivities of heteroaromatic cations containing a Group VIB element in nucleophilic reactions. Reactions of 9-phenyl-xanthylium, -thioxanthylium, and -selenoxanthylium salts with amines, sodium phenolate, and sodium benzenethiolate

Reactions of 9-phenylchalcogenoxanthylium salts (1a–c) with some nucleophiles have been examined in order to find the differences in reactivity in nucleophilic reactions. The chalcogenoxanthylium salts (1a–c) react with aniline in ether to give 9-anilino-9-phenylchalcogenoxanthenes (7a–c). However, in acetonitrile the xanthylium salt (1a) affords N,4-bis(9-phenylxanthen-9-yl)aniline (9a) together with the anilinoxanthene (7a)(at room temperature) or 9-(p-aminophenyl)-9-phenylxanthene (8a)(at reflux) and the sulphur (1b) and the selenium derivative (1c) affords only the anilino derivatives (7b,c), respectively.In the reactions with sodium phenolate, the thioxanthylium salt (1b) gave 9-phenoxy-9-phenylthioxanthene (13b), whereas the oxygen (1a) and the selenium congener (1c) gave O,4-bis(9-phenylchalcogenoxanthen-9-yl)phenols (15a,c) together with the 9-phenoxy derivatives (13a,c), respectively.The results show that the thioxanthylium salt (1b) gave the products formed on attack by the heteroatom of the ambident nucleophiles and the ratio of the carbon attack increased in the order (1a) > (1c) > (1b). This difference would be attributable to the properties of carbocations at the 9-position in the heteroaromatic cations (1a–c).