Basicity of silatranes (Review)

Abstract

The silatranes basicity study results are summarized. A large amount of experimental data obtained 25-40 years ago is critically reviewed. It is shown that the reactivity and physicochemical properties of silatranes are determined by their basicity, which depends on the stereoelectronic structure.Silatranes substituted at the silicon atom XSi(OCH 2 CH 2 ) 3 N (XSa) represent a widely studied class of chelate compounds of pentacoordinated silicon. Investigators of silatranes have been attracted primarily by their unique crystal and stereoelectronic structure and also by their high biological activity. The systematic and comprehensive investigations of silatranes started by M. G. Voronkov and co-workers as far back as 1963 continue to this day [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Of particular practical interest here is the specific biological activity of silatranes [11][12][13][14]. Their unique reactivity is due both to the electron density distribution and the spatial structure.During study of the structure and properties of silatranes by numerous physicochemical methods [1-10] greatest attention has been paid to the axial fragment of their molecule and particularly to the length of the transannular N→Si bond. Insufficient attention has been paid to the equatorial fragment of the molecule. Here account was rarely taken of the fact that there is a redistribution of electron density during the transition of the silicon atom from the tetravalent state to the pentacoordinated state; in the silatrane molecule the positive charge of the central silicon atom and the negative charge of the surrounding three oxygen atoms and also of the axial substituent are increased [15][16][17].During studies of the silatranes basicity the attention of investigators was turned first of all to the basicity of the nitrogen atom, which was always considered a stronger nucleophile than the oxygen atom. Nevertheless, the ability of silatranes to form onium complexes with the participation of the nitrogen or oxygen atoms has been the subject of heated discussions in the literature. By this time in a letter to the editor, which initiated the story of silatranes (described at that time by the systematic name 5-aza-2,8,9-trioxa-1-silabicyclo[3.3.3]undecanes), Frye and co-workers unexpectedly announced that silatranes are not neutralized by perchloric acid in anhydrous acetic acid, but are only slowly titrated by it at 100°C. This led said authors to conclusion that the nitrogen atom does not have basic properties due to the presence of the transannular donoracceptor N→Si bond in the silatranes [18].

Synthesis, characterization, electronic absorption and antimicrobial studies of N-(silatranylpropyl)phthalimide derived from phthalic anhydride

“…X-ray diffraction study supports a tricyclic caged structure involving hypervalent silicon atom centered in a trigonal-bipyramidal arrangement, surrounded equatorially by three endocyclic oxygens and axially by a substituent group and opposing nitrogen atom having lone pair directed towards the positively charged silicon in an electrostatically favorable arrangement [1,2]. Due to this arrangement, silatranes have attracted the attention of scientists worldwide who are keen to study silatranes by varying exocyclic substituent that affects the steric and electronic characteristics of the silatranes [3,4]. Recently, we have worked on the modification at axial position of silatranes with various derivatives [5][6][7].…”

Synthesis and Structural Characterization of Organopropylsilatranes Derived From Benzotriazole

“…In 13 C NMR spectra, the methylene carbon of propyl chain attached to silicon atom appeared as the most shielded carbon atom which is observed around δ ≈ 12.75-14.20 ppm for all silatranes. Other peak due to CCH 2 C is observed in region δ ≈ 24.15-25.50 ppm.…”

“…Organosilicon derivatives of nitrogen-containing heterocyclic compounds began to attract the attention of many investigators [13]. These studies made a valuable contribution to synthetic, theoretical, medicinal, and applied chemistry [14,15].…”

Synthesis and Characterization of Novel 1-Methanesulfonato Silatranes

“…It was shown that the silatrane systems with the larger rings allowed substituent effects associated with the axial position of the resulting trigonal bipyramidal structures (tbp) to be reflected in dramatically altering the Si-N bond interactions as well as in changes of the 29 Si chemical shifts [3][4][5][6][7]. The use of dimethyl sulfite and methyl methanesulfonate as alkylating, alkoxylating and acetylating reagent in synthesis is well known in literature [8,9].…”