Zwitterionic silicates, preparation

The preparation of similar zwitterionic silicates based on glycolic acid was also reported by Erchak and coworkers53,54. 

The acyclic (ammonioorganyl)trifluoro(organyl)silicates 17,22 18,22,19 20 20,20 2123 and 2220 are zwitterionic A55/-silicates with an S7F3C2 skeleton. As illustrated for 19 and 20 in Scheme 3, compounds 17-22 were prepared analogously to the syntheses of 4-16 (compare Scheme 2). 

Over the past few years, we have synthesized and structurally characterized a series of zwitterionic acyclic and spirocyclic X S/-silicates, such as compounds 1 [1] and 2 [2] (Scheme 1). These zwitterions contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. We have now succeeded in preparing a series of related zwitterionic monocyclic A. S i-silicates. We report here on the syntheses, properties, and crystal structures of compounds 3a/3b-5a/5b (Scheme 1) [3] (for reviews dealing with pentacoordinate silicon compounds, see [4—10]). 

The zwitterionic monocyclic X S/-silicates 3a-5a were synthesized according to Scheme 2 by reaction of [(dimethylammonio)methyl]trifluoro(methyl)silicate (6a) with one molar equivalent of the 0,0 -bis(trimethylsilyl) derivatives of benzohydroximic acid, oxalic acid or glycolic acid (formation of two molar equivalents of MesSiF). Compounds 3b-5b were prepared analogously starting from trifluoro(methyl)[(2,2,6,6-tetramethylpiperidinio)methyl]silicate (6b) (Scheme 2). 

The zwitterionic monocyclic A. Si-silicates la and lb were synthesized by reaction of the zwitterionic A, 5 -trifluorosilicate 4 with one molar equivalent of the 0,0 -bis(trimethylsilyl) derivatives of glycolic acid and 2-methyl lactic acid, respectively (Scheme 1). Compounds 2a and 2b were prepared analogously starting from the zwitterionic X Si-trifluorosilicate 5 (Scheme 1). As shown for compound la in Fig. 1, the zwitterionic X i i-silicates la, lb, 2a, and 2b are chiral and exist as pairs of enantiomers [(A)- and (C)-enantiomers]. All compounds were isolated as racemic mixtures. 

Summary Derivatives of a new class of zwitterionic spirocyclic A, 5/-silicates with an SiO C framework have been prepared. The syntheses and the properties of these molecular pentacoordinate silicon compounds are reported. The crystal structures of two of the zwitterions are described. 

Following the strategy used for the synthesis of compound 5 [8], the zwitterionic A, 5/-silicates 6-10 were prepared according to Scheme 2 (method a). 

Following the strategy described for the preparation of compound 4 [11], the zwitterionic 5/-silicate 6 was synthesized by reaction of [(dimethylamino)methyl]dimethoxy(phenyl)silane (8) [9] with salicylic acid (molar ratio 1 2) in acetonitrile at room temperature (method a) (Scheme 1). This synthesis involves cleavage of two Si-O bonds and one Si-C bond (formation of two molar equivalents of methanol and one molar equivalent of benzene). 

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