Stereochemistry of the reaction of cis,trans,cis-2,4,6,8-tetraisocyanato-2,4,6,8-tetramethylcyclotetrasiloxane with triphenylsilanol and 1,1,3,3-tetraphenyldisiloxane-1,3-diol

All-cis-2,4,6,8-tetramethyl-2,4,6,8-tetrakis(triphenylsiloxy)cyclotetrasiloxane (4) and syn-1,3,9,11-tetramethyl-5,5,7,7,13,13,15,15-octaphenyltricyclo[9.5.1.13,9]octasiloxane (5) were synthesized by the reaction of cis,trans,cis-[MeSi(NCO)O]4 (1) with Ph3SiOH (2) and [Ph2Si(OH)]2O (3), respectively, in the presence of pyridine for the sake of investigating the synthesis of ladder polysilsesquioxanes with perfect siloxane frameworks. Their stereostructures were confirmed by nuclear magnetic resonance spectra and X-ray crystallography, which revealed that 4 and 5 did not retain the stereostructure of the precursor 1. This result was caused by the racemization of 1 with pyridine, and a subsequent nucleophilic substitution reaction of 1 with 2 or 3, including inversion and retention of the configuration at the silicon atoms.

All-cis-2,4,6,8-tetramethyl-2,4,6,8-tetrakis(triphenylsiloxy)cyclotetrasiloxane (4) and syn-1,3,9,11-tetramethyl-5,5,7,7,13,13,15,15-octaphenyltricyclo[9.5.1.13,9]octasiloxane (5) were synthesized by the reaction of cis,trans,cis-[MeSi(NCO)O]4 (1) with Ph3SiOH and [Ph2Si(OH)]2O, respectively. 4 and 5 did not retain the stereostructure of the precursor 1 due to the inversion and retention of the stereochemical configuration during the production of an intermediate with a penta- and/or hexacoordinated silicon atom formed by the addition of base as a nucleophile to the silicon atom. And a subsequent nucleophilic substitution reaction of 1 included inversion and retention of the configuration at the silicon atoms.Figure optionsDownload as PowerPoint slideResearch highlights
► Isocyanatosilane transformed its conformation in the presence of pyridine.
► Isocyanatosilane transformed its conformation via hyper coordinated intermediate.
► All-cis-tetrakis(triphenylsiloxy)tetramethyltetrasiloxane was synthesized.
► Ladder oligosilsesquioxane with tricyclic system was synthesized.
► Ladder oligosilsesquioxane was arranged in syn-form.