Diastereoselective self-assembly of a pentacoordinate siliconate tetraanionic molecular square. A mechanistic investigation

Condensation of (+/-)-5,5',6,6'-tetrahydroxy-1,1'-spirobis(indane) (1) and phenyltrietlioxysilane with triethylamine in refluxing THF results in formation of a tetraanionic molecular square incorporating four pentavalent siliconates. The macrocyclization is stereoselective. Of the four possible macrocyclic tetramers, a single isomer, meso-2 (C-2h), is produced in high yield from racemic 1. Reaction intermediates leading to formation of macrocycle 2 were identified by electrospray time-of-flight mass spectrometry (ES-TOF-MS). The reaction proceeds by a path that deviates from conventional step-growth polymerization. The condensations produce linear dimers, trimers, and tetramers. The configurational flexibility of the pentacovalent siliconate and the angular connector, bisspirocatechol 1, predispose specific oligomers toward macrocyclization. At the linear tetramer stage, there are a total of six possible stereoisomers. Calculations reveal little difference in energy between the paths that lead to them. The RSSR and SSRR diastereomers undergo macrocyclization, and the product, meso-2 (C-2h), precipitates from solution. Crossover experiments establish the reversibility of the reaction, even after formation and precipitation of cyclic tetramer, The high reaction yields are accounted for by a self-correcting mechanism to produce what is found to be (computationally) the most stable (and most likely least soluble) diastereomer.