Synthesis of trioxy- and tetraoxysilatranes with all six-membered rings. Structure and dynamic NMR behavior

A series of silatranes containing all six-membered rings N[CH2(Me2C6H2)O](3 )SiR was prepared using tris(2-hydroxy-3,5-dimethylbenzyl)amine, 7, as the encapsulating agent in the case of 2 (CH2Cl), 4A (CH=CH2), 4B (CH=CH2), 5 ( CH2Ph), and 6 (CH2CH2(C5H4N)). Silatranes 1 (OC6H4-4-Br) and 3 (OC6H4-4-CMe 3) were synthesized by transesterification of a previously known silatrane B, where R = OMe. Their structures were established by X-ray analyses and c orrelated with Si-29 NMR data. VT H-1 NMR spectra reflected fluxional behav ior for all of these chiral molecules 1-6, The activation energies for enan tiomeric conversion of the clockwise and anticlockwise orientations of the propeller-like silatranes 1-6 increased as the Si-N distance decreased. Thi s trend paralleled an increase in structural rigidity associated with the r ing system as supported by ring torsional data supplied by the X-ray analys es. The degree of nitrogen donor interaction depends primarily on electrone gativity effects induced at silicon by the exocyclic ligand, Also, the Si-N distances, which are shown to linearly reflect the degree of trigonal bipy ramidal character, agreed with 29Si chemical shifts in indicating a corresp ondence between the solid and solution state structures.