Synthesis of Trioxy- and Tetraoxysilatranes with All Six-Membered Rings. Structure and Dynamic NMR Behavior1

A series of silatranes containing all six-membered rings N[CH 2 (Me 2 C 6 H 2 )O] 3 SiR was prepared using tris(2-hydroxy-3,5-dimethylbenzyl)amine, 7, as the encapsulating agent in the case of 2 (CH 2 Cl), 4A (CH CH 2 ), 4B (CH CH 2 ), 5 (CH 2 Ph), and 6 (CH 2 CH 2 (C 5 H 4 N)). Silatranes 1 (OC 6 H 4 -4-Br) and 3 (OC 6 H 4 -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 correlated with 29 Si NMR data. VT 1 H NMR spectra reflected fluxional behavior for all of these chiral molecules 1 − 6 . The activation energies for enantiomeric conversion of the clockwise and anticlockwise orientations of the propeller-like silatranes 1 − 6 increased as the Si−N distance decreased. This trend paralleled an increase in structural rigidity associated with the ring system as supported by ring torsional data supplied by the X-ray analyses. The degree of nitrogen donor interaction depends primarily on electronegativity effects induced at silicon by the exocyclic ligand. Also, the Si−N distances, which are shown to linearly reflect the degree of trigonal bipyramidal character, agreed with 29 Si chemical shifts in indicating a correspondence between the solid and solution state structures.