A new class of silatranes: Structure and dynamic NMR behavior

A new class of silatranes N[CH2(Me2C6H2)O](3)SiR was prepared by the reacti on of organosilanes with tris(2-hydroxy-4,6-dimethylbenzyl) amine (5) in th e case of 1, 3, and 4, and by the reaction of Si(OMe)4 with the same amine (5) to give 2. This is the first series of silatranes containing all six-me mbered rings. Their structures were established by X-ray analysis and corre lated with Si-29 NMR data. VT H-1 NMR spectra reflected fluxional behavior for the chiral molecules 1-3, whereas 4 was a rigid molecule. The activatio n energies for enantiomeric conversion of the clockwise and anticlockwise o rientations of the propeller-like silatranes 1-3 were statistically the sam e at: about 10 kcal/mol. A simultaneous pseudorotation of all three six-mem bered rings is suggested to cause the racemization process. The Si-N distan ces for 1-4 show a large dependence on the exocyclic substituent. Over an a pproximate electronegativity range of 2.3-3.0, the Si-N distance varies fro m 2.745(4) Angstrom for 1 to 2.025(4) Angstrom for 4. Thus, the influence o f substituent effects on nitrogen-silicon donor-acceptor interactions can b e readily studied in the absence of constraints imposed by the use of the s maller ring-containing silatranes.