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 reaction of organosilanes with tris(2-hydroxy-4,6-dimethylbenzyl) amine (5) in the 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-membered rings. Their structures were established by X-ray analysis and correlated 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 activation energies for enantiomeric conversion of the clockwise and anticlockwise orientations of the propeller-like silatranes 1-3 were statistically the same at: about 10 kcal/mol. A simultaneous pseudorotation of all three six-membered rings is suggested to cause the racemization process. The Si-N distances for 1-4 show a large dependence on the exocyclic substituent. Over an approximate electronegativity range of 2.3-3.0, the Si-N distance varies from 2.745(4) Angstrom for 1 to 2.025(4) Angstrom for 4. Thus, the influence of substituent effects on nitrogen-silicon donor-acceptor interactions can be readily studied in the absence of constraints imposed by the use of the smaller ring-containing silatranes.