Irreversible Rearrangement of Silyl Groups between Oxygen and Nitrogen in Tris(silyl)hydroxylamines

Abstract

Diorganofluorosilylation of 1,2-bis(trimethylsilyl)hydroxylamines at low temperatures gives exclusively O-(diorganofluorosilyl)-N,N-bis(trimethylsilyl)hydroxylamines 1a,b. The thermal rearrangement of 1a,b to the isomeric N-(diorganofluorosilyl)-N,O-bis(trimethylsilyl)hydroxylamines 2a,b is found to be irreversible, and the experimental data suggest an intramolecular course of the isomerization. Ab initio and density functional calculations for model compounds demonstrate the dyotropic course of the rearrangement. Anomeric stabilization of 2a,b by more favorable negative F−Si−N and O−Si−C hyperconjugation is important for the irreversibility of the reaction.

Thermal Isomerisation of Tris(silyl)hydroxylamines to Silylaminodisiloxanes − Experimental and Quantum Chemical Results

“…Energetically, the dyotropic TS is located at 33 kcal·mol Ϫ1 above the reactant. This is of the same order of magnitude as reported by Wolfgramm et al [5] The dyotropic TSdB is calculated to be 36 kcal·mol Ϫ1 above the reactant B ( Table 6). The isomer B1 is 1.5 kcal·mol Ϫ1 higher in energy than B.…”

“…(2) In 1998, we studied the rearrangement of O-(fluorosilyl)-N,N-bis(trimethylsilyl)hydroxylamines to the isomeric N-(fluorosilyl)-N,O-bis(trimethylsilyl)hydroxylamines and found it to be irreversible. [5] Ab initio and density functional calculations were consistent with the dyotropic course of the isomerisation. (3) In a thermal rearrangement, which proceeds at temperatures up to 200°C, tris(organosilyl)hydroxylamines undergo an intramolecular isomerisation involving the insertion of a silicon moiety into the bond between the nitrogen and the oxygen atom and the transfer of an organyl group from the silicon to the nitrogen atom, thereby forming silylaminodisiloxanes.…”

“…(5) The lithium salt of N,O-bis(tert-butyldimethylsilyl)hydroxylamine crystallises from THF as dimeric N,N-bis(tertbutyldimethylsilyl)hydroxylamide [7] and reacts with trifluoro-(phenyl)silane at 0°C with formation of N,N-bis(tert-butyldimethylsilyl)-O-[difluoro(phenyl)silyl]hydroxylamine (2). (6) Product 2 could be isolated following the removal of LiF in a centrifuge by virtue of the low reaction temperature and the bulky SiMe 2 CMe 3 groups.…”

“…It should be noted that the first calculations on dyotropic transition states in silylhydroxylamines were carried out by Müller. [5] Compared to the reactants A and B, the OϪN bonds are only slightly longer by 4.6 and 5.3 pm, respectively. In both transition states, the hydroxylamine SiϪO bonds are considerably stretched (by 39.7 pm and 46.1 pm for TSdA and TSdB, respectively).…”

“…The geometries are in very good agreement with the results for H 3 SiOϪN(SiH 3 ) 2 reported in ref. [5] In Table 3, the equilibrium structures of the eight silylaminodisiloxanes PA, PA1, PB, PB1, PC1, PC2, PD1, and PD2 are given. These structures may be compared with the structures of the diazasilacyclopropanes discussed in refs.…”