Intramolecular rearrangement of organosilyl groups between oxygen and nitrogen in aminosiloxanes - a joint experimental-theoretical study, part II

Lithium amino-di-tert-butylsilanolate reacts with halosilanes to give 1-silylamino-1,3-siloxanes (1-7). The tetrakis(1-silylamino)siloxane thermally condenses yielding a spirocyclic six-membered ring (8). One six-membered ring of 8 forms a boat and the other has a twist conformation. Lithium salts of amino-disiloxanes form silylamino-silanolates or amido-disiloxanes. The first includes a 1,3-silyl group migration from the oxygen to the nitrogen atom. The energies of the isomeric lithium salts of model compounds are calculated and show that the lithium-trimethylsilylamino-dimethylsilanolate III is 0.7 kcal/mol more stable than the isomeric lithium-1,3-disiloxaneamide V. Experiments show that the lithium salts of amino-1,3-disiloxanes, (Me₃C)₂SiNH₂-O-R (R = SiMe₃, SiMe₂Ph, SiF₂CMe₃) reacts with ClSiMe₃, FSiMe₂Ph or F₃SiCMe₃ under a 1,3-O-N-silyl group migration to give the 1-silylamino-1,3-disiloxanes 9-11. If the trimethylsilyl group is substituted by SiMeF₂, the difference between the isomers III′ and V′ is even smaller, 0.12 kcal/mol, and the barrier to reaction via the dyotropic transition state is calculated to be 10.1 kcal/mol. Interestingly, the fluorine atoms allow for two other isomers VI and VIII which are even lower in energy. The low difference in the energies of III and V respectively VI and VIII explains that in absence of steric and/or electronic restraints the lithium salts of amino-1,3-disiloxanes react halosilanes to give both isomeric silylamino-1,3-disiloxanes, e. g. the lithiated (Me₃C)₂SiNH₂-O-SiF₂CMe₃ reacts with F₂SiMe₂ or F₃SiPh to give the structural isomers 12, 13, and 14, 15.The silyl group migration can be prevented kinetically, e. g. the lithium salts of (Me₃C)₂SiNH₂-O-R (R = SiF(N(CHMe₂)₂)₂, SiH(CMe₃)₂) react with F₂SiMe₂ or F₂Si(CMe₃)₂ to 16 and 17. A thermodynamically prevented rearrangement is observed in the reaction of lithiated (Me₃C)₂SiNH₂-O-SiMe₃ with F₃SiR (R = CMe₃ (18), Ph (19), N(SiMe₃)₂ (20), C₆H₂ (CMe₃)₃ (21). 18-21 ((Me₃C)₂SiNHSiF₂R)-O-SiMe₃) are formed.LiF-elimination from (Me₃C)₂SiNHLiO-SiF₂Me leads to the formation of the eight-membered (SiOSiN)-ring 22. The most stable lithium salts of 1-silylamino-1,3-disiloxanes form amides. This explains that in further reactions with halosilanes, the new ligand is bonded with the nitrogen atom (28-30). In results of crystal structure determinations new lithium-1-fluorosilylamino-1,3-disiloxanes of 20, (21, 23-25) are presented. 23 crystallizes as tricyclic, 24 as an unknown pentacyclic, and 25, as monomeric compound. In 25 the shortest Si-N bond length (157.9 pm) with four coordinate silicon is found. Lithium salts of 1-fluorosilylamido-1,3-disiloxanes lose thermally LiF with formation of siloxane substituted cyclodisilazanes, 26 and 27. Crystal structures of 4, 8, 17, 20, 21, 22, 23, 24, 25, 26, 28 are presented.