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.