Hydride and fluoride transfer reactions accompanying nucleophilic substitution at pentacoordinate silicon

Article abstract of DOI:10.1021/ja00106a018

The syntheses of aminoazasilatranes of the type R₂NSi(R′NCH₂CH₂)₃N (R′ = H, R = H (2a), CH₃ (3a), CH₂CH₃ (4a), Si(CH₃)₃ (6a), R′ = CH₃, R = H (2b), CH₃ (3b), CH₂CH₃ (4b), Si(CH₃)₃ (6b) via nucleophilic substitution reactions of ClSi(R′NCH₂CH₂)₃N (R′ = H (7a), R′ = CH₃ (7b), respectively) with amide anions are reported. Reactivities of 7a and 7b toward other nucleophilic reagents such as alkyllithiums and Group 1 metal alkoxides are also described. It is found that the equatorial NR′ functionalities significantly influence the reaction pathways. With strong bases, lithiation of the equatorial NH hydrogens of 7a predominated along with some nucleophilic substitution products and hydride transfer product HSi(HNCH₂CH₂)₃N, 1a. With 7b, however, equatorial nitrogen lithiation is precluded and its reaction with nucleophiles can produce substantial amounts of nucleophilic substitution product as well as hydride transfer product HSi(CH₃NCH₂CH₂)₃N, 1b. The relative ratios of these products depend substantially on stereoelectronic factors, the nature of the nucleophilic reagents, and the reaction conditions. In the case of the reaction of 7b with BrC₆F₅/n-BuLi, three products, namely, C₆F₅Si(CH₃NCH₂CH₂) ₃N (13b), FSi(CH₃NCH₂CH₂)₃N (14b), and C₆F₅Si(CH₃NCH₂CH₂) ₂(ο-C₆F₄CH₃NCH ₂CH₂)N (15) formed in an approximate ratio of 1:2:1. The formation of 15 is attributed to perfluorobenzyne insertion into a Si-N_eq bond of (13b). Interestingly, the plane defined by the axial NSi₂ moiety in 6a is found to be fixed at the apical position of the silicon, providing an interesting example of pπ-dπ interaction between a pentacoordinate silicon and a nitrogen. However, the axial moiety in analogue 6b freely rotates around the apical Si-N bond due to steric interactions with nearby methyl groups on the cage.