Formation of Trichlorosiryl-Substituted Carbon-Centered Stable Radicals through the Use of π-Accepting Carbenes

摘要

Carbon or silicon radicals with silyl substituents are important intermediates in organometallic and organic chemistry. Already in 1970, Bassindale et al. had reported the persistent tris(trimethylsilyl)methyl radical, (Me3Si)3C·, which has a lifetime of several days at 298 K. The groups of Ingold, Apeloig, Bravo-Zhivotovskii, Lee, Sekiguchi, and others have shown that the lifetime of the radicals largely depends on the steric bulk of the substituents. In 2002, Sekiguchi et al. reported the first stable silicon-centered radical without any π conjugation. Several reports describe the preparation of this class of radicals. The most successful ones proceed through the photolytic or thermal cleavage of a Si—Si bond, and R3Si—SiHCl2 can be reacted with bulky reagents, such as (tBu)2MeSiLi, according to the Apeloig-Sekiguchi method. Several radical species of main-group elements, such as PN·~+, P_2~(·+), phosphinyl radical cations, HB~(·+), and ketenes with biradical character, were also stabilized by cyclic alkyl(amino) carbenes (cAACs). We have observed that the chemical response of cAACs toward silylenes is very different from that of N-heterocyclic carbenes (NHCs). Recently, we have demonstrated that NHC→SiCl2 1 reacts with Me2-cAAC in a redox reaction to form the biradical (Me2-cAAC·)2SiCl2 2 in a two-electron redox step. Consequently, we were curious to investigate the one-electron redox process. Therefore, we reacted cAAC→SiCl4 3 with KC8 in an equimolar ratio in n-hexane to yield stable radicals 4, of the general formula (cAAC·)-SiCl3.