An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

作者：Ahmad Syed Usman; Szilvasi Tibor; Irran Elisabeth; Inoue Shigeyoshi^*

来源：Journal of the American Chemical Society, 2015, 137(17): 5828-5836.

DOI：10.1021/jacs.5b01853

The silicon analogues of an acylium ion, namely, sila-acyliurn ions 2a and 2b [RSi(O)(NHC)(2)]Cl stabilized by two N-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO2 using the donor stabilized silyliumylidene cations la and 1b [RSi(NHC),]Cl (1a, 2a; R = m-Ter = 2,6-Mes(2)C(6)H(3), Mes = 2,4,6-Me3C6H2 and 1b, 2b; R = Tipp = 2,4,6-iPr(3)C(6)H(2)). Structurally, compound 2a features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions 2a and 2b with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of 2a to H2O afforded a stable silicon analogue of carboxylate anion as a dimer form, [m-TerSi(O)O](2)(2-)center dot 2[NHC-H](+) (3), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion 2b afforded cyclotetrasiloxanediol dianion [{TippSi(O)}(4){(O)OH}(2)](2-)center dot 2[NHC-H](+) (4). Metric and DFT (Density Functional Theory) evidence support that 2a and 2b possess strong Si=O double bond character, while 3 and 4 contain more ionic terminal Si-O bonds. Mechanistic details of the formation of different (SiO)(n) (n = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry.

• 出版日期2015-5-6