The Elusive 7-Silanorbornadien-7-ylium:  Synthesis and Characterization of Nitrilium and Oxonium Ions Deriving from 2,3-Benzo-7-silanorbornadien-7-ylium

Abstract

Silylnitrilium (6) and Silyloxonium (7) ions have been synthesized by reaction of 2,3-benzo-1,4,5,6,7-pentaphenyl-7-silanorbornadiene, 4, with trityl tetrakis(pentafluorophenyl)borate in benzene in the presence of acetonitrile and diethyl ether, respectively, and have been identified by NMR spectroscopy, supported by density functional calculations of the NMR chemical shift. Solvent-free 2,3-benzo-1,4,5,6,7-pentaphenyl-7-silanorbornadien-7-ylium, 3, could not be detected under the applied conditions (room temperature, aromatic hydrocarbons or -60 °C, dichloromethane). The results of density functional calculations for the model compound 2,3-benzo-7-phenyl-7-silanorbornadien-7-ylium, 8, and isomeric C 16 H 13 Si + species suggest that 3 undergoes in hydrocarbon solution a facile exothermic rearrangement to the highly reactive π-type complex PhSi + //tetraphenylnaphthalene.

29Si NMR Chemical Shift Tensor and Electronic Structure of 7-Silanorbornadienes

“…Characteristic for the 7-silanorbornadiene skeleton is the unusually deshielded resonance of the bridging silicon atom (δ 29 Si= 31.6-65.6, see Table 1). The identity of benzosilanorbornadienes 4b, c was confirmed by comparison of the analytical data with published data [13,18].…”

“…The newly synthesized silanorbornadienes 4a, d, e were prepared by [4+2] cycloaddition reactions of substituted siloles 6 with benzyne [13,14,17]. In variance, the 7-terphenylsubstituted silanorbornadiene 5a was prepared by a reaction sequence involving regioselective deprotonation of 9-silyl-dihydroanthracene 7 at carbon atom C 10 , followed by an intramolecular substitution reaction [15] (Scheme 1).…”

“…9,10-Dihydroanthracene, n-butyllithium (1.6 M in n-hexane) and diisopropylamine were obtained from commercial suppliers and were used without further purification. Lithiumdiisopropylamide (LDA) and 7-silanorbornadienes 4b, c were prepared as described in literature [13,17,18,33]. (C 6 H 3 -2,6-Mes 2 )SiHCl 2 was prepared according to a slightly modified literature procedure [34].…”

“…Structural solutions were obtained with direct methods using SHELXS-97 [35]. The crystallographic data is summarized in Table 3. 6.3.2 Synthesis of 2,3-Benzo-7-Silanorbornadienes 4a, d, e, General Procedure [13,15] A sample of the corresponding silole 6 (8.6 mmol) was suspended in 100 mL of THF and 0.63 g (26 mmol) of magnesium was added. A solution of 2.94 mL (26 mmol) of ortho-fluorobromobenzene in 20 mL of THF was then added slowly at room temperature.…”

“…Recent synthetic efforts yielded 7-silanorbornadienes with new substitution patterns and provided important information on the molecular structure of 7-silanorbornadienes [13][14][15][16]. This new data prompted us to reinvestigate the origin of the unusual 29 Si NMR chemical shift of 7-silanorbornadienes on the basis of the obtained structural data with the help of solid-state NMR measurements and density functional computations.…”