Synthesis and Crystal Structure of a Silyl‐Stabilized Allyl Cation Formed by Disruption of an Arene by a Protonation–Hydrosilylation Sequence

Abstract

Sly silyl caught in the act: Protonation of a mesitylene ring by the strongly acidic arenium carborane [CH(3)C(6)H(6)]- [CHB(11)Me(5)Br(6)] initiates a cascade reaction that results in a stable beta-silyl allyl cation (see picture, H yellow, C blue, silyl allyl group red). Remarkably, the driving force in the reaction suffices to disrupt a stable aromatic ring in favor of a cationic reactive intermediate.

Planar Chiral, Ferrocene‐Stabilized Silicon Cations

“…Siegel and co‐workers made a similar observation during the generation of a silylium ion decorated with a meta ‐terphenyl scaffold . Although a 29 Si NMR chemical shift of δ =79.1 ppm corresponded to an intramolecularly arene‐stabilized silicon cation, an additional signal was observed at δ =−4.2 ppm, which was eventually ascribed to a silicon‐stabilized allyl cation possibly emerging from a protonation/intramolecular hydrosilylation sequence . Repeating hydride abstraction from ( S p , Si S )‐ 3 b at lower temperature (−40 °C in CD 2 Cl 2 instead of room temperature in 1,2‐Cl 2 C 6 D 4 ) suppressed such side reactions, and the ferrocene‐stabilized silicon cation ( S p )‐ 10 b with a typical chemical shift of δ =112.5 ppm in the 29 Si NMR spectrum was formed (Scheme , bottom).…”

“…Hydride abstraction through Corey's method was applicable to all of them, but the outcome was different in each case. Aryl groups adjacent to the electron‐deficient silicon center displayed π interactions, leading to dynamic processes and, in the case of a naphthyl substituent, even to bond formation between the silicon atom and arene . The best result was obtained with a phenyl group in the proximity of the silicon atom for which a single resonance was detected at δ =117.5 ppm in the 29 Si NMR spectrum.…”

Reductive Elimination and Oxidative Addition of Hydrogen at Organostannylium and Organogermylium Cations

“…have found the same type of cationic hydrogenated aryl moiety in the reaction of mesityl terphenyl substituted dimethyl silicon hydride [Ar'SiHMe 2 ]w ith the strong acid [CH 3 C 6 H 6 ][CHB 11 Br 6 ][ Ar' = 2,6-(2',4',6'-trimethylphenyl)phenyl].I nt he proposed mechanism protonation of the mesityl moiety and formation of aW heland intermediate is followed by hydride transfer to give the cyclohexadiened erivative which reacts with the silylium cation to give an allyl cation. [21] In ap lausible mechanistic sequence for the formation of 7 (Scheme 5) the cationic GeH 2 unit of salt 6 transfers presumably in at ype of protonation and hydride transfer (A,i n Scheme 5) the hydrogen atoms to the carbon atoms C2 and C3 to give ac ationic Ge II compound and as ingle bond between carbon atomsC 2a nd C3 (B,i nS cheme 5). In the next step the cationic low valent germanium atom oxidatively adds aC ÀHb ond of ad ifluorobenzene molecule (C, D in Scheme5).…”

Carboranes in the chemist's toolbox

“…5,6 By this means, isolable C 6 H 7 + , n-C 6 H 13 + , and CMe 3 + salts of HCB 11 F 11 − have been generated from benzene, n-hexane, and (remarkably) n-butane, respectively. Among these are (n-C 4 H 9 ) 3 Sn + , 8 protonated porphyrins, 9 imidazolium salts, 10,11 C 59 N + (azafullerene), 12,13 silyl-stabilized allyl cations, 14 chloroalkanes, 15 ClR 2 + (chloronium ions, R = Me or Et), 16 oxonium ions, 17 R 3 E + ions where E is Ge, Sn, or Pb, 18 [19][20][21] Also noteworthy is the protonation of fullerenes, with the isolation of HC 60 + CB 11 H 5 Cl 6 − as a stable salt and its structural characterization via 13 C CPMAS spectroscopy, revealing rapid movement of the proton that renders all 60 carbon atoms equivalent on the NMR time scale. 7 The capability of protonating hydrocarbons under ambient conditions affords a powerful tool for investigation in several areas such as hydrocarbon reformation on zeolites, a process which normally occurs at high temperature and has been difficult to study.…”