Synthesis of a Counteranion-Stabilized Bis(silylium) Ion.
ABSTRACT: The preparation of a molecule with two alkyl-tethered silylium-ion sites from the corresponding bis(hydrosilanes) by two-fold hydride abstraction is reported. The length of the conformationally flexible alkyl bridge is crucial as otherwise the hydride abstraction stops at the stage of a cyclic bissilylated hydronium ion. With an ethylene tether, the open form of the hydronium-ion intermediate is energetically accessible and engages in another hydride abstraction. The resulting bis(silylium) ion has been NMR spectroscopically and structurally characterized. Related systems based on rigid naphthalen-n,m-diyl platforms can only be converted into the dications when the positively charged silylium-ion units are remote from each other (1,8 versus 1,5 and 2,6).
Project description:An in-depth experimental and theoretical study of the substituent exchange reaction of silylium ions is presented. Apart from the substitution pattern at the silicon atom, the selectivity of this process is predominantly influenced by the counteranion, which is introduced with the trityl salt in the silylium ion generation. In contrast to Müller's protocol for the synthesis of triarylsilylium ions under kinetic control, the use of Reed's carborane anions leads to contact ion pairs, allowing selective formation of trialkylsilylium ions under thermodynamic control. DFT calculations finally revealed an unexpected mechanism for the rate-determining alkyl exchange step, which is initiated by an unusual 1,2-silyl migration in the intermediate <i>ipso</i>-disilylated arenium ion. The resulting <i>ortho</i>-disilylated arenium ion can then undergo an alkyl transfer <i>via</i> a low-barrier five-centered transition state.
Project description:Novel silylium ions with N-heterocyclic imines were successfully synthesized. The reaction of trimethylsilyl imidazolin-2-imine Me?SiNIPr (NIPr = bis(2,6-diisopropylphenyl)-imidazolin-2-imino) with B(C?F?)? leads to dimeric imino-substituted silylium ions through a methyl group abstraction on the silicon atom. Meanwhile, the intermolecular imino-coordinated silylium ion is formed by using the less sterically crowded imine Me?SiNItBu (NItBu = bis(tert-butyl)-imidazolin-2-imino). Furthermore, the treatment of dimethylchlorosilane Me?(Cl)SiNIPr with AgOTf affords the contact ion pair Me?(OTf)SiNIPr by substitution of the chloride. A novel complex with the formula [Me?(DMAP)SiNIPr][OTf] was prepared by coordination with 4-dimethylamino-pyridine (DMAP). In the solid state, the DMAP adduct [Me?(DMAP)SiNIPr][OTf] contains a distinct [Me?(DMAP)SiNIPr]? moiety.
Project description:A metal-free, intermolecular syn-addition of hexamethyldisilane across simple alkenes is reported. The catalytic cycle is initiated and propagated by the transfer of a methyl group from the disilane to a silylium-ion-like intermediate, corresponding to the (re)generation of the silylium-ion catalyst. The key feature of the reaction sequence is the cleavage of the Si-Si bond in a 1,3-silyl shift from silicon to carbon. A central intermediate of the catalysis was structurally characterized by X-ray diffraction, and the computed reaction mechanism is fully consistent with the experimental findings.
Project description:Instead of yielding the desired non-classical silylium ions, the reactions of different alkenes/alkynes with several [Me3Si]+ sources mostly led to oligomerization, or - in the presence of Me3SiH - hydrosilylation of the alkenes/alkynes. Yet, from the reaction of 2-butyne with ion-like Me3Si-F-Al(ORF)3 (RF = C(CF3)3) the salt of the silylated tetramethyl cyclobutenyl cation [Me4C4-SiMe3]+[al-f-al]- 1 ([al-f-al]- = [(RFO)3Al-F-Al(ORF)3]-) was obtained in good yield (NMR, scXRD, Raman, and IR). All the experimental and calculated evidence suggest a mechanism in which 1 was formed via a non-classical silylium ion as an intermediate. The removal of the [Me3Si]+ moiety from the cation in 1 was investigated as a means to provide free tetramethyl cyclobutadiene (CBD). However, the addition of [NMe4]F, in order to release Me3SiF and form CBD, led to the unexpected deprotonation of the cation. The addition of 4-dimethylaminopyridine to remove the [Me3Si]+ cation as a Lewis acid/base adduct, led to an adduct with the four-membered ring in the direct neighborhood of the Me3Si group. By the addition of Et2O to a solution of 1, the [F-Al(ORF)3]- anion (and Et2O-Al(ORF)3) was generated from the [al-f-al]- counterion. Subsequently, the [F-Al(ORF)3]- anion abstracted the [Me3Si]+ moiety from [Me4C4-SiMe3]+, probably releasing CBD. However, due to the immediate reaction of CBD with [Me4C4-SiMe3]+ and subsequent oligomerization, it was not possible to use CBD in follow-up chemistry.
Project description:The hydride-bridged silylium cation [Et<sub>3</sub> Si-H-SiEt<sub>3</sub> ]<sup>+</sup> , stabilized by the weakly coordinating [Me<sub>3</sub> NB<sub>12</sub> Cl<sub>11</sub> ]<sup>-</sup> anion, undergoes, in the presence of excess silane, a series of unexpected consecutive reactions with the valence-isoelectronic molecules CS<sub>2</sub> and CO<sub>2</sub> . The final products of the reaction with CS<sub>2</sub> are methane and the previously unknown [(Et<sub>3</sub> Si)<sub>3</sub> S]<sup>+</sup> cation. To gain insight into the entire reaction cascade, numerous experiments with varying conditions were performed, intermediate products were intercepted, and their structures were determined by X-ray crystallography. Besides the [(Et<sub>3</sub> Si)<sub>3</sub> S]<sup>+</sup> cation as the final product, crystal structures of [(Et<sub>3</sub> Si)<sub>2</sub> SMe]<sup>+</sup> , [Et<sub>3</sub> SiS(H)Me]<sup>+</sup> , and [Et<sub>3</sub> SiOC(H)OSiEt<sub>3</sub> ]<sup>+</sup> were obtained. Experimental results combined with supporting quantum-chemical calculations in the gas phase and solution allow a detailed understanding of the reaction cascade.
Project description:Chemo- and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d- and f-block metal complexes. The cationic hydride of calcium [CaH]+ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti-Markovnikov products, while aryl-substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di- and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy- and siloxy-substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.
Project description:Diorganyl[2-(trimethylsilylethynyl)phenyl]silanes 1a-c and methyl-substituted phenylsilanes 1d and 1e were treated with a small amount of trityl tetrakis(pentafluorophenyl)borate (TPFPB) as an initiator in benzene to afford the corresponding benzosiloles (2a-e) in moderate to good yields. However, no reaction was observed for the reaction using [2-(1-hexynyl)phenyl]diisopropylsilane lf. The methyl substituent was tolerated under the reaction conditions and increased the yield of the corresponding benzosilole depending on the substitution position. From the result using 1f, the current reaction was found to require the trimethylsilyl group, which can stabilize intermediary alkenyl carbocations by the ?-silyl effect. The current reaction can be considered an intramolecular chain hydrosilylation of alkynylarylsilanes involving silyl cations as chain carriers. Therefore, the silyl cations generated by hydride abstraction from hydrosilanes 1 with the trityl cation causes intramolecular electrophilic addition to the C-C triple bond to form ethenyl cations, which abstract a hydride from 1 to afford benzosiloles 2 with the regeneration of the silyl cations.
Project description:Tertiary silane?1H , 2-[(diphenylsilyl)methyl]-6-methylpyridine, reacts with tris(pentafluorophenyl)borane (BCF) to form the intramolecular pyridine-stabilized silylium?1+ -HBCF. The corresponding 2-[(diphenylsilyl)methyl]pyridine, lacking the methyl-group on the pyridine ring, forms classic N(py)?B adduct 2H -BCF featuring an intact silane Si-H fragment. Complex?1+ -HBCF promotes cleavage of the C?O triple bond in carbon monoxide with double C-Csp2 bond formation, leading to complex?3 featuring a B-(diarylmethyl)-B-aryl-boryloxysilane fragment. Reaction with pinacol generates bis(pentafluorophenyl)methane?4 as isolable product, proving the transition-metal-free deoxygenation of carbon monoxide by this main-group system. Experimental data and DFT calculations support the existence of an equilibrium between the silylium-hydroborate ion pair and the silane-borane mixture that is responsible for the observed reactivity.
Project description:LCuOTf complexes [L = cyclic (alkyl)(amino)carbenes (CAACs) or N-heterocyclic carbenes (NHCs)] selectively promote the dehydrogenative borylation of C(sp)-H bonds at room temperature. It is shown that ?,?-bis(copper) acetylide and copper hydride complexes are the key catalytic species.
Project description:Variable temperature spectroscopic, kinetic, and chemical studies were performed on a soluble CrIIICl3(PNP) (PNP = bis(diarylphosphino)alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence. These studies indicate that treatment of CrIIICl3(PNP) with MAO leads to first replacement of chlorides with alkyl groups, followed by alkyl abstraction, and then reduction to lower-valent species. Reactivity studies demonstrate that the majority of the chromium species detected is not catalytically active.