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Martin Oestreich - One of the best experts on this subject based on the ideXlab platform.
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Silylium ion promoted 5 1 cycloaddition of aryl substituted vinylcyclopropanes and hydrosilanes involving aryl migration
Angewandte Chemie, 2020Co-Authors: Guoqiang Wang, Hendrik F. T. Klare, Vittorio Bonetti, Martin OestreichAbstract:A transition-metal-free (5+1) cycloaddition of aryl-substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self-regeneration of the Silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4- rather than 3-aryl-substituted silacyclohexane derivatives as major products. Various control experiments and quantum-chemical calculations support a mechanistic picture where a Silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane-to-cyclopropane rearrangement.
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Silylium‐Ion‐Promoted (5+1) Cycloaddition of Aryl‐Substituted Vinylcyclopropanes and Hydrosilanes Involving Aryl Migration
Angewandte Chemie (International ed. in English), 2020Co-Authors: Guoqiang Wang, Hendrik F. T. Klare, Vittorio Bonetti, Martin OestreichAbstract:A transition-metal-free (5+1) cycloaddition of aryl-substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self-regeneration of the Silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4- rather than 3-aryl-substituted silacyclohexane derivatives as major products. Various control experiments and quantum-chemical calculations support a mechanistic picture where a Silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane-to-cyclopropane rearrangement.
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Synthesis of a Counteranion‐Stabilized Bis(Silylium) Ion
Angewandte Chemie (International ed. in English), 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Elisabeth Irran, Guoqiang Wang, Martin OestreichAbstract: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).
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synthesis of a counteranion stabilized bis Silylium ion
Angewandte Chemie, 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Elisabeth Irran, Guoqiang Wang, Martin OestreichAbstract: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).
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Silylium-Ion-Promoted Ring-Opening Hydrosilylation and Disilylation of Unactivated Cyclopropanes.
Organic letters, 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Vittorio Bonetti, Guoqiang Wang, Martin OestreichAbstract:A Silylium-ion-promoted ring-opening hydrosilylation of unactivated cyclopropanes is reported. The reaction is facilitated by the γ-silicon effect, and the regioselectivity is influenced by various stabilizing effects on the carbenium-ion intermediates, including the β-silicon effect. The experimental observations are in accord with the computed reaction mechanism. The work also showcases the ability of Silylium ions to isomerize cyclopropyl to allyl groups, and the resulting α-olefins engage in a Silylium-ion-mediated disilylation with hexamethyldisilane.
Hendrik F. T. Klare - One of the best experts on this subject based on the ideXlab platform.
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Silylium ion promoted 5 1 cycloaddition of aryl substituted vinylcyclopropanes and hydrosilanes involving aryl migration
Angewandte Chemie, 2020Co-Authors: Guoqiang Wang, Hendrik F. T. Klare, Vittorio Bonetti, Martin OestreichAbstract:A transition-metal-free (5+1) cycloaddition of aryl-substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self-regeneration of the Silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4- rather than 3-aryl-substituted silacyclohexane derivatives as major products. Various control experiments and quantum-chemical calculations support a mechanistic picture where a Silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane-to-cyclopropane rearrangement.
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Silylium‐Ion‐Promoted (5+1) Cycloaddition of Aryl‐Substituted Vinylcyclopropanes and Hydrosilanes Involving Aryl Migration
Angewandte Chemie (International ed. in English), 2020Co-Authors: Guoqiang Wang, Hendrik F. T. Klare, Vittorio Bonetti, Martin OestreichAbstract:A transition-metal-free (5+1) cycloaddition of aryl-substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self-regeneration of the Silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4- rather than 3-aryl-substituted silacyclohexane derivatives as major products. Various control experiments and quantum-chemical calculations support a mechanistic picture where a Silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane-to-cyclopropane rearrangement.
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Synthesis of a Counteranion‐Stabilized Bis(Silylium) Ion
Angewandte Chemie (International ed. in English), 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Elisabeth Irran, Guoqiang Wang, Martin OestreichAbstract: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).
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synthesis of a counteranion stabilized bis Silylium ion
Angewandte Chemie, 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Elisabeth Irran, Guoqiang Wang, Martin OestreichAbstract: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).
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Silylium-Ion-Promoted Ring-Opening Hydrosilylation and Disilylation of Unactivated Cyclopropanes.
Organic letters, 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Vittorio Bonetti, Guoqiang Wang, Martin OestreichAbstract:A Silylium-ion-promoted ring-opening hydrosilylation of unactivated cyclopropanes is reported. The reaction is facilitated by the γ-silicon effect, and the regioselectivity is influenced by various stabilizing effects on the carbenium-ion intermediates, including the β-silicon effect. The experimental observations are in accord with the computed reaction mechanism. The work also showcases the ability of Silylium ions to isomerize cyclopropyl to allyl groups, and the resulting α-olefins engage in a Silylium-ion-mediated disilylation with hexamethyldisilane.
Thomas Müller - One of the best experts on this subject based on the ideXlab platform.
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single electron transfer reactions in frustrated and conventional Silylium ion phosphane lewis pairs
Angewandte Chemie, 2018Co-Authors: Anastasia Merk, Hendrik F. T. Klare, Martin Oestreich, Marc Schmidtmann, Henning Grosekappenberg, Marcelphilip Luecke, Christian Lorent, Matthias Driess, Thomas MüllerAbstract:Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylSilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylSilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu3 P, which does not react with either B(C6 F5 )3 or Silylium ions.
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Quantitative Assessment of the Lewis Acidity of Silylium Ions
Organometallics, 2015Co-Authors: Henning Großekappenberg, Matti Reißmann, Marc Schmidtmann, Thomas MüllerAbstract:The Lewis acidity of several aryl-substituted tetrylium ions was classified experimentally by applying the Gutmann–Beckett method and computationally by calculation of fluoride ion affinities (FIA) (tetrel elements = Si, Ge). According to these measures, tetrylium ions are significantly more Lewis acidic than boranes, and aryl-substituted Silylium borates are among the strongest isolable Lewis acids. A fine-tuning of the Lewis acidity of Silylium ions is possible by taking advantage of electronic and/or steric substituent effects.
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Cyclic silylated onium ions of group 15 elements.
Inorganic chemistry, 2015Co-Authors: Matti Reißmann, André Schäfer, Marc Schmidtmann, Michael Bolte, Robin Panisch, Thomas MüllerAbstract:Five- and six-membered cyclic silylated onium ions of group 15 elements I were synthesized by intramolecular cyclization of transient Silylium ions II. Silylium ions II were prepared by the hydride transfer reaction from silanes III using trityl cation as hydride acceptor. It was found that smaller ring systems could not be obtained by this approach. In these cases tritylphosphonium ions IV were isolated instead. Cations I and IV were isolated in the form of their tetrakispentafluorphenyl borates and characterized by multinuclear NMR spectroscopy and, in two cases, by X-ray diffraction analysis. Cyclic onium ions I showed no reactivity similar to that of isoelectronic intramolecular borane/phosphane frustrated Lewis pairs (FLPs). The results of DFT computations at the M05-2X level suggest that the strength of the newly formed Si–E linkage is the major reason for inertness of I[B(C6F5)4] versus molecular hydrogen.
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Silylium Ion/Phosphane Lewis Pairs
Organometallics, 2013Co-Authors: Matti Reißmann, André Schäfer, Sebastian Jung, Thomas MüllerAbstract:The reactivity of a series of Silylium ion/phosphane Lewis pairs was studied. TriarylSilylium borates 4[B(C6F5)4] form frustrated Lewis pairs (FLPs) of moderate stability with sterically hindered phosphanes 2. Some of these FLPs are able to cleave dihydrogen under ambient conditions. The combination of bulky trialkylphosphanes with triarylSilylium ions can be used to sequester CO2 in the form of silylacylphosphonium ions 12. The ability to activate molecular hydrogen by reaction of Silylium ion/phosphane Lewis pairs is dominated by thermodynamic and steric factors. For a given Silylium ion increasing proton affinity and increasing steric hindrance of the phosphane proved to be beneficial. Nevertheless, excessive steric hindrance leads to a breakdown of the dihydrogen-splitting activity of a Silylium/phosphane Lewis pair.
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Silylium ion phosphane lewis pairs
Organometallics, 2013Co-Authors: Matti Reismann, André Schäfer, Sebastian Jung, Thomas MüllerAbstract:The reactivity of a series of Silylium ion/phosphane Lewis pairs was studied. TriarylSilylium borates 4[B(C6F5)4] form frustrated Lewis pairs (FLPs) of moderate stability with sterically hindered phosphanes 2. Some of these FLPs are able to cleave dihydrogen under ambient conditions. The combination of bulky trialkylphosphanes with triarylSilylium ions can be used to sequester CO2 in the form of silylacylphosphonium ions 12. The ability to activate molecular hydrogen by reaction of Silylium ion/phosphane Lewis pairs is dominated by thermodynamic and steric factors. For a given Silylium ion increasing proton affinity and increasing steric hindrance of the phosphane proved to be beneficial. Nevertheless, excessive steric hindrance leads to a breakdown of the dihydrogen-splitting activity of a Silylium/phosphane Lewis pair.
Dieter Cremer - One of the best experts on this subject based on the ideXlab platform.
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Theory as a viable partner for experiment—The quest for trivalent Silylium ions in solution
Theoretical and Computational Chemistry, 1999Co-Authors: Carl-henrik Ottosson, Elfi Kraka, Dieter CremerAbstract:Publisher Summary This chapter presents the debate in connection with the nature of Silylium ions in condensed phases and the role that quantum chemical investigations played and still play in this debate. It is not so difficult to design a Silylium cation that by strong internal complexation should be unable to interact with solvent molecules. The question on the existence of a free Silylium cation in solution is approached in the chapter by clarifying the properties that a free Silylium cation in solution should have and the ways in which these properties can bedetermined by either experimental or theoretical means. After these basic considerations, the chapter discusses and evaluates the various attempts of preparing free Silylium cations in solution. There is a large overlap between the Silylium cation problem and the problem of investigating ion solvation in general. Chemists understand the various steps in a solvation process, the accompanying changes in the properties of the solute, and its consequences for chemical reactivity. Solvent–solute interactions span the broad spectrum from Van der Waals type interactions to bonding between the interacting molecules.
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theory as a viable partner for experiment the quest for trivalent Silylium ions in solution
Theoretical and Computational Chemistry, 1999Co-Authors: Carl-henrik Ottosson, Elfi Kraka, Dieter CremerAbstract:Publisher Summary This chapter presents the debate in connection with the nature of Silylium ions in condensed phases and the role that quantum chemical investigations played and still play in this debate. It is not so difficult to design a Silylium cation that by strong internal complexation should be unable to interact with solvent molecules. The question on the existence of a free Silylium cation in solution is approached in the chapter by clarifying the properties that a free Silylium cation in solution should have and the ways in which these properties can bedetermined by either experimental or theoretical means. After these basic considerations, the chapter discusses and evaluates the various attempts of preparing free Silylium cations in solution. There is a large overlap between the Silylium cation problem and the problem of investigating ion solvation in general. Chemists understand the various steps in a solvation process, the accompanying changes in the properties of the solute, and its consequences for chemical reactivity. Solvent–solute interactions span the broad spectrum from Van der Waals type interactions to bonding between the interacting molecules.
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TrimesitylSilylium cation verification of a free Silylium cation in solution by NMR chemical shift calculations
Chemical Physics Letters, 1997Co-Authors: Elfi Kraka, Carlos P. Sosa, Jürgen Gräfenstein, Dieter CremerAbstract:NMR chemical shift calculations at the SOS-DFPT/PW91/[9s6p2d/5s4pld/5s4pld/3s]//B3LYP/6-31G(d) level of theory were used to describe the trimesitylSilylium cation 1, recently synthesized in benzene solution and investigated by NMR spectroscopy. The conformation of cation 1 is characterized by mesityl rings rotated by 47° in a propeller-like form. Contrary to other Silylium cations investigated, cation 1 forms a weak Van der Waals complex 3 with benzene rather than a Wheland σ-complex. The calculated 29Si NMR chemical shifts for 1 and 3 are 226 and 227 ppm, compared to the experimental value of 225.5 ppm. The agreement between calculated and measured NMR chemical shifts provides evidence that cation 1 presents the first free Silylium cation synthesized.
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The Tris(9-borabicyclo[3.3.1]nonyl)Silylium Cation: A Suggestion for a Weakly Coordinated Silylium Cation in Solution
Organometallics, 1997Co-Authors: Carl-henrik Ottosson, Kálmán J. Szabó, Dieter CremerAbstract:The tris(9-borabicyclo[3.3.1]nonyl)Silylium cation, Si(9-BBN)3+ (3), was investigated by ab initio quantum chemical methods, where, in particular, its coordination ability with benzene was studied....
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Nature of the Si(SiMe3)3+ Cation in Aromatic Solvents
Organometallics, 1996Co-Authors: Carl-henrik Ottosson, Dieter CremerAbstract:The Silylium cation Si(SiMe3)3+ (1) was investigated by HF, B3LYP, PISA-HF, SCRF, and the NMR/ab initio/IGLO approach in the gas phase and in benzene solution employing the 6-31G(d) basis set. In the gas phase, the δ(29Si) value of the central Si atom of 1 is 920 ppm relative to that of TMS according to IGLO/[7s6p2d/5s4p1d/3s1p]//HF/6-31G(d) calculations, which is in line with the corresponding δ(29Si) values calculated for other silyl-substituted Silylium cations and results from large paramagnetic contributions to the δ(29Si) shift value. In benzene solution, 1 forms the Wheland σ-complex Si(SiMe3)3-C6H6+ (3), in which the Silylium cation character of 1 is lost despite the fact that 3 is the weakest Silylium cation−benzene complex so far investigated. According to NMR/ab initio/IGLO calculations, complex 3 is characterized by a weak covalent SiC bond of 2.293 A, a coordination energy of 13.6 kcal/mol, and a δ(29Si) value of 111 ppm. Steric blocking by three SiMe3 groups is not sufficient to prevent a si...
Elisabeth Irran - One of the best experts on this subject based on the ideXlab platform.
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Synthesis of a Counteranion‐Stabilized Bis(Silylium) Ion
Angewandte Chemie (International ed. in English), 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Elisabeth Irran, Guoqiang Wang, Martin OestreichAbstract: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).
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synthesis of a counteranion stabilized bis Silylium ion
Angewandte Chemie, 2020Co-Authors: Avijit Roy, Hendrik F. T. Klare, Elisabeth Irran, Guoqiang Wang, Martin OestreichAbstract: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).
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Catalytic Difunctionalization of Unactivated Alkenes with Unreactive Hexamethyldisilane through Regeneration of Silylium Ions
Angewandte Chemie (International ed. in English), 2019Co-Authors: Avijit Roy, Hendrik F. T. Klare, Elisabeth Irran, Stefan Grimme, Martin OestreichAbstract: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.
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Electrophilic Formylation of Arenes by Silylium Ion Mediated Activation of Carbon Monoxide.
Angewandte Chemie (International ed. in English), 2018Co-Authors: Lukas Omann, Hendrik F. T. Klare, Elisabeth Irran, Stefan Grimme, Martin OestreichAbstract:Arene-stabilized Silylium ions react with carbon monoxide rather than carbon monoxide adducts of Silylium ions reacting with arenes. This mechanism is supported by quantum-chemical calculations. Even sterically hindered mesitylene and electronically deactivated chlorobenzene engage in this electrophilic aromatic substitution. The Silylium ion mediated formylation corresponds to Gattermann-Koch reactions promoted by strong Bronsted acids. The resulting silylcarboxonium ion of the arenecarbaldehyde was crystallographically characterized, for the first time revealing the molecular structure of this synthetically important intermediate.
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Thermodynamic versus kinetic control in substituent redistribution reactions of Silylium ions steered by the counteranion
Chemical science, 2018Co-Authors: Lukas Omann, Hendrik F. T. Klare, Elisabeth Irran, Bimal Pudasaini, Mu-hyun Baik, Martin OestreichAbstract: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 Muller'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 ipso-disilylated arenium ion. The resulting ortho-disilylated arenium ion can then undergo an alkyl transfer via a low-barrier five-centered transition state.
