Silyl Cations Research Articles

Silyl Cations Stabilized by Pincer Type Ligands with Adjustable Donor Atoms

Novel E,C,E'‐pincer supported silyl cations (E, E' = O, S, Se, Au) were prepared in three steps starting from 2,6‐F2C6H3SiMe2H (1a) and 2,6‐Br2C6H3SiMe2H (1b), which were first converted in two complementary ways into 2,6‐(Ph2P)2C6H3SiMe2H (2). The oxidation of 2 with H2O2·urea, S8, and Se8 afforded 2,6‐(Ph2PE)2C6H3SiMe2H (3a, E = O; 3b, E = S; 3c, E = Se) and 2‐(Ph2PE)‐6‐(Ph2P)‐C6H3SiMe2H (4b, E = S; 4c, E = Se), which were reacted to the E,C,E‐supported silyl cations [2,6‐(Ph2PE)2C6H3SiMe2]+ (5a, E = O, counterion Br3–; 5b, E = S, counterion B(C6F5)4–; 5c, E = Se, counterion B(C6F5)4–), the E,C‐supported silyl cations [2‐(Ph2PE)‐6‐(Ph2P)C6H3SiMe2]+ (6b, E = S, not isolated; 6c, E = Se, not isolated), the O,C,S‐supported silyl cation [2‐(Ph2PS)‐6‐(Ph2PO)C6H3SiMe2]+ (7, counterion B(C6F5)4–) as well as the E,C,Au‐supported silyl cations [2‐(Ph2PAuC6F5)‐6‐(Ph2PE)C6H3SiMe2]+ (8b, E = S, counterion [B{3,5‐(CF3)2C6H3}4]–; 8c, E = Se, [B{3,5‐(CF3)2C6H3}4]–) using Br2, O2, S8, (tht)AuC6F5, Ph3C[B(C6F5)4] and Ph3C[B{3,5‐(CF3)2C6H3}4]. All compounds were characterized by multinuclear (1H, 13C, 19F, 29Si, 31P, 77Se) NMR spectroscopy, ESI MS spectrometry and X‐ray crystallography (2, 3a·H2O, 3b, 3c, 4b, 5a, 5c, 7, 8b, 8c). The gas phase structures of 2, 3a–c, 5a–c (fully optimized) and 8b, 8c (single‐point calculations) were studied at the B3PW91/6‐311+G(2df,p) level of theory. A set of real‐space bonding indicators (RSBIs) derived from the theoretically calculated electron and pair densities were analyzed utilizing the atoms‐in molecules (AIM) and electron‐localizability indicator (ELI‐D) space partitioning schemes.

A Mono-Substituted Silicon(II) Cation: A Crystalline "Supersilylene".

Mono‐coordinated silicon(II) cations are predicted to be reactive ambiphiles, combining the typically high Lewis acidity of silicon cations with nucleophilicity due to the presence of an electron pair at the same atomic centre. Here, a carbazole‐derived scaffold was used to isolate salts with a mono‐coordinated silicon(II) cation, [RSi]+ (R=bulky carbazolyl substituent), by halide abstraction from a base‐free halosilylene, RSiI, with Ag[Al(OtBuF)4]. Despite the bulk of the carbazolyl moiety, the silylenylium cation [RSi]+ retains high reactivity. It was shown to react with an amine to form three bonds at the silicon atom in one reaction which conforms with the notion of a “supersilylene”. The resulting silylium cation [RSi(H)NR′2]+ (in the formal oxidation state SiIV) obtained by oxidative addition of an NH bond at [RSi]+ is even more acidic than the silylenylium cation (SiII) due to the absence of a lone pair of electrons the silicon atom.

Synthesis of Boryl-Substituted Disilane, Disilene, and Silyl Cation

Reduction of (boryl)dibromosilane (boryl)SiHBr2 (1; boryl = B(NArCH)2, Ar = 2,6-iPr2C6H3) with 2 equiv of potassium graphite in DME at low temperature led to the isolation of bis(boryl)disilane (bo...

Silylium cation initiated sergeants-and-soldiers type chiral amplification of helical aryl isocyanide copolymers

Silylium cations act as new highly efficient metal-free single-component cationic initiators for the cationic polymerization and copolymerization of chiral or achiral aryl isocyanides, preparing optically active polymers and copolymers obeying “sergeants-and-soldiers” rule.

Chiral Memory in Silyl-Pyridinium and Quinolinium Cations.

Pyridine- and quinoline-stabilized silyl cations have been prepared, and their structure in condensed phases unambiguously assigned using 1H, 13C, 15N, 29Si, and 1H DOSY NMR as well as X-ray diffraction studies. Solid state structures thus show in both cases a stabilization of the cationic silicon center through an N-Si interaction and formation of a highly strained four-membered ring system. Chiral memory at the silicon atom in these heterocycle-stabilized silyl cations was also established, leading to various levels of selectivity depending on the nature of the heterocycle. Lowest energy conformations of the starting silanes obtained through DFT calculations, along with the isolation and characterization of the Si-centered chiral silyl cation intermediates, finally allowed to propose a plausible hypothesis as to the configurational stability of these silyl cations.

Photoredox Reaction of 2-Mercaptothiazolinium Salts with Silyl Enol Ethers.

A method for the generation of free radicals from thiazolinium salts upon photocatalytic reduction is described. The thiazolinium salts are generated by treatment with methyl triflate of 2-mercaptothiazolines, which can be readily obtained from alkyl bromides and tosylates via a nucleophilic substitution reaction or by hydrothiolation of alkenes. Silyl enol ethers were used to trap the radicals, furnishing ketones after successive single-electron oxidation and elimination of the silyl cation.

An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

A new NMR‐based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p‐fluorobenzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The 19F NMR chemical shifts and the 1 J(CF) coupling constants of these nitrilium ions vary in a predictable manner with the donor capability of the stabilizing group. The spectroscopic parameters are suitable probes for scaling the acidity of Lewis acids. These new probes allow for the discrimination between very similar Lewis acids, which is not possible with conventional NMR tests, such as the well‐established Gutmann–Beckett method.

Isolation, characterization and reactivity of three-coordinate phosphorus dications isoelectronic to alanes and silylium cations.

Three-coordinate main group Lewis acids are exceedingly important reagents in chemical synthesis. In contrast to the well-established chemistries of neutral group 13 and cationic group 14 species, isoelectronic group 15 element dications are unknown. In this work, we use stabilizing N-heterocyclic imine substituents to isolate and characterize phosphorandiylium dications ([R3P]2+) and show that the electrophilicity at the phosphorus atoms is controlled by the π-electron-donating ability of these subtituents. Structural, spectroscopic and theoretical results reveal that the phosphorus dications adopt a perfectly trigonal-planar geometry with the electron-deficient phosphorus centres being well separated from the borate anions. The reactivity of the dications reveal their exceptional Lewis acidity at phosphorus; the adjacent nitrogen atoms, however, are weakly basic, resulting in transformations such as chloride ion abstraction from Me3SiCl and the selective monodefluorination of trifluoromethyl groups.

Follow the Silyl Cation: Insights into the Vorbrüggen Reaction

To succeed in the development of a mechanistic model for the Vorbruggen reaction of thymine with an advanced furanose intermediate, a range of spectroscopic techniques were applied and found essent...

An acidic route to silicon cations

Inorganic Chemistry The simplest silicon cation, with a central Si atom bonded to just three hydrogens, has long eluded bulk synthesis. Wu et al. now report a straightforward route to this molecule by reacting a carborane acid with phenyl silane, producing benzene and the silylium carborane ion pair. A similar protocol offered efficient syntheses of primary and secondary silyl cations through acidic cleavage of Si–phenyl or Si–H bonds. All three products, characterized crystallographically and in solution, manifested weak coordination to bromine substituents of the carboranes. Science , this issue p. [168][1] [1]: /lookup/doi/10.1126/science.aax9184

A Simple and Effective Method of Determining Lewis Acidity by Using Fluorescence

Summary The strength of a Lewis acid is an important parameter in establishing its utility for practical applications. Here, we report a universal fluorescence-based method for accurately determining the strength of a range of Lewis acids. Utilizing a fluorescent dithienophosphole oxide architecture, we are able to emulate the structural motif of the widely employed Gutmann-Beckett method. This allows us to precisely correlate the optical response to the strength of a Lewis acid. In comparison to commonly used methods that employ 31P NMR chemical shifts or quantum-chemical calculations, our method strongly suggests that using fluorescent Lewis acid-base adducts (FLAs) is more reliable. Moreover, our method provides a direct measure of the Lewis acidity of a given solution, independent of the Lewis base, with direct implications to the overall utility of a Lewis acid in practical applications.

Silicon–Halogen Bond Activation in Mixed Si/Al Compounds and an Approach to Intramolecular Stabilized Silylium Ions

Alkenylchlorosilanes functionalized by coordinatively unsaturated Al atoms, R′′(R′)Si(Cl)C(AltBu2)=C(H)R 5 [R = tBu, cHex, 1‐Ad, Ph, 3,5‐(F3C)2C6H3; R′ = Mes, Ph, 4‐tBuC6H4, 4‐tBuOC6H4, 4‐Et2NC6H4, tBu; R′′ = Mes, CH(SiMe3)2, tBu], were obtained in good yields by hydroalumination of the respective alkynylchlorosilanes R′′(R′)Si(Cl)C≡C‐R 3 with tBu2Al‐H. They feature four‐membered, close to planar SiCAlCl heterocycles by intramolecular Al–Cl interactions. The activation of the Si–Cl bonds results in bond lengthening from about 209 pm in 3 to 220 to 225 pm in compounds 5. The 29Si NMR resonances show an increasing shift to a lower field and range from δ = 24.0 for 5a [R = tBu, R′ = R′′ = Mes] to δ = 68.1 for 5j [R = R′ = R′′ = tBu]. The latter value approaches those of solvent coordinated silyl cations such as Et3Si(C6H5Me)+. The related fluorides R′(tBuC≡C)Si(F)C(AltBu2)C = C(H)tBu 9 (R′ = Ph, Mes) were similarly obtained by hydroalumination of alkynylsilanes R′Si(F)(C≡C‐tBu)2 8. Compounds 9 are dimeric via Si–F–Al bridges and form eight‐membered C2Al2Si2F2 heterocycles. The 29Si NMR shifts of δ = –4.7 and –14.7 and Si–F bond lengths of about 170 pm indicate the presence of four‐coordinate, covalently bound Si atoms. Treatment of 8a with two equivalents of tBu2Al‐H led to H/F exchange and formation of tBu2Al‐F which is coordinated by monomeric 9a via Si–F–Al and Al–F–Al bridges to form a six‐membered SiCAlFAlF heterocycle.

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

AbstractInvited for the cover of this issue is the group of Nadia C. Mösch‐Zanetti at the University of Graz. The image depicts the victorious boron getting the hydride while the defeated molybdenum oxide is left with the silyl cation. Read the full text of the article at 10.1002/chem.201800226.

Chiral Atropisomeric Indenocorannulene Bowls: Critique of the Cahn–Ingold–Prelog Conception of Molecular Chirality

AbstractChiral corannulenes abound, but suffer generally from configurational lability associated with bowl‐to‐bowl inversion, thus obviating questions of stereogenicity and stereoelement construction. In contrast, peri‐annulated corannulenes show greatly increased barriers for bowl‐to‐bowl inversion; specifically indenocorannulenes invert on a time scale too slow to observe by normal NMR methods and raise the possibility of creating chiral atropisomeric bowl‐shaped aromatics. Two methods for preparing indenocorannulene from simple 2‐haloarylcorannulenes—silyl cation C–F activation, and Pd‐mediated C–Cl activation[5]—enable the synthesis of an array of such chiral atropisomeric indenocorannulenes. Resolution of the enantiomers by high‐performance liquid chromatography over chiral support phases motivates the study of chiroptical properties, the assignment of absolute “Cartesian” configuration, and the assessment of configurational stability. These studies bring into question any systematic assignment of nontrivial stereoelements (i.e. not the molecule in its entirety) and refute any assertion of congruence between “Cahn–Ingold–Prelog elements” and the physical or “Cartesian” basis of chirality.

New reactivity at the silicon bridge in sila[1]ferrocenophanes.

We describe two new types of reactivity involving silicon-bridged [1]ferrocenophanes. In an attempt to form a [1]ferrocenophane with a bridging silyl cation, the reaction of sila[1]ferrocenophane [Fe(η-C5H4)2Si(H)TMP] (12) (TMP = 2,2,6,6-tetramethylpiperidyl) towards the hydride-abstraction reagent trityl tetrakis(pentafluorophenyl)borate ([CPh3][B(C6F5)4]) was explored. This yielded the unusual dinuclear species [Fe(η-C5H4)2Si(TMP·H)(η-C5H3)Fe(η-C5H4)Si(H)TMP][B(C6F5)4] [13][B(C6F5)4] in low yield. The formation of [13]+ is proposed to involve abstraction of hydride from the silicon bridge in 12 with subsequent C-H bond cleavage of a cyclopentadienyl group by the resulting electrophilic transient silyl cation intermediate. We also explored the reaction of dimethylsila[1]ferrocenophane [Fe(η-C5H4)2SiMe2] (1) with the Au(i) species AuCl(PMe3). This was found to result in addition of the Au-Cl bond across the Cpipso-Si bond to yield the ring-opened species [1'-(chlorodimethylsilyl)-ferrocenyl](trimethylphosphine)gold(i), [Fe(C5H4SiMe2Cl){C5H4Au(PMe3)}] (14). This represents the first example of ring-opening addition of a metallocenophane with a reagent possessing a transition metal-halogen bond.

ChemInform Abstract: Cooperative Organocatalysis of Mukaiyama‐Type Aldol Reactions by Thioureas and Nitro Compounds

Abstractinvolving a nitro group‐mediated silyl cation transfer

Selective Hydrosilylation of Esters to Aldehydes Catalysed by Iridium(III) Metallacycles through Trapping of Transient Silyl Cations.

The combination of an iridium(III) metallacycle and 1,3,5-trimethoxybenzene catalyses rapidly and selectively the reduction of esters to aldehydes at room temperature with high yields through hydrosilylation followed by hydrolysis. The ester reduction involves the trapping of transient silyl cations by the 1,3,5-trimethoxybenzene co-catalyst, supposedly by formation of an arenium intermediate whose role was addressed by DFT calculations.

Intramolecular Chain Hydrosilylation of Alkynylphenylsilanes Using a Silyl Cation as a Chain Carrier.

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 β-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.

IPr3 Si3 Cl5 (+) : A Highly Reactive Cation with Silanide Character.

The reaction of a metastable SiCl2 solution with the sterically less-demanding carbene N,N-diisopropylimidazo-2-ylidene (IPr) yields the salt [(IPr3 Si3 Cl5 )(+) ]Cl(-) (1-Cl), containing a silyl cation with a Si3 backbone. Salt 1 is highly reactive, but it can be used as a reagent in deuterated dichloromethane, whereby dehalogenation with Me3 SiOTf (OTf=O3 SCF3 ) gives the dicationic silyl halide [(IPr3 Si3 Cl4 )](2+) 2. Quantum chemical calculations show that the HOMO is localized at the negatively charged central silicon atom of 1 and 2, and thus although both compounds are cations they are better described as silanides, which was also corroborated by NMR investigations.

Cationic Si-H-Si Bridges in Polysilanes: Their Detection and Targeted Formation in Stable Ion Studies.

The ionization of 1,1‐dihydridocyclopentasilane 7 has been found to yield the cyclic polysilanylsilyl cation 8 instead of the expected hydrogen‐substituted silylium ion 6. The silyl cation 8 is stabilized by the formation of an intramolecular Si−H−Si bridge, which also provides the thermodynamic driving force for its formation. In general, the preference for the formation of Si−H−Si bridges can be used to scavenge and identify transient intermediates in the Lewis acid induced rearrangement of polysilanes. The validity of this concept has been demonstrated for one central step in this chemistry, the ring‐contraction reaction of cyclohexasilanes to form silylcyclopentasilanes.