Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Solvent effect on simple and high yield synthesis of polydichlorophosphazene precursor Cl3PNSiMe3

The effect of solvent polarity on the reaction of PCl5 with (Me3Si)2NH has been investigated and it is found that nonpolar solvents promote the formation of Cl3PNSiMe3, whereas the more polar chloroform gives unusual phosphazenes at room temperature.

Nonclassical Applications of closo-Carborane Anions: From Main Group Chemistry and Catalysis to Energy Storage

Classically closo-carborane anions, particularly [HCB₁₁H₁₁]^- and [HCB₉H₉]^-, and their derivatives have primarily been used as weakly coordinating anions to isolate reactive intermediates, platforms for stoichiometric and catalytic functionalization, counteranions for simple Lewis acid catalysis, and components of materials like liquid crystals. The aim of this article is to educate the reader on the contemporary nonclassical applications of these anions. Specifically, this review will cover new directions in main group catalysis utilized to achieve some of the most challenging catalytic reactions such as C-F, C-H, and C-C functionalizations that are difficult or impossible to realize with transition metals. In addition, the review will cover the utilization of the clusters as dianionic C σ-bound ligands for coordination chemistry, ligand substituents for coordination chemistry and advanced catalyst design, and covalently bound spectator substituents to stabilize radicals. Furthermore, their applications as solution-based and solid-state electrolytes for Li, Na, and Mg batteries will be discussed.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

An unexpected Staudinger reaction at an N-heterocyclic carbene-carbon center

The previously unreported carbene-phosphine adduct (IPr)PCl2N3 [IPr = (HCNDipp)2C:; Dipp = 2,6-iPr2C6H3] was synthesized and used as a synthon toward the elusive dichlorophosphazene monomer unit, [Cl2P=N]. (IPr)PCl2N3 was found to undergo halide and azide abstraction when combined with various electrophiles and its thermolysis yielded the unexpected Staudinger reaction product (IPr=N)PCl2.

Synthesis of polyphosphazenes with different side groups and various tactics for drug delivery

Polyphosphazenes (PPZs) are hybrid polymers comprising a main chain containing nitrogen and phosphorous linked through interchanging single and double bonds, and side chains.

Electrochemical Oxidation of [1-X-12-I-CB11Me10-] Anions: Formation of Borenium Ylides [12-Dehydro-1-X-CB11Me10] and Iodonium Ylide Anions [{12-(1-X-CB11Me10-)}2I+]

Cyclic voltammograms of 12-iodinated icosahedral carborane anions [1-X-12-I-CB₁₁Me₁₀^-] (X = H, CH₃, C₂H₅, C₃H₇, C₄H₉, C₆H₁₃, and COOCH₃) show two one-electron anodic oxidation peaks at the Pt electrode in liquid SO₂. Oddly, the first is irreversible and the second partially reversible. Mass spectrometry of the principal anionic product of preparative anodic oxidation of [1-H-12-I-CB₁₁Me₁₁^-], identical with the anionic product of its reaction with [Et₃Si-H-SiEt₃]⁺ and/or Et₃Si⁺, allows it to be identified as the iodonium ylide anion [{12-(1-H-CB₁₁Me₁₀^-)}₂I⁺]. Its reversible oxidation to a neutral ylide radical [{12-(1-H-CB₁₁Me₁₀^•)}{12-(1-H-CB₁₁Me₁₀^-)}I⁺] is responsible for the second peak. A DFT geometry optimization suggests that both the ylide anion and the ylide radical are very crowded and have an unusually large C-I-C valence angle of ∼132°; they are the first compounds with two bulky highly methylated CB₁₁ cages attached to the same atom. Molecular iodine is another product of the electrolysis. We propose an electrode mechanism in which initial one-electron oxidation of [1-X-12-I-CB₁₁Me₁₀^-] is followed by a transfer of an iodine atom from the B-I bond to SO₂ to yield a weakly bound radical ISO₂^• which disproportionates into SO₂ and I₂. The other product is the borenium ylide [12-dehydro-1-X-CB₁₁Me₁₀], which has a strongly Lewis acidic naked vertex in position 12 that rapidly adds to another [1-X-12-I-CB₁₁Me₁₀^-] anion to form the observed stable ylide anion [{12-(1-X-CB₁₁Me₁₀^-)}₂I⁺]. In acetonitrile, where it presumably exists as a solvent adduct, [12-dehydro-1-X-CB₁₁Me₁₀] has been trapped with H₂O and, to a small extent, with MeOH, but not with several other potential trapping agents.

Polyphosphazene Based Star-Branched and Dendritic Molecular Brushes

A new synthetic procedure is described for the preparation of poly(organo)phosphazenes with star-branched and star dendritic molecular brush type structures, thus describing the first time it has been possible to prepare controlled, highly branched architectures for this type of polymer. Furthermore, as a result of the extremely high-arm density generated by the phosphazene repeat unit, the second-generation structures represent quite unique architectures for any type of polymer. Using two relativity straight forward iterative syntheses it is possible to prepare globular highly branched polymers with up to 30 000 functional end groups, while keeping relatively narrow polydispersities (1.2-1.6). Phosphine mediated polymerization of chlorophosphoranimine is first used to prepare three-arm star polymers. Subsequent substitution with diphenylphosphine moieties gives poly(organo)phosphazenes to function as multifunctional macroinitiators for the growth of a second generation of polyphosphazene arms. Macrosubstitution with Jeffamine oligomers gives a series of large, water soluble branched macromolecules with high-arm density and hydrodynamic diameters between 10 and 70 nm.

Polyphosphazenes for the Stille reaction: a new type of recyclable stannyl reagent

A random phosphazene copolymer {[N = P((CH2)7-Br)Ph]0.5[N = PMePh]0.5}n (2) and a block copolyphosphazene {[N = P((CH2)7-Br)Ph]0.5[N = PMePh]0.5}45-b-[N = P(O2C12H8)]55 (5), having a branch with two randomly distributed units, have been synthesized and used as precursors for the stannyl derivatives {[N = P((CH2)7-SnBu2An)Ph]0.5[N = PMePh]0.5}n (3) and {[N = P((CH2)7-SnBu2An)Ph]0.5[N = PMePh]0.5}45-b-[N = P(O2C12H8)]55 (6, An = p-MeOC6H4). Polymers 3 and 6 were tested as recyclable tin reagents in the Stille cross-coupling reaction with ArI, using various Pd catalysts and different experimental conditions. Polymer 6 can be recycled without a significant release of tin, but its efficiency decreased after three consecutive cycles. This effect was explained by studying the self-assembly of the polymer under the same conditions used for the catalytic experiments, which evidenced the progressive coalescence of the polymeric vesicles (polymersomes) leading to stable and bigger core-shell aggregates by the attraction of the [N = P(O2C12H8)] rich membranes, thus decreasing the accessibility of the tin active centers.

Polymers and the p-block elements

A survey of the state-of-the-art in the development of synthetic methods to incorporate p-block elements into polymers is given. The incorporation of main group elements (groups 13-16) into long chains provides access to materials with fascinating chemical and physical properties imparted by the presence of inorganic groups. Perhaps the greatest impedance to the widespread academic and commercial use of p-block element-containing macromolecules is the synthetic challenge associated with linking inorganic elements into long chains. In recent years, creative methodologies have been developed to incorporate heteroatoms into polymeric structures, with perhaps the greatest advances occurring with hybrid organic-inorganic polymers composed of boron, silicon, phosphorus and sulfur. With these developments, materials are currently being realized that possess exciting chemical, photophysical and thermal properties that are not possible for conventional organic polymers. This review focuses on highlighting the most significant recent advances whilst giving an appropriate background for the general reader. Of particular focus will be advances made over the last two decades, with emphasis on the novel synthetic methodologies employed.

Cations and dications of heavier group 14 elements in low oxidation states

This review gives an introduction to the synthesis, properties, and reactivity of the cations and dications of the heavier group 14 elements in their low oxidation state.

Copper-promoted cyanation of a boron cluster: synthesis, X-ray structure, and reactivity of 12-CN-closo-CHB11H10-

Microwave-assisted cross-coupling reactions of boron-iodinated derivatives of 1-carba-closo-dodecaborate(1-) (1) with CuCN is shown to cyanate boron vertices of this anion. Clusters with one or two CN groups can be prepared: syntheses of 12-CN-CHB11H10(-) (3) and 7,12-(CN)2-CHB11H9(-) (6) gave yields of 80% and 81%, respectively. The [Et4N](+) salts of 3 and 6 were characterized by NMR, IR, and mass spectroscopies, and the crystal structure of [Et4N]3 was determined by single-crystal X-ray diffraction. Hydrolysis of 3 gave the carboxylic acid 12-COOH-CHB11H10(-) (7).

A cooperative role for the counteranion in the PCl5-initiated living, cationic chain growth polycondensation of the phosphoranimine Cl3P═NSiMe3

The counteranion associated with the cationic initiator [Cl(3)P═N═PCl(3)](+) ([4](+)) generated during the PCl(5)-initiated living, cationic chain growth polycondensation of the N-silylphosphoranimine Cl(3)P═NSiMe(3) (3) to give poly(dichlorophosphazene), [N═PCl(2)](n) (2), has been found to have a dramatic effect on the polymerization. When the counteranion of [4](+) was changed from PCl(6)(-) or Cl(-) to the weakly coordinating anions [BAr*(F)(4)](-) and [BAr(F)(4)](-) (Ar*(F) = 3,5-{CF(3)}(2)C(6)H(3), Ar(F) = C(6)F(5)) instead of the polymerization of 3 being complete in 4-6 h, no reaction was observed after 24 h. Remarkably, the polymerization of 3 may be initiated by Cl(-) anions even in the absence of an active cation such as [4](+). However, in the presence of [4](+), the reaction proceeded significantly faster and allowed for molecular weight control. These results reveal that the currently accepted mechanism for the PCl(5)-initiated living polymerization of 3 needs to be revised to reflect the key role of the counteranion present.

Silylium ion-catalyzed challenging Diels-Alder reactions: the danger of hidden proton catalysis with strong Lewis acids

The pronounced Lewis acidity of tricoordinate silicon cations brings about unusual reactivity in Lewis acid catalysis. The downside of catalysis with strong Lewis acids is, though, that these do have the potential to mediate the formation of protons by various mechanisms, and the thus released Brønsted acid might even outcompete the Lewis acid as the true catalyst. That is an often ignored point. One way of eliminating a hidden proton-catalyzed pathway is to add a proton scavenger. The low-temperature Diels-Alder reactions catalyzed by our ferrocene-stabilized silicon cation are such a case where the possibility of proton catalysis must be meticulously examined. Addition of the common hindered base 2,6-di-tert-butylpyridine resulted, however, in slow decomposition along with formation of the corresponding pyridinium ion. Quantitative deprotonation of the silicon cation was observed with more basic (Mes)(3)P to yield the phosphonium ion. A deuterium-labeling experiment verified that the proton is abstracted from the ferrocene backbone. A reasonable mechanism of the proton formation is proposed on the basis of quantum-chemical calculations. This is, admittedly, a particular case but suggests that the use of proton scavengers must be carefully scrutinized, as proton formation might be provoked rather than prevented. Proton-catalyzed Diels-Alder reactions are not well-documented in the literature, and a representative survey employing TfOH is included here. The outcome of these catalyses is compared with our silylium ion-catalyzed Diels-Alder reactions, thereby clearly corroborating that hidden Brønsted acid catalysis is not operating with our Lewis acid. Several simple-looking but challenging Diels-Alder reactions with exceptionally rare dienophile/enophile combinations are reported. Another indication is obtained from the chemoselectivity of the catalyses. The silylium ion-catalyzed Diels-Alder reaction is general with regard to the oxidation level of the α,β-unsaturated dienophile (carbonyl and carboxyl), whereas proton catalysis is limited to carbonyl compounds.

Reactions of phosphine oxides with bromophosphoranimines; synthesis and unusual rearrangements of O-donor stabilized phosphoranimine cations

Reaction of phosphine oxides R(3)P═O [R = Me (1a), Et (1c), (i)Pr (1d) and Ph (1e)], with the bromophosphoranimines BrPR'R''P═NSiMe(3) [R' = R'' = Me (2a); R' = Me, R'' = Ph (2b); R' = R'' = OCH(2)CF(3) (2c)] in the presence or absence of AgOTf (OTf = CF(3)SO(3)) resulted in a rearrangement reaction to give the salts [R(3)P═N═PR'R''O-SiMe(3)]X (X = Br or OTf) ([4]X). Reaction of phosphine oxide 1a with the phosphoranimine BrPMe(2)═NSiPh(3) (5) with a sterically encumbered silyl group also resulted in the analogous rearranged product [Me(3)P═N═PMe(2)O-SiPh(3)]X ([8]X) but at a significantly slower rate. In contrast, the direct reaction of the bulky tert-butyl substituted phosphine oxide, (t)Bu(3)P═O (1b) with 2a or 2c in the presence of AgOTf yielded the phosphine oxide-stabilized phosphoranimine cations [(t)Bu(3)P═O·PR'(2)═NSiMe(3)](+) ([3](+), R' = Me (d), OCH(2)CF(3) (e)). A mechanism is proposed for the unexpected formation of [4](+) in which the formation of the donor-stabilized adduct [3](+) occurs as the first step.

Dinuclear silylium-enolate bifunctional active species: remarkable activity and stereoselectivity toward polymerization of methacrylate and renewable methylene butyrolactone monomers

Novel dinuclear silylium-enolate active species, consisting of an electrophilic silylium catalyst site and a nucleophilic silicon enolate initiating site that are covalently linked as single molecules, and their unique polymerization characteristics and kinetics are reported. Such unimolecular, bifunctional propagating species are conveniently generated from activation of ethyl- and oxo-bridged disilicon enolate (i.e., disilyl ketene acetal, di-SKA) compounds with [Ph(3)C][B(C(6)F(5))(4)]. Both the ethyl- and oxo-bridged dinuclear species are much more active for the polymerization of methyl methacrylate (MMA) than the mononuclear SKA-based active species, exhibiting an approximate rate enhancement by a factor of 12 and 44, respectively. The oxo-bridged silylium-enolate species is considerably more active and controlled than the ethyl-bridged one, with their differences being even more pronounced in polymerizing a renewable monomer, γ-methyl-α-methylene-γ-butyrolactone. The polymerization by the oxo-bridged silylium-enolate active species follows first-order kinetics in both monomer and silylium catalyst concentrations, indicating a unimolecular propagation mechanism which involves an intramolecular delivery of the polymeric enolate nucleophile to the monomer activated by the silylium ion electrophile being placed in proximity in the same catalyst molecule. Highly stereoregular poly(methyl methacrylate) (PMMA), with a syndiotacticity up to 92% rr, can be produced in quantitative yield using the oxo-bridged propagator at low temperature.

Phosphorus-nitrogen compounds. Part 20: Fully substituted spiro-cyclotriphosphazenic lariat (PNP-pivot) ether derivatives

The condensation reactions of partly substituted spiro-cyclotriphosphazenic lariat (PNP-pivot) ethers, N(3)P(3)[(o-NHPhO)(2)R]Cl(4) [where R=-CH(2)CH(2)- (1) and -CH(2)CH(2)OCH(2)CH(2)- (2)] with morpholine and 1,4-dioxa-8-azaspiro[4,5]decane (DASD) produce fully substituted morpholino (3 and 4) and 1,4-dioxa-8-azaspiro[4,5]deca (5 and 6) phosphazenes. These are the new examples of the spiro-cyclophosphazenic lariat ether derivatives with N(2)O(x) (x=2 and 3) donor type containing 11- and 14-membered macrocycles. The solid state structures of 3, 5 and 6 have been determined by X-ray diffraction techniques. Compound 3 has intermolecular N-H...O hydrogen bond, compound 5 has intra- and intermolecular N-H...O hydrogen bonds, while compound 6 has intramolecular N-H...O and O-H...N and intermolecular N-H...O and O-H...O hydrogen bonds. The correlations of the endocyclic (alpha) and exocyclic (alpha') NPN bond angles with deltaP(spiro) values are investigated. The structural investigations of 3-6 have been verified by elemental analyses, MS, FTIR, (1)H, (13)C and (31)P NMR, DEPT and HETCOR techniques.

H(+), CH(3)(+), and R(3)Si(+) carborane reagents: when triflates fail

For decades, triflic acid, methyl triflate, and trialkylsilyl triflate reagents have served synthetic chemistry well as clean, strong electrophilic sources of H(+), CH(3)(+), and R(3)Si(+), respectively. However, a number of weakly basic substrates are unreactive toward these reagents. In addition, triflate anion can express undesired nucleophilicity toward electrophilically activated substrates. In this Account, we describe methods that replace triflate-based electrophilic reagents with carborane reagents. Using carborane anions of type CHB(11)R(5)X(6)(-) (R = H, Me, X; X = Br, Cl), members of a class of notably inert, weakly nucleophilic anions, significantly increases the electrophilicity of these reagents and shuts down subsequent nucleophilic chemistry of the anion. Thus, H(carborane) acids cleanly protonate benzene, phosphabenzene, C(60), etc., while triflic acid does not. Similarly, CH(3)(carborane) reagents can methylate substrates that are inert to boiling neat methyl triflate, including benzene, phosphabenzenes, phosphazenes, and the pentamethylhydrazinium ion, which forms the dipositive ethane analogue, Me(6)N(2)(2+). Methyl carboranes are also surprisingly effective in abstracting hydride from simple alkanes to give isolable carbocation salts, e.g., t-butyl cation. Trialkylsilyl carborane reagents, R(3)Si(carborane), abstract halides from substrates to produce cations of unprecedented reactivity. For example, fluoride is extracted from freons to form carbocations; chloride is extracted from IrCl(CO)(PPh(3))(2) to form a coordinatively unsaturated iridium cation that undergoes oxidative addition with chlorobenzene at room temperature; and silylation of cyclo-N(3)P(3)Cl(6) produces a catalyst for the polymerization of phosphazenes that functions at room temperature. Although currently too expensive for widespread use, carborane reagents are nevertheless of considerable interest as specialty reagents for making reactive cations and catalysts.