The formation of methyl cations from acetone and 1,2-epoxypropane molecular cations

The technique of photoelectron—photoion coincidence spectroscopy is used to determine the breakdown curves of CH⁺₃ from acetone and 1,2-epoxypropane. The experimental appearance energies of CH⁺₃ from both isomers are found to be identical with the calculated thresholds for the formation of CH⁺₃ + CH₃ + CO, i.e. the secondary fragmentation of C₃H₆O⁺.     

Phosphazene cations

Cations derived from (NPCl2)3, hexachloro-cyclo-triphosphazene, the weakly basic precursor of phosphazene polymers, have been prepared using strongly electrophilic reagents based on carborane anions. N-protonated, N-methylated, and N-silylated adducts of (NPCl2)3 have been isolated and characterized by X-ray crystallographic and spectroscopic methods. The normally potent chloride-abstracting silyl reagents of the type R3Si(carborane) are unable to abstract chloride from (NPCl2)3, even though the coordinatively unsaturated N2P2Cl5+ cation is widely accepted as a reactive intermediate in the ring-opening polymerization of (NPCl2)3.     

Homopolyatomic and heteropolyatomic halogen cations

This review is not exhaustive in scope but rather focuses on the most recent years of work in the field of cationic halogens, covering the literature up to mid 1992. This area is still currently developing. The review summarizes structure, preparation, and reactivity of cationic halogens. Modern theoretical aspects (ab initio MO calculations) as well as the energetics (thermodynamics) of these species are also included. In particular, the symmetric and unsymmetric structures of heteropolyatomic cations, the charge delocalization and bond lengths alternation in npπ–npπ species (n < 2), and the dimerization by π*-π* interactions are discussed.     

Pyramidane and isoelectronic pyramidal cations

Ab initio STO-3G, 4-31G and semi-empirical MINDO/3 calculations were performed on a series of C₄H₄X compounds to analyze the possibility of existence of their nonclassical C_4V pyramidal isomeric forms. It was shown that with the four-electron group X (C, N⁺, P⁺, O²⁺, S²⁺) in an apex, C_4V pyramidal structures are indeed stable, i.e., conform to local minima on the corresponding potential energy surfaces. Relative energies, molecular geometries and charge density distribution for various isomeric forms of the C₄H₄X compounds are presented. The theoretically derived strategies for an experimental trapping of the pyramidane 1, X = C and the C_4V pyramidal thiophene dication 1, X = S²⁺ are discussed.     

Bridged and unbridged norbornyl cations

The solvolysis rates and products of 6-substituted 2-norbornyl-p-toluene-sulfonates $\text{1}$ and $\text{5}$ are controlled by 1,3-bridging, i.e. by the degree of bonding between C(6) and C(2) in the intermediate norbornyl cations.     

Cyclic distiba- and dibismadiazenium cations

Think positive, keep the anions away: Hitherto unknown mono- and dications of the type [XE(μ-NTerE)](+) and [E(2) (μ-NTer)(2) ](2+) (E=Sb, Bi; X=Cl, I) were studied and fully characterized for the first time. Salts bearing these highly reactive cations can be obtained from terphenyl-substituted cyclo-dipnictadiazanes [XE(μ-NR)](2) by halide abstraction or triflate substitution.     

Rotational equilibria in 1,2,6-trisubstituted pyridinium cations and reactions of 2-Isopropylpyrylium cations

2-Isopropyl-6-phenyl- and 2,6-diisopropyl-pyridiniums with bulky 1-substituents show temperature-variable NMR spectra which are interpreted in terms of restricted rotation. 2-Isopropyl-4,6-diphenylpyrylium can be deprotonated at the isopropyl group to give an anhydro base which forms new pyryliums with electrophiles.     

Complex hydrogen bonded cations - VI cations involving aliphatic bases

Hydrogen bond complexing between aliphatic bases and their cations has been examined by infrared spectroscopy. The results are: Morpholinium/morpholine - strong complex - asymmetric single minimum bond (a.s.m.). Piperidinium/piperidine - strong complex (a.s.m.). Piperidinium/NMe morpholine - complex (a.s.m.). NMe piperidinium/morpholine - weaker complex (a.s.m.). NMe piperidinium/NMe piperidine - no complexing. NMe morpholinium/NMe morpholine - no complexing. Triethylenediamine (TED). Complexes of type (TEDH⁺)_n, and TEDH⁺/TED have been distinguished.Models indicate the Me group retains the equatorial position in the tertiary bases on complexing, and the N β N distance is ? 2.5 Å.     

Concerning phosphirenium cations

Tetraphenyl-, tetraethyl-, and 1,1-diphenyl-2,3-diethylphosphirenium cations can be prepared by extension of a previously described procedure, but base-promoted cyclizations of 1,3-dihalophosphonium cations proceed to ring-opened products by an alternate pathway.     

Polyfluorinated radical cations

The synthesis of the first trifluoromethanesulfonate esters of the type CF₃SO₃(CH₂)_nO₃SCF₃ (n=1,2,3) are reported. The new compounds are prepared from Cl(CH₂)_nCl by substitutive electrophilic dehalogenation reactions with CF₃SO₂OX (x=Cl,Br). The extension of this reaction to HCCl₃ results in HC(O₃SCF₃)₃ but the compound is unstable at 22°.     

Dialkyl dichalcogen cations

One electron oxidation of dialkyl diselenides and ditellurides is achieved with NO(+)CF(3)SO(3)(-). According to ab initio calculations the radical cations have a trans stucture. In the solid state they dimerize to rectangular units [R-Te](4)(2+) and [R-Se](4)(2+). The long SeSe and TeTe bond is obviously due to a pi*-pi* interaction.     

Radical cations of aldehydes and ketones

We confirm the observation made by Snow and Williams in the previous letter that the quartet substructure observed in the ESR spectrum of acetaldehyde is due to hyperfine coupling to a single chlorine nucleus of the solvent, fluorotrichloromethane. Reasons for this reversible interaction, and its absence for other similar cations are discussed.     

Disilene,silylsilylene and their cations

An accurate examination of features of the ground state surfaces of Si₂H₄ and Si₂H ₄ ⁺ is reported; they are compared to C₂H₄ and C₂H ₄ ⁺ . For the neutral species, accurate SCF calculations show disilene to be planar, but silylsilylene has the lower energy, whereas at the correlated (CI, MP2, MP3, MP4(SD)) levels disilene becomes trans bent and has the lower energy by 6 kcal/mol. In view of a recent theoretical suggestion that this value should be 23 kcal/mol, we have used large basis sets in these investigations. Our calculations cannot support this large value. Similar investigations are reported for the cation, where the planar disilene structure is predicted to be the most stable. It may be very slightly twisted at high accuracy CI, but it is much lower in energy than the silylsilylene structure. Vibrational frequencies and infra-red intensities are also reported. Theoretical photoelectron spectra of C and Si systems are presented and compared with experiment.     

Synthesis and reactivity of indolylmethyl cations

The synthesis of diindolylmethyl cations by reaction of aldehydes and indoles is described, and their reaction with Grignard reagents, organo-lithium compounds and sodium borohydride, giving the corresponding diindolyl methanes, is reported. The analogous phenyl indolylmethyl cations could never be isolated, but their formation was proved indirectly.     

Microsolvation of small cations and anions

Recent advances in the theoretical treatment of microsolvation of small ions, a problem with practical implications in chemistry, physics, and biology, are exposed. In particular, we discuss sound stochastic approaches to sample complex energy landscapes and delve into the nature of bonding interactions that dictate both structural and energetical preferences. An in-depth analysis of the effect of formal charges in the surrounding network of solvent to solvent hydrogen bonds is also presented. The problem, as expected, is more complicated than simple definitions may forecast.     

Palladium-mediated functionalization of heteroaromatic cations: comparative study on quinolizinium cations

An efficient palladium-catalyzed cross-coupling reaction on heteroaromatic cations is described. A comparative study of the Stille and Suzuki reactions shows that only the Stille reaction is able to produce an efficient C-C bond formation between any of the four isomeric bromoquinolizinium bromides and a variety of stannanes. In the presence of the catalysts Pd(PPh3)4 or Pd2(dba)3P(o-Tol)3, vinyl, ethynyl, aryl, and heteroaryl groups are successfully incorporated into the quinolizinium system in satisfactory yields under mild reaction conditions. This procedure represents a marked improvement on the functionalization of this class of heteroaromatic cation.     

Nucleogenic silylium cations and their analogs

Radiochemical studies of nucleogenic trivalent cations of the 14th group elements (silicon and germanium) generated by the β-decay of tritium atoms incorporated into organometallic molecules are reviewed. The radiochromatographic analysis of the neutral labeled products of reactions of these cations with various nucleophiles combined with quantum chemical data allowed us to obtain new information about the reactivity and rearrangements of silylium and germylium cations.