Intrinsic Acidity of Dimethylhalonium Ions: Evidence for Hyperconjugation in Dimethylhalonium Ylides in the Gas Phase

Chemical Properties of Dialkyl Halonium Ions (R2Hal+) and Their Neutral Analogues, Methyl Carboranes, CH3-(CHB11Hal11), Where Hal = F, Cl

Chloronium cations in their salts (C_nH_2n+1)₂Cl⁺{CHB₁₁Cl₁₁^-}, with n = 1 to 3 and exceptionally stable carborane anions, are stable at ambient and elevated temperatures. The temperature at which they decompose to carbocations with HCl elimination (below 150 °C) decreases with the increasing n from 1 to 3 because of increasing ionicity of C-Cl bonds in the C-Cl⁺-C bridge. At room temperature, the salts of cations with n ≥ 4 [starting from t-Bu₂Cl⁺ or (cyclo-C₅H₁₁)₂Cl⁺] are unstable and decompose. With decreasing chloronium ion stability, their ability to interact with chloroalkanes to form oligomeric cations increases. It was shown indirectly that unstable salt of fluoronium ions (CH₃)₂F⁺(CHB₁₁F₁₁^-) must exist at low temperatures. The proposed (CH₃)₂F⁺ cation is much more reactive than the corresponding chloronium, showing at room temperature chemical properties expected of (CH₃)₂Cl⁺ at elevated temperatures.

The salts of chloronium ions R-Cl(+)-R (R = CH3 or CH2Cl): formation, thermal stability, and interaction with chloromethanes

The interaction of CH3Cl/CD3Cl or CH2Cl2/CD2Cl2 with the carborane acid H(CHB11Cl11) (abbreviated as H{Cl11}) generates the salts of CH3-{Cl11} and CH2Cl-{Cl11} and their deuterio analogs, respectively, which are analogs of the salts of asymmetric chloronium cations. Next, salts of chloronium cations CH3-Cl(+)-CH3, ClCH2-Cl(+)-CH2Cl, and ClCH2-Cl(+)-CH3 and their deuterio analogs were obtained from the above compounds. The asymmetric ClCH2-Cl(+)-CH3 cation was found to be unstable, and at ambient temperature, slowly disproportionated into symmetric cations (CH3)2Cl(+) and (CH2Cl)2Cl(+). At a high temperature (150 °C), disproportionation was completed within 5 minutes, and the resulting cations further decomposed into CH3-{Cl11} and CH2Cl-{Cl11}. The molecular fragment ClCH2-(X) of the compounds (X = {Cl11}, -Cl(+)-CH2Cl, or -Cl(+)-CH3) is involved in exchange reactions with CH2Cl2 and CHCl3, converting into CH3-(X) with the formation of chloroform and CCl4, respectively.

Quantitative assessment of the multiplicity of carbon-halogen bonds: carbenium and halonium ions with F, Cl, Br, and I

CX (X = F, Cl, Br, I) and CE bonding (E = O, S, Se, Te) was investigated for a test set of 168 molecules using the local CX and CE stretching force constants k(a) calculated at the M06-2X/cc-pVTZ level of theory. The stretching force constants were used to derive a relative bond strength order (RBSO) parameter n. As alternative bond strength descriptors, bond dissociation energies (BDE) were calculated at the G3 level or at the two-component NESC (normalized elimination of the small component)/CCSD(T) level of theory for molecules with X = Br, I or E = Se, Te. RBSO values reveal that both bond lengths and BDE values are less useful when a quantification of the bond strength is needed. CX double bonds can be realized for Br- or I-substituted carbenium ions where as suitable reference the double bond of the corresponding formaldehyde homologue is used. A triple bond cannot be realized in this way as the diatomic CX(+) ions with a limited π-donor capacity for X are just double-bonded. The stability of halonium ions increases with the atomic number of X, which is reflected by a strengthening of the fractional (electron-deficient) CX bonds. An additional stability increase of up to 25 kcal/mol (X = I) is obtained when the X(+) ion can form a bridged halonium ion with ethene such that a more efficient 2-electron-3-center bonding situation is created.

Halogen and chalcogen cation pools stabilized by DMSO. Versatile reagents for alkene difunctionalization

Halogen and chalcogen cations (X(+) = Br(+), I(+), ArS(+), and ArSe(+)) were generated by low-temperature electrochemical oxidation in the presence of dimethyl sulfoxide (DMSO) and were accumulated in the solution. DFT calculations indicated that DMSO stabilizes these cations by coordination. The complexes of I(+) with one and two DMSO molecules were observed by cold-spray-ionization MS analyses. The stability of the resulting cation pools of X(+) increased in the order of Br(+) < I(+) < ArS(+) < ArSe(+), which could be explained in terms of the electronegativity of X. The cation pools served as versatile reagents for organic synthesis; the reactions with alkenes gave β-X-substituted alkoxysulfonium ions, which were converted to the corresponding carbonyl compounds by the treatment with triethylamine, whereas the treatment with methanol gave the corresponding alcohols. The reactions with aminoalkenes and 1,6-dienes gave the cyclized products.