tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

A new type of C+⋯Hδ-(C=) bond in adducts of vinyl carbocations with alkenes

By X-ray diffraction analysis and IR spectroscopy, it was established here that vinyl carbocations C3H5+/C4H7+ with carborane counterion CHB11Cl11- form stable monosolvates C3H5+⋅C3H6/C4H7+⋅C4H8 with molecules of alkenes C3H6/C4H8. They contain molecular group =C+⋯Hδ--Cδ+= with a new type of bond formed by the H atom of the H-C= group of the alkene with the C atom of the C+=C group of the carbocation. The short C+----Cδ+ distance, equal to 2.44 Å, is typical of that of X----X in proton disolvates (L2H+) with an quasi-symmetrical X-H+⋯X moiety (where X = O or N) of basic molecule L. The nature of the discovered bond differs from that of the classic H-bond by an distribution of electron density: the electron-excessive Hδ- atom from the (=)C-H group of the alkene is attached to the C+ atom of the carbocation, on which the positive charge is predominantly concentrated. Therefore, it can be called an inverse hydrogen bond.

Spontaneous Transition of Alkyl Carbocations to Unsaturated Vinyl-Type Carbocations in Organic Solutions

It was found that alkyl carbocations, when their salts are dissolved in common organochlorine solvents, decompose to unsaturated vinyl-type carbocations that are stabler in solutions. This is a convenient method for obtaining salts of vinyl cations and their solutions for further research.

Chloride-Mediated Alkene Activation Drives Enantioselective Thiourea and Hydrogen Chloride Co-Catalyzed Prins Cyclizations

The mechanism of chiral hydrogen-bond donor (HBD) and hydrogen chloride (HCl) co-catalyzed Prins cyclizations was analyzed through a combination of experimental and computational methods and revealed to involve an unexpected and previously unrecognized mode of alkene activation. Kinetic and spectroscopic studies support the participation of a catalytically active HCl·HBD complex that displays reduced Brønsted acidity relative to HCl alone. Nevertheless, rate acceleration relative to the HCl-catalyzed background reaction as well as high levels of enantioselectivity are achieved. This inverse Brønsted correlation is ascribed to chloride-mediated substrate activation in the rate-limiting and enantiodetermining cyclization transition state. Density functional theory (DFT) calculations, distortion-interaction analysis, and quasiclassical dynamics simulations support a stepwise mechanism in which rate acceleration and enantioselectivity are achieved through the precise positioning of the chloride anion within the active site of the chiral thiourea to enhance the nucleophilicity of the alkene and provide transition-state stabilization through local electric field effects. This mode of selective catalysis through anion positioning likely has general implications for the design of enantioselective Brønsted acid-catalyzed reactions involving π-nucleophiles.

The Chloronium Cation [(C2H3)2Cl+] and Unsaturated C4-Carbocations with C=C and C≡C Bonds in Their Solid Salts and in Solutions: An H1/C13 NMR and Infrared Spectroscopic Study

Solid salts of the divinyl chloronium (C₂H₃)₂Cl⁺ cation (I) and unsaturated C₄H₆Cl⁺ and C₄H₇⁺ carbocations with the highly stable CHB₁₁Hal₁₁⁻ anion (Hal=F, Cl) were obtained for the first time. At 120 °C, the salt of the chloronium cation decomposes, yielding a salt of the C₄H₅⁺ cation. This thermally stable (up to 200 °C) carbocation is methyl propargyl, CH≡C-C⁺-H-CH₃ (VI), which, according to quantum chemical calculations, should be energetically much less favorable than other isomers of the C₄H₇⁺ cations. Cation VI readily attaches HCl to the formal triple C≡C bond to form the CHCl=CH-C⁺H-CH₃ cation (VII). In infrared spectra of cations I, VI, and VII, frequencies of C=C and C≡C stretches are significantly lower than those predicted by calculations (by 400–500 cm⁻¹). Infrared and ¹H/¹³C magic-angle spinning NMR spectra of solid salts of cations I and VI and high-resolution ¹H/¹³C NMR spectra of VII in solution in SO₂ClF were interpreted. On the basis of the spectroscopic data, the charge and electron density distribution in the cations are discussed.

IR-Spectroscopic and X-ray-Structural Study of Vinyl-Type Carbocations in Their Carborane Salts

The butylene carbocation in its salts with anions CHB₁₁F₁₁ ^- and CHB₁₁Cl₁₁ ^- forms isomers CH₂=C⁺-CH₂-CH₃ (I) and CH₃-C⁺=CH-CH₃ (II), which were characterized here by infrared (IR) spectroscopy and X-ray diffraction analysis. The strongest influence on the structure of the cations is exerted by geometric ordering of their anionic environment. In the crystalline phase, the cations uniformly interact with neighboring anions, and the C=C bond is located in the middle part of the cations forming a -CH=C⁺- moiety with the highest positive charge on it and the lowest νC=C frequency, at 1490 cm^-1. In the amorphous phase with a disordered anionic environment of the cations, contact ion pairs Anion^-···CH₂=C⁺-CH₂-CH₃ form predominantly, with terminal localization of the C=C bond through which the contact occurs. The positive charge is slightly extinguished by the anion, and the C=C stretch frequency is higher by ∼100 cm^-1. The replacement of the hydrogen atom in cations I/II by a Cl atom giving rise to cations CH₂=C⁺-CHCl-CH₃ and CH₃-C⁺=CCl-CH₃ means that the donation of electron density from the Cl atom quenches the positive charge on the C⁺=C bond more strongly, and the C=C stretch frequency increases so much that it even exceeds that of neutral alkene analogues by 35-65 cm^-1. An explanation is given for the finding that upon stabilization of the vinyl cations by polyatomic substituents such as silylium (SiMe₃) and t-Bu groups, the stretching C=C frequency approaches the triple-bond frequency. Namely, the scattering of a positive charge on these substituents enhances their donor properties so much that the electron density on the C=C bond with a weakened charge becomes much higher than that of neutral alkenes.

Stability of alkyl carbocations

The traditional and widespread rationale behind the stability trend of alkyl-substituted carbocations is incomplete.

The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

Superacidic media became famous in connection with carbocations. Yet not all reactive intermediates can be generated, characterized, and eventually isolated from these Brønsted acid/Lewis acid cocktails. The counteranion, that is the conjugate base, in these systems is often too nucleophilic and/or engages in redox chemistry with the newly formed cation. The Brønsted acidity, especially superacidity, is in fact often not even crucial unless protonation of extremely weak bases needs to be achieved. Instead, it is the chemical robustness of the aforementioned counteranion that determines the success of the protolysis. The advent of molecular Brønsted superacids derived from weakly coordinating, redox-inactive counteranions that do withstand the enormous reactivity of superelectrophiles such as silicon cations completely changed the whole field. This Perspective summarizes general aspects of medium and molecular Brønsted acidity and shows how applications of molecular Brønsted superacids have advanced from stoichiometric reactions to catalytic processes involving protons and in situ generated superelectrophiles.

Isomers of the Allyl Carbocation C3H5 + in Solid Salts: Infrared Spectra and Structures

Three isomers of the allyl cation C₃H₅ ⁺ were obtained in salts with the carborane anion CHB₁₁Cl₁₁ ^-. Two of them, angular CH₃-CH=CH⁺ (I) and linear CH₃-C⁺=CH₂ (II), were characterized by X-ray crystallography, and the third one, (CH₂CHCH₂)⁺ (III), is formed in an amorphous salt. The stretch vibration of the charged double bond C=C⁺ of I and II is decreased by 162 cm^-1 (I) or 76 cm^-1 (II) as compared to that of neutral propene. This result contradicts the prediction of DFT and MP2 calculations with the 6-311G++(d,p) basis set that the appearance of the positive charge on the C=C bond should increase its stretch vibration by 200 cm^-1 (I) or 210 cm^-1 (II). According to infrared spectra, the CC bonds in isomer III have one-and-a-half bond status. Isomers I and II in the crystal lattice are stabilized due to uniform ionic interactions with neighboring anions with partial transfer of a positive charge to them. Additional stabilization of II is provided by a weak hyperconjugation effect. Isomer III is stabilized in the amorphous phase due to ion paring with a counterion and a strong intramolecular hyperconjugation effect.

Unsaturated Vinyl-Type Carbocation [(CH3)2C=CH]+ in Its Carborane Salts

The isobutylene carbocation (CH3)2C=CH+ was obtained in amorphous and crystalline salts with the carborane anion CHB11Cl11 -. The cation was characterized by X-ray crystallography and IR spectroscopy. Its crystal structure shows a relatively uniform ionic interaction of the cation with the surrounding anions, with a slightly shortened distance between the C atom of the =CH group and the Cl atom of the anion, pointing to a higher positive charge on this group. In the amorphous phase, the asymmetric interaction of the cation with the anion increases, approaching ion pairing. This gives rise to a strong hyperconjugation between the two CH3 groups and the 2pz orbital of the central carbon sp2 atom (the red shift of the CH stretch is 150 cm-1); this effect stabilizes the cation. Over time, as the structure of the amorphous phase becomes more ordered, the hyperconjugation weakens and disappears in the crystalline phase with the disappearance of ion pairing. The carbocation stabilization in the crystalline phase is achieved due to the transfer of a portion of the charge to the neighboring anions, whereas the charge on the C=C bond becomes the strongest: the C=C stretch frequency drops to ∼160 cm-1 relative to neutral isobutylene. The collected IR spectra for the optimized cation under vacuum (in the 6-311G ++ (d, p) basis for all HF, MP2, and DFT calculations) predict that a positive charge on the C=C bond increases its stretching frequency; this computational result contradicts the experimental data, perhaps because it does not take into account the significant impact of the environment.

Stabilization of Saturated Carbocations in Condensed Phases

Based on the experimentally established mechanism of hyperconjugative stabilization of the simplest saturated carbocations [Stoyanov, E. S.; et al. PCCP, 2017, 19, 7270], the infrared spectra of t-alkyl⁺ and methyl-cyclo-pentyl⁺ carbocations were interpreted. This approach allows us to extract new information about the electronic state of (CH₃)₂C⁺R cations with R = H, CH₃, C₂H₅, C₄H₇, and CH(CH₃)₂, namely, the electron density distribution over the (CH₃)₂C group and the positive charge dispersion on the H atoms of this group. Thus, donation of the electron density to the empty 2p_z orbital of the sp² C atom occurs not only from one C-H bond oriented parallel to the 2p_z orbital but also equally from all other C-H and C-C bonds of the molecular group involved in hyperconjugation. This mechanism preserved the isoelectronic nature of this group toward the corresponding groups of the neutral alkanes. Hyperconjugation and polarization are closely linked in stabilization of carbocations: the strengthening of one effect weakens the second and vice versa without changing the efficiency of scattering of the positive charge in the carbocation. In the condensed phase, carbocations are additionally stabilized by the bulk effect and hydrogen bonding with the environment: increasing H-bonding strength increased hyperconjugation and decreased polarization. The contribution of all the effects on the stabilization of carbocations was evaluated.

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.

Stabilization of carbocations CH3+, C2H5+, i-C3H7+, tert-Bu+, and cyclo-pentyl+ in solid phases: experimental data versus calculations

Comparison of experimental infrared (IR) spectra of the simplest carbocations (with the weakest carborane counterions in terms of basicity, CHB₁₁Hal₁₁^-, Hal = F, Cl) with their calculated IR spectra revealed that they are completely inconsistent, as previously reported for the t-Bu⁺ cation [Stoyanov E. S., et al. J. Phys. Chem. A, 2015, 119, 8619]. This means that the generally accepted explanation of hyperconjugative stabilization of the carbocations should be revised. According to the theory, one CH bond (denoted as ) from each CH₃/CH₂ group transfers its σ-electron density to the empty 2p_z orbital of the sp² C atom, whereas the σ-electron density on the other CH bonds of the CH₃/CH₂ group slightly increases. From experimental IR spectra it follows that donation of the σ-electrons from the bond to the 2p_z C-orbital is accompanied by equal withdrawal of the electron density from other CH bonds, that is, the electrons are supplied from each CH bond of the CH₃/CH₂ group. As a result, all CH stretches of the group are red shifted, and IR spectra show typical CH₃/CH₂ group vibrations. Experimental findings provided another clue to the electron distribution in the hydrocarbon cations and showed that the standard computational techniques do not allow researchers to explain a number of recently established features of the molecular state of hydrocarbon cations.