Stabilization of Saturated Carbocations in Condensed Phases

Abstract

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 (CH)CR cations with R = H, CH, CH, CH, and CH(CH), namely, the electron density distribution over the (CH)C group and the positive charge dispersion on the H atoms of this group. Thus, donation of the electron density to the empty 2p orbital of the sp C atom occurs not only from one C-H bond oriented parallel to the 2p 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.

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

“…At the same time, other important results were obtained: (1) solvated saturated carbocations (C n H 2n+1 ) solv are less stable than unsaturated ones (C n H 2n−1 ) solv , and this property may facilitate the transition of the former into the latter; (2) vinyl cations are soluble in common organochlorine solvents without interaction with the solvent, i.e., there are no solvolysis reactions; (3) under the action of solvation in solutions, the CC bond of vinyl cations can spontaneously break yielding products with shorter and longer C n chains of carbon atoms. This work offers a simple way to obtain pure crystalline salts of certain stable vinyl carbocations, such as isobutylene + or chain butylene + , from cationic alkyne precursors whose synthesis has been developed [ 3 , 23 ]. In the case of the formation of unstable unsaturated carbocations (e.g., from methyl-cyclopentyl + ) that undergo further changes, it is now possible to characterize their properties by NMR spectroscopy in solvents common in chemical practice.…”

“…It forms from the alkyl cation of methyl-cyclopentyl + in a solvent medium and, being unstable, subsequently decomposes into butylene cation Z and other products (Scheme 1). The following findings support the notion that allyl cation Y + is formed: (i) at the first stage, the methyl-cyclopentyl + cation decomposes only into one unsaturated carbocation without the formation of other accompanying carbon-containing products; therefore, it can be a cation of composition С6Н9 + ; (ii) only the asymmetric vibration of the group with close-to-aromatic CC bonds-on which the positive charge is predominantly concentrated-can have a frequency lower than that of the C=C + stretch; (iii) the CH3 group of the cation, in contrast to methylcyclopentyl + [23], does not participate in hyperconjugation because it is bonded to the sp 2 C atom with a field pz orbital. The Y + cation is unstable and decomposes yielding two or more unsaturated carbocations.…”

“…As a counterion, the undecachlorocarborane anion, CHB 11 Cl 11 − , was chosen (hereinafter abbreviated as {Cl 11 − }, Figure S1 in Supplementary Materials ) because of its extreme stability and very low basicity, which promotes the formation of stable ionic salts with highly acidic cations [ 22 ]. Salts t-Bu + {Cl 11 − } and methylcyclopentyl + {Cl 11 − } were prepared as described in [ 3 , 23 ].…”

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

“…The facile access to carbocations when paired with carborate anions allowed their accurate structure determination and an in-depth analysis of their stabilization in the condensed phase. − The ability to isolate and store these reactive intermediates makes them valuable reagents (and even catalysts) in synthetic chemistry, surpassing the reactivity of commonly used triflates…”

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

“…Simple saturated carbocations (C 2 –C 7 ) have been studied experimentally in condensed phases [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ] and their infrared (IR) spectra have been interpreted [ 6 , 7 , 8 ]. Protonated arenium cations, starting with C 6 H 7 + , have been broadly studied by NMR spectroscopy in liquid superacids and as carborane salts [ 9 , 10 ].…”