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

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.

Interaction of Vinyl-Type Carbocations, C3H5+ and C4H7+ with Molecules of Water, Alcohols, and Acetone

X-ray diffraction analysis and IR spectroscopy were used to study the products of the interaction of vinyl cations C₃H₅⁺ and C₄H₇⁺ (Cat⁺) (as salts of carborane anion CHB₁₁Cl₁₁^-) with basic molecules of water, alcohols, and acetone that can crystallize from solutions in dichloromethane and C₆HF₅. Interaction with water, as content increased, proceeded via three-stages. (1) adduct Cat⁺·OH₂ forms in which H₂O binds (through the O atom) to the C=C⁺ bond of the cation with the same strength as seen in the binding to Na in Na(H₂O)₆⁺. (2) H⁺ is transferred from cation Cat⁺·OH₂ to a water molecule forming H₃O⁺ and alcohol molecules (L) having the CH=CHOH entity. The O- atom of alcohols is attached to the H atom of the C=C⁺-H moiety of Cat⁺ thereby forming a very strong asymmetric H-bond, (C=)C⁺-H⋅⋅⋅O. (3) Finally all vinyl cations are converted into alcohol molecule L and H₃O⁺ cations, yielding proton disolvates L-H⁺-L with a symmetric very strong H-bond. When an acetone molecule (Ac) interacts with Cat⁺, H⁺ is transferred to Ac giving rise to a reactive carbene and proton disolvate Ac-H⁺-Ac. Thus, the alleged high reactivity of vinyl cations seems to be an exaggeration.