Solvent Effects on the Equilibrium between Localized and Delocalized States of Thermochromic Semibullvalenes and Barbaralanes

Abstract

The position of the equilibrium between localized and delocalized states of thermochromic semibullvalenes and barbaralanes (see the Equation) depends strongly on the solvent. Dipolar aprotic solvents, particularly N,N'-dimethylpropylene urea, favor the delocalized, bishomoaromatic state (ΔH(0) =8 kJ mol(-1) (cyclohexane), ΔH(0) <0 kJ mol(-1) (N,N'-dimethylpropylene urea)).

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

“…For 1 , this is indeed true. When comparing experimental and calculated values, it should be noted that the values presented here are calculated in vacuum, while experimental Δ H ‡ were measured in CF 2 Cl 2 /CD 2 Cl 2 ,1 and furthermore that significant solvent effects have been reported44, 45 for substituted derivatives of 1 .…”

“…In both cases, a drastic increase in the rate to the Cope rearrangement was observed. The actual nature of the ground state of molecules based on structures like 4 , 8 and 14 , that is to which extent homoconjugation is present, is highly influenced by parameters like additional substituents, solvent and temperature 44, 45…”

The Equilibrium between Localized and Delocalized States of Thermochromic Semibullvalenes and Barbaralanes—Direct Observation of Transition States of Degenerate Cope Rearrangements

“…Unfortunately, gas-phase enthalpy differences are not directly available, and possible relations between DH 0 values measured for solutions and the structures of the compounds are obscured by solvent effects. [25] The values of DH 0 are very small indeed and in the range of van der Waals and other weak solute ± solvent interactions. [26] Only in the case of 1 a can the enthalpy difference DH 0 between A, A' ' and A* be compared with the barrier for the degenerate Cope rearrangement A > A' ', DG = (200 K) 14.4 kJ mol À1 (in chlorodifluoromethane/ [D 6 ]dimethyl ether).…”

An One-Pot Synthesis of Semibullvalenes and Its Mechanism

“…The dienophile 3-methyl-3-[2-(1-methylcyclopropyl)ethyl]cyclopropene (14), synthesized by an optimized procedure reported by Schuster, [39] was added and the reaction mixture was stirred until the characteristic red colour of the tetrazine disappeared (reaction times: see below). The solution was concentrated to dryness, and the crude material was purified by flash column chromatography (FC) and recrystallization to afford the product.…”