Experimental and Theoretical Study of Stabilization of Delocalized Forms of Semibullvalenes and Barbaralanes by Dipolar and Polarizable Solvents. Observation of a Delocalized Structure that Is Lower in Free Energy than the Localized Form

Abstract

[reaction: see text] UV/vis spectra of thermochromic semibullvalenes 1 and barbaralanes 2, which undergo rapid degenerate Cope rearrangements, display temperature-dependent shoulders (1b, 1d, 1e) or absorption maxima (1c, 2c, 2f) at the low-energy side of their strong UV bands. These long-wavelength absorptions are ascribed to Franck-Condon transitions from delocalized structures 1(deloc) and 2(deloc). Gibbs free energy differences, DeltaG*, between delocalized and localized forms were calculated from the temperature dependence of the long-wavelength absorptions. Dipolar and polarizable solvents strongly affect and even may reverse the relative stabilities of the localized and delocalized forms of 1c, 2c, and 2f. For example, DeltaG*(2c) = 8 kJ mol(-)(1) in cyclohexane, 2 kJ mol(-)(1) in dimethylformamide, and -3 kJ mol(-)(1) in N,N'-dimethylpropylene urea (DMPU), so that (2c(deloc))(DMPU) becomes the global minimum. In contrast to the case for 2c, the intensities of the long-wavelength shoulders of the yellow semibullvalenes 1b, 1d, and 1e are only moderately influenced by solvents, and the rates of Cope rearrangements of the nonthermochromic, colorless barbaralanes 2a and 2b, determined by NMR methods, are almost solvent-invariant. In search of the solute properties that are decisive in determining the influence of solvent upon DeltaG*, electrical dipole and quadrupole moments and molecular polarizabilities have been calculated using the B3LYP/6-31G* method and solvation energies have been computed with the conductorlike polarized continuum model (CPCM). The results of these calculations indicate that the solvent effects are due to the greater polarity and polarizability of the delocalized structures relative to the localized structures.

Application of the Cplex-isoelectronic theory to electrocyclisations, sigmatropic rearrangements, cheletropic reactions and antiaromaticityConsistent with Santilli's hadronic chemistry

“…the reverse to that found for the Cope) [69]. The process is also diamagnetic as measured by a SQUID magnetometer, consistent with the movement of isoelectron pairs [70]. Breaking the single bond in the 1 process is predicted to be a major factor, coupled to the strained bonds and the fixed cisoid conformation of the double bonds, involved in semibullvalene having the lowest activation energy for a Cope rearrangement with fluxional behaviour even at −110 • C. The advantage of breaking the single bond in the 1 process is consistent with the fact that the [5,5] sigmatropic shift is favoured over the [3,3] [71], as in the former system the single bond can break in the 1 process of the ADEP pathway to give syn FSED.…”

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 .…”

4,6‐Barbaralanedicarboxy‐2,8‐dicarboxylic anhydride and 1,5‐dimethyl‐4,6‐semibullvalenedicarboxy‐2,8‐dicarboxylic anhydride: examples of unusual barbaralanes and semibullvalenes that do not undergo the Cope rearrangement. They are locked as the closed tautomers

“…1 for the low temperature range À90 to +30°C in acetone-d 6 ) show insignificant changes in chemical shift for the putative exchanging carbons. The high temperature NMR spectra, recorded in dimethylsulfoxide-d 6 , show evidence for considerable decomposition of 5 during the experiment. At each temperature (whether heating or cooling the sample), the spectra are referenced to the methano bridge C-9 as this carbon would have identical chemical shift in both tautomers, 5 loc and 5′ loc .…”

“…B3LYP/6-31G* and B3PW91/6-31G* optimized energies (Hartrees) and free energy differences (kcal/mol) between the lowest energy conformer (12 loc UU) and the other conformers of semibullvalene monoanhydride 12 Experimental shifts for the parent barbaralane (no substituents) from reference, [23] fully resolved slow exchange limit chemical shifts were not reported. b Experimental shifts for barbaralane 5 recorded at 30°C in dimethyl sulfoxide-d 6 and acetone-d 6 .…”

“…Equilibrium imbalances in the Cope rearrangement of unsymmetrically substituted barbaralanes and semibullvalenes 6 are well known. [6] However, examples of compounds where the Cope rearrangement is inoperative, with one localized tautomer (6 loc or 6′ loc ) as the exclusive representative of that particular bridged-homotropilidene, are rare. Such examples include mono-7 and di-benzosemibullvalenes/barbaralanes 8, some substituted 4-bromo-semibullvalenes/barbaralanes 9, triphenylbarbaralane 10 and ditrifluoromethylsemibullvalenes 11a and 11b.…”