Synthesis and Properties of 1,3,5,7‐Tetra‐tert‐butyl‐s‐indacene

Abstract

According to quantum chemical calculations (MIND013 method) on s-indacene l [ll-a [12]annulene perturbed by two o-bonds-substituents can be expected to have a strong influence on the bonding situation of the 1271 perimeter. The enthalpy of formation for the localized structure 1A with C,,-symmetry and the corresponding structure of the 2,4,6,8-tetraalkyl derivatives is calculated ca. 10 kcal.mol-' less than that for the delocalized structure 1B 1 A a with D2,-symmetry. For the I ,3,5,7-tetraalkyl-s-indacenes this energy gap should only be 2 kcal.mol-', whereas in the case of 4,8-diamino-s-indacenes a completely delocalized n-electron system should be preferred. Even nitrile groups in the 2-and 6-position can be expected to lower the energy difference between 1A and 1B to ca. 6 kcal . mol -I. A similarly significant alkyl group effect, changing the bonding situation of the 1271 perimeter of 1, has so far never been reported in the case of other [4n]-n-electron systems. This effect is due to enhanced coupling between the n-electron system and the substituents in the case of the delocalized structure 1B. The quantum chemical predictions could be experimentally established in the case of the stable 4,8-bis(dimethylamino)-s-indacene and the 4-dimethylamino-2,6,8-tri-tert-butyl d e r i~a t i v e . '~.~~ These results encouraged us to check the calculations also for pure alkyl derivatives of 1, to gain more detailed information about the 12n-electron system. 1,3,5,7-Tetra-tertbutyl-s-indacene 4 seemed to be especially suitable for this purpose since it should be additionally stabilized kinetically by the bulky alkyl groups compared to the thermally extremely unstable 1 . ['I In analogy to the synthesis of 4,8-bis(dimethylamino)-sindacene, 4 can be obtained as red needles (decomp.

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Historical landmarks in the chemistry of these molecules are the synthesis of monocycles up to [30]annulene [163,164] and of the first bridged [10]annulene [165,166]. Due to angle strain and/or steric interactions between hydrogen atoms higher annulenes are characterized by a rich variety of non planar conformational and configurational isomers and of interconversion processes between them [6].…”

“…The s-indacene ground state has been described as due to bonding of two allyl radicals with a central benzene moiety (regarded in turn to be formed by two allyl radicals). Although s-indacene is a highly reactive molecule, the 1,3,5,7 tert-butyl derivative is a stable compound at room temperature [30]. X-ray diffraction data have been taken at room as well as at 100 K [30,31] consistent with a symmetric D 2h ring geometry.…”

“…Although s-indacene is a highly reactive molecule, the 1,3,5,7 tert-butyl derivative is a stable compound at room temperature [30]. X-ray diffraction data have been taken at room as well as at 100 K [30,31] consistent with a symmetric D 2h ring geometry. Coupling these data with previous results it appears that tert-butyl substituents play a minor role in determining the ring structure, limited to the increase of the 1-8a bond [212].…”

“…Going up with the ring size, while cyclooctatetraene is characterized by a non planar arrangement of the four π bonds [4,10,11], pentalene, where the ring is forced to planarity by crosslink [27,28], is equally well suited to model antiaromatic properties [29]. On the contrary, the more complex and formally antiaromatic structure of s-indacene peculiarly shows a bond equalized ring geometry [30,31].…”

From polysubstituted 1,5-dihydro-s-indacene lithium derivatives to polysubstituted hydro-s-indacenyl radicals by a new method involving a single electron transfer from an electron rich olefin

“…Polyalkyl-s-indacenes have been synthesized and studied for the last fifty years for their use as ligands or spacers in the synthesis of binuclear complexes, in conducting polymers or biological compounds [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. However, literature reports only some examples of s-indacenyl and substituted s-indacenyl radical ions [16,19] and to the best of our knowledge no examples of substituted s-indacenyl radicals.…”