Dynamic NMR (DNMR) spectroscopy of [R 1 C(R 2 )SO 2 R 3 ]Li (R 1 , R 2 = alkyl, phenyl; R 3 = Ph, tBu, adamantyl, CEt 3 ) in [D 8 ]THF has shown that the S-tBu, S-adamantyl, and S-CEt 3 derivatives have a significantly higher enantiomerization barrier than their S-Ph analogues. C α -S bond rotation is most likely the rate-determining step of the enantiomerization of the salts bearing a bulky group at the S atom and two substituents at the C α atom.

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.

The structures of the norbornenyl and norbornyl sulfones exo‐5, endo‐5 and endo‐6 have been determined experimentally, by X‐ray analysis, and theoretically by ab initio calculations (HF/6–31+G*). X‐ray crystal structure analyses of the lithiated allylic norbornenyl and norbornyl sulfones endo‐3/ent‐endo‐3·2diglyme and endo‐4/ent‐endo‐4·2diglyme revealed dimeric O–Li contact ion pairs devoid of C–Li bonds. The anions of endo‐3/ent‐endo‐3·2diglyme and endo‐4/ent‐endo‐4·2diglyme adopt both the endo conformation (C2–S) and are characterized by in the exo direction pyramidalized anionic C atoms. The degree of pyramidalization of the C2 atom of 3 is higher than that of 4. Ab initio optimizations (HF/6–31+G*) of the structures of the anions of methylenenorbornene I and methylenenorbornane II resulted in local minima featuring non‐planar C2 atoms which are pyramidalized in the exo direction in both cases, but to different degrees. In both cases cryoscopy of 3 and 4 in THF at –108.5 °C revealed approximately 1:1 mixtures of monomers and dimers. The sulfones exo‐5, endo‐5, exo‐6 and endo‐6 as well as the lithiosulfones 3 and 4 were studied by NMR spectroscopy. 1H‐NMR (400 MHz), 13C‐NMR (100 MHz) and 6Li‐NMR (44 MHz) spectroscopy of 3 and 4 at –100 °C in [D]8THF revealed in each case only one set of signals, independent of the configuration of the starting sulfones. This indicates in both cases that attainment of both the monomer‐dimer and the endo/exo equilibria of 3 and 4 is fast on the NMR time scale. According to 6Li{1H}‐ and 1H{1H}‐NOE experiments of 3 and 4 the monomeric and dimeric species endo‐3 and endo‐4, having endo anions, seem to be preferred in THF solution. Ab initio calculations of the anions of 3 and 4 resulted in structures endo‐3(–Li+), exo‐3(–Li+), endo‐4(–Li+) and exo‐4(–Li+) (HF/6–31+G*), whose atomic point charges were calculated by the method of Kollman et al. The C2 atoms of endo‐3(–Li+) and endo‐4(–Li+) are pyramidalized in the exo direction whereas the C2 atoms of exo‐3(–Li+) and exo‐4(–Li+) are pyramidalized in the endo direction. According to the calculations, the endo anions are more stable than the exo anions. There is good agreement between the optimized structures of the free anions and the experimentally determined structures of the anions of the contact ion pairs in the crystal. Reactions of 3 and 4 with DX, MeI, EtI, nPrI and nHeI occurred at the C2 atom under the selective formation of the corresponding endosulfones endo‐8a–e and endo‐9a–e, respectively, in all cases. Thus, an earlier report on the selective formation of the exosulfone exo‐9b in the reaction of 4 with MeI has to be revised. Product ratios were independent of the configuration of the starting sulfones and varied with the nature of the electrophile. Selectivities were highest in the case of the norbornyl species 4. Reaction of 3 with PhCHO occurred at the α position (C2) to afford the alcohols endo‐8f and exo‐8f (88:12) as single diastereomers and at higher temperatures at the γ position (C8), whereas reaction of ...

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