The racemization barriers of pentahelicene up to nonahelicene have been computed with A M I, MNDO, and PM3. All methods lead to Cs transition states which have lower energy than those with Civ symmetry. The barriers calculated by A M I match the experimental values best for all helicenes. The reliability of the results has been confirmed by ab initio methods using the B3LYP functional with the 3-21G basis set as implemented in the GAUSSIAN94 package. Furthermore, 12 methyl-substituted helicenes have been computed with the A M I method. The racemization barriers of 1-methyl-substituted penta-and hexahelicene are at least as high as that of the next higher unsubstituted helicene. A second methyl group in the l'-position increases the barrier further, while methyl groups in the 2-position do not have a severe influence on the racemization barrier.

The kinetics of the reactions of the vinyl cations 2 [PhC═C-(4-MeO-CH)] and 3 [MeC═C-(4-MeO-CH)] (generated by laser flash photolysis) with diverse nucleophiles (e.g., pyrroles, halide ions, and solvents containing variable amounts of water or alcohol) have been determined photometrically. It was found that the reactivity order of the nucleophiles toward these vinyl cations is the same as that toward diarylcarbenium ions (benzhydrylium ions). However, the reaction rates of vinyl cations are affected only half as much by variation of the nucleophiles as those of the benzhydrylium ions. For that reason, the relative reactivities of vinyl cations and benzhydrylium ions depend strongly on the nature of the nucleophiles. It is shown that vinyl cations 2 and 3 react, respectively, 227 and 14 times more slowly with trifluoroethanol than the parent benzhydrylium ion (Ph)CH, even though in solvolysis reactions (80% aqueous ethanol at 25 °C) the vinyl bromides leading to 2 and 3 ionize much more slowly (half-lives 1.15 yrs and 33 days) than (Ph)CH-Br (half-life 23 s). The origin of this counterintuitive phenomenon was investigated by high-level MO calculations. We report that vinyl cations are not exceptionally high energy intermediates, and that high intrinsic barriers for the sp ⇌ sp rehybridizations account for the general phenomenon that vinyl cations are formed slowly by solvolytic cleavage of vinyl derivatives, and are also consumed slowly by reactions with nucleophiles.

Treatment of aminoalkynes (ynamines), Me3Si-CC-NR2 [NR2 = N(CH2CH2)2NMe, N(CHMe-CH2)2CH2], with dialkylaluminum hydrides, H-AlR′2 (R′ = CH2 tBu, iBu, tBu), afforded by hydroalumination the corresponding alkenyl derivatives R′2Al-(Me3Si)CC(H)-NR2 (3 to 6) in which the opposite Lewis functionalities adopted a cis-arrangement with the aluminum and nitrogen atoms on the same side of the resulting CC double bonds. Intramolecular Al–N interactions gave four-membered AlC2N heterocycles, but ring strain may cause relatively long Al–N distances of 2.07 to 2.15 Å. These compounds represent a new type of Lewis acid–base pairs, and we observed C–H bond activation by opening of the Al–N bond upon treatment with H-CC-C6H5. Compound 7 was isolated, which had the proton attached to nitrogen and the anionic ethynyl group bonded to aluminum. The formation of 7 is reversible in solution with an increasing dissociation into the starting compounds at elevated temperatures. Two different structural motifs were obtained upon reaction of the Lewis acid–base pairs with dicyclohexylcarbodiimide, which inserted selectively into the bond between the aluminum and the vinylic carbon atom. In the first compound (8) an amidinato ligand was formed, which coordinated the metal atom by both nitrogen atoms to give an AlCN2 heterocycle. A six-membered heterocycle (9) resulted from the insertion of only one CN double bond of the carbodiimide into the Al–C(vinyl) bond. 9 had an unchanged interaction of the metal atom with the α-nitrogen atom of the former ynamine. The latter compound may be considered an intermediate, and rearrangement to yield an amidinato compound analogous to 8 was indeed observed with extended reaction time.

The NMR spectroscopic investigation of solutions of the iminium ions Me2N+C(H)Cl (1 + ), Me2N+ C(H)Ph (2a + ), and Et2N+CH2 (3 + ) in aprotic solvents shows that the replacement of the weakly nucleophilic counterions SbCl6 -, AlCl4 -, BF4 -, or CF3SO3 - by halide ions (I-, Br-, Cl-) causes a deshielding of the proton at the iminium carbon atom (Δδmax = +2.8 for 2a + in CD2Cl2) while the chemical shifts of all other 1H and 13C nuclei remain almost unaffected. The solvent dependence of these effects, 14N NMR investigations, and ab initio MO calculations suggest that iminium halides form CH···Y- hydrogen bonds (Y = Cl, Br, and I) and thus exist as contact ion pairs in acetonitrile, chloroform, dichloromethane, and dimethyl sulfoxide solutions. In liquid sulfur dioxide, specific anion−cation interactions cannot be observed due to the superior solvation of the anions by SO2.