Functionalized thiophene-based [7]helicene: chirooptical properties versus electron delocalization.

The functionalized, enantiomerically pure [7]helicene 1 derived from bis(benzodithiophene) functionalized with four heptyl groups is prepared from 1,8-dibromo-4,5-diheptylbenzo[1,2-b:4,3-b']dithiophene building block 2. Such [7]helicene structure, functionalized with bromines at the terminal positions of the helicene inner rim and multiple solubilizing alkyl groups, is an attractive building block for long [n]helicenes and oligo[7]helicenes. Chirooptical properties and the degree of electron delocalization are determined and compared to those of analogous carbon-sulfur [7]helicene and [7]helicenes derived from benzodithiophene to provide a correlation between chirooptical properties and the degree of electron delocalization. [7]Helicene 1 possesses a moderately increased electron delocalization, but its chirooptical properties are similar to those for analogous [7]helicenes with relatively lower electron delocalization, indicating that chirooptical properties are not significantly affected by electron delocalization for this series of [7]helicenes. Molecular structures of racemic [7]helicene 1 and its benzodithiophene building block 2 are confirmed by single-crystal X-ray analysis. Crystals of 2 are chiral and adopt the shape of long, flexible, flat needles that can be readily bent.

Multinuclear NMR study of the solution structure and reactivity of tris(trimethylsilyl)methyllithium and its iodine ate complex.

The extreme steric bulk of tris(trimethylsilyl)methyl derivatives (1-X) provides interesting structural and dynamic behavior for study. Dynamic NMR studies on 1-SePh and 1-I showed restricted rotation around the C-Si bonds of each trimethylsilyl groups. An extensive multinuclear NMR study of natural abundance and (6)Li and (13)C enriched 1-Li revealed three species in THF-containing solvents, a dimer 1T, and two monomers, the contact ion pair 1C, and solvent separated ion pair 1S. Observed barriers for interconversion of 1-Li aggregates were unusually high (DeltaG(double dagger) ca. 9 kcal/mol for exchange of 1S and 1C, DeltaG(double dagger)(41) = 16.4 kcal/mol for exchange of 1T with 1C and 1S), allowing for study of reactivity of each aggregate individually. We can show that 1S is at least 50 times as reactive as 1C and at least 5 x 10(10) times as reactive as 1T toward MeI. The large difference in reactivity allowed further study on the mechanism of the lithium-iodine exchange of 1-I with 1-Li and characterization of the intermediate iodine ate complex 4. Additional calibrations are presented for the sensitive yet chemically inert (13)C NMR chemical shift thermometer 1-H.

A rapid injection NMR study of the reaction of organolithium reagents with esters, amides, and ketones.

Unexpectedly high rates of reaction between alkyllithium reagents and amides, compared to esters and ketones, were observed by Rapid Inject NMR and competition experiments. Spectroscopic investigations with 4-fluorophenyllithium (ArLi, mixture of monomer and dimer in THF) and a benzoate ester identified two reactive intermediates, a homodimer of the tetrahedral intermediate, stable below -100 °C, and a mixed dimer with ArLi. Direct formation of dimers suggested that the ArLi dimer may be the reactive aggregate rather than the usually more reactive monomer. In contrast, RINMR experiments with ketones demonstrated that the ArLi monomer was the reactive species.