Spiraling up: Easy access to dibenzo[5]-, dibenzo[6]-, and dibenzo[7]helicenes as well as their functionalized derivatives includes Sonogashira and Suzuki-Miyaura couplings, desilylation, and [2+2+2] alkyne cycloisomerization. The simplicity of this non-photochemical approach combined with the potential for helicity control favors dibenzohelicenes over the parent helicenes for practical applications.

A nonphotochemical synthetic route to 3-hexahelicenol is reported. It involves a key [2+2+2] cycloisomerization of CH3O-substituted triyne that is readily available from 1-methoxy-3-methylbenzene and 1-bromo-2-(bromomethyl)naphthalene. Further functional group transformations led to 3-CO2CH3, 3-NH2, 3-PPh2, and 3-SC(O)N(CH3)2 substituted hexahelicenes.

The classical term ortho effect was quantitatively analyzed and decomposed into its real or supposed components, using the acidity of eleven 2-substituted benzoic acids as a sample. The substituent effects were evaluated by calculations at the B3LYP/6-311þG(d,p) level by means of isodesmic reactions, separately for the acid molecules and for their anions. An intramolecular hydrogen bond, affecting the acidity moderately, was found only in the case of 2-methoxy-and 2-dimethylaminobenzoic acids. For the other acids, the oftenemployed interpretation invoking a hydrogen bond is wrong: the pertinent form was not revealed by calculations or by infrared spectroscopy. Another widespread explanation relying on the steric inhibition of resonance of the COOH group is also not correct in most cases: only some acid molecules are nonplanar (with substituents tert-Bu, CH 2 Cl, CF 3 , NO 2 , Cl) and the steric inhibition exists also in the anions, thus partly cancelling this effect.The main part of the ortho effect consists of polar effects (inductive and resonance) transmitted through the ring and of purely steric effects. Their tentative separation was based on comparison with the 4-substituted isomers. Polar effects are most important for the acidity and are not very different in the ortho and para positions (rough estimate 0.8:1). Steric effects are strong but mostly of similar magnitude in the acid as in the anion; hence they are of restricted importance for the acidity. They may be better described as van der Waals interactions rather than as short-range field effects. The whole analysis leads to the conclusion that the acidity of ortho-substituted benzoic acids cannot be interpreted by a simple universal theory.Benzene ortho derivatives have received less attention compared to their meta and para isomers. While in the latter the dependence of the reactivity and properties on structure is simple and can be approximated by the Hammett equation, effects of ortho substituents are much more complex. 1,2 Mostly they have been assumed to arise from two contributions: one similar as in the meta and para isomers, called inductive effect and resonance, 1,3 the other commonly called steric. The latter was further divided into several tentative components, 2,4,5 which were not always exactly identified: primary steric effect, steric inhibition to resonance (SIR), short-range field effect, hydrogen bonds (HB), steric inhibition to solvation and others. In numerous treatments of this subject, certain of these effects were more or less stressed and sometimes they were expressed quantitatively by an empirical equation with limited success. 4 Note that some of these effects are only simplified models, sometimes poorly defined: for instance, one can hardly distinguish a short-range field effect from a primary steric effect. As an extreme opinion, reactivities of ortho derivatives were explained without referring to any steric effects at all, 3 either by a blend of inductive and resonance effects 3a,b or most simply, by correlation with the...

An original approach to helicene frameworks exploiting atom economic isomerization of appropriate energy-rich aromatic cis,cis-dienetriynes has been developed. The new paradigm provides nonphotochemical syntheses of helicenes based on the easy, convergent, and modular assembly of key cis,cis-dienetriynes and their nickel(0)-catalyzed [2+2+2] cycloisomerization. The potential of the methodology is underlined by the syntheses of the parent [5]helicene (2), 7,8-dibutyl[5]helicene (23), [6]helicene (24), and [7]helicene (25). The approach can be adapted to prepare functionalized helicenes as exemplified by the eight-step synthesis of 7,8-dibutyl-2,3-dimethoxy[6]helicene (34). Density functional theory (DFT) calculations showed that bis[2-((1Z)-1-buten-3-ynyl)phenyl]acetylene (1) and isomeric [5]helicene (2) differ enormously in the Gibbs energy content (DeltaG = -136.6 kcal x mol(-1)) to favor highly the devised intramolecular simultaneous construction of three aromatic rings.