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Examines the stereochemistry in palladium-catalyzed cyclization. Formation of vinyldihydrofuran; Intramolecular allylic substitution reaction; Racemization of enantiomerically pure allylic monocarbonate.

Abstract: The aromaticity of dicyclopenta‐fused acenes (DPAs) and polyacenes (PAs) of increasing size has been studied by evaluation with the GIMIC method at the DFT level of the magnetically‐induced currents (MICs), and by analyzing their spatial distributions. For these open‐shell singlet molecules, spin‐restricted and ‐unrestricted treatments provide very different MICs, the latter ones providing the most reliable solution. These MICs and the differences between spin‐restricted and ‐unrestricted treatments are interpreted in terms of the bond current strengths and the current gradients, which indicate the bond aromaticity and enable the spatial distributions of the diatropic and paratropic currents to be analyzed, respectively. In particular, they allow the rationalization of the MICs in correlation with the odd‐electron density distributions and their diradical characters. These calculations demonstrate that 1) in increasingly large PAs the bond current strengths get smaller and smaller than in benzene and get almost similar in the central and terminal rings, 2) for DPAs the MICs increase from dominant paratropic currents and antiaromaticity in the small compounds to diatropic currents and aromaticity in the larger ones, and 3) in the largest DPAs, the central rings are characterized by large diatropic currents and the terminal five‐membered rings, for which the odd‐electron densities are localized by weak ones. [ABSTRACT FROM AUTHOR]

Abstract: Tunability of the open‐shell character, charge asymmetry, and third‐order nonlinear optical (NLO) properties of covalently linked (hetero)phenalenyl dimers are investigated by using the density functional theory method. By changing the molecular species X and substitution position (i, j) for the linker part, a variety of intermonomer distances R and relative alignments between the phenalenyl dimers can be realized from the geometry optimizations, resulting in a wide‐range tuning of diradical character y and charge asymmetry. It is found that the static second hyperpolarizabilities along the stacking direction, γyyyy, are one‐order enhanced for phenalenyl dimer systems exhibiting intermediate y, a feature that is in good agreement with the “y–γ correlation”. By replacing the central carbon atoms of the phenalenyl rings with a boron or a nitrogen, we have also designed covalently linked heterophenalenyl dimers. The introduction of such a charge asymmetry to the open‐shell systems, which leads to closed‐shell ionic ground states, is found to further enhance the γyyyy values of the systems having longer intermonomer distance R with intermediate ionic character, that is, charge asymmetry. The present results demonstrate a promising potential of covalently linked NLO dimers with intermediate open‐shell/ionic characters as a new building block of highly efficient NLO systems. [ABSTRACT FROM AUTHOR]