Chirality-Controlled Self-Assembly of Amphiphilic Dibenzo[6]helicenes into Langmuir-Blodgett Thin Films.

Racemic and highly enantioenriched 3-methoxycarbonyl, 3-carboxy, and 3-hydroxymethyl derivatives of dibenzo[6]helicene were prepared. The Langmuir layers of these helicenes were formed at the air-water interface and transferred onto solid substrates to afford Langmuir-Blodgett films, which were then studied by ambient atomic force microscopy and (chir)optical spectroscopy. Significant differences were found in the behaviour of the Langmuir layers as well as in the morphology, UV/Vis, electronic circular dichroism (ECD), and fluorescence spectra of the Langmuir-Blodgett thin films depending on the molecular chirality and nature of the polar group.

Helicenes as Chirality-Inducing Groups in Transition-Metal Catalysis: The First Helically Chiral Olefin Metathesis Catalyst.

Helical chirality is a novel enantioselectivity-inducing property in transition-metal-catalyzed transformations. The principle is illustrated herein for the example of asymmetric olefin metathesis. This work reports the synthesis of the first helically chiral Ru-NHC alkylidene complex from an aminohelicene-derived imidazolium salt, which was ligated to the first generation Hoveyda-Grubbs catalyst.

Asymmetric Synthesis of Nonracemic 2-Amino[6]helicenes and Their Self-Assembly into Langmuir Films.

Alternative ways of preparing nonracemic 2-amino[6]helicene derivatives were explored. The enantioselective [2 + 2 + 2] cycloisomerization of a nonchiral triyne under Ni(cod)/( R)-QUINAP catalysis delivered the enantioenriched (+)-( P)-2-aminodibenzo[6]helicene derivative in 67% ee. An ultimate "point-to-helical" chirality transfer was observed in the cyclization of enantiopure triynes mediated by Ni(CO)(PPh) affording (-)-( M)- or (+)-( P)-7,8-bis( p-tolyl)hexahelicen-2-amine in >99% ee as well as its benzoderivative in >99% ee.

Oxahelicene NHC ligands in the asymmetric synthesis of nonracemic helicenes.

A straightforward approach to enantiopure 2H-pyran-modified amino[5]helicenes and amino[6]helicenes was developed. They were converted to 1,3-disubstituted imidazolium salts and used as NHC ligand precursors in the Ni-catalysed enantioselective [2+2+2] cycloisomerisation of aromatic triynes to obtain the model helicene derivatives in up to 86% ee.

2,2'-Biphenols via protecting group-free thermal or microwave-accelerated Suzuki-Miyaura coupling in water.

User-friendly protocols for the protecting group-free synthesis of 2,2'-biphenols via Suzuki-Miyaura coupling of o-halophenols and o-boronophenol are presented. The reactions proceed in water in the presence of simple additives such as K2CO3, KOH, KF, or TBAF and with commercially available Pd/C as precatalyst. Expensive or laboriously synthesized ligands or other additives are not required.