Sometimes, molecules need help making the right connections. When multiple ways exist to join organic fragments together, metal catalysts can direct the assembly process so that only certain structures form. Now, Shunpei Ishikawa and Kei Manabe from the RIKEN Advanced Science Institute in Wako and the University of Shizuoka, Japan, have developed a palladium-catalyzed procedure that couples aromatic rings in completely unexpected ways, thanks to a new molecular ligand with specially designed spatial attributes1.
Ishikawa and Manabe studied how to attach a benzene-based molecule to another aromatic ring containing an alcohol (–OH) group and two bromine (Br) atoms, located either beside (ortho) or far across from the –OH. Reactions that can link the rings at one of the Br sites, while leaving the other untouched, are extremely valuable to synthetic chemists for creating drug compounds and materials like liquid crystals. Because the ortho-Br is the geometrically and electronically least favored addition site, it is particularly difficult to establish couplings there.
The researchers designed a new series of molecular ligands, called dihydroxy-terphenylphosphines (DHTP), to enable ortho-selective aromatic couplings. DHTP consists of three benzene rings, linked end-to-end through rotationally flexible carbon–carbon (C–C) bonds; the first benzene contains a phosphorus group, while the third has dual –OH units. According to Manabe, DHTP ligands had the right geometric balance needed for this reaction.
“Catalysts should not be too flexible, and not too rigid,” says Manabe. “Our DHTP catalyst can rotate about the C–C bonds, making it flexible enough to fit its structure to the catalytic transition state.”
The researchers attached the DHTP ligand to the bromine-containing aromatic ring via a magnesium atom that bridges the molecules together through their respective –OH functionalities. Then, they added a palladium catalyst to the reaction, which they assumed would bind to DHTP through the phosphorus unit. In this geometric configuration, the palladium atom can only interact efficiently with the ortho-Br atom to initiate a catalytic cycle that yields ortho-coupled aromatic rings with 80–90% selectivity and few by-products—a complete reversal of the usual aromatic coupling.
The DHTP-based catalytic system improved upon the authors’ previous work2 by having two –OH groups on the ligand, instead of one; this way, there is always a magnesium atom located close to the palladium catalyst, even if a C–C bond rotation occurs. “For me, it is very interesting that introducing only one –OH group improves selectivity and reactivity to a great extent,” says Manabe.
The corresponding author for this highlight is based at the Manabe Initiative Research Unit, RIKEN Advanced Science Institute
1. Ishikawa, S. & Manabe, K. DHTP ligands for the highly ortho-selective, palladium-catalyzed cross-coupling of dihaloarenes with Grignard reagents: A conformational approach for catalyst improvement. Angewandte Chemie International Edition 49, 772–775 (2010)
2. Ishikawa, S. & Manabe, K. Oligoarene strategy for catalyst development: Hydroxylated oligoarene-type phosphines for palladium-catalyzed cross coupling. Chemistry Letters 36, 1302–1303 (2007)
Saeko Okada | Research asia research news
Brought to light – chromobodies reveal changes in endogenous protein concentration in living cells
21.09.2018 | NMI Naturwissenschaftliches und Medizinisches Institut an der Universität Tübingen
A one-way street for salt
21.09.2018 | Julius-Maximilians-Universität Würzburg
The building blocks of matter in our universe were formed in the first 10 microseconds of its existence, according to the currently accepted scientific picture. After the Big Bang about 13.7 billion years ago, matter consisted mainly of quarks and gluons, two types of elementary particles whose interactions are governed by quantum chromodynamics (QCD), the theory of strong interaction. In the early universe, these particles moved (nearly) freely in a quark-gluon plasma.
This is a joint press release of University Muenster and Heidelberg as well as the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt.
Then, in a phase transition, they combined and formed hadrons, among them the building blocks of atomic nuclei, protons and neutrons. In the current issue of...
Thin-film solar cells made of crystalline silicon are inexpensive and achieve efficiencies of a good 14 percent. However, they could do even better if their shiny surfaces reflected less light. A team led by Prof. Christiane Becker from the Helmholtz-Zentrum Berlin (HZB) has now patented a sophisticated new solution to this problem.
"It is not enough simply to bring more light into the cell," says Christiane Becker. Such surface structures can even ultimately reduce the efficiency by...
A study in the journal Bulletin of Marine Science describes a new, blood-red species of octocoral found in Panama. The species in the genus Thesea was discovered in the threatened low-light reef environment on Hannibal Bank, 60 kilometers off mainland Pacific Panama, by researchers at the Smithsonian Tropical Research Institute in Panama (STRI) and the Centro de Investigación en Ciencias del Mar y Limnología (CIMAR) at the University of Costa Rica.
Scientists established the new species, Thesea dalioi, by comparing its physical traits, such as branch thickness and the bright red colony color, with the...
Scientists have succeeded in observing the first long-distance transfer of information in a magnetic group of materials known as antiferromagnets.
An international team of researchers has mapped Nemo's genome, providing the research community with an invaluable resource to decode the response of fish to...
21.09.2018 | Event News
03.09.2018 | Event News
27.08.2018 | Event News
21.09.2018 | Physics and Astronomy
21.09.2018 | Life Sciences
21.09.2018 | Event News