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
Could this protein protect people against coronary artery disease?
17.11.2017 | University of North Carolina Health Care
Microbial resident enables beetles to feed on a leafy diet
17.11.2017 | Max-Planck-Institut für chemische Ökologie
The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.
Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...
Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.
That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...
Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.
During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....
The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.
Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...
Pillared graphene would transfer heat better if the theoretical material had a few asymmetric junctions that caused wrinkles, according to Rice University...
15.11.2017 | Event News
15.11.2017 | Event News
30.10.2017 | Event News
17.11.2017 | Physics and Astronomy
17.11.2017 | Health and Medicine
17.11.2017 | Studies and Analyses