By ResearchSEA
Friday, February 26, 2010
Palladium catalysts containing unique molecular ligands
couple aromatic rings together in surprising ways
 |
| A new molecular ligand, called DHTP, helps selectively generate
ortho-coupled aromatic rings (left) instead of the usual coupled
product (right). |
| Copyright : RIKEN 2010 |
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.
|
Link to the Manabe Initiative Research Unit, RIKEN Science
Institute
Journal information
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)
SOURCE
