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"This is a really hot area," Toste said. "If you
look at the most-cited articles in the Journal of the American
Chemical Society, many are about gold catalysis.
"With this class of gold catalysts, you can develop
a number of unprecedented reactions that have never been seen before."
In a review article appearing in the March 22,
2007, issue of Nature, Toste discusses the new field and proposes a
new theory for why gold has such unusual, and practical, catalytic
properties. So far, the hypothesis has successfully predicted the
behavior of gold catalysts in new chemical reactions.
"Our hypothesis really allows us to approach
catalysis in a new way, melding the two fields of theoretical
chemistry and synthetic chemistry," Toste said.
At the heart of his hypothesis is the special
theory of relativity, proposed by Albert Einstein 102 years ago and
typically thought of as applying only to cosmological questions. But
late UC Berkeley chemist Kenneth Pitzer showed some 70 years ago that
the theory comes into play in chemistry as well. Other researchers
have used so-called relativistic quantum mechanics to explain gold's
yellow color and why mercury is a liquid instead of a solid.
Toste now takes this explanation a step further,
crediting special relativity with making gold - and perhaps the
related and widely used catalyst platinum - act as both an acceptor
and a donor of electrons in a catalytic reaction. Typical metal
catalysts do one or the other, but not both.
One of the key tenets of relativity is that nothing
can travel faster than the speed of light. The reason for this is that
objects become heavier, or more massive, the faster they go, with the
mass approaching infinity as the object approaches the speed of light.
In an atom, where electrons race around the nucleus
like buzzing bees, the velocity of an electron doesn't get anywhere
near the speed of light until the atomic nucleus fills up with lots of
positively charged protons - the negatively charged electrons have to
move faster to keep from being pulled into the highly positive nucleus.
This occurs in the transition metals of the periodic table of elements,
metals ranging from tantalum and tungsten to platinum and gold. In a
gold atom, with 79 protons in the nucleus, the 79 electrons whip
around the nucleus at about half the speed of light.
The net effect is that gold's electrons are much
heavier and are pulled in closer to the nucleus, lowering the energy
levels and making the atom more compact. According to this hypothesis,
gold's s shells, which are its lowest energy spherically symmetric
electron shells, contract. This shields the electrons in outer,
asymmetric p and d orbits from the nuclear charge, allowing them to
expand slightly. In gold, the contraction of the outermost (6s) shell
and the expansion of the next-inner (5p) shell reduces the energy
difference between the two to the equivalent of a photon of blue
light. This allows gold to absorb blue light and, thus, look yellow.
Silver, because it exhibits a much less dramatic relativistic effect,
is unable to absorb any visible light and is totally reflective.
Toste proposes that this same shielding effect
allows the more tightly bound s shell to easily accept electrons from
other molecules, while the partly shielded d shell can easily donate
electrons to a reaction.
Thus, gold is able to participate in reactions both
as a donor and as an acceptor of electrons, which makes it
particularly useful in catalyzing reactions at carbon-carbon bonds,
the backbone of all organic molecules. According to Toste, a gold atom
can attach to carbon loosely, with a single bond or a double bond,
allowing flexibility in reactions that can lead to novel organic
molecules.
Using this model, he has accurately predicted the
products in various organic reactions. For example, a gold atom
attached to the chemical phosphine and dispersed homogeneously in a
liquid can efficiently convert alkynes to pyrroles, which are ring
structures found in many drugs. Gold-phosphine catalysis also can
create an unusual carbon triangle called cyclopropane that is used in
industrial organic synthesis.
"We can make cyclopropanes without the need for
explosive diazo compounds," Toste said.
Toste predicts that gold catalysts also will be
very useful in producing chemicals with a specific handedness, that is,
a left-handed molecule, but not its right-handed or mirror image. Such
stereoselective reactions are becoming more important because many
drugs come in right- and left-handed forms, but only one form is
effective in the body. The most efficient synthesis would produce only
the effective form, not its ineffective mirror image. He is tuning the
phospine attached to gold to affect this stereoselectivity.
"The future of gold catalysis still involves a lot
of theoretical work, and we need to understand more about how it works,"
Toste said. "But already, some of these reactions are being used by
medicinal chemists, and it's a really exciting field." |
