| MECHANISM OF ACTION |
Sanford and Groves also determined the mechanism of the reaction, which involves reactive Rh(II) radicals as intermediates. "A key advantage of the radical process is its wide tolerance for other reactive functional groups," Groves says. "At the end of the cycle, we get the cyclic organic product and the rhodium(I) anion in quantitative yield," he says. "To get back the starting hydride, we protonate the rhodium(I) anion with a mild acid to remake the rhodium(III) hydride. While what we have achieved here is a formal catalytic cycle, a truly catalytic system returns spontaneously to the active form to start another cycle. That is the goal for us now--to find a single set of conditions that will allow this protonation and still be compatible with the rest of the cycle. We have every reason to believe that such conditions can be found." PHOTO BY ADAM MATZGER PHOTO BY SHELLEY WESTER RADICALS Sanford (left) and Groves developed an intramolecular anti-Markovnikov reaction. Its mechanism--generation of a rhodium(II) radical and its anti-Markovnikov addition to a terminal olefin--is shown on the blackboard. -------------------------------------------------------------------------------- The technique currently doesn't produce the type of terminally functionalized addition products that are favored by industry. Instead, it uses an intramolecular nucleophilic displacement in the product-forming step, yielding cyclic products. Such a reaction is easier to carry out because it's favored entropically--that is, the nucleophile and substrate are favorably positioned to react with each other. |
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