| SUBJECT |
ALTHOUGH A LARGE fraction of the studies in chiral heterogeneous catalysis involve the surfaces of metals, metals aren't the only game in town. Porous materials such as zeolites and mesoporous solids (2 to 50-Å pore diameters) play a major role as catalyst supports in conventional heterogeneous catalysis. So some scientists have tried to determine whether or not the holey materials can play a special role in chiral catalysis. Just recently, a few research groups have shown that, indeed, they can. Sir John Meurig Thomas recalls that his interest in using mesoporous silica and other porous materials in asymmetric catalysis grew out of a research project on "heterogenizing" (immobilizing) homogeneous catalysts. Thomas, who is a professor of chemistry and former director at the Royal Institution of Great Britain, in London, and a professor of materials science at the University of Cambridge, coauthored a paper a decade ago in Nature that reported a procedure for anchoring achiral homogeneous titanium epoxidation catalysts to the interior walls of porous supports. His coauthor on that paper was Thomas Maschmeyer, who is now a chemistry professor at the University of Sydney, in Australia. "It occurred to us that if we could do the same with an organometallic chiral catalyst--meaning a catalyst in which chiral ligands are attached to a metal atom--then we could capitalize on the confinement in the pores to do asymmetric catalysis," Thomas notes. Not only would anchoring the compounds in a confined space boost enantioselectivity, but it would also enable researchers to recover the costly catalysts, in which the chiral ligands may be even more expensive than the precious metals. ORITO REACTION Platinum modified with cinchonidine, a chiral alkaloid, functions as an enantioselective hydrogenation catalyst. The idea is that the confining dimensions of the interior of a pore should be able to restrict the possible orientations that a bulky reactant can assume as it approaches a chiral catalytic center that's attached to the pore wall. If a reactant is nudged by its surroundings into the right orientation for a stereospecific reaction, then the reaction should proceed enantioselectively. It took a few years to prove the hypothesis, but now the effect has been demonstrated. In a recent study, Thomas, Robert Raja, Matthew D. Jones, Dewi W. Lewis, Brian F. G. Johnson, and their coworkers used a brominated trichlorosilane compound to anchor rhodium(I) diene-diamine and palladium(II) allyl-diamine catalysts to the inner walls of porous silica. The group then compared the performance of the surface-bound versions of the catalysts with that of the homogeneous forms. In separate test reactions involving asymmetric hydrogenation of phenylcinnamic acid and methyl benzoylformate, the group found that the tethered catalysts provide significantly greater enantiomeric excesses than their homogeneous counterparts [Angew. Chem. Int. Ed., 42, 4326 (2003)]. The group also found that attaching the chiral catalysts to nonporous silica reduced the enantioselectivity as compared with the same compounds attached inside the silica pores. "It shows that the magic of it all is confinement. The restricted approach of the reactant to the catalyst is the root cause of the enhancement in enantioselectivity," Thomas asserts. Going a step further to assess the importance of confinement, Thomas, Raja, David E. W. Vaughan, and coworkers compared the performance of catalysts that were fixed to silica samples with pore sizes ranging from 38 to 250 Å. They found that the catalysts' ability to steer reactions enantiospecifically diminished systematically with increasing pore size [J. Am. Chem. Soc., 125, 14982 (2003)]. The larger the pores, the less effective the support is in constraining the reactants' orientation. ONE SHORTCOMING of the anchoring technique is the large effort required to prepare the catalysts. So rather than anchoring the metal compounds to silica covalently by way of the brominated trichlorosilane tether, the team has adapted a noncovalent immobilization method that uses surface-bound triflate (CF3SO3-) counterions to secure the cationic catalyst compounds in place. ACE IN THE HOLE Confining a rhodium(I) organometallic catalyst (bottom molecule) within a silica pore restricts the possible orientations of a reactant molecule (methyl benzoylformate) on approach to the catalyst, thereby boosting enantioselectivity. COURTESY OF ROBERT RAJA Raja, who is a senior research associate in the chemistry department at Cambridge, points out that the noncovalent method is quick, convenient, and industrially relevant. Catalysts prepared via the newer method using nonordered mesoporous silica are robust and effective but roughly one-eighth the cost of earlier versions of the covalently tethered catalysts, he says. Underscoring chemical manufacturers' interest in the chiral catalysis developments, Raja notes that Bayer, which provided financial support for the research, has filed several international patents to protect the work. |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |