| RESEARCH |
professor at the Swiss Federal Institute of Technology, Zurich, presented a similar approach to creating modular ligands based on easily prepared substituted [2.2.2]-bicyclooctadienes. The ligands, also derived from terpenes, are useful for asymmetric catalysis involving late transition metals without the necessity of added phosphine groups. Initially, Carreira and coworkers tested the diene ligands in the iridium-catalyzed kinetic resolution of allylic carbonates and achieved up to 98% ee (J. Am. Chem. Soc. 2004, 126, 1628). "We've been trying to understand why these olefins function in the capacity that they do and, in certain processes, are capable of effecting transformations phosphines can't," Carreira said. "The interaction of an olefin with a metal center is a curious balance of donation and back donation." Although the olefin bonding energies can be weaker than those displayed by the more commonly used phosphines, strain in the diene ligand structure may help stabilize the metal complex. SECURED Tethering the (-6)-arene ring to the rest of the ligand makes for a more active, selective, and structurally versatile catalyst. Using other substituted variants of the dienes in rhodium-catalyzed reactions, Carreira and coworkers have conducted asymmetric conjugate additions of arylboronic acids to enals (J. Am. Chem. Soc. 2005, 127, 10850); to enones, enamides, and coumarins (Org. Lett. 2004, 6, 3873); and to unsaturated tert-butyl esters (Org. Lett. 2005, 7, 3821). The method gives rise to compounds incorporating diarylmethine stereogenic centers, otherwise challenging to prepare, but important in natural products and some notable drugs. Carreira also briefly described a new biaryl phosphorus and nitrogen, or P,N-containing, ligand. "We wanted a ligand scaffold structurally similar to QUINAP [1-(2-diphenylphosphino-1-naphthyl)isoquinoline] that, unlike QUINAP, would be easy to make, inexpensive, and much more amenable to modification," he said. His lab developed PINAP, based instead on phthalazine and 2-naphthol, which has reactivity comparable with that of QUINAP in hydroboration and C5N addition reactions (Angew. Chem. Int. Ed. 2004, 43, 5971). Using PINAP, Carreira noted, his lab has accomplished the first catalytic, enantioselective, conjugate addition involving direct use of terminal acetylenes (J. Am. Chem. Soc. 2005, 127, 9682). |
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