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chemistry professor at the University of North Carolina, Chapel Hill, also presented work on coupling reactions--namely, metal cyanide- and metallophosphite-catalyzed cross silyl benzoin reactions between acyl silanes and aldehydes. "Our goal was an enantioselective version of an -hydroxyketone synthesis," he said. And the researchers achieved a "proof of concept for the first nonenzymatic cross benzoin reaction that was enantioselective." Using potassium cyanide and a phase-transfer catalyst, his group initially found that racemic unsymmetrical aryl-, heteroaryl-, and alkyl-substituted benzoin adducts could be generated in moderate to excellent yield with complete regiocontrol (J. Am. Chem. Soc. 2005, 127, 1833). They then speculated that metallophosphites might offer the needed mechanistic functions as a nucleophile, anion stabilizing group, and leaving group in cross silyl benzoin reactions that involve phosphite addition and Brook rearrangement. The best metallophosphite catalyst, a tetra(o-fluorophenyl) Taddol phosphite, generated good yields (6588%) and enantiomeric excesses generally between 80 and 90% with aryl acylsilane and aryl or heterocyclic aldehyde substrates (J. Am. Chem. Soc. 2004, 126, 3070). Catalyst loadings of 5-20 mole % are not yet practical, Johnson pointed out; yield and selectivity can be addressed through modifying the tunable ligand structure. His group next worked on metallophosphite catalysis, facilitated by a diastereoselective retro [1,4] Brook rearrangement, in the acylation of ,ß-unsaturated amides to produce 1,4-dicarbonyls and achieved similar success (Angew. Chem. Int. Ed. 2005, 44, 2377). |
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