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Chemistry Seminars

May 3, 2012
Room 113, Lathrop Hall 11:00 a.m.

*Pre-seminar reception at 10:45 a.m.

Elizabeth A. Clizbe

Department of Chemistry
The College at Brockport, Brockport, NY

Metal-Catalyzed Allylic Substitution Reactions

The transition metal-catalyzed allylic substitution has proven to be a powerful and versatile cross-coupling for the construction of new stereogenic bonds. Specifically the allylic amination reaction provides a convenient method for preparing chiral non-racemic allylamines that can help in the construction of secondary metabolites and medicinally important agents, ie. the batzelladine alkaloid core. Ylides possess unique characteristics that allow them to be tuned for use as nucleophiles in allylic substitution. The regio- and enantiospecific rhodium-catalyzed allylic amination reaction was developed using the aza-ylide derived from 1-aminopyridinium iodide (1). Further synthetic utility was shown by the cleaving of the azanide product to the free amine in high yield.

In contrast to the rhodium substitution reaction utilizing enriched starting materials to afford chiral material, the iridium-catalyzed substitutions are able to incorporate linear substrates to proceed in an enantioselective manor. This reaction however often uses electronically biased substrates and has lower regioselectivities in comparison to rhodium; however, a highly regioselective version has been developed using diphenylsulfilimine in combination with a phosphoramidite ligand to afford sulfilimine products in high yields (2).

The asymmetric a-alkylation of ketone enolates represents one of the most important and challenging carbon-carbon bond forming reactions in total synthesis. Traditionally this transformation has been accomplished using chiral auxiliaries, though advances in enantioslective palladium-catalyzed allylic alkylations using unstabilized ketone enolates have provided methods for synthesizing ternary and quaternary stereogenic centers. With this knowledge in hand, the first enantioselective rhodium-catalyzed allylic alkylation was developed using the lithium enolate of an acyclic a-alkoxy aryl ketone with the allylic benzoate using the rhodium complex derived from Wilkinson's catalyst modified with BINOL-MeOP, to afford the protected chiral nonracemic homoallylic alcohol in excellent yield and enantioselectivities (3).

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