Synthetic Studies of Chiral Organometallics

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Many of today's drugs contain saturated nitrogen heterocyclic rings, thus, it is important to develop new ways to synthesize and modify nitrogen heterocycles. Likewise, it has become important to produce chiral drugs in enantiomerically pure form. For decades, the only stereoselective reactions known to organic chemists were additions of a fourth ligand to heterotopic faces of a trigonal atom. This category includes almost all stereoselective reactions in organic synthesis, and we have accumulated significant insight into the electonic, steric, and dynamic factors that influence these processes. In the past 10 years, significant advances in applying electrophilic substitutions of chiral organometallics (especially organolithiums) as new methods for asymmetric synthesis have been made. This reactivity mode is different from most other asymmetric syntheses in that it involves substitution of a metal attached to a stereogenic carbon atom. Substitution of the metal for an electrophile completes the enantioselective synthesis. Mechanistic understanding of developing methodology is necessary to advancing the field. When contemplating electrophilic substitutions of stereogenic carbanions, one must consider configurational stability, steric course, and aggregation state. If the electrophile is prochiral, two new stereocenters are formed and the question of diastereoselectivity arises. Answering these questions is not always possible; but without answers, mechanistic interpretation is speculative, at best. The specific aims of this project are: 1. To continue our investigation into the scope and limitations of alpha-aminoorganolithium electrophilic substitutions and sigmatropic rearrangements, with emphasis on stereoselectivity in compounds with multiple stereocenters. Explore further extensions of the methodology such as ring-closing metathesis. 2. To develop improved methods of preparation of chiral organometallics. 3. To further explore the solution and solid state structure of chiral organometallics. Evaluate the effects of ligating atoms and metal effects on configurational and structural dynamics of chiral carbanions. 4. To pursue our recent findings relating to possible stereoelectronic effects on transmetalation and the nature of heteroatom participation in tin-lithium exchanges using rapid injection NMR.