Highly Functionalized Piperidine Generation via Pyridine Repolarization and Hyper-Distorted Allyl Complexes of {TpW(NO)(PMe₃)}

Chapter 1 introduces the traditional organic chemistry of pyridine with an emphasis on its dearomatization. Organometallic methods of dearomatization are also discussed. Strategies for averting nitrogen coordination (i.e. kN) in favor of haptotropic (i.e. η² ,η⁴ , η⁶ ) pyridine coordination are discussed, as well as known modifications of these carbon-coordinated pyridines. The work previously performed by our group with {TpW(NO)(PMe₃)} and our strategy to utilize this fragment is introduced.

Chapters 2 and 3 report our findings on the large scale synthesis of η² pyridine complexes of tungsten, utilizing a borane-protection strategy to avert kN coordination. The reactivity of complexes that result from the removal of the borane and replacement with alternative electrophilic groups are investigated. In particular, we have found that an acetyl group provides an isolable N-acetylpyridinium complex, which allows for the mild regioand stereoselective modification of the pyridine ring with nucleophiles.

Chapters 4 and 5 report on the fundamentally new chemistry of pyridine that results from the coordination of the {TpW(NO)(PMe₃)}. Tandem electrophilic followed by nucleophilic additions and cycloadditions with 1,2-dihydropyridine (DHP) complexes are reported. These findings suggest that the metal coordination reverses the polarization of the pyridine ring carbons such that electrophiles add α-to-N rather than β-to-N. Importantly, we report the isolation of new 2-, 3-, or 4-substitituted piperidine compounds that result from this methodology.

Chapter 6 reports pyridine ring scission with nucleophiles capable of delivering 4 e- (2σ, 2π) to the pyridine ring. The resulting conjugated complexes were probed for fluorescent activity. While none was found for the metal complexes, photolysis liberated an organic cation that did display fluorescence.

Chapter 7 discusses our discovery of highly distorted allyl complexes of {TpW(NO)(PMe₃)}. Here we endeavor to understand the origin of the large distortions and orientations of allyls observed in crystal structures and reproduced by density functional theory (DFT) calculations. We propose that the nitrosyl is responsible for the distortion, while Tp is responsible for the orientation of the allyls distal to PMe₃.
 

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