Reactivity and synthetic studies of iron tricarbonyl-pentadienyl cations and diene complexes
The use of transition organometallic complexes as reagents for the synthesis of complex organic molecules has been under intensive growth and development for several decades, and many exemplary organic transformations of profound synthetic potential have been achieved. Particularly, the use of organoiron complexes has been recognized as a powerful tool in synthetic methodology due to their ability to react with nucleophiles and electrophiles in a stereoselective fashion. The first part of the dissertation concerns the study of nucleophilic attack at pentadienyl-Fe(CO)$\sb3$ cations. It is well known that pentadienyl cations show high reactivity, characteristic of carbocations, and in general, limited regiochemical control can be achieved by using the free pentadienyl cation. However, coordination of the pentadienyl with the Fe(CO)$\sb3$ moiety provides a substantially more manageable system in which reactivity, stereochemistry, and regiochemistry can be controlled. In most of the cases studied nucleophilic addition occurs at the dienyl ligand, and anti to the Fe(CO)$\sb3$ moiety of the acyclic $(\eta\sp5$-pentadienyl)Fe(CO)$\sb3$ cations to yield ($\eta\sp4$-substituted diene)Fe(CO)$\sb3$ complexes rather than attack at the CO ligands. A whole range of nucleophiles have been employed for this reaction, including heteroatoms, and more recently, carbon nucleophiles of greater relevance to organic synthesis. It is of prime importance to delineate the factors (i.e. steric and electronic) influencing the reactivity in order to be able to predict the outcome of the reaction with a variety of nucleophiles. This understanding will make this methodology a useful one in organic synthesis. My research goal was to continue the study of the reactivity of mono and disubstituted acyclic $(\eta\sp5$-pentadienyl)Fe(CO)$\sb3$ cations with a set of standard nucleophiles of wide range of reactivity including H$\sb2$O, PPh$\sb3$, malonate, NaBH$\sb3$CN, and methyl cuprates. The results of these studies has shed light on the relative effects of the substituents on the dienyl moiety. More interestingly, in the case of disubstituted systems, the studies have demonstrated the extent of competition, or cooperativity between substituents in directing nucleophilic attack at a particular site of the pentadienyl system. The second part of the dissertation involves model studies towards the synthesis of the C16-C24 fragment of Macrolactin A. An effective synthetic scheme was developed whereby long range stereodirecting effect of Fe(CO)$\sb3$ was achieved. Thus, the stereocenter at C23 of Macrolactin A could be introduced via organoiron methodology. The third part of the dissertation concerned the total synthesis of the recently isolated conjugated trienone Lignarenone-B. The synthesis was achieved in 28% overall yield. All reactions leading to the final product proceeded in moderate to good yields under mild conditions. The spectral data of the final product was identical to that obtained from the natural product.
Peter Thomas Bell,
"Reactivity and synthetic studies of iron tricarbonyl-pentadienyl cations and diene complexes"
(January 1, 1994).
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