Abstract:
A^-heterocyclic carbenes (NHCs) have become the focus of much interest as ancillary
ligands for transition metal catalysts in recent years. Their structural variability and strong
cy-donation properties have led to the preparation of demonstrably useful organometallic
catalysts.
Among the three general structural types of NHCs (imidazolylidenes, imidazolinylidenes,
and benzimidazolylidenes), benzimidazolylidenes are the least investigated because of the
limitation of current synthetic approaches. The preparation of chiral analogues is even more
challenging. Previously, our group has demonstrated an alternative approach to synthesizing
benzimidazolylidenes with a tetracyclic framework in three steps from 1,10-phenanthroline.
This thesis is focused on approaches to chiral benzimidazolylidenes derived from
substituted 1,10-phenanthrolines. A key step in the preparation of these ligands involves a
reduction of the pyridyl rings in 1,10-phenanthrolines. Chirality can be introduced to
phenanthrolines before, during, or after the reduction as illustrated by three approaches: 1) de
novo construction of the phenanthroline from chiral ketones with endo and exo faces to provide a
degree of diastereoselectivity during subsequent reduction; 2) introduction of substituents into
the 2- and 2,9- position of phenanthroline by nucleophilic aromatic substitution, followed by a
reduction-resolution sequence; and 3) use of the protected octahydrophenanthroline as a
substrate for chiral induction a to nitrogen.
