Titanium-Mediated Catalytic and Stoichiometric Carbon-Carbon Bond Formations

The focus of the proposed research activity is the study of ancillary ligand effects in titanium chemistry with the aim of developing new Ti(IV) electrophiles and storable Ti(II) reductants that mediate carbon-carbon bond formations by well-understood mechanisms. Hence throughout this proposal, an emphasis will be placed on discovery and use of ancillary ligands that afford well-characterized members of the families of titanium compounds generated. The proposed research activity includes several aspects: (1) Development of arene-aryloxide ligands (OAr-(bridge)-Ar') that exhibit hemilabile behavior in titanium chemistry, by tuning steric and electronic properties of the arene and aryloxide moieties, as well as the nature of the bridging group. The motivation is to take advantage of hemilabile behavior of arene-aryloxide ligands and variable electron donor ability of their arene and aryloxide moieties to stabilize different molecular geometries, generate open coordination site(s) at the metal center, and effect reactivity. (2) Development of titanium complexes of arene-aryloxide ligands for catalysis of ethylene trimerization to give 1- hexene, and polymerization of ethylene and a-olefins. (3) Development of storable and well-characterized Ti(II) complexes or synthetic equivalents for applications as alkyne cyclotrimerization catalysts or as stoichiometric reagents for reductive coupling reactions of unsaturated organic compounds. The potential of arene-aryloxide and calixarene-derived bis(aryloxide) ligands (Figure 3) to support Ti(II) and allow isolation of stable Ti(II)-TJ6- arene (titananorbornadiene) complexes will be investigated. (4) Studies aimed toward expanding the scope of novel bis(diimine)Ti-mediated reductive coupling of ketones and aromatic diimines to produce 6-(1-hydroxyalkyl)-2,2'-bipyridines and 2-(1-hydroxyalkyl)- l,10-phenanthrolines. Both improvement of the range of carbonyl compounds that can be employed in the reaction and development of an asymmetric version of the reaction will be examined. The intellectual merit of the proposed research lies in the investigation of the relationship between the structure of the titanium catalyst or reagent and its function, as well as of details of the reaction mechanism(s). Ti(IV) and Ti(II) species have been implicated in reaction mechanisms of a-olefin polymerization and selective trimerization of ethylene to yield 1-hexene, respectively. Moreover, the use of low-valent titanium species to mediate carbon-carbon bond forming processes is central to the fields of organic synthesis and organometallic chemistry, despite inadequate understanding of the structures and chemistry of low-valent titanium species. The proposed research will contribute a great deal of basic information about the synthesis, structure, and control of the chemical reactivity of highly electrophilic Ti(IV) and highly reducing Ti(II) species. These studies will provide better understanding of the correlation between steric and electronic properties of ancillary ligands and the efficacy of titanium compounds for promoting the reactions under study. The proposed study will have broader impacts in organic and organometallic chemistry by providing information that will greatly aid the development of new and highly selective transition metal catalysts or reagents that operate by well-understood mechanisms. The proposed research, which lies at the interface of organometallic and organic chemistry and catalysis, will provide multidisciplinary training to undergraduate, graduate, and postdoctoral researchers, as well as form the basis for facilitating the development of scientists that possess the knowledge and broad perspective necessary to pursue a spectrum of career opportunities. An equally important objective of the proposed research program is to utilize undergraduate research training to mentor students from groups traditionally under-represented in science by arousing their curiosity and attracting them to pursue graduate education in chemistry. This will be accomplished by working with Dr. Charlotte Carter (Dean of Arts and Sciences) at Stillman College (a historically black college) to attract two talented African-American sophomore or junior-level students each summer to participate in the proposed research project.