Exploration of Acyclic Diaminocarbenes as Transition Metal Ligands

by David Snead

Institution: University of Florida
Department: Chemistry
Year: 2010
Keywords: carbene, catalysis, chiral, organometallic, palladium, rhodium; Chemistry
Record ID: 1866862
Full text PDF: http://ufdc.ufl.edu/UFE0024905


Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy EXPLORATION OF ACYCLIC DIAMINOCARBENES AS TRANSITION METAL LIGANDS By David Robinson Snead May 2010 Chair: Sukwon Hong Major: Organic Chemistry Carbenes are an important class of spectator ligands, the most common of which are N-heterocyclic carbenes (NHCs). Lesser explored carbene ligands are acyclic diaminocarbenes(ADCs), which possess a degree of intrigue due to significant variations in electronic and steric parameters from NHCs. In this work, ADCs are explored, with a special emphasis on methods of complexation to metal centers. ADC ligands were built from chiral C_1-symmetric pyrrolidine subunits. Ureas 2-1 through 2-4 were synthesized from 2-substituted pyrrolidines, and X-ray analysis was obtained for these compounds. The location of the chiral substituents proximal to the oxygen atom of the carbonyl led to a reasonable hypothesis that these ADCs might be useful asymmetric ligands. Palladium complexes 2-15 and 2-18 were formed through oxidative addition of chloroamidinium precursors; however steric crowding created by the phosphine ligands caused the chiral groups to orient themselves away from the metal center, as observed by X-ray analysis. ADCs with substituents larger than benzyl were not able to be isolated with palladium, likely as a result of steric constraints. Complexes 2-15, 2-18, and 2-26 were tested in Suzuki reactions, but as the degree of ligand substitution increased, reactivity decreased. Access to a diversity of metal complexes from chloroamidiniums was restricted based on use of the oxidative addition methodology and also required the use of electron rich ligands like phosphines. As such, a new more general method of carbene generation was developed. Lithium-halogen exchange was applied to chloroamidiniums to give carbenoid intermediates. 13^C-NMR of the carbene in solution and formation of thiourea constitute proof, and after generation, Rh, Ir, Pd, and B complexes were produced. Interestingly, X-ray analysis of Rh-ADC complex 3-5 based on chiral C_1-symmetric pyrrolidine subunits demonstrated a change in conformational preference from the palladium compounds, as the methyl substituents were located proximal to the rhodium center. Rh-ADC complex 3-5 was tested in catalytic reactions and showed good reactivity in 1,4 conjugate addition of aryl boronic acids to cyclohexenone and in 1,2 addition of aryl boronic acids to aryl aldehydes. Notably, the ADC ligand performed better than NHC in 1,2 addition.