The Chemical Catalysis Program supports the research of Professor William D. Jones of the University of Rochester on the cleavage and formation of carbon-sulfur (C-S) and carbon-nitrogen (C-N) bonds in heterocycle compounds by homogeneous transition metal complexes. The studies with sulfur have applications in deep hydrodesulfurization (deep HDS) reactions while the studies with nitrogen focus on the use of electrophilic C-H bond activation in molecules containing directing groups for heterocycle synthesis. Specifically, the researchers obtain kinetic and thermodynamic information on the cleavage of strong and/or hindered C-X bonds (X = S or N), provide information about the mechanisms for C-X bond cleavage and formation, work to understand the mechanism of the bond cleavage reactions using density functional theory (DFT), and develop new catalytic systems for the manipulation of C-S and C-N bonds.
This research project is focused on the development of new synthetic routes to make nitrogen-containing heterocycles, which are fundamental building blocks in many biologically-important compounds. The transition metal-based catalysts may also have an impact on new routes towards intermediates of pharmaceutical importance. In learning how to selectively break carbon-sulfur versus carbon-hydrogen or carbon-carbon bonds, Professor Jones and his graduate and undergraduate students contribute to meeting new EPA standards for low sulfur emissions from fuels. Master's level graduate students from Hampton University are encouraged to participate in this research project and can be accepted directly into the Ph.D. Program at the University of Rochester. McNair Scholars and NSF REU students also contribute to the research efforts.
Major Activities: During the past year we have examined ruthenium, rhodium, iridium, platinum and palladium complexes for the cleavage of C-H, C-S, and C-O bonds in small organic molecules such as phenylpyridines, thiophenes, thioethers, and esters. We have examined the reactivity of the insertion products obtained. We have tried to produce heterocycles from the activated products. We have prepared new nickel PONOP complexes for C-N cleavage. Specific Objectives: (1) To examine complexes of (Cymene)RuII, Cp*RhIII, and Cp*IrIII containing xylyl isocyanide ligands to look for electrophilic concerted metalation-deprotonation activation of the C-H bond, and to see if cyclization can lead to the formation of indoles. (2) To find a precursor for [Pd(dippe)] to see if it can activate the C-S bond in thiophenes. (3) To react a precursor for [Pt(dippe)] with thioethers to look for C-S cleavage and to determine the scope and cleavage selectivity in these reactions. (4) To react a precursor for [Pt(dippe)] with esters to look for C-O cleavage and to determine the scope and cleavage selectivity in these reactions. (5) To use a nickel PONOP complex for bond cleavage reactions. Significant Results: (1) While we could synthesize cationic xylylisocyanide complexes of Ru, Rh, and Ir, in none of the electrophilic complexes was indole formation observed. (2) We have found that [Pd(dippe)] readily inserts into dibenzothiophene via C-S bond cleavage. (3) We have found that [Pt(dippe)] cleaves thioether C-S bonds via oxidative addition. (4) We have found that [Pt(dippe)] cleaves C-O bonds in esters via oxidative addition. (5) We have found that [Ni(PONOP)Cl]+ can cleave C-N bonds in t-butylisocyanide. Key Outcomes: (1) Since the cationic Ru, Rh, and Ir complexes of xylyl isocyanide do not cyclize, we conclude that the linearity of the M-C-N-C bond linkage must be keeping the methyl group too far from the metal for concerted metalation-deprotonation to take place. We will abandon this route. (2) Since the new precursor to [Pd(dippe)] reacts with dibenzothiophene, we will further explore the reactivity with other thiophenes. (3) We have found the Ph-S bonds can be cleaved at Pt0 with great difficulty (1 week, 140 °C). In contrast, allyl-S and vinyl-S bonds can be cleaved at Pt0 under mild conditions (100 °C, 20 min). Furthermore, the products disproportionate to give R-R coupled products. (4) We have found that Pt0 cleaves esters at the more difficult C-O bond, i.e. R-OC(=O)R', to give Pt(R)(O2CR') products. (5) Isocyanides bind to [Ni(PONOP)Cl]+ and undergo C-N heterolysis to give a nickel-cyanide and a carbonium ion. The latter is trapped by solvent or by chloride.
