Department of Chemistry
Northern Michigan University
Phone : (906) 227-1073
E-mail : firstname.lastname@example.org
1. The hydrogenation and hydroformylation of alkenes by metal carbonyl hydrides has been studied extensively for some time. The mechanism of the reaction for simple alkenes has been shown to be quite complex, involving p-complexes, s-sigma complexes, hydride transfer steps, and carbonyl insertion steps, all in equilibrium. The current research program has shown that this mechanism changes subtely to a free-radical mechanism when the alkene possesses conjugated (especially phenyl and carbonyl) groups by dissociating the s-complexes into allyl, phenyl, or a-carbonyl radicals and 17-electron metal carbonyl radical pairs. Such radicals tend to favor hydrogenation products over hydroformylation products, while the reverse is true for simple non-conjugated alkenes. The mechanism has been studied by a combination of kinetics (including determination of rates, rate constants, order of reaction, isotope effects, and solvent effects), isotopic labeling, product ratio determination (hydrogenation vs hydroformylation), and transition metal selectivity. So far, alkenes have been studied with cobalt, manganese, and rhodium carbonyl hydrides. Currently, homoallyl radicals are being studied as a potentially sensitive indicator of the selectivity for hydroformylation vs hydrogenation; the homoallyl radical has been shown to form readilly from vinylcyclopropyl alkenes.
"Stoichiometric hydrogenation of a,b-unsaturated ketones by HCo(CO)4," J. A. Roth and K. Grega, Journal of Organometallic Chemistry 1988, 342, 129.
2. The carbon-halogen bond in aromatic halides is well-known to be strong and unreactive. Few reagents are able to activate or displace chloride or bromide from such compounds. Some of the most environmentally long-lived man-made compounds are of this type, such as PCB's and DDT, to name just the most notorious. Such compounds have half-lives in the environment on the order of decades, mainly due to the hardiness of the aromatic ring system and its associated halogens. Certain anaerobic bacteria are able to slowly metabolize these compounds, removing the halogens (chlorines) in a moderately selective way. The research underway has discovered and compared some transition metal complexes which seem similar to the bacterial reactivity in their ability to hydrogenolyze aromatic chloride and bromide: the Ni(0)(Ph3P)4 complex has been thoroughly studied and appears to mimic some of the bacterial strains discovered so far. The mechanism of hydrogenolysis is a two step single electron transfer (SET) process from the metal of the complex, followed by borohydride (or simple hydride) displacement as the source of hydrogen. The use of Ni(0) in this reaction appears to be stoichiometric, rather than catalytic, however, and other complexes have also been explored: Ni(0)(Bu3P)4 seems to behave much like the Ph3P complex, but has a somewhat higher turnover number (still being studied) and could be catalytic; Pd(0)(Ph3P)4 has also been used, but seems to decompose under the reaction conditions and only gives slow or no yields. Other similar complexes are yet to be studied, in the search for a truly catalytic transition metal complex capable of hydrogenolyzing aromatic halides smoothly under mild conditions.
"Hydrogenolysis of Polychlorinated Biphenyls by Sodium Borohydride with Homogeneous and Heterogeneous Nickel Catalysts," J. A. Roth, S. R. Dakoji, R. C. Hughes, and R. E. Carmody, Environmental Science and Technology 1994, 28, 80.