Development of novel cyclopentadienone based transition metal complexes for catalytic and electrocatalytic transfer hydrogenation of polar bonds
Keywords: Chemistry.

Chapter 1. Cyclopentadienone-ligated transition metal (Shvo-type) complexes are known to catalyze hydrogenation and transfer hydrogenation of polar bonds through outer-sphere metal ligand bifunctional mechanism. Herein, select examples of Shvo-type analogs and their synthesis, applications in hydrogen transfer reactions, specifically transfer hydrogenation, are presented. Chapter 2. The reaction of commercially available tetraphenylcyclopentadienone with Co2(CO)8 led to the formation of a rare mixed-valence tricobalt complex [Ph4C4CO]Co(CO)2]2[μ-Co(MeCN)2]. Treatment of this tricobalt cobalt complex with proton sources such as water and alcohols yields monomeric cobalt complex [Ph4CpOH]Co(CO)2 via protonolysis. In contrast to the conventional Pauson-Khand reaction to generate cyclopentadienone ligated cobalt complexes, this method benefits from both mild reaction conditions and high yields. Phosphite substituted monomeric cobalt complexes [Ph4C4OH]Co(CO)[P(OR)3] were also synthesized, and protonation of these complexes gave rise to the transient formation of cobalt hydride species. Chapter 3. The molybdenum tetraphenylcyclopentadienone complex (C5Ph4O)Mo(CO)3(CH3CN) 1 is an effective precatalyst for the transfer hydrogenation of aldehydes, allylic alcohols, ketones and imines under mild conditions with either 2-propanol or formic acid as reducing reagent. Mechanistic studies implicate that these molybdenum cyclopentadienone complexes can be reduced to the corresponding hydroxycyclopentadienyl Mo hydrides. These complexes, by virtue of the hydroxyl group on the cyclopentadienyl ligand, are more reactive and chemoselective than the analogous cyclopentadienyl molybdenum complexes for the reduction of ketones, aldehydes and imines. Chapter 4. New synthetic routes have been developed to unprecedented tetraphenylcyclopentadienone complexes of group VI metals (Cr, Mo, W), analogs of the Ru Shvo-type complexes, using a newly designed bidentate diphenylphosphinomethyl-tetraphenylcyclopentadienone ligand. All synthesized compounds were found to be effective precatalyst for transfer hydrogenation of benzaldehyde with 2-propanol. The phosphine-tethered Mo complex showed superior catalytic activity in transfer hydrogenation than the previously reported molybdenum tetraphenylcyclopentadienone complex (C5Ph4O)Mo(CO)3(CH3CN). Furthermore, the rare monomeric ruthenium complex with the bidentate diphenylphosphinomethyl-tetraphenylcyclopentadienone ligand was also synthesized. Enantiomers of axially chiral Ru analog were isolated and shown to mediate the asymmetric hydrogenation of secondary imines with modest enantioselectivities. Chapter 5. Pentamethylcyclopentadienyl(Cp*)iridium(III) chloride catalyzed acceptorless dehydrogenation of -tetralone is reported. Cp*iridium chloride showed higher activity compared to other Cp*iridium complexes having bromide, iodide or hydride or complexes without a Cp ring. The desired product, naphthol was obtained in up to 71% yield from -tetralone. Not only -tetralone but dihydrocoumarin, dihydroquinolinone, dimethylcyclohexanone, dihydrobenzofuran, and 1-isochromanone could be applied to this dehydrogenation by Cp* iridium catalyst, albeit the conversion stayed moderate. The catalytic turnover was not limited by the increased concentration of the product but by catalyst decomposition. Chapter 6. The electrochemistry of the previously reported cyclopentadienone molybdenum complex (C5Ph4O)Mo(CO)3(CH3CN) 1a, and the cationic hydroxycyclopentadienyl molydebnum complex [(C5Ph4OH)Mo(CO)3(CH3CN)]+[BF4]- 2a is described. The cyclic voltammetry and NMR measurements reveal that chemical reduction of the cationic complex 2a, generated in situ with 1a and excess proton source such as HBF4 etherate in o-difluorobenzene yields the active hydroxylcyclopentadienyl molybdenum hydride species (C5Ph4OH)Mo(H)(CO)3 1b. The mechanistic studies suggest that such process occurs through a one-electron reduction of 2a followed by an intermolecular disproportionative hydrogen atom transfer. The chemical reduction of 2a under the same conditions in acetonitrile leads to decomposition. Molybdenum complex 1a has been demonstrated to serve as a precatalyst for electroreduction of benzaldehyde to benzyl alcohol (TON=5.5) in the presence of excess triethylammonium triflate with an onset potential at -0.9 V versus Fc0/+.