Alex McSkimming

Bio‐Inspired Catalytic Imine Reduction by Rhodium Complexes with Tethered Hantzsch Pyridinium Groups: Evidence for Direct Hydride Transfer from Dihydropyridine to Metal‐Activated Substrate

Herein, we report a conceptually new approach to the catalytic reduction of unsaturated substrates, demonstrated for imine hydrogenation, based on mimicry of biological processes in which hydride is directly transferred from dihydronicotinamide adenine dinucleotide (phosphate) (NAD(P)H) cofactor to an enzyme-activated substrate. NAD(P)H is Nature s hydride carrier. 3] In many (de)hydrogenase enzymes that catalyze direct hydride transfer to/ from NAD(P)/NAD(P)H, the substrate is polarized and thus activated by binding to a metal ion. Classic examples are alcohol dehydrogenases (Zn active site) and acetohydroxy acid isomeroreductase hydrogenases (with an (Mg)2 or (Mn)2 active site). [5] Our aim in this research was to prepare and test a new design for a homogeneous catalyst in which an unnatural organo-transition-metal center is tethered to an organohydride donor (OHD). The design incorporates the main features of an (de)hydrogenase enzyme and its NAD(P)H cofactor into one molecule. We envisaged that the close proximity of cofacial, linked metal and OHD centers would facilitate both regeneration of the OHD through the intermediacy of metallo-hydride species and the rapid transfer hydride from the OHD to a metal-bound, and thus activated, unsaturated substrate. We targeted a [Cp*Rh(NN)L] (NN = diimine; L = halido, n = 1; L = solvato co-ligand, n = 2) complex, as these are the most commonly used catalysts for regeneration of NAD(P)H from NAD(P). Electrolytic reduction of [Cp*Rh(NN)L] affords the corresponding Rh complex, which is rapidly protonated at low pH to give the active hydrido–Rh species for hydride transfer to NAD(P). Conveniently, catalytic regeneration of OHDs using [Cp*Rh(NN)L] can be driven directly by electricity, by light and a photosensitizer, or by renewable chemical reductants, such as formate. We employed a Hantzsch ester, such as 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (HEH), as the OHD center, because of their wide use as mimics for NAD(P)H in transfer hydrogenations of unsaturated substrates, such as imines or enones. 11] Typically, a Brønsted or Lewis acid catalyst is required to activate the substrate and to organize it and HEH for stereoselective hydride transfer. Of note here is the recent disclosure by Zhou et al. on ruthenium-complex-catalyzed HEH regeneration using dihydrogen at high pressures for the asymmetric transfer hydrogenation of cyclic oximines catalyzed by a chiral phosphoric acid. We believed that the flexible electron source and the mild reduction conditions for [Cp*Rh(NN)L] catalysis of OHD regeneration would circumvent the demand for dihydrogen pressure and lead to a more convenient, greener, process. Moreover, substrate coordination and activation at the metal center would obviate the need for an expensive phosphoric acid catalyst. To test our ideas, three complexes were synthesized: 1 and 2, which have orthoor meta-phenyl-bridged pyridinium (HE) and [Cp*Rh(NN)Cl] centers, respectively, and 3, which lacks an HE substituent, as a control (Figure 1). The

Macrocyclic Bis(phenanthroline‐pyrrole): A Convenient One‐Pot Synthesis, Structure(s), Spectroscopic, and Redox Properties, and the Binding of Amine Guests, Protons, and Lanthanide Ions

This paper reports a convenient, one-pot, easily scalable and readily modifiable synthesis of a novel large-ring bis(1,10-phenanthrolinyl-2,5-pyrrole) macrocycle, H2LMC, and describes its spectroscopic and electrochemical properties, protonation, cooperative amine binding, electrocatalysis of the oxidation of primary amines, photosensitization of the decomposition of dichloromethane, and the first lanthanide complexes of the hexaaza-dianion LMC(2-) including the novel dimer, [(NO3)(LMC)Eu(μ-OH)Eu(LMC )(H2 O)2]·2py.

Bio‐Inspired Transition Metal–Organic Hydride Conjugates for Catalysis of Transfer Hydrogenation: Experiment and Theory

Taking inspiration from yeast alcohol dehydrogenase (yADH), a benzimidazolium (BI(+) ) organic hydride-acceptor domain has been coupled with a 1,10-phenanthroline (phen) metal-binding domain to afford a novel multifunctional ligand (L(BI+) ) with hydride-carrier capacity (L(BI+) +H(-) ⇌L(BI) H). Complexes of the type [Cp*M(L(BI) )Cl][PF6 ]2 (M=Rh, Ir) have been made and fully characterised by cyclic voltammetry, UV/Vis spectroelectrochemistry, and, for the Ir(III) congener, X-ray crystallography. [Cp*Rh(L(BI) )Cl][PF6 ]2 catalyses the transfer hydrogenation of imines by formate ion in very goods yield under conditions where the corresponding [Cp*Ir(L(BI) )Cl][PF6 ] and [Cp*M(phen)Cl][PF6 ] (M=Rh, Ir) complexes are almost inert as catalysts. Possible alternatives for the catalysis pathway are canvassed, and the free energies of intermediates and transition states determined by DFT calculations. The DFT study supports a mechanism involving formate-driven RhH formation (90 kJ mol(-1) free-energy barrier), transfer of hydride between the Rh and BI(+) centres to generate a tethered benzimidazoline (BIH) hydride donor, binding of imine substrate at Rh, back-transfer of hydride from the BIH organic hydride donor to the Rh-activated imine substrate (89 kJ mol(-1) barrier), and exergonic protonation of the metal-bound amide by formic acid with release of amine product to close the catalytic cycle. Parallels with the mechanism of biological hydride transfer in yADH are discussed.