Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)⁺ Analogues

Iron­(II) hydride complexes of the “piano-stool” type, Cp*­(P-P)­FeH (P-P = dppe (<b>1H</b>), dppbz (<b>2H</b>), dppm (<b>3H</b>), dcpe (<b>4H</b>)) were examined as hydride donors in the reduction of <i>N</i>-benzylpyridinium and acridinium salts. Two pathways of hydride transfer, “single-step H^–” transfer to pyridinium and a “two-step (e^–/H^•)” transfer for acridinium reduction, were observed. When 1-benzylnicotinamide cation (BNA⁺) was used as an H^– acceptor, kinetic studies suggested that <b>BNA</b>^<b>+</b> was reduced at the C6 position, affording 1,6-BNAH, which can be converted to the more thermally stable 1,4-product. The rate constant <i>k</i> of H^– transfer was very sensitive to the bite angle of P–Fe–P in Cp*­(P-P)­FeH and ranged from 3.23 × 10^–3 M^–1 s^–1 for dppe to 1.74 × 10^–1 M^–1 s^–1 for dppm. The results obtained from reduction of a range of <i>N</i>-benzylpyridinium derivatives suggest that H^– transfer is more likely to be charge controlled. In the reduction of 10-methylacridinium ion (<b>Acr</b>^<b>+</b>), the reaction was initiated by an e^– transfer (ET) process and then followed by rapid disproportionation reactions to produce <b>Acr</b>_<b>2</b> dimer and release of H₂. To achieve H^• transfer after ET from [Cp*­(P-P)­FeH]⁺ to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium (<b>PhAcr</b>^<b>+</b>) was selected as an acceptor. More evidence for this “two-step (e^–/H^•)” process was derived from the characterization of <b>PhAcr^•</b> and [<b>4H</b>]^<b>+</b> radicals by EPR spectra and by the crystallographic structure confirmation of [<b>4H</b>]^<b>+</b>. Our mechanistic understanding of fundamental H^– transfer from iron­(II) hydrides to NAD⁺ analogues provides insight into establishing attractive bio-organometallic transformation cycles driven by iron catalysis

Iron-Catalyzed 1,2-Selective Hydroboration of <i>N</i>‑Heteroarenes

A N₂-bridged diiron complex [Cp*­(Ph₂PC₆­H₄S)­Fe]₂­(μ-N₂) (<b>1</b>) has been found to catalyze the hydroboration of <i>N</i>-heteroarenes with pinacolborane, giving <i>N</i>-borylated 1,2-reduced products with high regioselectivity. The catalysis is initiated by coordination of <i>N</i>-heteroarenes to the iron center, while the B–H bond cleavage is the rate-determining step

Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)⁺ Analogues

Iron­(II) hydride complexes of the “piano-stool” type, Cp*­(P-P)­FeH (P-P = dppe (<b>1H</b>), dppbz (<b>2H</b>), dppm (<b>3H</b>), dcpe (<b>4H</b>)) were examined as hydride donors in the reduction of <i>N</i>-benzylpyridinium and acridinium salts. Two pathways of hydride transfer, “single-step H^–” transfer to pyridinium and a “two-step (e^–/H^•)” transfer for acridinium reduction, were observed. When 1-benzylnicotinamide cation (BNA⁺) was used as an H^– acceptor, kinetic studies suggested that <b>BNA</b>^<b>+</b> was reduced at the C6 position, affording 1,6-BNAH, which can be converted to the more thermally stable 1,4-product. The rate constant <i>k</i> of H^– transfer was very sensitive to the bite angle of P–Fe–P in Cp*­(P-P)­FeH and ranged from 3.23 × 10^–3 M^–1 s^–1 for dppe to 1.74 × 10^–1 M^–1 s^–1 for dppm. The results obtained from reduction of a range of <i>N</i>-benzylpyridinium derivatives suggest that H^– transfer is more likely to be charge controlled. In the reduction of 10-methylacridinium ion (<b>Acr</b>^<b>+</b>), the reaction was initiated by an e^– transfer (ET) process and then followed by rapid disproportionation reactions to produce <b>Acr</b>_<b>2</b> dimer and release of H₂. To achieve H^• transfer after ET from [Cp*­(P-P)­FeH]⁺ to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium (<b>PhAcr</b>^<b>+</b>) was selected as an acceptor. More evidence for this “two-step (e^–/H^•)” process was derived from the characterization of <b>PhAcr^•</b> and [<b>4H</b>]^<b>+</b> radicals by EPR spectra and by the crystallographic structure confirmation of [<b>4H</b>]^<b>+</b>. Our mechanistic understanding of fundamental H^– transfer from iron­(II) hydrides to NAD⁺ analogues provides insight into establishing attractive bio-organometallic transformation cycles driven by iron catalysis

Radical-Smiles Rearrangement by a Vitamin B2-Derived Photocatalyst in Water

Herein, we report a catalytic radical-Smiles rearrangement system of arene migration from ether to carboxylic acid with riboflavin tetraacetate (RFT), a readily available ester of natural vitamin B2, as the photocatalyst and water as a green solvent, being free of external oxidant, base, metal, inert gas protection, and lengthy reaction time. Not only the known substituted 2-phenyloxybenzoic acids substrates but also a group of naphthalene- and heterocycle-based analogues was converted to the corresponding aryl salicylates for the first time. Mechanistic studies, especially a couple of kinetic isotope effect (KIE) experiments, suggested a sequential electron transfer-proton transfer processes enabled by the bifunctional flavin photocatalyst