A Broadly Applicable Strategy for Entry into Homogeneous Nickel(0) Catalysts from Air-Stable Nickel(II) Complexes

A series of air-stable nickel complexes of the form L[subscript 2]Ni(aryl) X (L = monodentate phosphine, X = Cl, Br) and LNi(aryl)X (L = bis-phosphine) have been synthesized and are presented as a library of precatalysts suitable for a wide variety of nickel-catalyzed transformations. These complexes are easily synthesized from low-cost NiCl[subscript 2]·6H[subscript 2]O or NiBr[subscript 2]·3H[subscript 2]O and the desired ligand followed by addition of 1 equiv of Grignard reagent. A selection of these complexes were characterized by single-crystal X-ray diffraction, and an analysis of their structural features is provided. A case study of their use as precatalysts for the nickel-catalyzed carbonyl-ene reaction is presented, showing superior reactivity in comparison to reactions using Ni(cod)[subscript 2]. Furthermore, as the precatalysts are all stable to air, no glovebox or inert-atmosphere techniques are required to make use of these complexes for nickel-catalyzed reactions.National Institute of General Medical Sciences (U.S.) (GM63755)National Science Foundation (U.S.). Graduate Research Fellowshi

Understanding the Unusual Reduction Mechanism of Pd(II) to Pd(I): Uncovering Hidden Species and Implications in Catalytic Cross-Coupling Reactions

The reduction of Pd­(II) intermediates to Pd(0) is a key elementary step in a vast number of Pd-catalyzed processes, ranging from cross-coupling, C–H activation, to Wacker chemistry. For one of the most powerful new generation phosphine ligands, P<i>t</i>Bu₃, oxidation state Pd­(I), and not Pd(0), is generated upon reduction from Pd­(II). The mechanism of the reduction of Pd­(II) to Pd­(I) has been investigated by means of experimental and computational studies for the formation of the highly active precatalyst {Pd­(μ-Br)­(P<i>t</i>Bu₃)}₂. The formation of dinuclear Pd­(I), as opposed to the Pd(0) complex, (<i>t</i>Bu₃P)₂Pd was shown to depend on the stoichiometry of Pd to phosphine ligand, the order of addition of the reagents, and, most importantly, the nature of the palladium precursor and the choice of the phosphine ligand utilized. In addition, through experiments on gram scale in palladium, mechanistically important additional Pd- and phosphine-containing species were detected. An ionic Pd­(II)­Br₃ dimer side product was isolated, characterized, and identified as the crucial driving force in the mechanism of formation of the Pd­(I) bromide dimer. The potential impact of the presence of these side species for <i>in situ</i> formed Pd complexes in catalysis was investigated in Buchwald–Hartwig, α-arylation, and Suzuki–Miyaura reactions. The use of preformed and isolated Pd­(I) bromide dimer as a precatalyst provided superior results, in terms of catalytic activity, in comparison to catalysts generated <i>in situ</i>

Iridium-Catalyzed C–H Borylation of Heterocycles Using an Overlooked 1,10-Phenanthroline Ligand: Reinventing the Catalytic Activity by Understanding the Solvent-Assisted Neutral to Cationic Switch

The preformed catalyst [Ir­(Cl)­(COD)­(1,10-phenanthroline)] (<b>2</b>; COD = cyclooctadiene) was found to be highly effective in a model reaction for the borylation of N-Boc-indole at the 3-position with B₂pin₂ (pin = pinacolato) as the borylating agent to give consistently 99% yield with 0.5 mol % catalyst loading. The corresponding in situ formed catalyst from [Ir­(Cl)­(COD)]₂ and 1,10-phenanthroline provided very inconsistent results for the same reaction (0–94% conversion). We propose this to be due to the competing formation of a catalytically inactive cationic complex, [Ir­(COD)­(1,10-phenanthroline)]⁺Cl^– (<b>1</b>), in a noncoordinating solvent such as octane. Complexes <b>1</b> and <b>2</b> were characterized using solid-state NMR (¹³C and ³⁵Cl) in conjunction with XPS to be cationic and neutral, respectively. The X-ray crystal structure of a pentavalent neutral Ir complex, [Ir­(Cl)­(COD)­(2,2′-bipyridine)] (<b>3</b>), was also obtained for comparison purposes. Using catalyst <b>2</b>, the total synthesis of <i>Meridianin G</i> was accomplished in 87% overall isolated yield in a one-pot, three-step process

Metal-free coupling of saturated heterocyclic sulfonylhydrazones with boronic acids

The coupling of aromatic moieties with saturated heterocyclic partners is currently an area of significant interest for the pharmaceutical industry. Herein, we present a procedure for the metal-free coupling of 4-, 5-, and 6-membered saturated heterocyclic p-methoxyphenyl (PMP) sulfonylhydrazones with aryl and heteroaromatic boronic acids. This procedure enables a simple, two-step synthesis of a range of functionalized sp2−sp3 linked bicyclic building blocks, including oxetanes, piperidines, and azetidines, from their parent ketones. ■ INTRODUCTIO

Catalytic Direct Cross-Coupling of Organolithium Compounds with Aryl Chlorides

<p>Palladium-catalyzed direct cross-coupling of aryl chlorides with a wide range of (hetero)aryl lithium compounds is reported. The use of Pd-PEPPSI-IPent or Pd-2(dba)(3)/XPhos as the catalyst allows for the preparation of biaryl and heterobiagl compounds In high yields under mild conditions (room temperature to 40 degrees C) with short reaction times.</p>