A Three-Coordinate Copper(II) Amide from Reductive Cleavage of a Nitrosamine

Three-Coordinate Copper(II) Alkynyl Complex in C-C Bond Formation: The Sesquicentennial of the Glaser Coupling

Copper(II) alkynyl species are proposed as key intermediates in numerous Cu-catalyzed C-C coupling reactions. Supported by a β-diketiminate ligand, the three-coordinate copper(II) alkynyl [Cu^II]-C≡CAr (Ar = 2,6-Cl₂C₆H₃) forms upon reaction of the alkyne H-C≡CAr with the copper(II) tert-butoxide complex [Cu^II]-O^tBu. In solution, this [Cu^II]-C≡CAr species cleanly transforms to the Glaser coupling product ArC≡C-C≡CAr and [Cu^I](solvent). Addition of nucleophiles R'C≡C-Li (R' = aryl, silyl) and Ph-Li to [Cu^II]-C≡CAr affords the corresponding C_sp-C_sp and C_sp-C_sp2 coupled products RC≡C-C≡CAr and Ph-C≡CAr with concomitant generation of [Cu^I](solvent) and {[Cu^I]-C≡CAr}^-, respectively. Supported by density functional theory (DFT) calculations, redox disproportionation forms [Cu^III](C≡CAr)(R) species that reductively eliminate R-C≡CAr products. [Cu^II]-C≡CAr also captures the trityl radical Ph₃C· to give Ph₃C-C≡CAr. Radical capture represents the key C_sp-C_sp3 bond-forming step in the copper-catalyzed C-H functionalization of benzylic substrates R-H with alkynes H-C≡CR' (R' = (hetero)aryl, silyl) that provide C_sp-C_sp3 coupled products R-C≡CR via radical relay with ^tBuOO^tBu as oxidant.

A reagent for heteroatom borylation, including iron mediated reductive deoxygenation of nitrate yielding a dinitrosyl complex

4,4'-Bipyridyl is shown to be a catalyst for transfer of pinacolboryl groups from (Bpin)₂ to nitrogen heterocycles and to Me₃SiN₃. Using stoichiometric (Bpin)₂(pyrazine) or (Bpin)₂(bipyridine) in an analogous manner, an aromatic nitro group is deoxygenated and subsequently borylated, and four-fold deoxygenation of (DIM)Fe(NO₃)₂(MeCN) to yield the dinitrosyl complex (DIM)Fe(NO)₂ is facile. The co-product O(Bpin)₂ is the quantitative fate of the removed oxo groups. With borylation of both nitrogen heterocycles and doubly deoxygenating two nitrates coordinated to a single metal center, broad spectrum methodology is demonstrated.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

A Stable Aminyl Radical Coordinated to Cobalt

A family of cobalt complexes bearing the trop₂ NH [bis(5-H-dibenzo[a,d]cyclohepten-5-yl)-amine] and 2,2'-bpy (2,2'-bipyridine) chelate ligands were prepared and fully characterized. The compounds [Co(trop₂ N)(bpy)], [Co(trop₂ NH)(bpy)]⁺ , and [Co(trop₂ N)(bpy)]⁺ are cobalt complexes interrelated by one-electron redox processes and/or proton transfer. Two limiting resonance structures can be used to describe the paramagnetic complex [Co(trop₂ N)(bpy)]⁺ : [Co^II (trop₂ N^- )(bpy)]⁺ (Co^II amido) and [Co^I (trop₂ N^⋅ )(bpy)]⁺ (Co^I -aminyl radical). Structural data, DFT calculations, and reactivity toward H-abstraction indicate a slightly higher contribution of the aminyl radical form to the ground state of [Co(trop₂ N)(bpy)]⁺ . The results described here complete the series of Group 9 metal aminyl radical complexes bearing the diolefin amine ligand trop₂ NH.

Reactivity Study of Unsymmetrical β-Diketiminato Copper(I) Complexes: Effect of the Chelating Ring

β-Diketiminato copper(I) complexes play important roles in bioinspired catalytic chemistry and in applications to the materials industry. However, it has been observed that these complexes are very susceptible to disproportionation. Coordinating solvents or Lewis bases are typically used to prevent disproportionation and to block the coordination sites of the copper(I) center from further decomposition. Here, we incorporate this coordination protection directly into the molecule in order to increase the stability and reactivity of these complexes and to discover new copper(I) binding motifs. Here we describe the synthesis, structural characterization, and reactivity of a series of unsymmetrical N-aryl-N'-alkylpyridyl β-diketiminato copper(I) complexes and discuss the structures and reactivity of these complexes with respect to the length of the pyridyl arm. All of the aforementioned unsymmetrical ß-diketiminato copper(I) complexes bind CO reversibly and are stable to disproportionation. The binding ability of CO and the rate of pyridyl ligand decoordination of these copper(I) complexes are directly related to the competition between the degree of puckering of the chelate system and the steric demands of the N-aryl substituent.

Isolation of an unusual [Cu6] nanocluster through sequential addition of copper(i) to a polynucleating ligand

Addition of 2 equiv. of CuI to a [Cu]₄ multimetallic complex results in cluster formation leading to the isolation of a rare bicapped tetrahedral [Cu₆I₂] cluster that is stabilised by two conformationally constrained polynucleating ligands.

Asymmetric dimerization of aniline-ruthenium-dioxolene complex driven by stepwise PCET

The base-assisted oxidation of an aniline-Ru-quinone complex produced an anilinyl radical-Ru-semiquinone. Furthermore, base-assisted oxidation of the radical complex resulted in selective C-N bond formation through an intermolecular coupling between nitrogen and carbon atoms at the para-position of the anilinyl ligand.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

While β-diketiminate (BDI or ‘nacnac’) ligands have been widely adopted to stabilize a wide range of metal ions in multiple oxidation states and coordination numbers, in several occurrences these ligands do not behave as spectators and participate in reactivity.

Metal-Chelating N,N'-Bis(4-dimethylaminophenyl)acetamidinyl Radical: A New Chromophore for the Near-Infrared Region

A new non-innocent ligand redox system, N,N'-bis(4-dimethylaminophenyl) substituted acetamidinato/acetamidinyl, has been designed and described by example of structurally and spectroscopically characterized ruthenium complexes. The hitherto unreported ligand is responsible for rather intense and narrow absorptions in the near-infrared region of the one- and two-electron oxidized forms. The spectroscopic, computational, and first structural characterization of an amidinyl radical complex adds to the list of established N-based radical ligands.

Copper-catalyzed sulfonamides formation from sodium sulfinates and amines

A new and convenient method for the construction of sulfonamides via a copper-catalyzed oxidative coupling between sodium sulfinates and amines with 1 atm O2 or DMSO as the oxidant was described. This method provides efficient and robust synthesis of functional sulfonamides in good yields and excellent chemoselectivity. And detailed mechanistic studies showed that this transformation may go through a single electron transfer (SET) pathway.

Benzylation of nitroalkanes using copper-catalyzed thermal redox catalysis: toward the facile C-alkylation of nitroalkanes

The C-alkylation of nitroalkanes under mild conditions has been a significant challenge in organic synthesis for more than a century. Herein we report a simple Cu(I) catalyst, generated in situ, that is highly effective for C-benzylation of nitroalkanes using abundant benzyl bromides and related heteroaromatic compounds. This process, which we believe proceeds via a thermal redox mechanism, allows access to a variety of complex nitroalkanes under mild reaction conditions and represents the first step toward the development of a general catalytic system for the alkylation of nitroalkanes.

Cu(I)-amido complexes in the Ullmann reaction: reactions of Cu(I)-amido complexes with iodoarenes with and without autocatalysis by CuI

A series of Cu(I)-amido complexes both lacking ancillary ligands and containing 1,10-phenanthroline (phen) as ancillary ligand have been prepared. These complexes react with iodoarenes to form arylamine products, and this reactivity is consistent with the intermediacy of such complexes in catalytic Ullmann amination reactions. The stoichiometric reactions of the Cu(I)-amido complexes with iodoarenes are autocatalytic, with the free CuI generated during the reaction serving as the catalyst. Such autocatalytic behavior was not observed for reactions of iodoarenes with copper(I) amidates, imidates, or phenoxides. The selectivity of these complexes for two sterically distinct aryl halides under various conditions imply that the autocatalytic reaction proceeds by forming highly reactive [CuNPh(2)](n) lacking phen. Reactions with radical probes imply that the reactions of phen-ligated Cu(I)-amido complexes with iodoarenes occur without the intermediacy of aryl radicals. Density functional theory calculations on the oxidative addition of iodoarenes to Cu(I) species are consistent with faster reactions of iodoarenes with CuNPh(2) species lacking phen in DMSO than reactions of iodoarenes with LCuNPh(2) in which L = phen. The free-energy barrier computed for the reaction of PhI with (DMSO)CuNPh(2) was 21.8 kcal/mol, while that for the reaction of PhI with (phen)CuNPh(2) was 33.4 kcal/mol.

Mechanism of nitric oxide reactivity and fluorescence enhancement of the NO-specific probe CuFL1

The mechanism of the reaction of CuFL1 (FL1 = 2-{2-chloro-6-hydroxy-5-[(2-methylquinolin-8-ylamino)methyl]-3-oxo-3H-xanthen-9-yl}benzoic acid) with nitric oxide (NO) to form the N-nitrosated product FL1-NO in buffered aqueous solutions was investigated. The reaction is first-order in [CuFL1], [NO], and [OH(-)]. The observed rate saturation at high base concentrations is consistent with a mechanism in which the protonation state of the secondary amine of the ligand is important for reactivity. This information provides a rationale for designing faster-reacting probes by lowering the pK(a) of the secondary amine. Activation parameters for the reaction of CuFL1 with NO indicate an associative mechanism (DeltaS(double dagger) = -120 +/- 10 J/mol.K) with a modest thermal barrier (DeltaH(double dagger) = 41 +/- 2 kJ/mol; E(a) = 43 +/- 2 kJ/mol). Variable-pH electron paramagnetic resonance experiments reveal that, as the secondary amine of CuFL1 is deprotonated, electron density shifts to yield a new spin-active species having electron density localized on the deprotonated amine nitrogen atom. This result suggests that FL1-NO formation occurs when NO attacks the deprotonated secondary amine of the coordinated ligand, followed by inner-sphere electron transfer to Cu(II) to form Cu(I) and release of FL1-NO from the metal.