Catalytic N-H Bond Formation Promoted by a Ruthenium Hydride Complex Bearing a Redox-Active Pyrimidine-Imine Ligand.

The synthesis of a piano-stool ruthenium hydride, [(η5-C5Me5)Ru(PmIm)H] (PmIm = (N-(1,3,5-trimethylphenyl)-1-(pyrimidin-2-yl)ethan-1-imine), for the dual purpose of catalytic dihydrogen activation and subsequent hydrogen atom transfer for the formation of weak chemical bonds is described. The introduction of a neutral, potentially redox-active PmIm supporting ligand was designed to eliminate the possibility of deleterious C(sp2)-H reductive coupling and elimination that has been identified as a deactivation pathway with related rhodium and iridium catalysts. Treatment of [(η5-C5Me5)RuCl2]n with one equivalent PmIm ligand in the presence of zinc and sodium methoxide resulted in the isolation of the diruthenium complex, [(η5-C5Me5)Ru(PmIm)]2, arising from the C-C bond formation between two PmIm chelates. Addition of H2 to the ruthenium dimer under both thermal and blue light irradiation conditions furnished the targeted hydride, [(η5-C5Me5)Ru(PmIm)H], which has a relatively weak DFT-calculated Ru-H bond dissociation free energy (BDFE) of 47.9 kcal/mol. Addition of TEMPO to [(η5-C5Me5)Ru(PmIm)H] generated the 17-electron metalloradical, [(η5-C5Me5)Ru(PmIm)], which was characterized by EPR spectroscopy. The C-C bond forming process was reversible as the irradiation of [(η5-C5Me5)Ru(PmIm)]2 generated [(η5-C5Me5)Ru(PmIm)H] and a piano-stool ruthenium complex containing an enamide ligand derived from H-atom abstraction from the PmIm chelate. Equilibration studies were used to establish an experimental estimate of the effective Ru-H BDFE, and a value of 50.8 kcal/mol was obtained, in agreement with the observed loss of H2 and the DFT-computed value. The ruthenium hydride was an effective catalyst for the thermal catalytic hydrogenation of TEMPO, acridine, and a cobalt-imido complex and for the selective reduction of azobenzene to diphenylhydrazine, highlighting the role of this complex in catalytic weak bond formation using H2 as the stoichiometric reductant.

Synthesis of an elusive, stable 2-azaallyl radical guided by electrochemical and reactivity studies of 2-azaallyl anions.

The super electron donor (SED) ability of 2-azaallyl anions has recently been discovered and applied to diverse reactivity, including transition metal-free cross-coupling and dehydrogenative cross-coupling processes. Surprisingly, the redox properties of 2-azaallyl anions and radicals have been rarely studied. Understanding the chemistry of elusive species is the key to further development. Electrochemical analysis of phenyl substituted 2-azaallyl anions revealed an oxidation wave at E 1/2 or E pa = -1.6 V versus Fc/Fc+, which is ∼800 mV less than the reduction potential predicted (E pa = -2.4 V vs. Fc/Fc+) based on reactivity studies. Investigation of the kinetics of electron transfer revealed reorganization energies an order of magnitude lower than commonly employed SEDs. The electrochemical study enabled the synthetic design of the first stable, acyclic 2-azaallyl radical. These results indicate that the reorganization energy should be an important design consideration for the development of more potent organic reductants.

Expanding the Rare-Earth Metal BINOLate Catalytic Multitool beyond Enantioselective Organic Synthesis.

Shibasaki's rare earth alkali metal BINOLate (REMB) framework has provided chemists with a general catalyst platform to access a range of enantioenriched small molecules from the single, commercially available pro-ligand (R)- or (S)-BINOL. A defining feature of these heterobimetallic frameworks is the high level of catalyst tunability, achieved through the simple modulation of the central rare-earth cation and peripheral alkali metal cations. While this family of multifunctional catalysts displays impressive generality and catalytic capability, detailed mechanistic understanding of these complex, multimetallic systems was lacking prior to our investigations. This backdrop served as initial inspiration for our investigations of this privileged class of complexes over the past decade, which have led to new and exciting advances in catalysis and beyond.In this Account, we describe our investigations using Shibasaki's framework focusing on the central metal-ion, the BINOLate ligands, and the secondary sphere cations. Our studies began with an investigation into the Lewis acidity of the complexes, where we demonstrated that Lewis bases readily coordinate to REMB frameworks when lithium occupies the secondary coordination sphere. This observation was contrasted by the complexes containing sodium or potassium in the secondary coordination sphere, as the rare earth cation is evidently less accessible for substrate binding. Our efforts in understanding the ligand exchange of the complexes enabled the discovery that associative processes dominate the mechanism of ligand exchange and LA/LA (Lewis acid/Lewis acid) and LA/BB (Lewis acid/Brønsted base) catalysis by the REMB frameworks. Replacing metal cations in the secondary coordination sphere with the N,N,N',N'-tetramethylguanidinium cation delivered an effective precatalyst that is air and water stable over the course of 6 months.To expand the reactivity of the REMB, we investigated the ability of UIV cations to occupy the primary coordination sphere and ZnEt+ and Cu(DBU)+ cations to occupy the secondary coordination sphere. Synthesizing the REMB complexes using the thiol congener monothioBINOL provided an unusual anionic REMB framework, driven by the oxophilicity of the lithium cations. Using the REMB as a platform for investigating the CeIII/CeIV redox couple, we demonstrated that, while oxidative cerium functionalization is observed in the case of lithium containing REMBs, salt elimination is observed in the sodium, potassium, and cesium containing REMBs. Furthermore, we found that while the rate of heterogeneous electron transfer for CeIII was ks(CsI) > ks(KI) > ks(NaI) > ks(LiI), the rates of reaction with the oxidant trityl chloride trended in the opposite order with kobs(LiI) ≫ kobs(NaI) > kobs(KI) > kobs(CsI). We attribute this to the ability to form inner-sphere complexes with the oxidant, rather than differences in redox potential or reorganization energies.Applying our knowledge in ligand exchange and redox behavior of Ce containing REMB complexes, we detailed the mechanism for oxidation of the heterochiral cerium REMB frameworks, reiterating the importance of the formation of inner-sphere complexes in the oxidation chemistry of cerium. There are many different avenues for both organic and inorganic investigation of Shibasaki's REMB framework, and our works have demonstrated the richness of the structural chemistry and properties of this framework that inform mechanism and properties of these privileged catalysts.

Isolation and characterization of a covalent CeIV-Aryl complex with an anomalous 13C chemical shift.

The synthesis of bona fide organometallic CeIV complexes is a formidable challenge given the typically oxidizing properties of the CeIV cation and reducing tendencies of carbanions. Herein, we report a pair of compounds comprising a CeIV - Caryl bond [Li(THF)4][CeIV(κ2-ortho-oxa)(MBP)2] (3-THF) and [Li(DME)3][CeIV(κ2-ortho-oxa)(MBP)2] (3-DME), ortho-oxa = dihydro-dimethyl-2-[4-(trifluoromethyl)phenyl]-oxazolide, MBP2- = 2,2'-methylenebis(6-tert-butyl-4-methylphenolate), which exhibit CeIV - Caryl bond lengths of 2.571(7) - 2.5806(19) Å and strongly-deshielded, CeIV - Cipso 13C{1H} NMR resonances at 255.6 ppm. Computational analyses reveal the Ce contribution to the CeIV - Caryl bond of 3-THF is ~12%, indicating appreciable metal-ligand covalency. Computations also reproduce the characteristic 13C{1H} resonance, and show a strong influence from spin-orbit coupling (SOC) effects on the chemical shift. The results demonstrate that SOC-driven deshielding is present for CeIV - Cipso 13C{1H} resonances and not just for diamagnetic actinide compounds.

Correction: Synthesis of novel copper-rare earth BINOLate frameworks from a hydrogen bonding DBU-H rare earth BINOLate complex.

Correction for 'Synthesis of novel copper-rare earth BINOLate frameworks from a hydrogen bonding DBU-H rare earth BINOLate complex' by Grace B. Panetti, et al., Dalton Trans., 2018, 47, 14408-14410.

Synthesis of novel copper-rare earth BINOLate frameworks from a hydrogen bonding DBU-H rare earth BINOLate complex.

The preparation of a novel H-bonding DBU-H+ BINOLate Rare Earth Metal complex enabled the synthesis of the first copper-Rare Earth Metal BINOLate complex (CuDBU-REMB). CuDBU-REMB was compared to the analogous Li complex using X-ray crystallography and Exchange NMR spectroscopy (EXSY). The results provide insight into the role of the secondary metal cation in the framework's stabilization.

Visible-Light-Mediated Umpolung Reactivity of Imines: Ketimine Reductions with Cy2NMe and Water.

A novel carbanionic reactivity of imines mediated by photoredox catalysis is demonstrated. The umpolung imine reactivity is exemplified by proton abstraction from water as a key step in the reduction of benzophenone ketimines to amines (up to 98% yield). Deuterium is introduced into amines efficiently using D2O as an inexpensive deuterium source (≥95% D ratio). The mechanism of this unusual transformation is probed.

Synthesis of Diarylated 4-Pyridylmethyl Ethers via Palladium-Catalyzed Cross-Coupling Reactions.

The direct arylation of weakly acidic sp3-hybridized C-H bonds via deprotonated cross-coupling processes (DCCP) is a challenge. Herein, a Pd(NIXANTPHOS)-based catalyst for the mono arylation of 4-pyridylmethyl 2-aryl ethers to generate diarylated 4-pyridyl methyl ethers is introduced. Furthermore, under similar conditions, the diarylation of 4-pyridylmethyl ethers with aryl bromides has been developed. These methods enable the synthesis of new pyridine derivatives, which are common in medicinally active compounds and in application in materials science.

Transition-Metal-Free Radical C(sp3)-C(sp2) and C(sp3)-C(sp3) Coupling Enabled by 2-Azaallyls as Super-Electron-Donors and Coupling-Partners.

The past decade has witnessed the rapid development of radical generation strategies and their applications in C-C bond-forming reactions. Most of these processes require initiators, transition metal catalysts, or organometallic reagents. Herein, we report the discovery of a simple organic system (2-azaallyl anions) that enables radical coupling reactions under transition-metal-free conditions. Deprotonation of N-benzyl ketimines generates semistabilized 2-azaallyl anions that behave as "super-electron-donors" (SEDs) and reduce aryl iodides and alkyl halides to aryl and alkyl radicals. The SET process converts the 2-azaallyl anions into persistent 2-azaallyl radicals, which capture the aryl and alkyl radicals to form C-C bonds. The radical coupling of aryl and alkyl radicals with 2-azaallyl radicals makes possible the synthesis of functionalized amine derivatives without the use of exogenous radical initiators or transition metal catalysts. Radical clock studies and 2-azaallyl anion coupling studies provide mechanistic insight for this unique reactivity.