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Described herein are studies toward the core modificationof cyclicaliphatic amines using either a riboflavin/photo-irradiation approachor Cu-(I) and Ag-(I) to mediate the process. Structural remodeling ofcyclic amines is explored through oxidative C-N and C-Cbond cleavage using peroxydisulfate (persulfate) as an oxidant. Ring-openingreactions to access linear aldehydes or carboxylic acids with flavin-derivedphotocatalysis or Cu salts, respectively, are demonstrated. A complementaryring-opening process mediated by Ag-(I) facilitates decarboxylativeCsp(3)-Csp(2) coupling in Minisci-type reactionsthrough a key alkyl radical intermediate. Heterocycle interconversionis demonstrated through the transformation of N-acyl cyclic aminesto oxazines using Cu-(II) oxidation of the alkyl radical. These transformationsare investigated by computation to inform the proposed mechanisticpathways. Computational studies indicate that persulfate mediatesoxidation of cyclic amines with concomitant reduction of riboflavin.Persulfate is subsequently reduced by formal hydride transfer fromthe reduced riboflavin catalyst. Oxidation of the cyclic aliphaticamines with a Cu-(I) salt is proposed to be initiated by homolysisof the peroxy bond of persulfate followed by alpha-HAT from thecyclic amine and radical recombination to form an alpha-sulfateadduct, which is hydrolyzed to the hemiaminal. Investigation of thepathway to form oxazines indicates a kinetic preference for cyclizationover more typical elimination pathways to form olefins through Cu-(II)oxidation of alkyl radicals., Journal of the American Chemical Society, 145 (20), ISSN:0002-7863, ISSN:1520-5126

Highly modular and rational syntheses of titanium compounds containing ditelluride, terminal telluride, and bis(telluride) structural motifs are disclosed in this study. Titanate anions bearing two

We employed the chemical potential neutralization principle to demonstrate that fractional electrons are involved in the electro-inductive effect as well as the vibrational Stark effect. By the chemical potential model, we were able to deduce that the frontier molecular orbitals of immobilized molecules can provide valuable insight into these effects. To further understand and quantify these findings, we introduced fractional charge density functional theory (FC-DFT), a canonical ensemble approach for open systems. This method allows for the calculation of electronic energies, nuclear gradients, and the Hessian matrix of fractional electronic systems. To correct the spurious delocalization error commonly found in approximate density functionals for small systems, we imposed the Perdew-Parr-Levy-Balduz (PPLB) condition through linear interpolation of two adjacent integer points (LI-FC-DFT). Although this approach is relatively simple in terms of molecular modeling, the results obtained through LI-FC-DFT calculations predict the same trend seen in experimental reactivity and the frequency change of immobilized molecules.

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Oxidative carbene organocatalysis, which proceeds via single electron transfer (SET) pathways, has been limited by the moderately reducing properties of deprotonated Breslow intermediates BI−s derived from thiazol-2-ylidene 1 and 1,2,4-triazolylidene 2. Using computational methods, we assess the redox potentials of BI−s based on ten different types of known stable carbenes and report our findings concerning the key parameters influencing the steps of the catalytic cycle. From the calculated values of the first oxidation potential of BI−s derived from carbenes 1 to 10, it appears that apart from the diamidocarbene 7, all the others are more reducing than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. We observed that while the reducing power of BI−s significantly decreases with increasing solvent polarity, the redox potential of the oxidant can increase at a greater rate, thus facilitating the reaction. The cation, associated with the base, also plays an important role when a non-polar solvent is used; large and weakly coordinating cations such as Cs+ are beneficial. The radical-radical coupling step is probably the most challenging step due to both electronic and steric constraints. Based on our results, we predict that mesoionic carbene 3 and abnormal NHC 4 are the most promising candidates for oxidative carbene organocatalysis.

The cationic Ru-H complex [(C6H6)(PCy3)(CO)RuH]+BF4- (1) was found to be an effective catalyst for the dehydrative C-H coupling reaction of phenols and aldehydes to form 2-alkylphenol products. The coupling reaction of phenols with branched aldehydes selectively formed 1,1-disubstituted benzofurans, while the coupling reaction with salicylaldehydes yielded xanthene derivatives. A normal deuterium isotope effect was observed from the coupling reaction of 3-methoxyphenol with benzaldehyde and 2-propanol/2-propanol-d8 (kH/kD = 2.3 ± 0.3). The carbon isotope effect was observed on the benzylic carbon of the alkylation product from the coupling reaction of 3-methoxyphenol with 4-methoxybenzaldehyde (C(3) 1.021(3)) and on both benzylic and ortho-arene carbons from the coupling reaction with 4-trifluorobenzaldehdye (C(2) 1.017(3), C(3) 1.011(2)). The Hammett plot from the coupling reaction of 3-methoxyphenol with para-substituted benzaldehydes p-X-C6H4CHO (X = OMe, Me, H, F, Cl, CF3) displayed a V-shaped linear slope. Catalytically relevant Ru-H complexes were observed by NMR from a stoichiometric reaction mixture of 1, 3-methoxyphenol, benzaldehyde, and 2-propanol in CD2Cl2. The DFT calculations provided a detailed catalysis mechanism featuring an electrophilic aromatic substitution of the aldehyde followed by the hydrogenolysis of the hydroxy group. The calculations also revealed a mechanistic rationale for the strong electronic effect of the benzaldehdye substrates p-X-C6H4CHO (X = OMe, CF3) in controlling the turnover-limiting step. The catalytic C-H coupling method provides an efficient synthetic protocol for 2-alkylphenols, 1,1-disubstituted benzofurans, and xanthene derivatives without employing any reactive reagents or forming wasteful byproducts.

Refluxing Mo(CO)6 in the presence of the phosphine-functionalized α-diimine ligand Ph2PPrDI allowed for substitution and formation of the dicarbonyl complex, (Ph2PPrDI)Mo(CO)2. Oxidation with I2 followed by heating resulted in further CO dissociation and isolation of the corresponding diiodide complex, (Ph2PPrDI)MoI2. Reduction of this complex under a N2 atmosphere afforded the corresponding bis(dinitrogen) complex, (Ph2PPrDI)Mo(N2)2. The solid-state structures of all three compounds were found to feature a tetradentate chelate and cis-monodentate ligands. Notably, the addition of CO2 to (Ph2PPrDI)Mo(N2)2 is proposed to result in head-to-tail CO2 coupling to generate the corresponding metallacycle and ultimately a mixture of (Ph2PPrDI)Mo(CO)2 and the bis(oxo) dimer, [(κ3-Ph2PPrDI)Mo(O)(μ-O)]2. Computational studies have been performed to gain insight into the reaction and evaluate the importance of cis-coordination sites for selective head-to-tail CO2 reductive coupling, CO deinsertion, disproportionation, and stepwise CO2 deinsertion.

We disclose herein a directing group-assisted nickel-catalyzed intermolecular C(sp3)–H amidation using organic azides as nitrene precursors. With the installation of an electronically tailored dire...

We report a copper-catalyzed enatiotopic-group-selective allylation of gem-diborylalkanes with allyl bromides. The combination of copper(I) bromide and H8-BINOL derived phosphoramidite ligand proved to be the most effective catalytic system to provide various enantioenriched homoallylic boronate esters, containing a boron-substituted stereogenic center that is solely derived from gem-diborylalkanes, in good yields with high enantiomeric ratios under mild conditions. Experimental and theoretical studies have been conducted to elucidate the reaction mechanism, revealing how the enatiotopic-group-selective transmetalation of gem-diborylalkanes with chiral copper complex occurs to generate chiral α-borylalkyl-copper species for the first time. Additional synthetic applications to the synthesis of various chiral building blocks are also included.

Copper-catalyzed [5 + 1] cycloadditions of N-aromatic zwitterions have been accomplished by chelation-assisted 1,2-dearomative addition of electron-deficient terminal alkynes. The unique modular sk...

Herein we report the synthesis of substituted indolizidines and related N-fused bicycles from simple saturated cyclic amines through sequential C─H and C─C bond functionalizations. Inspired by the Norrish–Yang Type II reaction, C─H functionalization of azacycles is achieved by forming α-hydroxy-β-lactams from precursor α-ketoamide derivatives under mild, visible light conditions. Selective cleavage of the distal C(sp(2))─C(sp(3)) bond in α-hydroxy-β-lactams using a Rh-complex leads to α-acyl intermediates which undergo sequential Rh-catalyzed decarbonylation, 1,4-addition to an electrophile, and aldol cyclization, to afford N-fused bicycles including indolizidines. Computational studies provide mechanistic insight into the observed positional selectivity of C─C cleavage, which depends strongly on the groups bound to Rh trans to the phosphine ligand.

The regioselective, direct alkylation of electron-deficient N-heteroarenes is, in principle, a powerful and efficient way of accessing alkylated N-heteroarenes that are important core structures of many biologically active compounds and pharmaceutical agents. Herein, we report a ZnMe2-promoted, direct C2- or C4-selective primary and secondary alkylation of pyridines and quinolines using 1,1-diborylalkanes as alkylation sources. While substituted pyridines and quinolines exclusively afford C2-alkylated products, simple pyridine delivers C4-alkylated pyridine with excellent regioselectivity. The reaction scope is remarkably broad, and a range of C2- or C4-alkylated electron-deficient N-heteroarenes are obtained in good yields. Experimental and computational mechanistic studies imply that ZnMe2 serves not only as an activator of 1,1-diborylalkanes to generate (α-borylalkyl)methylalkoxy zincate, which acts as a Lewis acid to bind to the nitrogen atom of the heterocycles and controls the regioselectivity, but also as an oxidant for rearomatizing the dihydro-N-heteroarene intermediates to release the product.

Multiple pathogenic elements, including reactive oxygen species, amyloidogenic proteins, and metal ions, are associated with the development of neurodegenerative disorders. We report minimalistic redox-based principles for preparing compact aromatic compounds by derivatizing the phenylene moiety with various functional groups. These molecular agents display enhanced reactivities against multiple targets such as free radicals, metal-free amyloid-β (Aβ), and metal-bound Aβ that are implicated in the most common form of dementia, Alzheimer's disease (AD). Mechanistic studies reveal that the redox properties of these reagents are essential for their function. Specifically, they engage in oxidative reactions with metal-free and metal-bound Aβ, leading to chemical modifications of the Aβ peptides to form covalent adducts that alter the aggregation of Aβ. Moreover, the administration of the most promising candidate significantly attenuates the amyloid pathology in the brains of AD transgenic mice and improves their cognitive defects. Our studies demonstrate an efficient and effective redox-based strategy for incorporating multiple functions into simple molecular reagents.

Cytochrome c (Cyt c) is an important, multifunctional protein for controlling cell fate. Emerging evidence suggests a potential role of Cyt c in the amyloid pathology associated with Alzheimer’s disease (AD); however, the interaction between Cyt c and Abeta with the consequent impact on the aggregation and toxicity of Abeta is not known. Here we report the discovery that Cyt c can directly bind to Abeta and alter the aggregation and toxicity profiles of Abeta in a peroxide-dependent manner. Cyt c redirects Abeta peptides into less toxic, off-pathway amorphous aggregates in the presence of hydrogen peroxide (H2O2), whereas it accelerates Abeta fibrillization without H2O2. Such effects can be achieved by three possible mechanisms, including the complexation between Cyt c and Abeta, the oxidation of Abeta by Cyt c and H2O2, and the H2O2-mediated modification of Cyt c. Our studies demonstrate a new function of Cyt c as a modulator against Abeta amyloidogenesis.

Herein, the mechanism of catalytic allylic C-H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that the allylic C-H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidants in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetry experiments concomitantly support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand proceeding through a Rh(IV) intermediate. Stoichiometric oxidation and bulk electrolysis of the proposed π-allyl intermediate are also reported to support these analyses. Lastly, evidence supporting the amination of an allylic acetate intermediate is presented. We show that Cp*Rh(III)

The oxidations of 1,1′-di-p-anisolyl-2-cymantrenylbutene (3b) and 1,1′-di-p-hydroxyphenyl-2-cymantrenylbutene (3c) were investigated by electrochemical and spectroscopic experiments and by density ...

N,N-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl2 in the presence of PyEtPDI afforded the six-coordinate Co(II) salt, [(PyEtPDI)CoCl][Cl]. Upon adding 2 equiv of NaEt3BH, hydride transfer to one chelate imine functionality was observed, resulting in the formation of (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co. Single-crystal X-ray diffraction and density functional theory calculations revealed that this compound possesses a low-spin Co(II) ground state featuring antiferromagnetic coupling to a singly reduced imino(pyridine) moiety. Importantly, (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co was found to catalyze the dihydroboration of nitriles using HBPin with turnover frequencies of up to 380 h-1 at ambient temperature. Stoichiometric addition experiments revealed that HBPin adds across the Co-Namide bond to generate a hydride intermediate that can react with additional HBPin or nitriles. Computational evaluation of the reaction coordinate revealed that the B-H addition and nitrile insertion steps occur on the antiferromagnetically coupled triplet spin manifold. Interestingly, formation of the borylimine intermediate was found to occur following BPin transfer from the borylated chelate arm to regenerate (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co. Borylimine reduction is in turn facile and follows the same ligand-assisted borylation pathway. The independent hydroboration of alkyl and aryl imines was also demonstrated at 25 °C. With a series of N,N-diborylamines in hand, their addition to carboxylic acids allowed for the direct synthesis of amides at 120 °C, without the need for an exogenous coupling reagent.

The ring-opening of 2-methylfuran and 2,3-dihydro-5-methylfuran catalyzed by the Lewis acid catalyst tris(pentafluorophenyl)borane in the presence of hydrosilanes was studied using quantum chemical methods. In a previous study, it was suggested that the stereoselective formation of the product is due to a nucleophilic vinylic substitution (SNV) during the reaction. Our calculations show that the pathway involving the SNV reaction is energetically not accessible. Instead, the intramolecular C-O bond cleavage is found to be much more favorable in energy for the ring opening reaction. The experimentally observed excellent stereoselectivity toward the Z-isomer product originates from an intrinsic preference of the furan ring to couple the C-O bond cleavage with a disrotatory motion of the oxygen and carbon fragments. This stereoselective feature is naturally programmed into the furan ring manifold and should be generally exploitable for engineering stereoselective ring-opening processes of bioderived furans.

Understanding the stability and reactivity of the propagating species is critical in living polymerization. Therefore, most living olefin metathesis polymerizations require the stabilization of the catalyst by coordination of external ligands containing Lewis basic heteroatoms, e.g., phosphines and pyridines. However, in some cases, chemists postulated that the propagating metal carbene could also be stabilized by olefin chelation. Here, we disclose that stable 16-electron olefin-chelated Ru carbenes play a key role in previously reported living/controlled ring-opening metathesis polymerization of endo-tricyclo[4.2.2.0^(2,5)]deca-3,9-diene and cyclopolymerization of 1,8-nonadiynes using Grubbs catalysts. We successfully isolated these propagating species during polymerization and confirmed their olefin-chelated structures using X-ray crystallography and NMR analysis. DFT calculations and van ’t Hoff plots from the equilibrium between olefin-chelated Ru carbenes and 3-chloropyridine (Py)-coordinated carbenes revealed that entropically favored olefin chelation overwhelmed enthalpically more stable Py-coordinated Ru carbenes at room temperature. Therefore, olefin chelation stabilized the propagating species and slowed down the propagation relative to initiation, thereby lowering polydispersity. This finding provides a deeper understanding of the olefin metathesis polymerization mechanism using Grubbs catalysts and offers clues for designing new controlled/living polymerizations.

A novel dynamic kinetic resolution (DKR) of tetrasubstituted alkenyl cyanohydrins prepared from the corresponding α,β-unsaturated aldehydes is described. The deprotonation of a geometrical mixture of tetrasubstituted alkenyl cyanohydrins with sodium diisopropylamide (NaDA) affords the allylic anions, which enables the equilibration of the E- and Z-olefins to permit the selective functionalization of the E-adduct. Theoretical studies indicate that the nature of the alkali metal cation is a critical component to lowering the barrier for interconversion between the two geometrical isomers, which provides the mechanistic basis for the DKR reaction. In addition, we demonstrate that the DKR reaction can be combined with a transition metal-catalyzed allylic substitution to generate a stereodefined E-tetrasubstituted olefin and quaternary center in a single cross-coupling reaction.

We report a Michael-type cyanation reaction of coumarins by using CO2 as a catalyst. The delivery of the nucleophilic cyanide was realized by catalytic amounts of CO2, which forms cyanoformate and ...

Designing new chromophores by tuning their molecular structures and optimizing their photophysical properties leads to suitable photochromic features. Herein, we report a series of anthraquinone (AQ)-based photosensitizers that undergoes excited-state intramolecular hydrogen transfer and effectively oxidizes amyloidogenic peptides, which significantly affects the subsequent aggregation pathways. DFT calculations showed that the appropriate position of the hydroxyl groups in the AQ backbone and the consequent intramolecular hydrogen transfer can facilitate the energy transfer to triplet oxygen. Biochemical and biophysical investigations confirmed that these photoactive chemical reagents are able to oxidatively modify both metal-free amyloid-β (Aβ) and metal-bound Aβ, thereby redirecting their on-pathway aggregation into off-pathway as well as disassembling their pre-formed aggregates. Moreover, the in vivo histochemical analysis of Aβ species produced upon photoactivation of the most promising candidate demonstrated that they do not aggregate into toxic oligomeric or fibrillar aggregates in the brain. Overall, our combined computational and experimental studies validate a light-based approach for designing small molecules as chemical reagents targeting and controlling amyloidogenic peptides associated with neurodegenerative disorders.

Intracellular C-terminal cleavage of the amyloid precursor protein (APP) is elevated in the brain of Alzheimer’s disease (AD). Emerging evidence proposes a pathological relationship between the production of a C-terminal APP fragment, called APP-C31, and the toxicity induced by amyloid-beta (Abeta) that is a major contributor towards AD; however, the interaction between the two peptides and the consequent impact of APP-C31 on Abeta-related toxicity were unknown thus far. Here we report the discovery that APP-C31 facilitates the aggregation of Abeta and aggravates its toxicity at the intracellular level, with escalating neurodegeneration. APP-C31 forms a hetero-dimer with Abeta through the contacts onto the N-terminal and self-recognition regions of Abeta and induces its conformational transition accelerating amyloid fibrillization. APP-C31 promotes the perinuclear and intranuclear deposition of enlarged Abeta aggregates and, consequently, damages the nucleus leading to apoptosis. Abeta-induced degeneration of neurites in human neurons is also intensified by APP-C31. Our studies demonstrate a new function of APP-C31 as an intracellular factor of the proteopathy found in AD.

Intracellular C-terminal cleavage of the amyloid precursor protein (APP) is elevated in the brain of Alzheimer’s disease (AD). Emerging evidence proposes a pathological relationship between the production of a C-terminal APP fragment, called APP-C31, and the toxicity induced by amyloid-beta (Abeta) that is a major contributor towards AD; however, the interaction between the two peptides and the consequent impact of APP-C31 on Abeta-related toxicity were unknown thus far. Here we report the discovery that APP-C31 facilitates the aggregation of Abeta and aggravates its toxicity at the intracellular level, with escalating neurodegeneration. APP-C31 forms a hetero-dimer with Abeta through the contacts onto the N-terminal and self-recognition regions of Abeta and induces its conformational transition accelerating amyloid fibrillization. APP-C31 promotes the perinuclear and intranuclear deposition of enlarged Abeta aggregates and, consequently, damages the nucleus leading to apoptosis. Abeta-induced degeneration of neurites in human neurons is also intensified by APP-C31. Our studies demonstrate a new function of APP-C31 as an intracellular factor of the proteopathy found in AD.

Pd-catalyzed nucleophilic fluorination reactions are important methods for the synthesis of fluoroarenes and fluoroalkenes. However, these reactions can generate a mixture of regioisomeric products that are often difficult to separate. While investigating the Pd- catalyzed fluorination of cyclic vinyl triflates, we observed that the addition of a substoichiometric quantity of TESCF3 significantly improves both the efficiency and the regioselectivity of the fluorination process. Herein, we report a combined experimental and computational study on the mechanism of this transformation focused on the role of TESCF3. We found that in the absence of additives such as TESCF3, the transmetalation step produces predominantly the thermodynamically more stable trans isomer of the key LPd(vinyl)F complex (L = biaryl monophosphine ligand). This intermediate, rather than undergoing reductive elimination, preferentially reacts through an intramolecular β-deprotonation to form a Pd-cyclohexyne intermediate. This undesired reactivity is responsible for the low efficiency (11% yield) and poor regioselectivity (1.8:1) of the catalytic reaction. When TESCF3 is added to the reaction mixture, the cis-LPd(vinyl)F complex is instead formed, through a pathway involving an unusual dearomatization of the ligand by nucleophilic attack from a trifluoromethyl anion (CF3–). In contrast to the trans isomer, this cis-LPd(vinyl)F complex readily undergoes reductive elimination to provide the vinyl fluoride product with desired regioselectivity, without the generation of Pd-cyclohexyne intermediates.

Enantioenriched molecules bearing indole-substituted stereocenters form a class of privileged compounds in biological, medicinal, and organic chemistry. Thus, the development of methods for asymmetric indole alkylation is highly valuable in organic synthesis. Traditionally, achieving N-selectivity in indole alkylation reactions is a significant challenge, since there is an intrinsic preference for alkylation at C3, the most nucleophilic position. Furthermore, selective and predictable access to either N- and C3-alkylated chiral indoles using catalyst control has been a long-standing goal in indole functionalization. Herein, we report a ligand-controlled regiodivergent synthesis of N- and C3-alkylated chiral indoles that relies on a polarity reversal strategy. In contrast to conventional alkylation reactions in which indoles are employed as nucleophiles, this transformation employs electrophilic indole derivatives, N-(benzoyloxy)indoles, as coupling partners. N- or C3-alkylated indoles are prepared with high levels of regio- and enantioselectivity using a copper hydride catalyst. The regioselectivity is governed by the use of either DTBM-SEGPHOS or Ph-BPE as the supporting ligand. Density functional theory (DFT) calculations are conducted to elucidate the origin of the ligand-controlled regiodivergence.

We describe different modes of dimerization of various α',γ-dioxyenone derivatives with potential applications to the synthesis of high-order securinega alkaloids. We learned that the relative stereochemical relationship between α'- and γ-hydroxyl groups of the α',γ-dihydroxyenone derivative determines the mode of dimerization. While cis-α',γ-dioxyenone 26 provided the Rauhut-Currier-type (RC-type) dimer 31 upon reaction with TBAF, trans-α',γ-dihydroxyenone 34 afforded dimeric tetrahydrofuran derivative 41 under the same reaction conditions. We also noticed that the protection of the γ-hydroxyl group drastically changes the reaction outcomes. While cis-α'-oxy-γ-OPiv-enone 49 did not show any reactivity in the presence of TBAF, trans-α'-hydroxy-γ-OPiv-enone 45 produced the RC-type dimer 46 under the same reaction conditions. Computational analysis revealed the detailed mechanism of the latter transformation.

A strategy for expedient synthesis of 3-substituted chromones from easily available o-hydroxyarylenaminones and diazo compounds has been developed. Carefully conducted experimental and computational studies led us to propose an uncommon mechanistic pathway involving the hydroxyl group assisted alkylation of enaminones with in situ generated gold carbenes.

The direct amination of C-H bonds with ammonia is a challenge in synthetic chemistry. Herein, we present a copper-mediated approach that enables a chelation-assisted aromatic C-H bond amination using aqueous ammonia. A key strategy was to use soft low-valent Cu(I) species to avoid the strong coordination of ammonia. Mechanistic investigations suggest that the catalysis is initiated by a facile deprotonation of bound ammonia, and the C-N coupling is achieved by subsequent reductive elimination of the resultant copper-amido intermediate from a Cu(III) intermediate that is readily generated by disproportionation of low-valent copper analogues. This mechanistic postulate was supported by a preliminary kinetic isotope effect study and computations. This new chelation-assisted, copper-mediated C-H bond amination with aqueous ammonia was successfully applied to a broad range of substrates to deliver primary anilines. Moreover, the mild conditions required for this transformation allowed the reaction to operate even under substoichiometric conditions to enable a late-stage application for the preparation of pharmaceutical agents.

Quantum chemical molecular modeling has become a standard tool in organometallic chemistry. In particular, density functional theory calculations are now indispensable for investigating the mechanism of even complex reactions and deliver precise energies of intermediates and transition states. Because software packages have become user-friendly and are widely available, even nonexperts can now produce high-quality computer models. In this tutorial, we highlight nontrivial mistakes, misconceptions, and misinterpretations often encountered when producing models of a chemical reaction that can lead to wrong conclusions. The reasons for these errors are conceptually explained in simple terms, and remedies are offered.

Combined computational and experimental studies elucidate the mechanism and suggest rational design and optimization strategies of a bis(phosphine)-supported iridium-catalyst for methane monoborylation. The activation of the C–H bond in methane via oxidative addition using tris(boryl) iridium(III) complexes bearing bis-chelating supporting ligands is modeled computationally. This model shows that the use of the soft Lewis base ligand such as 1,2-bis(dimethylphosphino)ethane (dmpe) lowers the activation barrier of the rate-determining step as it facilitates polarization of the metal-center, lowering the barrier of the oxidative addition to afford a seven-coordinate iridium(V) intermediate. The experimental optimization of this reaction using high-throughput methods shows that up to 170 turnovers can be achieved at 150 °C (500 psi) within 16 h using bis(pinacolato)diboron, a well-defined homogeneous and monomeric catalyst (dmpe)Ir(COD)Cl that is readily available from commercial precursors, with selectivity...

Uranium complexes in the +3 and +4 oxidation states were prepared using the anionic PN– (PN– = (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) ligand framework. New complexes include the halide starting materials, (PN)2UIIII (1) and (PN)2UIVCl2 (2), which both yield (PN)2UIV(N3)2 (3) by reaction with NaN3. Compound 3 was reduced with potassium graphite to produce a putative, transient uranium–nitrido moiety that underwent an intramolecular C–H activation to form a rare example of a parent imido complex, [K(THF)3][(PN)UIV(═NH)[iPr2P(C6H3Me)N(C6H2Me2CH2)]] (4). Calculated reaction energy profiles strongly suggest that a C–H insertion becomes unfavorable when a reductant is present, offering a distinctively different reaction pathway than previously observed for other uranium nitride complexes.

Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed. On the basis of a combined experimental and computational study, a precise mechanistic proposal was developed and rational optimization strategies were identified. The epoxide ring-opening, which requires an iodide as a nucleophile, was enhanced by utilizing an immonium functionality that can form an ion pair with iodide, making the ring-opening process intramolecular. The CO2 activation and cyclic carbonate formation were catalyzed by the concerted action of two hydrogen bonds originating from two phenolic groups placed at the claw positions of the scorpionate scaffold. Electronic tuning of the hydrogen bond donors allowed to identify a new catalyst that can deliver >90% yield for a variety of epoxide substrates within 7 h at room temperature under a CO2 pressure of only 10 bar, and is highly recyclable.

The oxidative electrochemical behavior of 1,1′-diphenyl-2-cymantrenylbutene (1), a cymantrene analogue of the breast cancer drug ferrocifen, was shown to involve the sequential electron-transfer series 1/1+/12+ in dichloromethane/0.05 M [NBu4][B(C6F5)4] (E1/2 values 0.78 and 1.18 V vs ferrocene). By a combination of spectroscopic and computational techniques, it was shown that the cymantrene functionality plays an important role in dissipating the positive charges in the oxidized compounds and is therefore an active participant in the redox events. The redox-active orbital goes from roughly equal degrees of organometallic and π-organic (diphenylolefin) makeup in 1 to increasingly organic based fractions in 1+ and 12+. Structural changes mimicking those of oxidized tetrakis(aryl)ethylenes accompany the one-electron oxidations. There is sufficient unpaired electron density on the manganese center in 1+ to allow for oxidatively induced ligand exchange of one or more of the carbonyl ligands with donor ligands...

A pyridinium zwitterion substrate is employed with two different types of transition metal catalysts to develop a regiodivergent cycloaddition. The pyridinium zwitterion is a highly reactive dipolar substrate that can undergo a dipolar cycloaddition with various reactants. It has multiple reaction sites, and the chemoselectivity is determined by the electronic demand of the catalyst–substrate complex. The reaction with nucleophilic Pd reagents affords fused N-heterocyclic compounds via regioselective [4 + 2] cycloaddition. The origin of the site selectivity and the mechanism of this reaction are investigated in this combined experimental and computational study. We found that the pyridinium zwitterion plays a completely different role in the palladium(0)-catalyzed [4 + 2] cycloaddition reaction and in the rhodium(II)-catalyzed [5 + 3] cycloaddition, which was examined experimentally in a previous study. The frontier molecular orbitals of the pyridinium substrate and activated catalyst complex reveal that ...

Stereochemical control of electronically excited states is a long-standing challenge in photochemical synthesis, and few catalytic systems that produce high enantioselectivities in triplet-state photoreactions are known. We report herein an exceptionally effective chiral photocatalyst that recruits prochiral quinolones using a series of hydrogen-bonding and π–π interactions. The organization of these substrates within the chiral environment of the transition metal photosensitizer leads to efficient Dexter energy transfer and effective stereoinduction. The relative insensitivity of these organometallic chromophores towards ligand modification enables the optimization of this catalyst structure for high enantiomeric excess (ee) at catalyst loadings as much as 100-fold lower than the optimal conditions reported for analogous chiral organic photosensitizers.

The complex (PNP)Ti═CHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2–) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti(CH2tBu)], which reacts with 2 equiv of quinoline (Q) at room temperature to form H3CtBu and a Ti(IV) species where the less hindered C2═N1 bond of Q is ruptured and coupled to another equivalent of Q. The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti═N[C18H13N] (1). Under photolytic conditions, intramolecular C—H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti(CH2tBu)] results in regioselective cleavage of the C1═N2 and C1—H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti(CH2tBu)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti═N[C10H9N] (4...

A new protocol for the synthesis of color-tunable fluorescent 3,5-diarylimidazo[1,2-a]pyridines has been achieved via palladium-catalyzed C-H amination of pyridinium zwitterions. Based on experimental results and computational analysis, we extracted a high correlation of photophysical properties with the theoretical concept and predicted emission wavelengths of 3,5-diarylimidazo[1,2-a]pyridines. The emission wavelengths of imidazo[1,2-a]pyridines increase as a function of the electron-withdrawing nature of the substituent on the C5-aryl group of imidazo[1,2-a]pyridine as a result of inductive effects on the LUMO levels. Varying the substituent on the C3-aryl group imidazo[1,2-a]pyridine changes the HOMO levels. Combining these two sites, the HOMO and LUMO levels can be tuned fairly decoupled from each other. This conceptual trend is demonstrated across a series where the C3 and C5 positions were functionalized independently and then utilizes a combination strategy where both sites are used to prepare fluorophores with a large window of emission wavelengths. In view of the biological properties of imidazo[1,2-a]pyridines, the developed method provides an efficient approach for understanding and preparing strongly fluorescent bioprobes.

The mechanism of water oxidation performed by a recently discovered manganese pyridinophane catalyst [Mn(Py2NtBu2)(H2O)2]2+ is studied using density functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a MnV-bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O–O bond formation is found on the quintet potential energy surface and involves an intramolecular coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin MnIII ion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a critical role for catalysis. It enables facile electron transfer between the oxo m...

The mechanism of catalytic allylic C–H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that allylic C–H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidant in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetryexperiments support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand. Lastly, evidences supporting the amination of an allylic acetate intermediate is presented. Both nucleophilic substitution catalyzed by Ag+that behaves as a Lewis acid catalyst and an inner-sphere amination catalyzed by Cp*Rh are shown to be viable for the last step of the allylic amination reaction.

The mechanism of catalytic allylic C–H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that allylic C–H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidant in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetryexperiments support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand. Lastly, evidences supporting the amination of an allylic acetate intermediate is presented. Both nucleophilic substitution catalyzed by Ag+that behaves as a Lewis acid catalyst and an inner-sphere amination catalyzed by Cp*Rh are shown to be viable for the last step of the allylic amination reaction.

We recently reported a bis(imino)pyridine (or pyridine diimine, PDI) manganese precatalyst, (Ph2PPrPDI)Mn (1), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for 1-mediated hydrosilylation and propose two different mechanisms through which catalysis is achieved. Aldehyde hydrosilylation turnover frequencies (TOFs) of up to 4900 min–1 have been realized, the highest reported for first row metal-catalyzed carbonyl hydrosilylation. Additionally, 1 has been shown to mediate formate dihydrosilylation with leading TOFs of up to 330 min–1. Under stoichiometric and catalytic conditions, addition of PhSiH3 to (Ph2PPrPDI)Mn was found to result in partial conversion to a new diamagnetic hydride compound. Independent preparation of (Ph2PPrPDI)MnH (2) was achieved upon adding NaEt3BH to (Ph2PPrPDI)MnCl2 and single-crystal X-ray diffraction analysis revealed this complex to possess a capped trigonal bipyramidal solid-state geometry. W...

MnV═O functionalities are important in synthetic and bioinorganic chemistry, being relevant to both C–H activation and the O–O bond formation steps in enzymatic water oxidation, for example. The triplet and quintet spin states are believed to be active in these reactions, but they have only been sparingly characterized experimentally. Density functional theory (DFT) gives varying results, depending on the exchange-correlation functional employed, leading to ambiguity about whether the triplet MnV═O is better represented as MnIV–O•. While recent CASPT2 studies confirmed that the MnIV–O• character is exaggerated by hybrid functionals, questions still remain about the nature of this bonding. Using high-level wave function methods, we investigated the fundamental relationship between the spin polarization, diradical character, and the physical oxidation state assignments. We conclude that, in terms of formal oxidation assignment, these species are best described as being between the MnV═O and MnIV–O• extremes...

The reactivity of metal sites in proteins is tuned by protein-based ligands. For example, in blue copper proteins such as plastocyanin (Pc), the structure imparts a highly elongated bond between the Cu and a methionine (Met) axial ligand to modulate its redox properties. Despite extensive study, a complete understanding of the contribution of the protein to redox activity is challenged by experimentally accessing both redox states of metalloproteins. Using infrared (IR) spectroscopy in combination with site-selective labeling with carbon-deuterium (C-D) vibrational probes, we characterized the localized changes at the Cu ligand Met97 in the oxidized and reduced states, as well as the Zn(II) or Co(II)-substituted, the pH-induced low-coordinate, the apoprotein, and the unfolded states. The IR absorptions of (d3-methyl)Met97 are highly sensitive to interaction of the sulfur-based orbitals with the metal center and are demonstrated to be useful reporters of its modulation in the different states. Unrestricted Kohn-Sham density functional theory calculations performed on a model of the Cu site of Pc confirm the observed dependence. IR spectroscopy was then applied to characterize the impact of binding to the physiological redox partner cytochrome (cyt) f. The spectral changes suggest a slightly stronger Cu-S(Met97) interaction in the complex with cyt f that has potential to modulate the electron transfer properties. Besides providing direct, molecular-level comparison of the oxidized and reduced states of Pc from the perspective of the axial Met ligand and evidence for perturbation of the Cu site properties by redox partner binding, this study demonstrates the localized spatial information afforded by IR spectroscopy of selectively incorporated C-D probes.

A new general method of controlling the chemical reactivity of molecules is proposed. Instead of tuning the electronic properties of reactive molecules by employing functional groups that impose different inductive effects to a molecule, we propose to immobilize the parent molecule onto an electrode, for example by installing a thiol group to the molecule and forming a self-assembled monolayer on a gold surface. By applying a voltage the electronic property of the immobilized molecule can be tuned, as is commonly done by decorating the molecule with electron-donating and electron-withdrawing groups. As a proof of principle, it is shown that the base-catalyzed saponification of benzoic ester can be shut down completely by applying a negative voltage, while it can be accelerated by using a positive voltage. Furthermore it is shown that the Suzuki-Miyaura cross-coupling reaction can be affected by the voltage when the arylhalide substrate of the reaction is immobilized on a gold electrode.