Selective Excitation 1D-NMR Experiments for the Assignment of the Absolute Configuration of Secondary Alcohols

Routine selective excitation experiments, easy to set up on modern NMR spectrometers, allow for the determination of the absolute configuration of chiral secondary alcohols by double derivatization directly in the NMR tube. As a general method, TOCSY1D with selective excitation of the α proton in the MPA esters and with a short mixing time reveals only the nearby protons in the coupling network. Typically, the analysis takes less than 30 min. A longer mixing time, selective excitation of other signals, or NOESY1D experiments can be used for measuring Δδ<i>RS</i> of other protons

Complex Hydroindoles by an Intramolecular Nitrile-Intercepted Allylic Alkylation Cascade Reaction

Bisnucleophilic reagents derived from malononitrile, ketones, benzaldehydes, and nitromethane can react with bisallylic electrophiles via a nitrile-intercepted allylic alkylation cascade reaction to yield complex hydroindole architectures. Also noteworthy is that the only stoichiometric byproducts from the preparation and reaction of the bisnucleophile and biselectrophile are water, acetic acid, and bicarbonate, making it a potentially “green” platform for multistep complex molecule synthesis. These scaffolds can be converted into hydrooxindoles by a unique olefin isomerization followed by Witkop–Winterfeldt-like oxidation

Complex Hydroindoles by an Intramolecular Nitrile-Intercepted Allylic Alkylation Cascade Reaction

Bisnucleophilic reagents derived from malononitrile, ketones, benzaldehydes, and nitromethane can react with bisallylic electrophiles via a nitrile-intercepted allylic alkylation cascade reaction to yield complex hydroindole architectures. Also noteworthy is that the only stoichiometric byproducts from the preparation and reaction of the bisnucleophile and biselectrophile are water, acetic acid, and bicarbonate, making it a potentially “green” platform for multistep complex molecule synthesis. These scaffolds can be converted into hydrooxindoles by a unique olefin isomerization followed by Witkop–Winterfeldt-like oxidation

¹⁹F NMR Characterization of the Encapsulation of Emerging Perfluoroethercarboxylic Acids by Cyclodextrins

Legacy perfluoroalkyl substances (PFASs) are known environmental pollutants with serious adverse health effects. Perfluoroethercarboxylic acids (PFECAs), emerging PFASs now being substituted for legacy PFASs, have recently been detected in the environment. Cyclodextrins (CDs) have been proposed as agents for the remediation of problematic pollutants, including legacy PFASs. The current study uses ¹⁹F NMR spectroscopy to measure the complexation of mono-, di-, and triether PFECAs by CDs for eventual environmental applications. Eight PFECAs were characterized by ¹⁹F and ¹³C NMR. The change in chemical shift of individual fluorines upon complexation of CDs at various stoichiometric ratios was used to determine the host–guest association constants. All studied PFECAs were most strongly encapsulated by β-CD, with association constants from 10²–10⁵ M^–1 depending on chain length and number of ether functionalities. ¹⁹F–¹H heteronuclear Overhauser effect spectroscopy (HOESY) NMR experiments were performed for the β-CD complexes of two branched monoethers, PFPrOPrA (“GenX”) and PFDMMOBA, to elucidate the structural details of the complexes, determine the specific orientation, and position of β-CD along the PFAS chain, and assess the roles of hydrogen-bonding and PFECA branching on the host–guest interactions. The results give new understanding into the fundamental nature of the host–guest complex between cyclodextrins and perfluorinated surfactants

A New ONO^3‑ Trianionic Pincer-Type Ligand for Generating Highly Nucleophilic Metal–Carbon Multiple Bonds

Appending an amine to a CC double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF₃–ONO^3‑ trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten–alkylidene and −alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF₃–ONO^3‑ trianionic pincer tungsten–alkylidene [CF₃–ONO]­WCH­(Et)­(O^<i>t</i>Bu) (<b>2</b>) and −alkylidyne {MePPh₃}­{[CF₃–ONO]­WC­(Et)­(O^<i>t</i>Bu)} (<b>3</b>) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes <b>2</b> and <b>3</b>. Exhibiting unique nucleophilic reactivity, <b>3</b> reacts with MeOTf to yield [CF₃–ONO]­WC­(Me)­(Et)­(O^<i>t</i>Bu) (<b>4</b>), but the bulkier Me₃SiOTf silylates the <i>tert</i>-butoxide, which subsequently undergoes isobutylene expulsion to form [CF₃–ONO]­WCH­(Et)­(OSiMe₃) (<b>5</b>). A DFT calculation performed on a model complex of <b>3</b>, namely, [CF₃–ONO]­WC­(Et)­(O^<i>t</i>Bu) (<b>3</b>′), reveals the amide participates in an enamine-type bonding combination. For complex <b>2</b>, the Lewis acids MeOTf, Me₃SiOTf, and B­(C₆F₅)₃ catalyze isobutylene expulsion to yield the tungsten–oxo complex [CF₃–ONO]­W­(O)­(^<i>n</i>Pr) (<b>6</b>)

Enlightening the Well-Controlled Photochemical Behavior of 1,1-Dicyanomethylene-3-Indanone-Functionalized π‑Conjugated Molecules

1,1-Dicyanomethylene-3-indanone (INCN) is a popular electron acceptor showcased in hundreds of push–pull oligomers, including some of the best nonfullerene acceptor (NFA) materials used in small molecule-based bulk-heterojunction (BHJ) organic photovoltaics (OPVs). Consequences of the configuration (i.e., Z or E) and conformation (i.e., s-cis or s-trans) of the exocyclic olefin that conjugates INCN to π-conjugated molecules have largely been ignored. Two recent reports have implicated Z/E photoisomerization in the photodegradation of popular NFAs like IT-4F when subjected to broad spectrum irradiation. Here, we elucidate through experiments and complementary ground- and excited-state computations the photochemical behavior of a family of eight INCN-functionalized donor–acceptor molecules varying in aryl and heteroaryl substitution, alkyl group substitution, and halogen functionalization on the INCN unit. Well-controlled Z/E photoisomerization using selective wavelengths of excitation spanning the ultraviolet and visible regions is observed in all cases yielding a range of Z/E photostationary state (PSS) distributions with no evidence of a previously reported photooxidation. Z/E photoisomerization followed by sequential pericyclic reactions, consistent with one recent literature report, is identified for just one target molecule upon irradiation at 454 nm. The alkyl group positioning on the thiophene ring neighboring the INCN is found to bias the conformational preferences of the target molecules and modulate access to this reaction pathway. All eight molecules undergo facile Z/E photoswitching over numerous cycles upon selective excitation. Overall, the work reveals the well-controlled photochemical behavior of INCN-functionalized π-systems and encourages their use in the design of future functional and organic materials and photoswitches

Synthesis and Characterization of Group 4 Trianionic ONO^3– Pincer-Type Ligand Complexes and a Rare Case of Through-Space ¹⁹F–¹⁹F Coupling

This report describes the synthesis and characterization of a new series of group 4 complexes supported by a trianionic ONO^3– pincer-type ligand. Treating TiCl₄ with the proligand [CF₃–ONO]­H₃ (<b>1</b>) and NEt₃ in benzene afforded {[CF₃–ONO]­TiCl₃}­{HNEt₃}₂ (<b>2</b>). By means of a lithium transmetalation route, the neutral monochloride complex [CF₃–ONO]­TiCl­(THF) (<b>3</b>) was synthesized in 91% yield. The analogous Hf­(IV) derivative could not be obtained using this method. Instead, transmetalation with thallium­(I) resulted in the formation of the seven-coordinate complex [CF₃–ONHO]­HfCl₂(THF)₂ (<b>4-(THF)</b>_<b>2</b>), which was characterized by combustion analysis and X-ray crystallography. Applying vacuum to <b>4-(THF)</b>_<b>2</b> liberated the THF ligands to provide the five-coordinate THF-free complex [CF₃–ONHO]­HfCl₂ (<b>4</b>). Alkylation of complex <b>4</b> with alkyllithium or Grignard reagents resulted in a mixture of unidentifiable products. However, access to the neutral complex <b>3</b> enabled the subsequent preparation of organotitanium complexes [CF₃–ONO]­TiR­(THF) (<b>5-R</b>; R = Me, Bn, Mes). Single-crystal X-ray analysis of <b>5-Me</b> indicated that the organotitanium complexes are mononuclear. Single-crystal X-ray diffraction and NMR studies in solution confirmed that complex <b>5-Mes</b> exhibits rare through-space ¹⁹F–¹⁹F coupling (5 Hz)

Modeling Biological Copper Clusters: Synthesis of a Tricopper Complex, and Its Chloride- and Sulfide-Bridged Congeners

The synthesis and characterization of a family of tricopper clusters housed within a tris­(β-diketimine) cyclophane ligand (H₃<b>L</b>) that bear structural similarities to biological copper clusters are reported. In all complexes, each Cu atom is held within the N₂-chelate of a single β-diketiminate arm. Reaction of <b>L</b>^3– with CuCl affords an anionic complex containing a μ₃-chloride donor in the central cavity, whereas there is no evidence for bromide incorporation in the product of the reaction of <b>L</b>^3– with CuBr (Cu₃<b>L</b>). Cu₃<b>L</b> reacts with elemental sulfur to generate the corresponding air-stable mixed-valent (μ₃-sulfido)­tricopper complex, Cu₃­(μ₃-S)<b>L</b>, which represents the first example of a sulfide-bridged copper cluster in which each metal center is both coordinatively unsaturated and held within a N-rich environment. The calculated LUMO is predominantly Cu–S π* in character and delocalized over all three metal centers, which is consistent with the isotropic ten-line absorption (<i>g</i> ∼ 2.095, <i>A</i> ∼ 33 G) observed at room temperature in EPR spectra of the one-electron chemically reduced complex, [Cu₃­(μ₃-S)<b>L</b>]⁻

Synthesis and Characterization of Tungsten Alkylidene and Alkylidyne Complexes Supported by a New Pyrrolide-Centered Trianionic ONO^3– Pincer-Type Ligand

Synthetic protocols for a pyrrolide-centered ONO^3– trianionic pincer-type ligand are presented. Treating (^<i>t</i>BuO)₃WC^<i>t</i>Bu with the proligand [pyr-ONO]­H₃ (<b>2</b>) results in the formation of the trianionic pincer alkylidene complex [pyr-ONO]­WCH^<i>t</i>Bu­(O^<i>t</i>Bu) (<b>3</b>). Addition of a mild base to complex <b>3</b> provides the trianionic pincer alkylidyne complex {MePPh₃}­{[pyr-ONO]­WC^<i>t</i>Bu­(O^<i>t</i>Bu)} (<b>4</b>). All new compounds were characterized by NMR spectroscopy, combustion analysis, and, in the case of complex <b>4</b>, single-crystal X-ray crystallography. DFT calculations performed on <b>4</b> provide insight into its electronic structure and indicate that the HOMO is ligand-based and localized on the pyrrolide π orbitals

Fast “Wittig-Like” Reactions As a Consequence of the Inorganic Enamine Effect

The tungsten alkylidyne [CF₃–ONO]­WCC­(CH₃)₃(THF)₂ (<b>3</b>) {where CF₃–ONO = (MeC₆H₃[C­(CF₃)₂O])₂N^3–} supported by a trianionic pincer-type ligand demonstrates enhanced nucleophilicity in unusually fast “Wittig-like” reactions. Experiments are designed to provide support for an inorganic enamine effect that is the origin of the enhanced nucleophilicity. Treating complex <b>3</b> with various carbonyl-containing substrates provides tungsten-oxo-vinyl complexes upon oxygen atom transfer. The rates of reactivity of <b>3</b> are compared with the known alkylidyne (DIPP)₃WCC­(CH₃)₃ (DIPP = 2,6-diisopropylphenoxide). In all cases (except acetone), complex <b>3</b> exhibits significantly faster overall rates than (DIPP)₃WCC­(CH₃)₃. New oxo-vinyl complexes are characterized by NMR, combustion analysis and single crystal X-ray diffraction. Treating <b>3</b> with acid chlorides provides the tungsten oxo chloride species [CF₃–ONO]­W­(O)Cl (<b>4</b>) and disubstituted alkynes. In the case of acetone the oxo-vinyl complex results in two rotational isomers <b>10</b>_{<i><b>syn</b></i>} and <b>10</b>_{<i><b>anti</b></i>.} The rate of isomerization was determined for the forward and reverse directions and was complimented with DFT calculations