Understanding Relationships between Free Volume and Oxygen Absorption in Ionic Liquids.

Understanding the factors that govern gas absorption in ionic liquids is critical to the development of high-capacity solvents for catalysis, electrochemistry, and gas separations. Here, we report experimental probes of liquid structure that provide insights into how free volume impacts the O2 absorption properties of ionic liquids. Specifically, we establish that isothermal compressibility─measured rapidly and accurately through small-angle X-ray scattering─reports on the size distribution of transient voids within a representative series of ionic liquids and is correlated with O2 absorption capacity. Additionally, O2 absorption capacities are correlated with thermal expansion coefficients, reflecting the beneficial effect of weak intermolecular interactions in ionic liquids on free volume and gas absorption capacity. Molecular dynamics simulations show that the void size distribution─in particular, the probability of forming larger voids within an ionic liquid─has a greater impact on O2 absorption than the total free volume. These results establish relationships between the ionic liquid structure and gas absorption properties that offer design strategies for ionic liquids with high gas solubilities.

Metal-Organic Phase-Change Materials for Thermal Energy Storage.

The development of materials that reversibly store high densities of thermal energy is critical to the more efficient and sustainable utilization of energy. Herein, we investigate metal-organic compounds as a new class of solid-liquid phase-change materials (PCMs) for thermal energy storage. Specifically, we show that isostructural series of divalent metal amide complexes featuring extended hydrogen bond networks can undergo tunable, high-enthalpy melting transitions over a wide temperature range. Moreover, these coordination compounds provide a powerful platform to explore the specific factors that contribute to the energy density and entropy of metal-organic PCMs. Through a systematic analysis of the structural and thermochemical properties of these compounds, we investigated the influence of coordination bonds, hydrogen-bond networks, neutral organic ligands, and outer-sphere anions on their phase-change thermodynamics. In particular, we identify the importance of high densities of coordination bonds and hydrogen bonds to achieving a high PCM energy density, and we show how metal-dependent changes to the local coordination environment during melting impact the entropy and enthalpy of metal-organic PCMs. These results highlight the potential of manipulating order-disorder phase transitions in metal-organic materials for thermal energy storage.

Ni-Catalyzed Regioselective Alkylarylation of Vinylarenes via C(sp3)-C(sp3)/C(sp3)-C(sp2) Bond Formation and Mechanistic Studies.

We report a Ni-catalyzed regioselective alkylarylation of vinylarenes with alkyl halides and arylzinc reagents to generate 1,1-diarylalkanes. The reaction proceeds well with primary, secondary and tertiary alkyl halides, and electronically diverse arylzinc reagents. Mechanistic investigations by radical probes, competition studies and quantitative kinetics reveal that the current reaction proceeds via a Ni(0)/Ni(I)/Ni(II) catalytic cycle by a rate-limiting direct halogen atom abstraction via single electron transfer to alkyl halides by a Ni(0)-catalyst.

Ni-Catalyzed Regioselective ß,δ-Diarylation of Unactivated Olefins in Ketimines via Ligand-Enabled Contraction of Transient Nickellacycles: Rapid Access to Remotely Diarylated Ketones.

We disclose a [(PhO)3P]/NiBr2-catalyzed regioselective ß,δ-diarylation of unactivated olefins in ketimines with aryl halides and arylzinc reagents. This diarylation proceeds at remote locations to the carbonyl group to afford, after simple H+ workup, diversely substituted ß,δ-diarylketones that are otherwise difficult to access readily with existing methods. Deuterium-labeling and crossover experiments indicate that diarylation proceeds by ligand-enabled contraction of transient nickellacycles.

Ni-catalysed regioselective 1,2-diarylation of unactivated olefins by stabilizing Heck intermediates as pyridylsilyl-coordinated transient metallacycles.

We report a Ni-catalysed diarylation of unactivated olefins in dimethylpyridylvinylsilane by intercepting Heck C(sp3)-NiX intermediates, derived from aryl halides, with arylzinc reagents. This approach utilizes a modifiable pyridylsilyl moiety as a coordinating group that plays a dual role of intercepting oxidative addition species to promote Heck carbometallation, and stabilizing the Heck C(sp3)-NiX intermediates as transient metallacycles to suppress ß-hydride elimination, and facilitate transmetalation/reductive elimination. This method affords 1,2-diarylethylsilanes, which can be readily oxidized to 1,2-diarylethanols that occur as structural motifs in 3-aryl-3,4-dihydroisocoumarin and dihydrostilbenoid natural products.

Ni-Catalyzed Regioselective Dicarbofunctionalization of Unactivated Olefins by Tandem Cyclization/Cross-Coupling and Application to the Concise Synthesis of Lignan Natural Products.

We disclose a (terpy)NiBr2-catalyzed reaction protocol that regioselectively difunctionalizes unactivated olefins with tethered alkyl halides and arylzinc reagents. The reaction shows an excellent functional group tolerance (such as ketones, esters, nitriles, and halides) and a moderate to good level of diastereoselectivity. The current cyclization/cross-coupling also tolerates molecules containing base-sensitive racemizable stereocenters, which are preserved without racemization during the reaction. This cyclization/cross-coupling provides a rapid access to (arylmethyl)carbo- and heterocyclic scaffolds, which occur widely as structural cores in various natural products and bioactive molecules. In order to show synthetic utility and generality, we have applied this new method in gram-scale quantities to the concise synthesis of six lignan natural products containing three different structural frameworks. We further conducted mechanistic investigations with radical probes and selectivity studies, which indicated that the current reaction proceeds via a single electron transfer (SET) process.

Ni-Catalyzed Regioselective 1,2-Dicarbofunctionalization of Olefins by Intercepting Heck Intermediates as Imine-Stabilized Transient Metallacycles.

We disclose a strategy for Ni-catalyzed dicarbofunctionalization of olefins in styrenes by intercepting Heck C(sp3)-NiX intermediates with arylzinc reagents. This approach utilizes a readily removable imine as a coordinating group that plays a dual role of intercepting oxidative addition species derived from aryl halides and triflates to promote Heck carbometalation and stabilizing the Heck C(sp3)-NiX intermediates as transient metallacycles to suppress ß-hydride elimination and facilitate transmetalation/reductive elimination steps. This method affords diversely substituted 1,1,2-triarylethyl products that occur as structural motifs in various natural products.

Copper-Catalyzed Dicarbofunctionalization of Unactivated Olefins by Tandem Cyclization/Cross-Coupling.

We present a strategy that difunctionalizes unactivated olefins in 1,2-positions with two carbon-based entities. This method utilizes alkyl/arylzinc reagents derived from olefin-tethered alkyl/aryl halides that undergo radical cyclization to generate C(sp3)-Cu complexes in situ, which are intercepted with aryl and heteroaryl iodides. A variety of (arylmethyl)carbo- and heterocycles (N, O) can be synthesized with this new method.

The copper-catalysed Suzuki-Miyaura coupling of alkylboron reagents: disproportionation of anionic (alkyl)(alkoxy)borates to anionic dialkylborates prior to transmetalation.

We report the first example of Cu(I)-catalysed coupling of alkylboron reagents with aryl and heteroaryl iodides that affords products in good to excellent yields. Preliminary mechanistic studies with alkylborates indicate that the anionic (alkoxy)(alkyl)borates, generated from alkyllithium and alkoxyboron reagents, undergo disproportionation to anionic dialkylborates and that both anionic alkylborates are active for transmetalation to a Cu(I)-catalyst. Results from a radical clock experiment and the Hammett plot imply that the reaction likely proceeds via a non-radical pathway.

General Copper-Catalyzed Coupling of Alkyl-, Aryl-, and Alkynylaluminum Reagents with Organohalides.

We report the first example of a very general Cu-catalyzed cross-coupling of organoaluminum reagents with organohalides. The reactions proceed for the couplings of alkyl-, aryl-, and alkynylaluminum reagents with aryl and heteroaryl halides and vinyl bromides, affording the cross-coupled products in good to excellent yields. Both primary and secondary alkylaluminum reagents can be utilized as organometallic coupling partners. These reactions are not complicated by ß-hydride elimination, and as a result rearranged products are not observed with secondary alkylaluminum reagents even for couplings with heteroaryl halides under "ligand-free" conditions. Radical clock experiment with a radical probe and relative reactivity study of Ph3Al with two haloarenes, 1-bromonaphthalene and 4-chlorobenzonitrile, having two different redox potentials indicates that the reaction does not involve free aryl radicals and radical anions as intermediates. These results combined with the result of the Hammett plot obtained by reacting Ph3Al with iodoarenes containing p-H, p-Me, p-F, and p-CF3 substituents, which shows a linear curve (R(2) = 0.99) with a ρ value of +1.06, suggest that the current transformation follows an oxidative addition-reductive elimination pathway.

Ligand-Free Copper-Catalyzed Negishi Coupling of Alkyl-, Aryl-, and Alkynylzinc Reagents with Heteroaryl Iodides.

Reported herein is an unprecedented ligand-free copper-catalyzed cross-coupling of alkyl-, aryl-, and alkynylzinc reagents with heteroaryl iodides. The reaction proceeds at room temperature for the coupling of primary, secondary, and tertiary alkylzinc reagents with heteroaryl iodides without rearrangement. An elevated temperature (100 °C) is required for aryl-heteroaryl and alkynyl-heteroaryl couplings.

Copper-catalysed cross-coupling: an untapped potential.

Copper is emerging as a viable catalytic metal for cross-coupling reactions to construct carbon-carbon (C-C) bonds. Recent revelations that Cu-catalysts can execute with high efficacy the cross-couplings of a variety of organometallic reagents, including organomagnesium, organoboron, organosilicon, organoindium and organomanganese, with alkyl, aryl and heteroaryl halides clearly demonstrate the versatility of Cu-based catalytic systems in conducting these reactions. In addition, Cu-catalysts are exhibiting a unique reactivity pattern that allows ligandless cross-coupling for aryl-heteroaryl and heteroaryl-heteroaryl bond formation, a transformation that generally requires special custom-designed ligands with Pd-catalysts. This review summarises early discoveries and subsequent advancements made in the area of Cu-catalysed cross-couplings of organometallic reagents with organohalides to form C-C bonds.

Copper-catalysed cross-coupling of arylzirconium reagents with aryl and heteroaryl iodides.

An unprecedented Cu(I)-catalysed cross-coupling of arylzirconium reagents with aryl and heteroaryl iodides is reported. Mechanistic studies with a Cp2ZrAr2 complex revealed that Cp2Zr(Ar)(Cl) is the reactive species that undergoes transmetalation with (PN-1)CuI. In addition, experiments with radical probes indicated that the reaction proceeds via a non-radical pathway.

Copper-catalyzed coupling of triaryl- and trialkylindium reagents with aryl iodides and bromides through consecutive transmetalations.

An efficient copper(I)-catalyzed coupling of triaryl and trialkylindium reagents with aryl iodides and bromides is reported. The reaction proceeds at low catalyst loadings (2 mol%) and generally only requires 0.33 equivalents of the triorganoindium reagent with respect to the aryl halide as all three organic nucleophilic moieties of the reagent are transferred to the products through consecutive transmetalations. The reaction tolerates a variety of functional groups and sterically hindered substrates. Furthermore, preliminary mechanistic studies that entailed the synthesis and characterization of potential reaction intermediates offered a glimpse of the elementary steps that constitute the catalytic cycle.

Copper-catalyzed Suzuki-Miyaura coupling of arylboronate esters: transmetalation with (PN)CuF and identification of intermediates.

An efficient Cu(I)-catalyzed Suzuki-Miyaura reaction was developed for the coupling of aryl- and heteroarylboronate esters with aryl and heteroaryl iodides at low catalyst loadings (2 mol %). The reaction proceeds under ligand-free conditions for aryl-heteroaryl and heteroaryl-heteroaryl couplings. We also conducted the first detailed mechanistic studies by synthesizing [(PN-2)CuI]2, [(PN-2)CuF]2, and (PN-2)CuPh (PN-2 = o-(di-tert-butylphosphino)-N,N-dimethylaniline) and demonstrated that [(PN-2)CuF]2 is the species that undergoes transmetalation with arylboronate esters.

Copper-catalyzed Hiyama coupling of (hetero)aryltriethoxysilanes with (hetero)aryl iodides.

A Cu(I)-catalyzed Hiyama coupling was achieved, which proceeds in the absence of an ancillary ligand for aryl-heteroaryl and heteroaryl-heteroaryl couplings. A P,N-ligand is required to obtain the best product yields for aryl-aryl couplings. In addition to facilitating transmetalation, CsF is also found to function as a stabilizer of the [CuAr] species, potentially generated as an intermediate after transmetalation of aryltriethoxysilanes with Cu(I)-catalysts in the absence of ancillary ligands.

3,4-Di-amino-pyridinium hydrogen malonate.

In the title salt, C5H8N3 (+)·C3H3O4 (-), the 3,4-di-amino-pyridinium cation is almost planar, with an r.m.s. deviation of 0.02 Å. The conformation of the hydrogen malonate anion is stabilized by an intra-molecular O-H⋯O hydrogen bond, which generates an S(6) ring. In the crystal, N-H⋯O hydrogen bonds link cations and anions into layers parallel to the ab plane.