Rare-Earth Metalation of Calix[4]pyrrole/Calix[4]arene Free of Alkali-Metal Companions

The redox transmetalation/protolysis (RTP) reactions of ytterbium or neodymium metal with calix[4]­H₄ (5,11,17,23-tetra-<i>tert</i>-butylcalix­[4]­arene-25,26,27,28-tetrol) in the presence of bis­(pentafluorophenyl)mercury under ultrasonication yielded [Ln^III(calix­[4]­H)­(thf)]₂ (<b>1</b>, Ln = Yb; <b>2</b>, Ln = Nd). The characterization of both <b>1</b> and <b>2</b>, including an X-ray single-crystal structural determination for <b>2</b>, suggests triple deprotonation of the macrocyclic ligand on metalation. The related RTP reaction of H₄N₄Et₈ (<i>meso</i>-octaethylcalix­[4]­pyrrole) with ytterbium metal and Hg­(C₆F₅)₂ at ambient temperature, however, resulted in quadruple deprotonation and afforded the ytterbium­(II) calix[4]­pyrrolide complex [Yb₂(N₄Et₈)­(thf)₄] (<b>3</b>) in good yield. Subsequent oxidation of <b>3</b> by dioxygen generated the novel tetranuclear ytterbium­(III) complex [Yb₄(μ-O)₂(N₄Et₈)₂(thf)₂] (<b>4</b>). The structures of the ytterbium­(II) complex <b>3</b> and the ytterbium­(III) complex <b>4</b> incorporate endo <b>(3)</b> and endo/exo (<b>4</b>) pyrrolide sandwich and half-sandwich units, respectively, with metal centers η¹ bound by nitrogen and η⁵ bonded by pyrrolide rings. The RTP reaction of lanthanum metal using diphenylmercury in place of bis­(pentafluorophenyl)mercury gave the triply deprotonated and N-confused pyrrolide (with an alkyl substituent of one pyrrolide ring migrated to a β-position) macrocyclic complex [La₂(HN₃N′Et₈)₂] (<b>5</b>). The triple deprotonation of the macrocyclic ligand H₄N₄Et₈ was also achieved through its reaction with 3 molar equiv of potassium metal, giving colorless crystals of [{K₃(HN₄Et₈)­(thf)­(PhMe)₂}_n] (<b>6</b>). However, an attempt to isolate the corresponding partially deprotonated calix[4]­pyrrolide ytterbium­(III) complex through the metathesis reaction of potassium precursor <b>6</b> with ytterbium triiodide was unsuccessful

Rare-Earth Metalation of Calix[4]pyrrole/Calix[4]arene Free of Alkali-Metal Companions

The redox transmetalation/protolysis (RTP) reactions of ytterbium or neodymium metal with calix[4]­H₄ (5,11,17,23-tetra-<i>tert</i>-butylcalix­[4]­arene-25,26,27,28-tetrol) in the presence of bis­(pentafluorophenyl)mercury under ultrasonication yielded [Ln^III(calix­[4]­H)­(thf)]₂ (<b>1</b>, Ln = Yb; <b>2</b>, Ln = Nd). The characterization of both <b>1</b> and <b>2</b>, including an X-ray single-crystal structural determination for <b>2</b>, suggests triple deprotonation of the macrocyclic ligand on metalation. The related RTP reaction of H₄N₄Et₈ (<i>meso</i>-octaethylcalix­[4]­pyrrole) with ytterbium metal and Hg­(C₆F₅)₂ at ambient temperature, however, resulted in quadruple deprotonation and afforded the ytterbium­(II) calix[4]­pyrrolide complex [Yb₂(N₄Et₈)­(thf)₄] (<b>3</b>) in good yield. Subsequent oxidation of <b>3</b> by dioxygen generated the novel tetranuclear ytterbium­(III) complex [Yb₄(μ-O)₂(N₄Et₈)₂(thf)₂] (<b>4</b>). The structures of the ytterbium­(II) complex <b>3</b> and the ytterbium­(III) complex <b>4</b> incorporate endo <b>(3)</b> and endo/exo (<b>4</b>) pyrrolide sandwich and half-sandwich units, respectively, with metal centers η¹ bound by nitrogen and η⁵ bonded by pyrrolide rings. The RTP reaction of lanthanum metal using diphenylmercury in place of bis­(pentafluorophenyl)mercury gave the triply deprotonated and N-confused pyrrolide (with an alkyl substituent of one pyrrolide ring migrated to a β-position) macrocyclic complex [La₂(HN₃N′Et₈)₂] (<b>5</b>). The triple deprotonation of the macrocyclic ligand H₄N₄Et₈ was also achieved through its reaction with 3 molar equiv of potassium metal, giving colorless crystals of [{K₃(HN₄Et₈)­(thf)­(PhMe)₂}_n] (<b>6</b>). However, an attempt to isolate the corresponding partially deprotonated calix[4]­pyrrolide ytterbium­(III) complex through the metathesis reaction of potassium precursor <b>6</b> with ytterbium triiodide was unsuccessful

A Rapid Aza-Bicycle Synthesis from Dendralenes and Imines

The diene-transmissive 2-fold Diels–Alder sequence between carbon-based dienophiles and [3]­dendralenes is becoming an established method for polycarbocycle synthesis. Here, we demonstrate for the first time that imines are competent participants in intermolecular formal [4 + 2] cycloadditions with dendralenes. After a second Diels–Alder process with a carbadienophile, hexahydro- and octahydro-isoquinoline structures are formed. The formal aza-Diels–Alder reaction, which requires Lewis acid promotion, proceeds in high regio- and stereoselectivity under optimized conditions. ωB97XD/Def2-TZVP//M06-2X/6-31+G­(d,p) calculations reveal a stepwise ionic mechanism for the formal aza-dienophile cycloadditions and also explain an unexpected Z → E olefin isomerization of a non-reacting CC bond in the first formal cycloaddition

Brønsted Acid Cocatalysis in Copper(I)-Photocatalyzed α‑Amino C–H Bond Functionalization

We have exploited a bis­(1,10-phenanthroline)­copper­(I) visible light photocatalyst (VLP), [Cu­(dap)₂]⁺, to effect the direct α-C–H functionalization of amines. To our knowledge, this represents the first example of the oxidation of amines that are ultimately incorporated into synthetic targets by a copper­(I) VLP. We have utilized this approach to rapidly prepare unprecedented octahydroisoquinolino­[2,1-<i>a</i>]­pyrrolo­[3,4-<i>c</i>]­quinoline frameworks and exploited this process to synthesize a novel aglycone analogue of the natural product incargranine B. Most significantly, our studies suggest that the presence of trifluoroacetic acid (TFA) is crucial in mediating the aerobic oxidative quenching of a putative photoexcited copper­(I) species involved in the catalytic cycle

Rare-Earth Metalation of Calix[4]pyrrole/Calix[4]arene Free of Alkali-Metal Companions

The redox transmetalation/protolysis (RTP) reactions of ytterbium or neodymium metal with calix[4]­H₄ (5,11,17,23-tetra-<i>tert</i>-butylcalix­[4]­arene-25,26,27,28-tetrol) in the presence of bis­(pentafluorophenyl)mercury under ultrasonication yielded [Ln^III(calix­[4]­H)­(thf)]₂ (<b>1</b>, Ln = Yb; <b>2</b>, Ln = Nd). The characterization of both <b>1</b> and <b>2</b>, including an X-ray single-crystal structural determination for <b>2</b>, suggests triple deprotonation of the macrocyclic ligand on metalation. The related RTP reaction of H₄N₄Et₈ (<i>meso</i>-octaethylcalix­[4]­pyrrole) with ytterbium metal and Hg­(C₆F₅)₂ at ambient temperature, however, resulted in quadruple deprotonation and afforded the ytterbium­(II) calix[4]­pyrrolide complex [Yb₂(N₄Et₈)­(thf)₄] (<b>3</b>) in good yield. Subsequent oxidation of <b>3</b> by dioxygen generated the novel tetranuclear ytterbium­(III) complex [Yb₄(μ-O)₂(N₄Et₈)₂(thf)₂] (<b>4</b>). The structures of the ytterbium­(II) complex <b>3</b> and the ytterbium­(III) complex <b>4</b> incorporate endo <b>(3)</b> and endo/exo (<b>4</b>) pyrrolide sandwich and half-sandwich units, respectively, with metal centers η¹ bound by nitrogen and η⁵ bonded by pyrrolide rings. The RTP reaction of lanthanum metal using diphenylmercury in place of bis­(pentafluorophenyl)mercury gave the triply deprotonated and N-confused pyrrolide (with an alkyl substituent of one pyrrolide ring migrated to a β-position) macrocyclic complex [La₂(HN₃N′Et₈)₂] (<b>5</b>). The triple deprotonation of the macrocyclic ligand H₄N₄Et₈ was also achieved through its reaction with 3 molar equiv of potassium metal, giving colorless crystals of [{K₃(HN₄Et₈)­(thf)­(PhMe)₂}_n] (<b>6</b>). However, an attempt to isolate the corresponding partially deprotonated calix[4]­pyrrolide ytterbium­(III) complex through the metathesis reaction of potassium precursor <b>6</b> with ytterbium triiodide was unsuccessful

Rare-Earth Metalation of Calix[4]pyrrole/Calix[4]arene Free of Alkali-Metal Companions

The redox transmetalation/protolysis (RTP) reactions of ytterbium or neodymium metal with calix[4]­H₄ (5,11,17,23-tetra-<i>tert</i>-butylcalix­[4]­arene-25,26,27,28-tetrol) in the presence of bis­(pentafluorophenyl)mercury under ultrasonication yielded [Ln^III(calix­[4]­H)­(thf)]₂ (<b>1</b>, Ln = Yb; <b>2</b>, Ln = Nd). The characterization of both <b>1</b> and <b>2</b>, including an X-ray single-crystal structural determination for <b>2</b>, suggests triple deprotonation of the macrocyclic ligand on metalation. The related RTP reaction of H₄N₄Et₈ (<i>meso</i>-octaethylcalix­[4]­pyrrole) with ytterbium metal and Hg­(C₆F₅)₂ at ambient temperature, however, resulted in quadruple deprotonation and afforded the ytterbium­(II) calix[4]­pyrrolide complex [Yb₂(N₄Et₈)­(thf)₄] (<b>3</b>) in good yield. Subsequent oxidation of <b>3</b> by dioxygen generated the novel tetranuclear ytterbium­(III) complex [Yb₄(μ-O)₂(N₄Et₈)₂(thf)₂] (<b>4</b>). The structures of the ytterbium­(II) complex <b>3</b> and the ytterbium­(III) complex <b>4</b> incorporate endo <b>(3)</b> and endo/exo (<b>4</b>) pyrrolide sandwich and half-sandwich units, respectively, with metal centers η¹ bound by nitrogen and η⁵ bonded by pyrrolide rings. The RTP reaction of lanthanum metal using diphenylmercury in place of bis­(pentafluorophenyl)mercury gave the triply deprotonated and N-confused pyrrolide (with an alkyl substituent of one pyrrolide ring migrated to a β-position) macrocyclic complex [La₂(HN₃N′Et₈)₂] (<b>5</b>). The triple deprotonation of the macrocyclic ligand H₄N₄Et₈ was also achieved through its reaction with 3 molar equiv of potassium metal, giving colorless crystals of [{K₃(HN₄Et₈)­(thf)­(PhMe)₂}_n] (<b>6</b>). However, an attempt to isolate the corresponding partially deprotonated calix[4]­pyrrolide ytterbium­(III) complex through the metathesis reaction of potassium precursor <b>6</b> with ytterbium triiodide was unsuccessful

The Diastereoselective Synthesis of Pyrroloindolines by Pd-Catalyzed Dearomative Cycloaddition of 1-Tosyl-2-vinylaziridine to 3‑Nitroindoles

An efficient, diastereoselective synthesis of densely functionalized pyrroloindolines is reported. The reaction proceeds via cycloaddition of a vinylaziridine-derived Pd-stabilized 1,3-dipole to electron-deficient 3-nitroindoles. The reactions give the <i>trans</i> diastereoisomer with high selectivity; however, when a 4-substituent is present on the indole ring, a reversal of diastereoselectivity is observed

Pd-Catalyzed Dearomative [3 + 2] Cycloaddition of 3‑Nitroindoles with 2‑Vinylcyclopropane-1,1-dicarboxylates

A <i>trans</i>-diastereoselective Pd-catalyzed dearomative [3 + 2] cycloaddition between vinylcyclopropane dicarboxylates and 3-nitroindoles has been developed. The reaction provides densely functionalized cyclopenta­[<i>b</i>]­indolines with versatile vinyl and nitro-groups. The addition of a halide additive was found to be critical for the diastereoselectivity of the reaction, which is proposed to be a result of a rapid π-σ-π interconversion between the intermediates allowing for Curtin–Hammett control. A switch in diastereoselectivity to afford products with the vinyl and nitro groups <i>cis</i> to each other is observed with a 4-substituted 3-nitroindole

New Chemistry from an Old Reagent: Mono- and Dinuclear Macrocyclic Uranium(III) Complexes from [U(BH₄)₃(THF)₂]

A new robust and high-yielding synthesis of the valuable U^III synthon [U­(BH₄)₃(THF)₂] is reported. Reactivity in ligand exchange reactions is found to contrast significantly to that of uranium triiodide. This is exemplified by the synthesis and characterization of azamacrocyclic U^III complexes, including mononuclear [U­(BH₄)­(L)] and dinuclear [Li­(THF)₄]­[{U­(BH₄)}₂(μ-BH₄)­(L^Me)] and [Na­(THF)₄]­[{U­(BH₄)}₂(μ-BH₄)­(L^A)­(THF)₂]. The structures of all complexes have been determined by single-crystal X-ray diffraction and display two new U^III₂(BH₄)₃ motifs

Theoretical Investigation into the Mechanism of Cyanomethylation of Aldehydes Catalyzed by a Nickel Pincer Complex in the Absence of Base Additives

Density functional theory (DFT) was used to study the reaction mechanism of cyanomethylation of aldehydes catalyzed by nickel pincer complexes under base-free conditions. The C-bound cyanomethyl complex, which was initially thought to be the active catalyst, is actually a precatalyst, and in order for the catalytic reaction to commence, it has to convert to the less-stable N-bound isomer. The carbon–carbon bond formation then proceeds via direct coupling of the N-bound isomer and the aldehyde to give a zwitterionic intermediate with a pendant alkoxide function, which is further stabilized by hydrogen-bonding interaction with water molecules (or alcohol product). The N-bound alkoxide group of the zwitterionic intermediate is subsequently substituted by MeCN via an associative mechanism, followed by deprotonation of the coordinated MeCN to afford the final product. It was found that the transition structure for the exchange reaction (substitution of MeCN for the alkoxide group) is the highest energy point on the catalytic cycle, and its energy crucially influences the catalyst efficiency. The Ni complexes ligated by bulky and weak trans-influencing pincer ligands are not appropriate catalysts for the cyanomethylation reaction due to the involvement of very-high-energy transition structures for the exchange reaction. In contrast, benzaldehydes with electron-withdrawing substituents are capable of stabilizing the exchange reaction transition structure due to the increased stability of the zwitterionic intermediate, leading to acceleration of the catalytic reaction