Post-Synthetic Modification of Calix[4]arene Framework by Iridium-Mediated Alkyne Insertion into an Inert C-C Bond: A Novel Strategy for Unsymmetrical Macrocycles.

As a novel synthetic method for unsymmetrical macrocycles, we herein developed a post-synthetic modification of calix[4]arenes by insertion of a terminal alkyne into an inert C(methylene)-C(aryl) bond of the macrocyclic framework. In this transformation, C-iridated calix[4]arenes, readily synthesized through C-H bond activation of the parent calix[4]arene, undergoes C(methylene)-C(aryl) bond cleavage followed by insertion of an alkyne to provide a ring-expanded calix[4]arene complex. Removal of the iridium metal from the resulting complex was readily accomplished by a simple treatment with an acid. The developed sequential method affords novel unsymmetrical, monofunctionalized macrocyclic compounds via 3 steps from the parent calix[4]arene in good yield. The unique unsymmetrical structures of the alkyne-inserted macrocycles were evaluated by X-ray single crystal analyses. On the basis of theoretical calculations, we propose a reaction mechanism involving an uncommon C-C bond cleavage step through δ-carbon elimination for the ring enlargement process.

Vinylidene rearrangements of internal borylalkynes via 1,2-boryl migration.

Vinylidene rearrangement of alkynes is a well-established and powerful method for alkyne transformations, while use of borylalkynes has remained largely unexplored. This paper describes vinylidene rearrangements of internal borylalkynes using a cationic ruthenium complex. This rearrangement is applicable to alkynes with both tri-(B(pin), B(dan)) and tetracoordinate (B(mida)) boryl groups, and the reaction rate is dramatically affected by the Lewis acidity of the boryl group. Mechanistic study revealed that the rearrangement proceeds via 1,2-boryl migration regardless of the coordination number of the boron center. The migration mode was elucidated by theoretical calculations to indicate that the migration of the tricoordinate boryl groups is an electrophilic process in contrast to the previous vinylidene rearrangements of internal alkynes with two carbon substituents.

Binuclear Complexes Supported by a Tetrapyridyl Ligand with a Bending Anthraquinodimethane Linker.

A tetrapyridyl ligand with a bending anthraquinodimethane linker has been synthesized, and its complexation with coinage metals has been examined. The treatment of the ligand with Ag(I) and Au(I) cations afforded binuclear complexes, wherein the two metal centers were in close proximity to the inside space of the ligand. X-ray analyses corroborated with theoretical calculations indicated that the ligand has reasonable flexibility toward a bending deformation of the linker moiety to provide a ligand pocket suitable for the proximal binuclear complexes, even though such deformations accompany a non-negligible amount of energetic cost. On the other hand, treatment with 2 equiv of Cu(I) salt afforded a binuclear complex, in which both copper atoms were coordinated at the periphery of the ligand.

One-pot Syntheses of Benzo- and Benzofuran-fused Iridaoxabenzenes via CH Bond Activations of Alkyl-bridged Diphenol Derivatives.

One-pot syntheses of new π-extended metallaaromatic compounds have been developed by utilizing Ir-mediated CH bond activation of ethylene- or ethylidene-bridged diphenol derivatives. Depending on the bridging alkyl groups, two types of iridaoxabenzenes, both of which are doubly fused with benzo and benzofuran units, have been obtained. Studies on their structures and electronic characters indicate that both complexes have an aromatic character on the iridaoxacycles, and their π-conjugated systems are fully delocalized over the whole molecular skeletons. These novel metallaaromatic complexes exhibited some reactivities which are distinct from those reported for the non-fused metallaaromatic compounds.

Inorganic salt-assisted assembly of anionic π-conjugated rings enabling 7Li NMR-based evaluation of antiaromaticity.

The 1H NMR-based estimation of antiaromaticity in anionic molecules is challenging because of the difficulty in separately evaluating NMR shielding effects due to paratropic ring currents and negative charges. Herein we propose a novel approach for the 7Li NMR-based evaluation of antiaromaticity enabled by inorganic salt-assisted clusterization, which is serendipitously found during our studies on hyperconjugative antiaromaticity. Reduction of a dibenzo[b,f]silepin (1) with lithium afforded the dilithium salt [(Li+)2(thf)5][12-] (2), which is expected to have antiaromatic character with a pseudo-16π-electron system involving hyperconjugation between the anionic π-clouds and the σ*(Si-Me) orbitals, although the evaluation of its antiaromaticity by NMR was difficult. Compound 2 reacted with 0.2 equivalents of O2 gas to form a trimeric cluster [(Li+)(solv.)n][(12-)3(Li9O25+)] (3), which can be understood as a supramolecular complex composed of three molecules of [Li+]2[12-] and two Li2O salts. X-ray diffraction analysis revealed that the [Li9O2]5+ core is surrounded by three dibenzosilepinyl dianions (12-), with multiple Li-π coordinations. The trimeric structure is maintained in a toluene solution according to the 1H and 7Li{1H} NMR spectra, and of particular interest are the significant downfield shifts of 7Li{1H} NMR signals of the Li9O2 core (δ(7Li) = 6.3, 4.4). These explicit downfield shifts are reasonably explained by the paratropic ring currents of the dianionic dibenzosilepin rings, which was supported by theoretical studies.

A new strategy for hyperconjugative antiaromatic compounds utilizing negative charges: a dibenzo[b,f]silepinyl dianion.

Herein we propose a new strategy for hyperconjugative antiaromatic compounds utilizing negative charges and design the 5,5-diphenyldibenzo[b,f]silepinyl dianion (pseudo 16π-electron system) in which negative hyperconjugation occurs between the anionic π-cloud and the σ*(Si-Ph) orbital. Essentially, reduction of the dibenzo[b,f]silepin with lithium readily generated a dilithium salt of the dibenzosilepinyl dianion, and its hyperconjugative antiaromaticity has been evidenced by the upfield shifts of 1H NMR signals and theoretical calculations, including large NICSzz values and ACID plots.

Synthesis and structures of diaryloxystannylenes and -plumbylenes embedded in 1,3-diethers of thiacalix[4]arene.

New types of diaryloxystannylenes and -plumbylenes have been synthesized and structurally characterized. Lappert's diaminostannylene and -plumbylene E[N(SiMe3)2]2 (E = Sn, Pb) react with 1,3-diethers of tetrathiacalix[4]arene bearing benzyl or SiiPr3 groups to give the corresponding diaryloxystannylenes and -plumbylenes embedded in thiacalix[4]arene, respectively. X-ray diffraction analysis revealed that these tetrylenes exist as monomers in the solid-state. The structure of the thiacalix[4]arene scaffold is highly dependent on the substituents on the phenolic oxygen atoms. The benzyl derivatives adopt apparently C2-symmetric structures with a strong intramolecular SnOBn coordination, whereas the SiiPr3 derivatives have a distorted thiacalix[4]arene skeleton with relatively weak intramolecular EOSiiPr3 and ES interactions. On the basis of the 119Sn{1H} and 207Pb NMR studies in solution, the tin and lead atoms in the benzyl derivatives are strongly coordinated by the ethereal oxygen atoms, whereas those in the SiiPr3 derivatives are weakly coordinated by the bridging sulfur atoms or ethereal oxygen atoms. The tetrylene complexes of thiacalix[4]arene presented here do not show any isomerization in a temperature range from 20 to 110 °C, which is in sharp contrast to the fact that related diaryloxygermylenes and -stannylenes embedded in calix[4]arene underwent isomerization under heating conditions. This difference could be attributable to the ring size of thiacalix[4]arene larger than that of calix[4]arene.

Reductive Formation of a Vanadium(IV/V) Oxide Cluster Complex [V8O19(4,4'- tBubpy)3] Having a C3-Symmetric Propeller-Shaped Nonionic V8O19 Core.

A novel C3-symmetric propeller-shaped vanadium(IV/V) oxide cluster complex, [V8O19(4,4'- tBubpy)3] (V8'), has been synthesized from the reaction of the windmill-shaped vanadium(V) oxide cluster complex [V8O20(4,4'- tBubpy)4] (V8) with PPh3 under N2, whereas refluxing V8 in methanol or ethanol under N2 provides tetranuclear oxido(alkoxido)vanadium(IV/V) complexes [V4O6(OR)6(4,4'- tBubpy)2] [R = Me (V4'-Me) and Et (V4'-Et)]. The mixed-valent vanadium(IV/V) clusters V8' and V4' are converted back to V8 under O2. Interconversions of V4' and the oxido(alkoxido)vanadium(V) complexes [V4O8(OMe)4(4,4'- tBubpy)2] (V4) and [V7O17(OEt)(4,4'- tBubpy)3] (V7-Et) are also presented.

P-C reductive elimination in Ru(ii) complexes to convert triarylphosphine ligands into five- or six-membered phosphacycles.

Rare examples of P-C reductive elimination in ruthenium complexes to generate phosphonium salts are presented. Triarylphosphines are converted into benzophospholium or phosphaphenalenium ligands via cyclometalation and 1,2-insertion of an alkyne followed by P-C reductive elimination. The intermediate in each step was successfully characterized using NMR and X-ray diffraction studies.

Direct transformation of 2-acetylpyridine oxime esters into α-oxygenated imines in an Ir(iii) complex.

2-Acetylpyridine oxime esters are transformed into the corresponding α-oxygenated imines in an Ir(iii) complex under mild conditions, where the selective metal coordination of the imino group effectively promotes tautomerization of the oxime esters into the N-oxyenamines and the subsequent rearrangement.

A ruthenium tellurocarbonyl (CTe) complex with a cyclopentadienyl ligand: systematic studies of a series of chalcogenocarbonyl complexes [CpRuCl(CE)(H2IMes)] (E = O, S, Se, Te).

The first tellurocarbonyl complex with a half-sandwich structure [CpRuCl(CTe)(H2IMes)] was synthesized by a ligand substitution reaction. The practically complete series of the CpCE complexes [CpRuCl(CE)(H2IMes)] (E = O, S, Se, Te) were systematically explored. The tellurium atom in the CTe complex could be smoothly replaced with lighter chalcogen atoms.

Synthesis and Interconversion of V4, V7, and V8 Oxide Clusters: Unexpected Formation of Neutral Heptanuclear Oxido(alkoxido)vanadium(V) Clusters [V7O17(OR)(4,4'-(t)Bubpy)3] (R = Et, MeOC2H4).

The octanuclear oxidovanadium(V) complex [V8O20(4,4'-(t)Bubpy)4] (1; 4,4'-(t)Bubpy = 4,4'-di-tert-butyl-2,2'-bipyridine) was synthesized in multigram-scale quantities by the reaction of VOSO4 with 4,4'-(t)Bubpy in the presence of sodium benzoate under O2. The reaction of 1 with MeOH under air afforded the tetranuclear oxidovanadium(V) complex [V4O8(OMe)4(4,4'-(t)Bubpy)2] (2) selectively. In contrast, treatment of 1 with EtOH or MeOC2H4OH under air gave the heptanuclear oxidovanadium(V) complexes [V7O17(OR)(4,4'-(t)Bubpy)3] (R = Et (3a), MeOC2H4 (3b)) having an unprecedented neutral V7O17(OR) core. Although 3a and 3b are relatively stable in CH2Cl2, slow diffusion of Et2O into the solution of 3a in CH2Cl2 afforded 1 as yellow crystals. Complex 2 was also converted into 1 by recrystallization from CHCl3-CH2Cl2/Et2O. (1)H and (51)V NMR as well as electrospray ionization mass spectrometry studies of the behavior of 2 in CD2Cl2 suggested the formation of the methoxido analogue of 3 (3c, R = Me), which is considered to be an intermediate species in the conversion of 2 to 1. The present results provide a rare example of interconversion among neutral vanadium(V) oxide cluster complexes by changing the solvent systems.

Competition between vinylidene rearrangement and 1,2-insertion of carbon-disubstituted internal alkynes at a Cp*Ir(III) complex.

Competition between vinylidene rearrangement/1,1-insertion and 1,2-alkyne insertion into the Ir-Ar bond of [Cp*Ir(ppy-F4)](+) was observed on reaction with diarylacetylenes. The former process afforded the iridacycle 2via the subsequent 1,4-Ir migration, whereas the latter led to the pyridoisoquinolinium complex 4. Detailed analysis revealed that 4 isomerizes to 2 by heating at 50 °C.

Remote rearrangement of the metal center in a (η6-C6Me6)Ru(II) complex.

Reaction of [(η(6)-C6Me6)RuCl(Ph)(PMe3)] with internal alkynes in the presence of NaBAr(F)4 gave rise to the 1,4-Ru migration to form the o-vinylaryl complex, providing the first example of 1,4-metal migration of a group 8 metal center; in one case further isomerization to an η(3)-allyl complex was observed.

A novel octanuclear vanadium(V) oxide cluster complex having an unprecedented neutral V8O20 core functionalized with 4,4'-di-tert-butyl-2,2'-bipyridine.

A novel octanuclear vanadium(V) oxide cluster complex, [V8O20(4,4'-(t)Bubpy)4] (1), was synthesized and characterized by single-crystal X-ray structure analysis to reveal that 1 has an unprecedented neutral V8O20 core. An unexpected interconversion between 1 and the methoxo(oxo)vanadium(V) cluster complex, [V4O8(OMe)4(4,4'-(t)Bubpy)2] (2), was observed upon changes in the solvent systems.

1,4-Metal migration in a Cp*Rh(III) complex.

The reaction of [Cp*RhCl(Ph)(PR3)] (R = Ph or Me) with internal alkynes in the presence of NaBAr(F)4 affords the o-vinylaryl complex with a Rh-(vinyl CH) agostic interaction by way of the alkyne insertion followed by 1,4-rhodium migration, providing the first example of the 1,4-migration of a Rh(III) center.