Stereospecific and Stereoselective Rhodium(I)‐Catalyzed Intramolecular [2+2+2] Cycloaddition of Allene‐Ene‐Ynes: Construction of Bicyclo[4.1.0]heptenes

Treatment of the allene‐ene‐yne substrates with [{RhCl(CO)₂}₂] effected the intramolecular [2+2+2]‐type ring‐closing reaction to produce various of tri‐ and tetracyclic derivatives containing a cyclopropane ring. The reaction is highly stereoselective as well as stereospecific with good to excellent yields.     

Gold-catalyzed transannular [4+3] cycloaddition reactions

In the presence of a catalytic amount of Au(I) and Ag(I) salts, the 14-membered furanophane II with an allene function located across the ring undergoes both a transannular [4+3] and [4+2] cycloaddition. Secondary rearrangement products containing a tricyclic ring system were isolated when the catalyst was PtCl₂.     

Stereoselective Rhodium‐Catalysed [2+2+2] Cycloaddition of Linear Allene–Ene/Yne–Allene Substrates: Reactivity and Theoretical Mechanistic Studies

Allene–ene–allene ( 2 and 5 ) and allene–yne–allene ( 3 and 7 ) N‐tosyl and O‐linked substrates were satisfactorily synthesised. The [2+2+2] cycloaddition reaction catalysed by the Wilkinson catalyst [RhCl(PPh₃)₃] was evaluated. Substrates 2 and 5 , which bear a double bond in the central position, gave a tricyclic structure in a reaction in which four contiguous stereogenic centres were formed as a single diastereomer. The reaction of substrates 3 and 7 , which bear a triple bond in the central position, gave a tricyclic structure with a cyclohexenic ring core, again in a diastereoselective manner. All cycloadducts were formed by a regioselective reaction of the inner allene double bond and, therefore, feature an exocyclic diene motif. A Diels–Alder reaction on N‐tosyl linked cycloadducts 8 and 10 allowed pentacyclic scaffolds to be diastereoselectively constructed. The reactivity of the allenes on [2+2+2] cycloaddition reactions was studied for the first time by density functional theory calculations. This mechanistic study rationalizes the order in which the unsaturations take part in the catalytic cycle, the reactivity of the two double bonds of the allene towards the [2+2+2] cycloaddition reaction, and the diastereoselectivity of the reaction.     

A mononuclear manganese(III) complex of an asymmetric macrocyclic ligand with a ring contraction [MnHL2(ClO4)](ClO4) · 2.5H2O

The template reaction of sodium 2,6-diformyl-4-chlorophenolate and N,N-bis(2-aminoethyl)–hydroxyethylamine followed by in situ transmetallation of Mn(ClO₄)₂ results in a mononuclear manganese(III) complex of one 21-membered asymmetric 2:2 Schiff base macrocycle, in which a hydroxyethyl group of the amine has been eliminated and ring contraction at one chamber of the macrocycle has occurred to form an imidazolidine ring. An X-ray study indicates that the geometry about the manganese(III) ion is distorted octahedral. The electrochemical behavior of this complex in MeCN has been studied by cyclic voltammetry.     

(4R)‐3‐Allenyl‐4‐(di­phenyl­methyl)­oxazolidin‐2‐one,an unsubstituted allen­amide

The first X‐ray structure of an unsubstituted allen­amide, C₁₉H₁₇NO₂, is reported. The solid‐state phase supports the notion that a key minimum conformation of allen­amides can be invoked to rationalize the observed stereochemical outcomes in many of our methodological studies employing allen­amides. This minimum conformation involves two important factors, i.e. having approximate coplanarity between the planes of the oxazolidinone ring and the internal olefin, and having the allene moiety facing away from the carbamate carbonyl group. The C—N—C=C torsion angle that quantifies this approximate coplanarity between the plane of the oxazolidinone ring and that of the internal olefin, as determined from this crystallographic study, is −19.1 (2)°. A minimized structural calculation, which determined this angle to be −16.1°, is in close agreement. Additional structural features include a probable π–π interaction between the allene moiety and a benzene ring, and non‐classical hydrogen bonding in the form of weak C—H⋯O interactions that are responsible for the formation of two‐dimensional networks.     

Synthesis and crystal structure of core-modified benziporphyrin: thia-p-benziporphyrin

The core-modified 5,20-phenyl-10,15-tolyl-thia-p-benziporphyrin (SBzP) can be prepared from the condensation of 1,4-bis(α-hydroxyl-benzyl)benzene with 5,10-ditolyl-16-thia-5,10,15,17-tetrahydrotripyrrin using BF₃·OEt₂ as catalyst. Spectroscopic studies suggest an aromatic macrocycle with a rapid flipping phenylene ring. SBzP exhibits a tilted phenylene ring and crystal packing shows dimeric structure with two SBzP rings linked by hydrogen bonding and π-π interaction. TFA acidified SBzPH₂²⁺ has a saddle-shaped dication porphyrin ring with two solvated trifluoroacetate and two solvated trifluoroacetic acid linked by hydrogen bondings.     

Crystal structures of rhodacyclopentane derivatives

Two complexes containing a rhodacyclopentane ring, viz. (acac)Rh(C₆H₈)(py)₂ and (acac)₂Rh₂(C₆H₈)(PPh₃) have been studied by X-ray diffraction. The ring forms through oxidative coupling of two allene molecules giving the structure A with trivalent, octahedrally-coordinated rhodium. In the second complex the unsaturated hydrocarbon part is also π-bonded to another Rh atom, and one acac group acts as a five-electron donor.     

Porphyrins Like Boron After All

The central core and the macrocycle skeleton of porphyrins both offer room for boron, and a gap in the chemistry of nonmetal-containing porphyrins has now been filled. In one case a four-membered B₂O₂ ring coordinates to a porphyrin cavity that has been distorted to a rectangle ( A ), and in the other case four boron atoms are located in the meso positions of a tetrathiaporphyrinogen ( B ).     

Effect of Solvent and Protonation/Deprotonation on Electrochemistry,Spectroelectrochemistry and Electron‐Transfer Mechanisms of N‐Confused Tetraarylporphyrins in Nonaqueous Media

A series of N‐confused free‐base meso‐substituted tetraarylporphyrins was investigated by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1 M tetra‐n‐butylammonium perchlorate (TBAP) and added acid or base. The investigated compounds are represented as (XPh)₄NcpH₂, in which “Ncp” is the N‐confused porphyrin macrocycle and X is a OCH₃, CH₃, H, or Cl substituent on the para position of each meso‐phenyl ring of the macrocycle. Two distinct types of UV/Vis spectra are initially observed depending upon solvent, one corresponding to an inner‐2H form and the other to an inner‐3H form of the porphyrin. Both forms have an inverted pyrrole with a carbon inside the cavity and a nitrogen on the periphery of the π‐system. Each porphyrin undergoes multiple irreversible reductions and oxidations. The first one‐electron addition and first one‐electron abstraction are located on the porphyrin π‐ring system to give π‐anion and π‐cation radicals with a potential separation of 1.52 to 1.65 V between the two processes, but both electrogenerated products are unstable and undergo a rapid chemical reaction to give new electroactive species, which were characterized in the present study. The effect of the solvent and protonation/deprotonation reactions on the UV/Vis spectra, redox potentials and reduction/oxidation mechanisms is discussed with comparisons made to data and mechanisms for the structurally related free‐base corroles and porphyrins.     

Inhibition of metal ion complexation by an N2O2-donor macrocycle by incorporation of an intramolecularly hydrogen-bonding pendant phenolic group

The synthesis and X-ray structure of a new pendant-arm derivative of a parent 14-membered N₂O₂-donor macrocycle bearing an N-substituted 6-methylene-2,4-dimethylphenol group is presented. A single crystal X-ray study shows a strong intramolecular hydrogen-bond between the pendant phenol proton and the tertiary nitrogen of the macrocyclic ring, which may inhibit its ability to bind with selected transition and post-transition metal ions.     

Structures and Dynamic Solution Behavior of Cationic,Two‐Coordinate Gold(I)–π‐Allene Complexes

A family of seven cationic gold complexes that contain both an alkyl substituted π‐allene ligand and an electron‐rich, sterically hindered supporting ligand was isolated in >90 % yield and characterized by spectroscopy and, in three cases, by X‐ray crystallography. Solution‐phase and solid‐state analysis of these complexes established preferential binding of gold to the less substituted C?C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C?C bond. Kinetic analysis of intermolecular allene exchange established two‐term rate laws of the form rate=k₁[complex]+k₂[complex][allene] consistent with allene‐independent and allene‐dependent exchange pathways with energy barriers of ΔG^≠₁=17.4–18.8 and ΔG^≠₂=15.2–17.6 kcal mol^?1, respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG^≠=8.9–11.4 kcal mol^?1) intramolecular exchange of the allene π faces through η¹‐allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)₂o‐binaphthyl}Au(η²‐4,5‐nonadiene) ]⁺SbF₆^? ( 5 ), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (ΔG^≠_298K=17.4 kcal mol^?1), which ruled out the participation of a η¹‐allylic cation species in the low‐energy π‐face exchange process for this complex.     

Intramolecular cycloaddition/cycloreversion of (E)-3β,17β-diacetoxy-5,10-secoandrost-1(10)-en-5-one

Treatment of 3β,17β-diacetoxy-5,10-secoandrost-1(10)-en-5-one with BF₃·Et₂O was shown to proceed with cleavage of the macrocycle and formation of a new compound containing a cyclopentenone ring. Based on DFT calculations, an intramolecular Lewis acid promoted [2+2]cycloaddition, followed by a cycloreversion of the intermediate oxetane, is proposed as a possible reaction mechanism.     

The reaction of allene with dicarbonylchlororhodium: Formation of new π-allylic complexes of rhodium(III)

The reaction of allene with [Rh(CO)₂Cl]₂ to give a new π-allylic complex of rhodium(III) containing an allene dimer is described; the reactions of this complex with PPh₃ and AsPh₃ are reported.     

The reaction of allene with β-diketonatoiridium(I) complexes: formation and crystal structure of a new bis-β-allylic derivative of iridium(III)

The reaction of allene with (Hfacac)Ir(η-C₈H₁₄)₂ to give a new bis-η-allylic complex of iridium(III) containing an allene tetramer is described; the X-ray structure of this compound is reported.     

Synthesis and conformational analysis of a novel carbohydrate-fused bis-crown ether: crown-CyPLOS

A novel sugar-based macrocycle consisting of a phosphate-linked 12-membered disaccharide ring (cyclic phosphate-linked oligosaccharide, CyPLOS), fused to two 18-crown-6 ether residues, is here described. The synthesis of the target compound has been accomplished in 23% overall yield for 11 reaction steps, exploiting phosphoramidite chemistry for the dimerization and a classical phosphotriester methodology for the cyclization reaction. NMR-based conformational analysis studies have been carried out on the fully deprotected macrocycle, showing a characteristic arched-structure with C₂-symmetry.     

Remarkable synthesis and structure of allene type zerumbone

The ring expansion of zerumbone to a 12-membered ring was studied via a ring opening system or a ring closure system of zerumbone. We succeeded in the synthesis of a zerumbone derivative with 12-membered ring, an allene type zerumbone. For the first time, a Doering-LaFlamme allene synthesis method was adopted and the structure was confirmed by monocrystal X-ray diffraction. It was obtained in total 27.7% yield from zerumbone. We believe that this compound is not only an important building block in synthesizing the BC ring of paclitaxel, but also plays an important role in a novel structure formation and a reactive discovery.     

An unusual oligomerization/oxidation reaction of a 3‐boron‐substituted 1‐phenylbuta‐1,3‐diene produces 6,9,16,19‐tetraphenyl‐5,15‐distyryl‐3,13,25,26‐tetraoxa‐2,12‐diborapentacyclo[16.2.2.28,11.12,5.112,15]hexacosa‐1(20),7,10,17‐tetraene

The unusual title macrocyclic structure, C₆₀H₅₄B₂O₄, has been isolated from exposure of 3‐BF₃‐1‐phenylbuta‐1,3‐diene to both air and moisture in an attempt to obtain crystals of the starting butadiene compound. Formation of the macrocycle from six molecules of the starting butadiene material is rationalized and its structural features are compared with those of other B(OR)₂‐substituted cyclohexane and benzene ring containing structures. Molecules reside on crystallographic centers of inversion and there are no intermolecular interactions of note in the crystal structure.     

Sterically Crowded Trianglimines—Synthesis,Structure, Solid‐State Self‐Assembly,and Unexpected Chiroptical Properties

The chiral, triangular‐shape hexaimine macrocycles (trianglimines), bearing bulky alkynyl or aryl substituents were synthesized and studied by means of experimental and theoretical methods. The macrocyclization reactions are driven by the extraordinary stability of the trianglimine ring and provided products with high yields. Electrostatic repulsion between imine nitrogen atoms and the substituents forced an anti conformation of the aromatic linkers. Although the DFT‐optimized structure of 7 is D₃ symmetrical, in the crystal, the macrocycle adopts a bowl‐like molecular shape. The macrocycle self‐assembles into tail‐to‐tail dimers by mutual interdigitation of aromatic moieties. In contrast, macrocycle 8 adopts a rigid pillararene‐like conformation. The nature of the substituent significantly affects the electronic properties of the linker. As a result, unexpectedly high exciton Cotton effects are observed in the electronic circular dichroism (ECD) spectra. The origin of these effects was subject of an in‐depth study.     

N‐Fusion Approach in Construction of Contracted Carbaporphyrinoids: Formation of N‐Fused Telluraporphyrin

Insertion of PCl₃ into 5,10,15,20‐tetraaryl‐21‐telluraporphyrin leads to a phosphorus complex of N‐fused dihydrotelluraporphyrin with an inverted tellurophene ring. Its CNN coordination core places the macrocycle in the family of contracted carbaporphyrinoids. A cycle of direct transformations affords an elegant triangle of three mutually convertible N‐fused porphyrinoids, with distinct spectroscopic features: antiaromatic, nonaromatic and aromatic. The nonaromatic species has a dome shaped skeleton which forms in the solid state a ball and socket structure with C₆₀.     

Coamplexation of mercury(II) compounds by crown ethers in organic media

Complexes between crown ethers having ring sizes of 15–24 atoms and 5 to 8 oxygens with HgX₂ species (X = Cl, I, CN, SCN) have been prepared and studied. Interactions of the substituents X bound to mercury with groups or fragments in the crown compound are essential in determining the characteristics and stability of the complexes formed. Complexes with crowns of 18C6 ring sizes are the most favourable except for HgX₂ compounds for which the size of X is larger than the macrocycle ring, as is the case for X = CF₃.