Square planar complexes of nickel(II), palladium(II) and platinum(II) with 1-(2-hydroxyphenyl)-3,5-diphenylformazan

Summary A novel series of formazan complexes of general formula FoML [H₂Fo = 1-(2-hydroxyphenyl)-3,5-diphenylformazan; M = Ni^II, Pd^II or Pt^II; L = NH₃, py and Ph₃P] are described. The formazan nickel(II) system shows linkage isomerism; one isomer, A, contains an unusual five-membered formazan chelate ring, whereas the other, isomer B, has the usual six-membered ring.¹³C n.m.r., u.v. and i.r. spectra are presented and interpreted. From these the palladium and platinum complexes appear to contain the six-membered ring of the B type isomer.     

Chromium Tricarbonyl η6-Complexes of (η5-Cyclopentadienyl)(η4-1,3-Diphenylcyclobutadiene)Cobalt

The reaction of (η⁵-cyclopentadienyl)(η⁴-1,3-diphenylcyclobutadiene)cobalt ( I ) with excess Cr(CO)₃py₃ in BF₃OEt₂ yielded two identified heterometallic compounds. Compounds II and III with one and two phenyl rings complcxed with Cr(CO)₃ fragment(s), respectively. These compounds were characterized by mass, infrared, ¹H and ¹³C NMR spectra and elemental analysis. The crystal structure of II was determined. The Cr(CO)₃ fragment bends inward toward the cyclobutadicne ring due to its electron-withdrawing ability, in accord with Hunter's postulate. A sharp line due to the non-complexed phenyl ring was observed in the ¹HNMR spectrum, which implies that five protons are magnetically equivalent. The chemical shifts of two protons of the cyclobutadiene ring decreased from I to II then to III , possibly because of diminished deshielding effect from the phenyl ring in (arene)Cr(CO)₃.     

Re-examination of the C NMR spectrum of xanthene: a revised correlation for the carbocyclic ring JCH meta-coupling constants in fused oxygen heterocycles

¹³C Chemical shifts and J_CH coupling constants are reported for xanthene and for some dimedone adducts containing a xanthene ring. A revised correlation pattern for the carbocyclic ring ³J_CH meta-couplings in fused oxygen heterocycles is proposed in which the magnitude of the interactions alternate around the ring.     

A comparison of crystallographic and NMR data for thieno[2,3‐b:4,5‐b′]dipyridine and its monohydroperchlorate salt

X‐ray crystallographic studies of thieno[2,3‐b:4,5‐b′]dipyridine ( 1 ) and its monohydroperchlorate salt ( 1a ) show that 1 is protonated at N1 in ring A and not at N6 in ring C. In each compound individual rings are planar, but there is a small dihedral angle‐of‐twist between the A and C rings. On going from 1 to 1a the largest changes in bond angles and bond lengths occur in ring A. ¹H and ^l3C nmr spectra of 1 plus the ¹³C nmr spectrum of 1a are reported.     

Synthesis of Hydrodipyrrins Tailored for Reactivity at the 1- and 9-Positions

A collection of 33 hydrodipyrrins (9 targets, 21 intermediates, and 3 byproducts) has been prepared. The hydrodipyrrins (dihydrodipyrrins, tetrahydrodipyrrins, and hexahydrodipyrrins) contain a pyrrole ring and a geminal-dimethyl substituted 1-pyrroline (or pyrrolidine) ring. The α-substituents on the pyrrole ring (H, Br, CHO) and pyrroline ring (H, CH₃, CH(OR)₂, OMe, SMe) provide different reactivity combinations (Nu⁻, E⁺) and 0, 1, or 2 carbon atoms (which can give rise to the bridging meso-carbons in hydroporphyrins). Straightforward access to various hydrodipyrrins should facilitate development of syntheses of diverse hydroporphyrins.     

A study of the reaction mechanism of 3-nitro-4-R-furoxans formation by nitrosation of dipotassium salts of 1-hydroxyimino-2,2-dinitro-1-R-ethanes

The mechanism proposed earlier for explanation of the furoxan ring formation in the nitrosation of dipotassium salts of 1-hydroxyimino-2,2-dinitro-1-R-ethanes with NaNO₂/AcOH was confirmed experimentally by determining the ionization constants of the dinitromethyl and oxime fragments in the starting dipotassium salt and by examining ¹H, ¹³C, ¹⁴N, and ¹⁵N NMR and mass spectra of isomeric 3(4)-nitro-4(3)-R-furoxans with the ¹⁵N(5) and ¹⁵N(2) ring atoms, respectively, and 3,4-diarylfuroxan with both ¹⁵N-labeled ring atoms. A comparative analysis of the IR and Raman spectra of the ¹⁵N-labeled furoxan derivatives obtained and their unlabeled analogs allowed refinement of the literature data on interpretation of the vibrational spectra of furoxan derivatives.     

Chair and boat conformations in the a ring of 4,4-dimethyl-3-keto steroids

The conformation of the A ring in 4.4-dimethylandrostan-3-ones in solution was determined to be a chair from the coupling observed in the ¹H NMR spectrum. The dipole moment of the 3,17-dione agrees with the dipole moment calculated for the proposed conformation. Force field calculations predict that in 4,4-dimethylandrost-5-en-3-ones the A ring prefers a non-chair conformation, in agreement with the dipole moment determined for the Δ⁵- 3.17-dione and NMR results.     

Nitrogen bridgehead compounds. Part 35. Structures of α-formyl-2,3-polymethylene-3,4-dihydroquinazolin-4-ones

The structures of the title compounds bearing a five-, six- or seven-membered A ring have been investigated by uv and ¹H and ¹³C nmr spectroscopy. The imine-enol-enamine (I-II-III) tautomerism of these compounds depends greatly on the ring size. A significant solvent-dependence is observed only for the five-membered-ring compounds 1 and 2 , which in ethanolic solution exist predominantly in the imine form I, and in chloroform solution in the enol form II. The compounds with a six-membered A ring, 3 and 4 , are mainly in the enamine form III. On protonation, 3 and 4 change into the E and Z isomeric mixture of the enol tautomer II. The seven-membered-ring compound 5 is a mixture of the imine I and the enamine III tautomers.     

Reactivity of cycloalkanes in hydrogen abstraction with different acceptors

The reactions of some acceptors (^?CH₃, ^?OOH, ^?CCl₃, O₃, and Br^?) with saturated cyclic hydrocarbons, viz., cyclohexane, cycloheptane, and cyclooctane were studied by the DFT methods based on B3LYP and PBE0 functionals, the method based on the double hybrid functional B2PLYP, a combined ONIOM approach (CCSD:B3LYP)) and the coupled-cluster method (CCSD) using the 6-31+G**, aug-cc-pVDZ, Midi-X, and SVP basis sets. A specific feature of these reactions is that their rates depend on the excess ring energy, although no ring opening occurs in all cases.     

Structure and reactivity of steroids—VI : Long range effects in a series of Δ1,3,5(10)-estratriene compounds

The kinetics of the oxidation of 17β-hydroxy-Δ^1,3,5(10)-estratrienes which differ in D ring size, in C₃, substituent and in the character of fusion of the B and C rings by CrO₃ in AcOH has been studied. An increase in size of the D ring in the substrate diminishes reactivity of the 17β-hydroxy group in the oxidation. It has been found that the reactivities of the 17β and 17β-hydroxy groups of the Δ^1,3,5(10)-estratriene derivatives depend upon the electronic nature of the substituent at C³, explained by a long range effect operating from the A to D ring. The pK'a's of phenolic steroids have been measured and it was shown that introduction of the Δ⁸⁽⁹⁾ double bond increases the degree of dissociation of the phenolic hydroxyl. Expansion of the D ring and also reduction of the 17(17a)-keto group to 17β (17aβ) hydroxy group decreases the degree of dissociation of the phenolic hydroxyl as a result of the long range effect from the D to A ring. A linear correlation has been established between the oxidation and dissociation rate constants, confirming the existence of an interaction between C₃ and C_17(17a) at the expense of the long range effects from the A to D ring and vice versa. The long range effect apparently arises from a combination of inductive and conformational effects of the substituent. The kinetics of enzymic oxidation of 17β-hydroxy-Δ^1,3,5(10)-estratrienes by soluble 17β-estradiol dehydrogenase from human placenta has been investigated. In the case of 17β-hydroxy-Δ^1,3,5(10)-estratrienes of natural configuration, a linear correlation has been established between the values of the maximum rate constant and the oxidation rate constant. An assumption can thus be made about the major role of the contribution of the specific reactivity of the substrate to that of the enzyme-substrate complex, involving the size of the D ring and the long range effect of the substituent at C₃ upon the reaction center at C_17(17a). For 17β-hydroxy-Δ^1,3,5,(10)-estratrienes of the 8-isoconfiguration it is the specific reactivity of the substrate that makes the major contribution to the maximum rate values; this specific activity is a result of the substrate's configuration, which is responsible for the stability of the enzyme-substrate complex.     

Trangmolins A–F with an Unprecedented Structural Plasticity of the Rings A and B: New Insight into Limonoid Biosynthesis

The absolute stereostructures of trangmolins A–F ( 1 – 6 ), limonoids with three new and one known topologies of the rings A and B, were unambiguously determined by NMR spectroscopic investigations, single‐crystal XRD analysis, and quantum‐chemical electronic circular dichroism calculations. Compounds 1 – 3 contain a hexahydro‐1H‐inden‐4‐one motif, compound 4 comprises a hexahydro‐2,6‐methanobenzofuran‐7‐one cage, and compound 5 consists of a hexahydro‐2H‐2,8‐epoxychromene scaffold. The C1?C30 linkage in 1 – 3 and the C3?C30 connection in 4 form two unprecedented types of ring A/B‐fused carbobicyclic cores: viii and ix . The oxidative cleavage of the C2?C3 bond in 5 and heterocyclization in 4 and 5 constitute the unprecedented tricyclic 6/6/5 ring A/B¹/B²‐ and 6/5/6 ring A¹A²/B‐fused topologies, respectively, which are uncovered, for the first time, in the construction of limonoid architectures. The diverse cyclization patterns of 1 – 6 reveal an unparalleled structural plasticity of rings A and B in limonoid biosynthesis.     

Synthèse et stéréochimie de perhydrocyclopenta[b]furannones-3-cis et perhydrocyclopenta[b]furannols-3-cis

Synthetic route to new cis-perhydrocyclopenta[b]furan-3-ones and cis-perhydrocyclopenta[b]furan-3-ols is described. Their configurations and conformations were inferred by ¹H and ¹³C nmr spectroscopy. A certain rigidity is associated with the bond common to the two fused ring. The conformation of tetrahydrofuran-3-one ring appears to be a composite of the C, and C₂ forms depending on the C-2 substitution.     

Chain-ring-chain tautomerism in 2-aryl-substituted hexahydropyrimidines and 1H-2,3-dihydroperimidines. Does it appear?

A series of 2-aryl substituted hexahydropyrimidines and perimidines were synthesized from aromatic aldehydes and substituted 1,3-propanediamines or 1,8-naphthalenediamine. The ¹H and ¹³C NMR spectra showed that 2-arylperimidines and 2-aryl-4,4,6-trimethylhexahydropyrimidines exist exclusively in ring forms even in DMSO solutions, whereas 2-aryl-4-methylhexahydropyrimidines undergo chain-ring-chain tautomerism with a good linear correlation between the ring-chain equilibrium constants (log K, where K=[ring]/[chain]) and the Hammett-Brown σ⁺ parameters of the aromatic substituents. 4,4,6-Trimethylhexahydropyrimidines underwent complete and irreversible ring opening in CF₃COOH solutions giving two different chain forms.     

CONFORMATION OF EIGHT-MEMBERED DIOXATHIASILOCIN HETEROCYCLES IN SOLUTION

Abstract

The dibenzo[d,g][1,3,6,2]dioxathiasilocin derivatives 3a-f were prepared by the reaction of the thiobisphenols 1a-b with the corresponding dichlorosilanes 2a-e using triethylamine as an acid acceptor. The free energy of activation for ring inversion of the 2,4,8,10-tetra-tert-butyl-substituted 3a was determined by variable temperature ¹H NMR to be 13.9 kcal/mol. The ¹H NMR spectral data of 3a requires that the ring conformer possess a σ plane of symmetry passing through the silicon and bridging sulfur atoms. In the variable temperature ¹H NMR spectra of the 2,4,8,10-tetramethyl-substituted 3b no evidence was observed for the slowing of ring inversion at-55^oC, suggesting that the energy of activation for ring inversion is less than 10.9 kcal/mol. The ¹H NMR spectral data of 3e-f indicates the presence of equilibrating conformational isomers. The results of this study supports the suggestion that steric factors are a major contributor to the barrier of ring inversion for the dibenzo-[d, g][1,3,6,2]dioxathiasilocin ring system.     

Intramolecular Friedel–Crafts alkylation with a silylium-ion-activated cyclopropyl group: formation of tricyclic ring systems from benzyl-substituted vinylcyclopropanes and hydrosilanes

A trityl-cation-initiated annulation of benzyl-substituted vinylcyclopropanes (VCPs) with hydrosilanes is reported. Two Si–C(sp³) bonds and one C(sp²)–C(sp³) bond are formed in this process where an intramolecular 6-endo-tet Friedel–Crafts alkylation of a silylium-ion-activated cyclopropane ring is the rate-determining key step. The reaction mechanism is proposed based on computations and is in agreement with experimental observations. The new reaction leads to an unprecedented silicon-containing 6/6/5-fused ring system. A phenethyl-substituted VCP derivative yields another unknown tricycle having 6/6/6 ring fusion by reacting in a related but different way involving a 6-exo-tet ring closure.

Downstream to alkene hydrosilylation, the opening of the cyclopropane ring in benzyl-substituted VCPs is interlinked with an S_EAr of the aryl group.     

The twinned crystal structure of tripotassium benzene‐1,3,5‐tris(trifluoroborate)

The title compound, 3K⁺·C₆H₃B₃F₉³⁻, crystallizes as discrete anions and cations which are connected by K...F and K...π interactions. Two of the –BF₃ residues attached to the aromatic ring adopt a conformation with all F atoms out of the plane of the aromatic ring, whereas the third residue has an almost synperiplanar conformation for one of the F—B—C—C torsion angles. It is remarkable that only one of the K⁺ cations interacts with the arene ring and that only one side of the aromatic ring coordinates to a K⁺ cation. As a result, a sandwich structure does not occur. All K⁺ ions show a coordination mode that cannot be conveniently described with a polyhedron. The anions are located in the (102) planes with the K⁺ cations located between these planes. The investigated crystal was a nonmerohedral twin with the fractional contribution of the minor twin component being 0.405 (4). The title compound is the first example of a structure containing a benzene ring substituted with three –BF₃ groups. Only eight other structures have been reported in which a benzene ring carries at least one –BF₃ group. Just five of these contain a K⁺ ion, but in none of these is the K⁺ ion coordinated to the aromatic ring.     

黄酮类化合物与α-羟乙基过氧自由基反应的脉冲辐解研究

由于脂质过氧化反应(LPO)是导致人体疾病(如肝炎、肝硬化、动脉硬化、脑溢血等)的主要原因, 而黄酮类化合物是一类很强的过氧化反应抑制剂, 因此有必要研究其化学结构与过氧化反应的关系及其抗氧化机理.本文选择α-羟乙基过氧自由基为脂质过氧自由基的模拟物, 采用脉冲辐解方法研究了乙醇溶液中4种典型的黄酮类化合物(槲皮素、芦丁、儿茶素以及黄岑甙)与α-羟乙基过氧自由基的反应动力学, 测得其反应活性顺序为:芦丁>槲皮素>黄岑甙>儿茶素. 同时以黄酮体和邻苯二酚为黄酮类化合物不同结构特征的模型化合物, 用脉冲辐解法测得二者与α-羟乙基过氧自由基的反应速率常数分别为(1.7±0.1)×10⁶和(2.9±0.1)×10⁵ mol^-1·dm³·s^-1.实验结果表明, 在黄酮类化合物与α-羟乙基过氧自由基的反应中, A环C₅位的羟基, C环C₂＝C₃或B-C环的大π键和B环邻二羟基共存时清除α-羟乙基过氧自由基活性最好, 而且C环C₂＝C₃或B-C环大π键的清除活性好于B环邻二羟基, 同时C环是否含有C₃-醣甙结构对清除作用没有明显影响. 因此我们推测在黄酮类化合物抑制脂质过氧化反应过程中, 起主要作用的是C环C₂＝C₃或B-C环的大π键与脂质过氧自由基的双键加成反应.     

Facile Self‐Assembly of Metallo‐Supramolecular Ring‐in‐Ring and Spiderweb Structures Using Multivalent Terpyridine Ligands

A series of metallo‐supramolecular ring‐in‐ring structures was generated by assembling Cd^II ions and the multivalent terpyridine ligands ( L^1‐3 ) composed of one 60°‐bent and two 120°‐bent bis(terpyridine)s with varying alkyl linker lengths. The mechanistic study for the self‐assembly process excluded an entropically templated pathway and showed that the intramolecularly complexed species is the key intermediate leading to ring‐in‐ring formation. The next‐generation superstructure, a spiderweb, was produced in quantitative yield using the elongated decakis(terpyridine) ligand ( L⁵ ).     

Stereochemistry and bonding in N-substituted-2-phenyl-3-cyanoaziridines

A series of ten N-alkyl(aralkyl)-2-phenyl-3-cyanoaziridines has been synthesized to continue investigations of the molecular stereochemistry and bonding of functionalized aziridines. Substantial spectroscopic evidence is presented which indicates the presence of stereoselective hyperconjugation between the phenyl and nitrile groups and the aziridine ring. The ¹H and ¹³C nmr chemical shifts are rationalized in terms of the interactions of the substituents on the aziridine ring, and how these interactions vary with the steric bulk of the nitrogen substituents. Various stereoselective coupling constants (¹H-¹H and ¹³C-¹H) are also reported.     

High‐Resolution Photoelectron Imaging of IrB3−: Observation of a π‐Aromatic B3+ Ring Coordinated to a Transition Metal

In a high‐resolution photoelectron imaging and theoretical study of the IrB₃^? cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B₃ ring coordinated with the Ir atom. The isomer with the higher EA consists of a B₃ ring with a bridge‐bonded Ir atom (C_s , ²A′), and the second isomer features a tetrahedral structure (C_3v , ²A₁). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C_3v isomer involves significant covalent Ir?B bonding and weak ionic bonding with charge transfer from B₃ to Ir, and can be viewed as an Ir–(η³‐B₃⁺) complex. This study provides the first example of a boron‐to‐metal charge‐transfer complex and evidence of a π‐aromatic B₃⁺ ring coordinated to a transition metal.