Anti Mode Research Articles

2D molecular square grid of Cobalt(II) with tridentate phenylglycinate and mandelate: structure and magnetism

The hydrothermal synthesis, single crystal X-ray structures and magnetic properties of two layered cobalt-carboxylate complexes, ∞ 2[Co II(O 2C ∗CH(OH)C 6H 5) 2] ( 1) and ∞ 2[Co II(O 2C ∗CH(NH 2)C 6H 5) 2] ( 2), where O 2CCH(OH)C 6H 5 is mandalate and O 2CCH(NH 2)C 6H 5 is phenylglycinate, are described. Pale pink crystals of 1 and 2 were obtained by the reaction of cobalt nitrate and the enantiomer-pure acids at 120 °C. In each case, the structure consists of stacks of quasi square-grid polymeric sheets consisting of carboxylato- bridges, M–O–C–O–M, and the presence of both d- and l-enantiomers of the ligands segregated on each face of the layer. The ligands exhibit both chelating and bridging functions with the carboxylate group adopting an anti–anti mode. The magnetic properties are characteristic of weakly interacting paramagnets where the moments are elevated by an important orbital contribution via spin-orbit coupling.

Coordination networks generated from transition metal chlorides and a flexible double betaine

The new flexible double betaine 1,4-bis(2-picolyloxyl)benzene- N, N′-diacetate (L) has been synthesized and used to generate five complexes with divalent metal chlorides: [M(H 2O) 6]Cl 2·L·2H 2O (M=Mn ( 1), Co ( 2)), [Cu(μ-L)Cl 2] ∞ ( 3), {[Zn(μ-L)Cl 2]·2H 2O} ∞ ( 4), and {[Cd 2(μ-Cl) 2(μ-L)Cl 2(H 2O) 2]·2H 2O} ∞ ( 5). Isomorphous compounds 1 and 2 have a hydrogen-bonded three-dimensional network in which adjacent L ligands overlap through π–π stacking of the pyridine and benzene rings. Compounds 3, 4, and 5 display three different types of one-dimensional polymeric structures. Bridged by L acting in the anti mode, compound 3 exhibits an infinite zigzag chain in which consecutive, approximately planar L ligands are arranged in a crossed manner when viewed along the chain direction, the distance between adjacent copper(II) ions being about 13 Å. In contrast, compound 4 features a spring-like infinite helical chain with zinc ions lying on the axis and bridged by L acting in the syn fashion, and the Zn(II)⋯Zn(II) separation is approximately 9 Å. In compound 5, the building unit is a centrosymmetric Cd 2Cl 2 moiety, and adjacent dimeric units are linked by the centrosymmetric L ligand acting in the anti mode to give an infinite zigzag chain running along the c axis; such chains are further connected through hydrogen bonds to form a three-dimensional network.

Synthesis, crystal structure and properties of a tetrametallic 3-ferrocenyl-2-crotonic acid-bridged manganese(II) complex [Mn2(phen)4(FCA)2](ClO4)2.H2O

The title complex [Mn2(phen)4(FCA)2](ClO4)2·H2O (1) (FCA = dianion of 3-ferrocenyl-2-crotonic acid, phen = 1,10-phenanthroline) has been prepared, and its structure determined by single crystal X-ray diffraction analysis. The structure consists of a dinuclear cation [Mn2(phen)4(FCA)2]2+, non-coordinated perchlorate anions and a water molecule. The two MnII ions are separated by 4.374 A in the cation and are dicarboxylate-bridged by carboxylate ligands containing ferrocenyl units. Each FCA is bound to two MnII ions through carboxylate oxygens with the syn–anti bridging mode. The MnII ion is coordinated in an octahedral N4O2 geometry by two chelate phen ligands and two μ-carboxylate oxygen atoms. Electrochemical properties of (1) are discussed.

Synthesis and crystal structure of a terephthalato-bridged copper(II) complex [Cu2(tpt)(bpy)2(H2O)4] [Cu2(tPt)3(bpy)2(H2O)2

A terephthalato-bridged copper(II) complex [Cu2(tpt)(bpy)2(H2O)4][Cu2(tpt)3(bpy)2 (H2O)2] (1) (tpt = terephthalate, bpy =; 2,2'-bipyridine) was synthesized by the reaction of Cu(ClO4)2 ⋅ 6H2O and terephthalic acid in the presence of 2,2'-bipyridine in H2O solution. Crystal structure of 1 was determined by X-ray diffraction analysis in the triclinic space group P\({\bar 1}\), with a = 10.7327(10) A, b = 11.1658(11) A, c = 17.3768(16) A, α = 108.063(2)○, β = 91.064(2)○, γ = 116.721(2)○, V = 1737.5(3) A3, and Z = 2. Both the anion and the countercation in 1 are copper(II) complexes linked by terephthalate bridges in the bis(monodentate) (syn–anti) mode. The anion and the countercation are also linked by two hydrogen bonds between coordinating water molecules and terephthalate ligand.

5'-N-substituted carboxamidoadenosines as agonists for adenosine receptors.

Novel as well as known 5'-N-substituted carboxamidoadenosines were prepared via new routes that provided shorter reaction times and good yields. Binding affinities were determined for rat A1 and A2A receptors and human A3 receptors. EC50 values were determined for cyclic AMP production in CHO cells expressing human A2B receptors. On all receptor subtypes relatively small substituents on the carboxamido moiety were optimal. Selectivity for the A3 receptor was found for several analogues (1a, 1d, 1h, and 1k). On A1 receptors a number of compounds, but not 5'-N-ethylcarboxamidoadenosine (NECA, 1b), showed small GTP shifts, which could be indicative of lower intrinsic activities at the A1 receptor. At the A2B receptor, derivatives 1i-k with modified ethyl substituents had reduced activities compared to the A2B reference agonist NECA (1b). Thiocarboxamido derivatives (8b and 8c) displayed considerable although decreased A2B receptor activity. The X-ray structure determination of compound 8b was carried out. Due to intramolecular hydrogen bonding between the carboxamido NH and the purine N3 in the crystal structure, the ribose moiety of this compound is in a syn conformation. However, theoretical calculations support that NECA (1b), and less so 8b, can readily adopt both the syn and the anti conformation, therefore not excluding the proposed anti mode of binding to the receptor.

Magnetic Anisotropy and Ferromagnetic Resonance in Co/Pb Multilayers

The magnetic properties of sputtered Co/Pb multilayers have been studied by vibrating-sample magnetometer (VSM), torque magnetometer and ferromagnetic resonance spectroscopy (FMR). Magnetic measurements show that with decreasing Pb layer thickness the perpendicular anisotropy field H u increases from 5.7 to 8.3 kOe. The interfacial anisotropy is obtained to be as small as K s = 0.006 erg/cm 2 . It is demonstrated that the increase of H u with decreasing Pb layer thickness is due to the variation of texture in h.c.p. Co along the [00.2] direction with spacer thickness. The in-plane magnetic hysteresis loops show a higher saturation field and smaller remanence ratio for thin spacer films which cannot be accounted for solely by H u and indicates the existence of antiferromagnetic (AFM) coupling between Co layers through Pb spacer. The interlayer coupling is also implied by the observation of spin wave resonance (SWR) in perpendicular FMR. From the SWR spectrum the effective exchange constant is obtained as A eff = 3.4 x 10 -7 erg/cm. A resonance peak appears on the high-field side of the uniform FMR mode, which is attributed to the AFM coupling of adjacent ferromagnetic layers producing an optical mode. The Pb layer thickness (t Pb ) dependence of the separation of the resonance fields, ΔH, between the optical mode anti uniform mode in the perpendicular FMR spectrum is fitted by ΔH = 2.80 exp (-(t Pb - 1))/1.78 (kOe) with a characteristic thickness of 1.78 nm. Thus, the coupling strength (J = M s t Co ΔH/2), in proportion to ΔH, decreases exponentially with Pb layer thickness, which is consistent qualitatively with the variation of the in-plane remanence ratio with spacer thickness.

Xanthine-7-Ribosides as Adenosine AlReceptor Antagonists: Further Evidence for Adenosine's Anti Mode of Binding

Abstract The synthesis and A1 adenosine receptor affinity of some xanthine-7-ribosides is described. It appears that these compounds are A, receptor antagonists. The orientation of the ribose moiety, as determined by ′H-NMR spectroscopy and theoretical chemical calculations, is compared with the orientation of the ribose in the agonist adenosine. Implications for the syn vs anti modes of binding to the receptor are discussed.

Syntheesis and stereochemistry of acidolysis of some cyclohept-2-enylstannames and -silanes

A range of methyl-substituted cyclohept-2-enyltrimethylstannanes and trimethylsilanes have been synthesised and characterised by 1H, 13C and 119Sn (or 29Si) nmr spectroscopy. Key conformational characteristics of some of these compounds are discussed. Acidolysis of these stannanes and silanes (with CF 3COOD) proceeds cleanly to provide the 2H-subatituted methylcycloheptene resulting from allylic rearrangement i.e. regiospecific γ-attack by the electrophile (S E'). Detailed examination of the 2H nmr spectra of the methylcycaloheptenes establishes a highly preferred, if not specific, γ- anti mode of electrophile delivery. Thus, this γ- anti-S E' process may form the basis of synthetically useful applications in what is sometimes an awkward ring system for functionalisation.

On the Stereochemistry of Organocopper Mediated Conversion of Propargylic Esters to Allenes

AbstractUsing a vitamin D‐related steroidal CD fragment as a stereochemical probe, the effect of various factors on the stereochemical course of the organocopper induced conversion of propargylic esters to allenes has been studied. In all cases, an anti mode of attack by the organocopper species was found to be preferred. The effectiveness of Gilman‐type reagents (R2CuLi) versus that of the Lipshutz‐type higher‐order mixed cuprate systems (R2CuCNLi2) in promoting the reaction was also studied.

Stereochemical aspects of the formation and trifluoroacetolysis of some allylic bis(trimethylsilyl)cyclohexenes

The stereochemistry of the 3,4-bis(trimethylsilyl)cyclohexene resulting from disilylation of 1,3-cyclohexadiene is concluded to be cis on the basis of 1H and 13C nuclear magnetic resonance spectra and hydrogenation to Cis-1,2-bis(trimethylsilyl)cyclohexane. The higher boiling cis- and trans-3,6-bis(trimethylsilyl)cyclohexenes could be separated from the above 3,4-isomer but not from each other by careful spinning band distillation. However, the cis,trans compositions of various fractions were established by 13C NMR spectra and chromatographic characteristics. Trifluoroacetolysis (CF3COOD in chloroform) of the cis 3,4-isomer proceeds to yield cyclohexene-d2 as the final product, and analysis of 2H NMR spectra and intermediates demonstrates the importance of a facile 1,2-trimethylsilyl shift in the presumed intermediate ion. Trifluoroacetolysis of the mixtures of the 3,6-disilyl isomers proceeds regiospecifically to yield cyclohex-3-enyltrimethylsilane. A preferred anti mode of attack by the electrophile is indicated for the cis isomer, but syn and anti modes are about equally preferred in the trans isomer. This is attributed to steric congestion by the trimethylsilyl group in the γ-carbon region, hindering anti approach by the electrophile.

Dehydration of alcohols over alumina: Effect of sodium impregnation on the mode of elimination

Dehydration of secondary alcohols has always been the key to studying reaction mechanisms of reactions catalyzed by oxides. The γ-alumina-promoted reactions always afforded a low S 12 (1-olefin/2-olefin) and high S ct ( Cis/trans). These were shown to proceed by the “anti” mode of elimination of elements of water. The effect of sodium impregnation has been studied, in detail, to understand the mode of elimination operative over these doped catalysts. Studies were conducted with 2-butanol and threo-3-methyl-2-pentanol over these catalysts. It appears that although sodium poisoning affected selectivity S 12, it did not show any variation in S ct , suggesting no variation in the mode of elimination. The studies conducted with threo-3-methyl-2-pentanol confirm that only the anti mode of elimination is operative on these catalysts.

Dehydrobromination of bromoalkanes over cabosil and alkali-ion-exchanged cabosilsI. A stereochemical study

The stereochemistry of dehydrobromination of 2-bromobutane and 2,3-dibromobutane has been determined over cabosils, amorphous silicon dioxide prepared by a dry process, to which various amounts of Cs and Li were loaded by an ion-exchange method. Stereochemistry, product distribution, and Arrhenius parameters were essentially the same as those obtained for silica gels prepared by an ordinary wet process. This fact confirmed the idea that the steric course is controlled by the acid-base properties of catalysts: The steric course of dehydrobromination of 2-bromobutane is mainly anti mode over basic solid surfaces and syn mode over acidic surfaces. The reaction rates via anti mode increased in proportion to the number of Cs ions loaded on silica. Basic sites responsible for anti elimination are likely the oxide ions associated with electron-donating Cs ion. A possible mechanism of anti elimination, which does not necessitate any particular pore structure, was discussed on the basis of the experimental results.

Stereochemical Studies of Elimination Reactions of 2-Bromobutane and 2,3-Dibromobutane over Bivalent-Ion Exchanged Silica Gels

Abstract Elimination reactions of 2-bromobutane and 2,3-dibromobutane over silica gels in which silanol protons on the surface were exchanged with bivalent ions such as Ba(II), Ca(II), Mg(II), and Ni(II) have been investigated. Dehydrobromination of 2-bromobutane was found to proceed mainly with syn elimination. Debromination by anti mode and non-stereospecific dehydrobromination were observed for 2,3-dibromobutane. The catalytic activities of these silica gels exhibit reversed volcano-type patterns when they are plotted against the electronegativity of metal ion together with those of alkali-ion exchanged silica gels. Reaction intermediates consistent with the stereochemistry observed are a s-butyl cation and a bromonium ion, respectively for 2-bromobutane and 2,3-dibromobutane.

Stereochemical Studies of Elimination Reactions of 2-Bromobutane and 2,3-Dibromobutane over Alkali-Ion Exchanged Silica Gels

Abstract The stereochemistry (anti or syn mode) of elimination reactions of 2-bromobutane and 2,3-dibromobutane over alkali-ion exchanged silica gels (Li, Na, K, and Cs) was determined at 100–300 °C from the products of the diastereomeric isomers of each alkyl bromide. They were erythro- and thero-2-bromobutanes-3-d1 and meso- and dl-2,3-dibromobutanes. The steric course of dehydrobromination of 2-bromobutane changed nearly reversibly between syn and anti mode by the partial exchange of a surface proton with K+ or Cs+ ion, although the surface structure seemed to remain unchanged by the exchange procedure. On the other hand, the exchange with Na+ or Li+ ion did not change the steric course. Dehydrobromination of 2,3-dibromobutane proceeded by anti mode over Na-, K-, and Cs–SiO2, while SiO2 and Li–SiO2 were not active. The poisoning effect of acidic and basic reagents as well as the indicator test revealed that basic sites played important roles over K- and Cs–SiO2 where anti elimination prevailed, and that weakly acidic sites were responsible for syn elimination.

EFFECTS OF ALKALI-CATION EXCHANGE ON THE STEREOCHEMISTRY OF β-ELIMINATION OF ALKYL HALIDES OVER SILICA GEL

Abstract Dehydrobromination of 2-bromobutane and 2,3-dibromobutane proceeded mainly by anti mode over K- and Cs-exchanged silica gel, but syn mode over silica and Li-silica. Upon re-exchange of K and Cs with proton the steric course returned to syn mode. Role of acid-base properties of solid surface and substrate in controlling the steric course was suggested.

Stereoselectivity of the elimination reactions of 2,3-dihalobutanes and 2-bromobutane over alkaline earth metal oxides and alumina

Abstract The stereochemistry ( anti or syn mode) of dehydrohalogenation of 2,3-dibromo-butane, 2,3-dichlorobutane and 2-bromobutane over MgO, CaO, SrO and Al 2 O 3 was determined by use of a pulse technique from the products of the diastereomeric isomers of each compound at 100–250 °C. The elimination of hydrogen halide proceeded selectively by anti mode over fresh surface of these oxides below 200 °C for dibromobutane and below 250 °C for dichlorobutane, reflecting the surface basicity. Anti mode was also favored in the case of 2-bromobutane at about 100 °C. However, due to the alteration of surface basicity by hydrogen halide formed, the stereoselectivity decreased with repeated pulses. Present results agree with the idea deduced previously in the case of silica gels that the acid-base properties of catalyst and substrate are the prime factor controlling the steric course of reactions. Stereochemistry of debromination and halogen-substitution reactions of dibromobutane, the accompanying reactions, was also determined to be anti mode and retention, respectively.

Stereoselectivity of the elimination reactions of alkyl halides over silica gel and alkali-treated silica gel

The stereochemistry ( anti or syn mode) of dehydrohalogenation of 2-bromobutane, 2,3-dichlorobutane and 2,3-dibromobutane over various solid acids and bases, such as silica gel and alkali salt-impregnated silica gels, at 95–300 °C was determined from the products of the diastereomeric isomers of each compound. The steric course of dehydrobromination of 2-bromobutane at low temperatures was mainly anti mode over KOHSiO 2, K 2CO 3SiO 2 and syn mode over SiO 2. At high temperatures, syn elimination was the favored mode over most catalysts studied. The reactions of 2,3-dihalobutanes proceeded by anti elimination over those solid surfaces. From the variation in the stereochemical reactions of the catalysts and reactants, the ease of β-proton elimination, which increases with the acidity of β-proton and basicity of solid, was found to be the prime factor determining the steric course of reaction. The reaction mechanism is discussed on the basis of the stereochemistry, isotope effects and the relative reactivity of various alkyl halides.