Showing papers in "Helvetica Chimica Acta in 1978"

Hydrogen Evolution from Water by Visible Light, a Homogeneous Three Component Test System for Redox Catalysis

Abstract: Reference LPI-ARTICLE-1978-012doi:10.1002/hlca.19780610740View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Cyclodextrin chemistry. Selective modification of all primary hydroxyl groups of α- and β-cyclodextrins

Abstract: Two efficient methods are described for the selective modification of all six primary hydroxyl groups of α-cyclodextrin (α-CD, 11). One, using an indirect strategy, involves protection of all 18 hydroxyl functions as benzoate esters, followed by selective deprotection of the six primary alcohol groups. The other, using a direct strategy, involves selective activation of the primary hydroxyl groups via a bulky triphenylphosphonium salt, which is then substituted by azide anion as the reaction proceeds. A number of modified α-cyclodextrin derivatives have been prepared and fully characterized, among which are: the useful intermediate α-cyclodextrin-dodeca (2, 3) benzoate (3); hexakis (6-amino-6-deoxy)-α-cyclodextrin hexahydrochloride (7); hexakis (6-amino-6-deoxy)-dodeca (2, 3)-O-methyl-α-cyclodextrin hexahydrochloride (9), hexa (6)-O-methyl-α-cyclodextrin (13). The direct substitution is shown to be even more efficient for β-cyclodextrin (16), giving the heptakis (6-azido-6-deoxy)-β-CD-tetradeca (2, 3)acetate (17), while the indirect strategy fails. The compounds are characterized by extensive use of 13C- and 1H-NMR. spectroscopy. The steric and statistical problems of selective polysubstitution reactions for the cyclodextrins are discussed, and possible reasons for the observed differences in reactivity between α- and β-cyclodextrins are examined. The dodecabenzoate 3 presents a very marked solvent effect on physical properties (IR. and NMR. spectra, optical rotation); the effects observed may be ascribed to an unusually strong intramolecular network of hydrogen bonds which severely distorts the α-cyclodextrin ring and lowers the symmetry from six-fold to three-fold.

Structural Studies of Crystalline Enamines

Abstract: Crystal structure analyses of five crystalline enamines together with recently published structural data for two other enamines reveal varying degree of pyramidality at the enamine nitrogen atom. The pyramidality appears to be most pronounced in enamines from piperidine and morpholine, less so in enamines derived from prolinoid amines. A pyrolidine enamine obtained from a derivative of cyclohexane-1,4-dione seems to have a virtually planar enamine group. The implications of these findings for current stereochemical problems in enamine chemistry are discussed.

On Metal‐Atom Clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali‐metal molecules

Abstract: The investigation of electronic and structural parameters of metal molecules as a function of size may be decisive for understanding and control of heterogeneous catalysis with finely divided metals. Metal-atom clusters can be prepared by several methods, most of which yield a molecular mixture only. Expansion of an atomic vapour into vacuum through a supersonic nozzle creates a complex cluster-spectrum which has been investigated by photoionization using a mass-spectrometer as selective detector. Broad band single photoionization (PI.) yields the abundances and ionization thresholds for Nax(x⩽16), Kx(x⩽12) and NaxKy(x+y⩽6). Rough indications about other deactivation channels of excited alkali-molecules are obtained from an analysis of the photoionization efficiency (PIE.) curves. Two-photon PIE. curves with narrow-band (laser) light sources give an accurate photoionization threshold value and detailed information on ionization processes. Two-photon ionization spectra via a real intermediate state reached by laser excitation are equivalent to normal absorption spectra, if the exciteation step is controlled by the true transition probabilities. By investigation of the power dependence of the ion current of Na as a function of the wavelength of the tunable cw dye laser (excitation) and of the ionizing Ar+- or Kr+-laser, conditions under which true spectra are obtained have been clarified, in good agreement with a photon-kinetic model of the processes involved. Vibronic and rovibronic spectra of several transitions in Na2 and K2 have been measured. Similarly a spectrum of Na3 has been determined mass-selectively. The scope of the new method for an absorption spectroscopy in molecular beams is discussed.

Is water critical to the formation of micelles in apolar media

Abstract: Photon correlation studies in H2O/sodium di-2-ethylhexylsulfosuccinate (AOT)/i-C8H18-systems, pertinent IR. investigations, and vapor pressure osmometric measurements with alkylated quaternary ammonium di-2-ethylhexylsulfosuccinates strongly suggest water to be pre-requistite for the micellization in apolar media.

Structural Studies of 1,8‐Disubstituted Naphthalenes as Probes for nucleophile‐electrophile interactions

Abstract: Results of crystal structure analyses of seven 1, 8-disubstituted naphthalenes (2a, 8-(N,N-dimethylamino)-1-naphthyl methyl ketone; 2b, 8-(N, N-dimethylamino)naphthalene-1-carboxylic acid; 2c, methyl 8-(N, N-dimethylamino)naphthalene-1-carboxylate; 2d, 8-methoxy-1-naphthyl methyl ketone; 2e, 8-methoxynaphthalene-1-carboxylic acid; 2f, N, N-dimethyl-8-methoxynaphthalene 1-carboxamide; 2g, N, N-dimethyl-8-hydroxynaphthalene-1-carboxamide) with a nucleophilic centre (N(CH3)2, OCH3, OH) at one of the peri positions and an electrophilic centre (carbonyl C) at the other are described. All seven molecules show a characteristic distortion pattern: the exocyclic bond to the electrophilic centre is splayed outward, and the one to the nucleophilic centre is splayed inward; the carbonyl C is displaced from the plane of its three bonded atoms towards the nucleophile. This distortion pattern differs from that found in other 1,8-disubstituted naphthalenes and is interpreted as an expression of incipient nucleophilic addition to a carbonyl group. The crystal structure of 2b contains an ordered arrangement of equal numbers of amino acid and zwitterionic molecules.

The kinetics of the bimolecular sel-reaction of t-butyl radicals in solution. I. Termination rates

Abstract: Termination rate constants 2Kt for the self-reaction of t-butyl radicals in twelve different solvents are determined as a function of temperature by kinetic electron spin resonance. In the entire range measured, 7. 107 < 2Kt < 2 · 1010 dm3 mol−1 s−1, they are well described by the von Smoluchowski equation for diffusion controlled reactions, using a spin statistical factor, a temperature and solvent independent, isotropic reaction diameter, and diffusion coefficients for t-butyl estimated following empirical or semiempirical prescriptions. Methods for the prediction of diffusion coefficients are critically discussed, and rules for the estimation of termination rate constants of steric unhindered alkyl radicals are suggested.

The Reaction of SnCl2 with the Complexes cis‐PtCl2 (phosphorus ligand)2 as studied by 31P and 195Pt NMR

Abstract: The reactions of SnCl2 with the complexes cis-PtCl2P2, P=trialkyl and triaryl phosphines and phosphites, in CDCl3 and CD2Cl2 have been studied using 31P and 195Pt NMR. methods. For equimolar amounts of Sn and Pt, products of the type PtCl(SnCl3)P2 are formed with the complex geometry depending on the nature of P. For PPEt3 and P(OPh)3, further addition of SnCl2 produces trans-Pt(SnCl3)2 complexes. The syntheses of trans-PtCl(SnCl3) (PEt3)2 and trans-Pt(SnCl3)2 (OPh3)2 are described.

Structures of Addition Products of Acetylenedicarboxylic Acid Esters with Various Dinucleophiles. An application of C, H‐spin‐coupling constants

Abstract: Heterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by 13 C-NMR. In particular, C,H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C,C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C,H-spin coupling is again demonstrated in the present series.

Cryptates. XXV. Stability and selectivity of cation inclusion complexes of polyaza‐macrobicyclic ligands. Selective complexation of toxic heavy metal cations

Abstract: We have measured the stability constants of the cryptate complexes formed by ligands 1–4 with alkali, alkaline-earth, transition metal and toxic heavy metal cations. Stabilities and selectivities of complexation of the alkali and alkaline-earth cations are less pronounced in 1–4 than in the parent compounds 5 and 6 and decrease as the number of nitrogen sites increase. Remarkable complexation properties are found towards transition metal and toxic heavy metal cations. The intramolecular cavity of ligands 1–3 is too large for small cations like Co2+, Ni2+, Zn2+ so that the complexes formed are comparatively weak; however these cations are strongly complexed by ligand 4 whose intramolecular cavity has a much smaller size, compatible with their ionic radius. On the other hand, ligands 1–4 all form highly stable cryptates with Cd2+, Hg2+, Pb2+. Thus by the combined operation of the two structural parameters, cavity size and nature of the binding sites, cryptands 2 et 3 present very high selectivities for the complexation of these toxic heavy metal cations with respect to the biologically important ones Na+, K+, Mg2+, Ca2+, Zn2+. The selectivities of ligand 2 for Cd2+, Hg2+ and Pb2+ with respect to Zn2+ are as high as 106, 1018 and 109 respectively. They are much more pronounced than those of previously known complexing agents. Cryptands like 2 and 3 thus present a unique selectivity sequence of special interest in detoxication (decorporation, depollution). Further structural elaboration may allow to design ligands which present a given selectivity pattern of potential use in “cryptatotherapy” and “environment pollution control”. The results also provide evidence for the existence, at low pH, of protonated complexes which probably participate in an acid catalysed process for dissociation of the complexes.

An Enantioselective Total Synthesis of Natural (+)‐Luciduline

Abstract: The naturally occurring Lycopodium alkaloid (+)-luciduline (1) has been synthesized from (R)-5-methyl-cyclohex-2-en-1-one (12) by a sequence of seven steps (s. Scheme 6) in 33% overall yield. The key step 46 presumably involves a transient nitrone 5 which undergoes a highly regioselective intramolecular addition to a non-polarized olefinic bond.

Metal Complexes with Macrocyclic Ligands. XI†. Ring size effect on the complexation rates with transition metal ions

Abstract: The 12-16 membered tetraazamacrocycles 1-6 were synthesized, their protonation constants and complexation kinetics measured at 25° and I = 0.50. The results of Table 1 Show that pK is strongly influenced by the ring size whereas pK and pK are relatively insensitive to it. This can be understood in terms of electrostatic interactions of the positive charges when located on adjacent amino groups. The kinetics of complex formation between the macrocyclic ligands and several transition metal ions have been studied by pH-stat and stopped-flow techniques and the results have been analyzed as bimolecular reactions between the metal ion and the different protonated species of the ligands. The rate constants, given in Table 2, show that the macrocycles react less rapidly than analogous open chain amines. However, for a given protonated species of the ligand the rate of complexation follows the order Cu2+ > Zn2+ > Co2+ > Ni2+ which parallels the sequence of their water exchange rates. For the diprotonated tetraamines LH reacting with Cu2+ the slower rates seem to be mainly a consequence of electrostatic interactions, since a correlation between logk and pK exists. For LH+, however, the complexation rates of a metal ion with the different macrocycles are all in one order of magnitude and do not depend in a regular way on the ring size or the basicity of the ligand. It is therefore suggested that in this case other factors such as unfavourable preequilibria must be considered as important.

Eine einfache Veresterungsmethode im Eintopf‐Verfahren

Abstract: A simple one-pot method for esterification A one-pot procedure for esterifications of primary, secondary and tertiary alcohols and of phenols in high yield is described. Reaction of oxalylchloride with dimethylformamide in a suitable solvent affords dimethylformamide imidechloride; addition of an acid to the suspension of this reagent furnishes an activated carboxy-group derivative, which is then transformed to an ester by addition of pyridine and an alcohol ro a phenol. Reaction temperatures needed for esterification are low, usually 0° to room temperature, reaction times are short.

Spectra of Organic Molecules in Thin Films

Abstract: In order to investigate molecules and their interactions at interfaces we have developed a new spectroscopic technique based on integrated optics. A significant increase in sensitivity has been predicted and observed. We report here the details of the method and some preliminary spectra of organic dye molecules on thin films.

Synthesis and properties of chiral macrotricyclic ligands. Complexation and transport of chiral molecular cations and anions

Abstract: The optically active macrotricyclic cryptands 1 and 2, containing the binaphthyl group as chiral unit, have been synthesized. These compounds contain lateral cavities which may serve as anchoring sites for polar groups of the substrates and a central, chiral cavity large enough for including more or les completely the bulk of a molecular ion. Their complexation properties towards molecular ions give evidence for the occurrence of two types of processes: (a) direct complexation of primary ammonium cations, like phenylethyl ammonium chloride, occurs with either 1 or 2; (b) cascade binding, involving first complexation of an alkali cation followed by pairing with a molecular anion, takes place with 2. Process (b) may be considered as a metallo-receptor model system where binding of an anionic substrate is dependent on initial binding of a cation. In both cases (a) and (b) weak resolution of chiral racemic substrates has been observed by extraction and transport (through a bulk liquid membrane) experiments. This indicates that in the complex the bulk of the substrate should be located close to the chiral unit and therefore more or less in the central cavity. In the case of cryptand 2, the resolution achieved for the (±)-mandelate anion is markedly affected by the nature of the complexed cation.

CH-Acidität in α-Stellung zum N-Atom in N,N-dialkylamiden mit sterisch geschützter Carbonylgruppe zur nucleophilen minoalkylierung

Abstract: CH-Acidity in α-position to the N-Atom ofN, N-Dialkylamides withSterically ProtectedCarbonyl Groups Contribution to the Nucleophilic Amino Alkylation Sterically protected amides 1 such as the 2,4,6-triisopropyl-benzoic acid derivatives 3, 8b and 10 undergo readily H/Li-exchange with s-butyllithium at the CH3N- or CH2N-groups. The resulting organolithium compounds (cf. 9, 11) are alkylated and hydroxyalkylated with primary haloalkanes, aldehydes, and ketones under chain elongation in the amine position of the amides. The (E/Z)-rotamers of the dialkylamides 7 and 8are separated by chromatography; the amides 4–6, 12, and 13 formally derived from β-hydroxyamines are obtained in the (Z)-form only. The configurational (E/Z)-assignments follow from NMR. and IR. data. The erythro and threo configuration of the two diastereomeric amides 12a and 12b are tentatively concluded from Eu(fod)3-1H-NMR.-shift experiments. The results strongly suggest that the H/Li-exchange takes place regioselectively at the CHN group which is in cis-position to the CO double bond (14). The methyl 2,4,6-tri(t-butyl)benzoate (18) can also be deprotonated to the lithium acyloxymethanide 19 which is trapped by alkylation with 1-iodooctane (20). – The steric protection of the carbonyl groups in the products 4–8, 10, 12, 13, and 20 prevents their ready hydrolysis to amines and alcohols, respectively. Therefore, triphenylacetic acid derivatives 21 rather than 2,4,6-triisopropylbenzoic acid derivatives for use in the electrophilic substitution of equation (1) are recommended. The trityl group in 21 may be considered a C-leaving-group (CC protective group, cf. 22, 23). The acetamide 25 reacts readily (26) and then with electrophiles to give products 27a–c. As shown in the Table, the amides 27 are cleaved under a variety of conditions with formation of triphenylmethane. LiAlH4 produces a tertiary amine, CH3Li a secondary amine, and dissolving alkali metals/naphthalene under aprotic conditions mixtures of secondary amine and its formamide (hydrolysed by acid treatment). Thus the overall process (2) is feasible.

Delayed Fluorescence from Upper Excited Singlet States Sn (n > 1) of the Aromatic Hydrocarbons 1,2‐benzanthracene, fluoranthene, pyrene, and chrysene in methylcyclohexane

Abstract: Delayed fluorescence (DF.) spectra of 1,2-benzanthracene, fluoranthene, pyrene, and chrysene in methylcyclohexane were measured at −80° up to the wavenumber corresponding to the energy of two triplets. With all four compounds a weak DF. Sn S0 from the highest state Sn accessible by triplet-triplet annihilation (TTA) was found. Lifetimes of Sn calculated (a) from the ratio of the DF's Sn S0 and S1 S0 and (b) from the difference in line-width of the 0,0-transitions Sn,0 S0,0 and S1,0 S0,0 agree reasonably well. This indicates that population of the highest accessible excited singlet state is the dominating primary process in the excited singlet channel of TTA. There is no evidence for excimer formation being the first step in TTA. DF. spectra extend up to the energy of two triplets. With pyrene the intensity distribution in the hot-band region of the DF. S2 S0 suggests that in TTA the same vibrational selection rules are valid as in one-photon absorption S2 S0 and that vibrational relaxation within the S2 manifold is slow compared with internal conversion S2 ↝ S1. The experimental technique is described in detail and experimental difficulties arising from impurities and photoproduct formation are discussed.

Résonance magnétique nucléaire de 13C et 17O d'éthers aliphatiques. Effets γ entre les atomes d'oxygène et de carbone

Abstract: The 13C- and 17O-chemical shifts of 31 aliphatic ethers are measured and discussed. The 17O-chemical shifts of the ethers ROR′ correlate with chemical shifts for the methylene groups of the corresponding alkanes RCH2R′. The constant of proportionality can be related to the orbital expansion term 〈r−3〉2p. The δc for carbon atoms can also be correlated with δc for the corresponding alkanes. The origin of the correlation is discussed taking into account the conformational modifications resulting from introduction of an oxygen atom in an alkyl chain.

Synthesen von Moniliformin, einem Mycotoxin mit Cyclobutendion-Struktur

Abstract: Syntheses of Moniliformin, a Mycotoxine with a Cyclobutenedione Structure Six different routes to 3-hydroxy-3-cyclobutene-1, 2-dione (4), the free acid of the mycotoxine Moniliformin (=alkali salt of 4) are described. A common feature of all pathways is the synthesis of cyclobutanes having the oxidation level 6. Moniliformin precursors which are easily transformed to 4 by acid catalysed hydrolysis include [2+2]-cycloadducts of ketene with tetraalkoxy-olefins, 3,4,4-trialkoxycyclobutenes, derivatives of polyfluorinated cyclobutenes, the brominated [2+2]-cycloadduct of ethyl vinyl ether and dichloroketene, tetrabromocyclobutanone, [2+2]-photocycloadducts of dichlorovinylenecarbonate with dichloroethylenes, and the dimer of chloroketene. The most convenient synthesis via the [2+2]-cycloadduct of tetraethoxyethylene and ketene (14b) is reported in detail and produces 4 in four simple steps in 57% overall yield. In addition, two new syntheses of squaric acid (56) are described.

CIDNP.‐Investigation of Photoinduced Polymerization

Abstract: The photochemistry and some initiator characteristics of the polymerization initiators ω,ω-dimethoxy-ω-phenyl-acetophenone (A), ω,ω-diethoxy-acetophenone (B) and ω,ω-diisopropoxy-acetophenone (C) have been studied by 1H- and 13C-chemically-induced nuclear polarization (CIDNP.) experiments. The primary reaction of initiator A is a Norrish-type I cleavage, while for B and CNorrish-type I and Norrish-type II cleavages are of comparable importance. Three different recombination products could be detected for initiator A which correspond to the three canonical resonance forms of the substituted benzyl radical. In a number of polymerization experiments the competition between reactions among initiator radicals and reactions of initiator radicals with the monomer acrylic acid methyl ester was studied at low concentrations of the monomer. These experiments give insight into the first steps of the polymerization process. The positive sign of the α-hydrogen hyperfine couplings of the dimethoxy-methyl- and the diisoproproxymethyl-radicals could be established, in agreement with the deviation of these radicals from planarity. A slow square-wave light-modulation technique has been employed in 13C-FT.-experiments to measure absolute CIDNP.-intensities.

A New Type of Claisen Rearrangement Involving 1,3-Dipolar Intermediates. Preliminary communication†

Abstract: During the reaction of allyl ethers, allyl sulfides and allyl selenides with in situ prepared dichloroketene (1), 2 competing pathways are observed. Besides [2+2]-cycloaddition, an unprecedented [3,3]-sigmatropic (Claisen) rearrangement via a 1,3-dipolar intermediate takes place. It leads to O-, S- or Se-esters of α,α-dichloro-γ, δ-unsaturated acids containing an inverted allylic group. Starting from cyclic n-membered α-vinyl-substituted ethers, lactones with n+4-membered rings are formed. A very facile synthesis of the naturally occurring macrolides (±)-phoracantholide I (10) and (±)-phoracantholide J (11) illustrates the synthetic utility of this new ‘ketene’ Claisen rearrangement.

Racematspaltung und Bestimmung der absoluten Konfiguration von 2, 6-disubstituierten Bicyclo [3.3.1]nonanen

Abstract: Resolution and Determination of the Absolute Configuration of 2,6-Disubstituted Bicyclo[3.3.1]nonanes (±)-endo, endo-Bicyclo [3.3.1]nonane-2,6-diol was resolved via diastereomeric camphanic acid esters. Conversion of the (+)-enantiomer 2via (+)-5 and (+)-6 as key intermediates gave (+) methyl 3-(3-oxocyclohexyl)-propionate (7) which independently could be prepared also from the known (+)-(R)-3-oxo-cyclohexane-carboxylic acid (8). These chemical correlations establish the absolute configuration of (+)-2, (+)-5 and (+)-6 as well as that of (+)-bicyclo [3.3.1]nonane-2,6-dione (1) obtained by oxidation of (+)-2. The chiroptical properties of 1 and 6 are discussed.

On the Mechanism of Magnetic Quenching of Fluorescence in Gaseous State

Abstract: The pressure dependence of fluorescence quenching of gaseous glyoxal has been measured in the presence of a magnetic field of 1–8 kG. Below 5 kG both the collision free lifetime and the collisional quenching constant were found to be dependent upon magnetic field strength. Above 5 kG the Collisional quenching constant turned out to be nearly equal to the value without magnetic field and the collision free lifetime took a constant value larger than that without magnetic field. The magnetic enhancement of the intramolecular radiationless transition of gaseous molecules has been studied theoretically. The phenomenon is shown to be explained by considering two mechanisms, mechanisms I and II. Mechanism I is due to the interaction of a primary state with secondary states through the Zeeman hamiltonian. Mechanism II is due to the shift and broadening of appropriate rovibronic levels by the Zeeman effect.

Vitispiranes, Important Constitutents of Vanilla Aroma

Abstract: The preparation of the vitispiranes 9 and 10, identified among the volatiles of vanilla, from the theaspiranes 1 and 2via the intermediates 4 and 5 and the allyl alcohols 7 and 8, respectively, is described. The theaspiranes 1 and 2 can be obtained from the compounds 11–15 or 24–26.

Michael-Addition von Thiocarbonsäureestern Anwendung bei der Synthese von (±)-Jasminketolacton

Abstract: Michael addition of carbothioates. Application to the synthesis of (±)-jasmine ketolactone It is shown that the lithium enolate of S-t-butyl thioacetate adds to 2-cyclopentenone in the β-position and that fluoride ions catalyze the 1, 4-addition of the trimethylsilyl enol ether of S-t-butyl thioacetate (5) to 2-cyclopentenone (4) to give 6. These novel versions of the Michael addition have been applied to a synthesis of jasmonoid compounds. Cleavage of the trimethylsilyl enol ether in 6 with tetrabutylammonium fluoride produced the corresponding ketone enolate which could be trapped in situ by alkylation with 1-bromo-5-(2′-tetrahydropyranoxy)-2-pentyne (7) to form 8. Removal of the alcohol protecting group in 8, followed by partial hydrogenation of the triple bond over Lindlar palladium and mercury ion promoted hydrolysis of the carbothioate moiety in 9, led to 5′-hydroxy jasmonic acid (10, Scheme 3). 10 was converted into the S-(2-pyridyl) carbothioate and cyclized in dilute benzene solution under the influence of silver ion to give (±)-jasmine ketolactone (1, Scheme 4), a component of the essential oil of Jasminum grandiflorum, in 72% yield. Similarly, methyljasmonate (2, Scheme 2) was obtained from 6 by the reaction with 1-bromo-2-pentyne and tetrabutylammonium fluoride followed by methanolysis and partial hydrogenation of the triple bond.

Über 3,3,6,9,9‐Pentamethyl‐2,10‐diaza‐bicyclo‐[4.4.0]‐1‐decen und einige seiner Derivate. Über synthetische Methoden. 13. Mitteilung

Abstract: 3,3,6,9,9-Pentamethyl-2,10-diaza-bicyclo[4.4.0]-1-decen and some of its derivatives A simple synthesis for the bicyclic amidine 1 (Scheme 3) is described. This base and the salts which were prepared from it show solubility characteristics which make the amidine a potentially useful reagent for salt formation of carboxylic acids and related proton complexes of bidentate ligands. Among the derivatives made from 1 are the sterically strongly hindered N-alkylated amidines 11, 12 and 14 (Scheme 5), as well as the stable crystalline N1-oxidoamidine-N2-oxyl radical 2 (Scheme 6). The ability of the latter to serve as a paramagnetic chelating ligand for metal ions is illustrated by the preparation of a corresponding nickel(II) complex. The radical is also a source for the α-nitronyl-nitrosonium cation 4 which shows in its reactivity towards conjugated dienes and olefines some of the expected resemblance to singlet oxygen.

Chemically modified silicon dioxide surfaces. Wetting properties of compact n-alkyl-dimethylsiloxy layers, and their mixed surface crystals. The Question of Wetting of Molecularly Rough and of Swollen Surfaces†

Abstract: By means of contact angle measurements, it has been shown that compact alkyl-dimethyl-siloxy layers can be obtained on the surface of acid leached glass using the method previously developed for fume silica. Subsequently, a series of densest possible alkyl-dimethylsiloxy layers were prepared having non-branched alkyl substituents, CzH2z+1, (with z=1, 2, 3, 6, 10, 14, 18 and 22) and wetting angles were measured as a function of the temperature. On surfaces coated with longer alkyl substituents the formation of mixed crystals, consisting of the sparse chemically bonded alkyl layer and a n-alkane acting as a wetting agent, has been observed at low temperatures. At higher temperatures such surfaces appeared to be «molten», and at the same time swollen by the wetting agent. Equations have been derived to deduce the free surface energies and to account for the wetting properties of rough and swollen surfaces. Application of the results to the data presented in this paper revealed that glass surfaces covered with short alkyl chains (methyl, ethyl and propyl) are rough and those covered with long chains (tetradecyl, octadecyl and docosyl) are swollen. Surfaces formed by hexyl- and decyl-dimethylsiloxy substituents exhibit intermediate properties.

Vibrational Spectra of Anhydrous Lanthanum, Europium, Gadolinium, and Dysprosium Nitrates and Oxinitrates

Abstract: IR. and Raman spectra of LnONO3 (50–4,000 cm−1, LnLa, Gd, Eu, and Dy) are reported and discussed. The low frequency region of the spectra reflects the cubic structure of these compounds. The dimensions of the cubic unit cells determined by X-Ray powder diagrams are: 12.81 ± 0.05 A for EuONO3, 12.69 ± 0.05 A for GdONO3, and 12.67 ± 0.05 A for DyONO3. The vibrational frequencies of the nitrato group are consistent with a bidentate nitrate of C2v symmetry. The synthesis of anhydrous Ln (NO3)3 (LnLa, Gd, Eu, and Dy) by dehydration of the corresponding penta- or hexahydrates is described. The IR. and Raman spectra (50-4,000 cm−1) are analysed. Splitting of the bands point to a complex structure of these compounds. All six vibrational modes of the nitrato group are observed and the data are again consistent with bidentate NO−3 moieties. Finally, an analytical control of the purity of Ln (NO3)3 is suggested.

Inductive, Hyperconjugative and Frangomeric Effects in the Solvolysis of 1‐Substituted 3‐Bromoadamantanes. Polar effects IV

Abstract: Three kinds of polar substitutent effects are observable in the solvolyses of 1-R-substituted 3-bromoadamantanes (VI). This follows from the relationship between products, rate constants k in 80% ethanol, and the inductive substituent constants σ of the substituent R. Alkyl groups and electron-attracting substituents at C (1) control the rate by their inductive effects alone, since logk correlates closely with σ. However, rates are higher than predicted on the basis of the respective σ values when conjugating (+ M)-substituents or electrofugal groups are attached to C(1). These exalted substituent effects are attributed to CC-hyperconjugative relay of positive charge from the cationic center at C(3) to the substituent at C(1). When the substituent is a strong electron donor (e.g. O− and S−), accelerated substitution gives way to heterolytic fragmentation, rates and products then being controlled by the frangomeric effect.

A New Method for the Construction of Macrolides Stereoselective Synthesis of (±)‐Phoracantholide J. Preliminary communication

Abstract: Starting from 5-chloro-2-pentanone (1) the naturally occurring 10-membered lactone phoracantholide J (8a) has been synthesized as its racemate in a sequence of six steps (Scheme 2). Salient features of the syntheis include an internal selenium assisted acetal formation (45) and a stereoselective Claisen rearrangement (678). This general synthetic strategy offers an alternative approach towards the construction of macrocyclic lactones.