摘要:

A series of yttrium, ytterbium, and lanthanum hexa-or heptacoordinate complexes were prepared and their catalytic activities tested in the hetero Diels–Alder reaction between crotonaldehyde and ethyl vinyl ether. It was found that a pKabelow 7 of the ligand was necessary but not sufficient for catalytic activity. It was determined that the ligand should possess a perfluoromethyl-β-diketonate functionality. Other factors such as bite angle and hinging motion may also play a determining role.Keywords: lanthanide, catalyst, hetero Dielsis–Alder, yttrium, ytterbium, lanthanum.

ISSN:0008-4042    

DOI:10.1139/v97-125

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The scope of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include cyanide anion as the nucleophile. Highest yields of adducts were obtained when the alkene or diene has an oxidation potential less than ca. 1.5 V (SCE). No adducts were obtained from 2-methylpropene (9), oxidation potential 2.6 V. Oxidation of cyanide anion, by the radical cation of the alkene or diene, can compete with the combination. With the alkenes, 2,3-dimethyl-2-butene (2) and 2-methyl-2-butene (10), both nitriles and isonitriles were obtained; isonitriles were not detected from the reactions involving the dienes, 2-methyl-1,3-butadiene (11), 2,3-dimethyl-1,3-butadiene (12), 4-methyl-1,3-pentadiene (13), 2,4-dimethyl-1,3-pentadiene (14), and 2,5-dimethyl-2,4-hexadiene (6). The specificity, nitrile versus isonitrile, is explained in terms of the Hard-Soft-Acid-Base (HSAB) principle. The photo-NOCAS reaction also occurs with the allene, 2,4-dimethyl-2,3-pentadiene (15), cyanide combining at the central carbon. Factors influencing the regiochemistry of the combination step, Markovnikov versusanti-Markovnikov, have been defined. Cyanide anion adds preferentially to the less alkyl-substituted, less sterically hindered, end of an unsymmetric alkene or conjugated diene radical cation, forming the more heavily alkyl-substituted radical intermediate. High-levelabinitiomolecular orbital calculations (MP2/6-31G*//HF/6-31G*) have been used to determine the effect of alkyl substitution on the stability of the intermediates, β-cyano and β-isocyano alkyl radicals, and products, alkyl cyanides and isocyanides. The more heavily alkyl-substituted radical is not necessarily the more stable. The product ratio (Markovnikov versusanti-Markovnikov) must be kinetically controlled.Keywords: photochemistry, radical ions, electron transfer, nitriles, isonitriles.

ISSN:0008-4042    

DOI:10.1139/v97-126

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

A simple procedure for the asymmetric synthesis of lignans via chiral β-benzyl-γ-butyrolactones has been developed. The key benzylbutyrolactone intermediates were efficiently synthesized using a six-step procedure, starting from 3,4-(methylenedioxy)cinnamic acid. The key step in this sequence was a highly diastereoselective alkylation of anN-acyloxazolidinone enolate. The resulting β-benzyl-γ-butyrolactones were subsequently transformed into the benzylidene lignans gossypifan and savinin (hibalactone) via aldol condensation–dehydration reactions, and into the dibenzylbutyrolactone lignan 4′-demethylyatein, through alkylation. Oxidation of 4′-demethylyatein with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) affordedcis-andtrans-benzylidenebenzylbutyrolactones, whereas oxidation with DDQ/TFA gave 4′-demethyl-deoxyisopodophyllotoxin.Keywords: lignans, synthesis, asymmetric, biosynthesis, oxidation, benzylbutyrolactones.

ISSN:0008-4042    

DOI:10.1139/v97-127

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The hydrolysis constants of trimethyl- and triethyltin(IV) ions and the stability constants for the complexation of trimethyltin(IV) with trimethylamine,N-substituted glycines (N,N-dimethylglycine andN,N,N-trimethylglycine), and aminopolycarboxylate ions (N-methyliminodiacetate, nitrilotriacetate, and ethylenediamine-N,N′-diacetate) were determined by the potentiometric method at 25 °C andI = 0.l mol dm−3((H, Na)ClO4and (H, Na)NO3). The structural parameters of Me3Sn(OH2)2, Et3Sn(OH2)2, and Me3Sn(OH2)(OH) in solution were determined by the EXAFS method. The coordination structures of trimethyltin(IV) complexes withN-substituted glycines, trimethylamine, and aminopolycarboxylate ions are discussed on the basis of the electronic interaction between the donor atom and Sn(IV).Keywords: organotin(IV), EXAFS, structure, potentiometry.

ISSN:0008-4042    

DOI:10.1139/v97-128

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The mechanism of the hydrolysis of acetylimidazole in aqueous perchloric, sulfuric, and hydrochloric acid mixtures has been determined. Benzoylimidazole was also studied in the latter two acids. The method of analyzing the available data, pseudo-first-order reaction rate constants as a function of acid concentration and, in one case, temperature, is the excess acidity method, here applied to the same reaction in the three different acid media, allowing their comparison. The reaction is not acid catalyzed; the rates decrease with increasing acidity. The substrate reacts in the form that is monoprotonated on the imidazole ring; it is 100% protonated at acidities much lower than those used here. Acetylimidazole is shown to become diprotonated at high acidity, protonating on the carbonyl oxygen, but the diprotonated form is not reactive. The hydrolysis involves the reversible addition of one water molecule to the substrate to give a tetrahedral intermediate; at low acidities the decomposition of this hydrate is the rate-determining step, but as the acidity increases and the water activity decreases its formation becomes rate limiting. Hydroxide catalysis was also observed in dilute perchloric acid, but this is swamped by nucleophilic catalysis by the acid anion in HCl and H2SO4.Keywords: acylimidazoles, excess acidity, hydrolysis, protonation, tetrahedral intermediate.

ISSN:0008-4042    

DOI:10.1139/v97-129

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The fundamental and overtone vibrational spectra of Ba(NO2)2•X2O (X=H,D) have been interpreted using the local mode model. The calculated parameters are consistent with the structure, which has one strong hydrogen bond and one weak bifurcated hydrogen bond. Evidence for Fermi resonance between stretch and bend features having similar energies is discussed. Evidence for the breakdown of the simple Morse oscillator model, which was quite successful forhydrogen bonds, is also presented.Keywords: overtone, spectra, near-infrared, hydrates, local-mode model.

ISSN:0008-4042    

DOI:10.1139/v97-130

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The cuprous iodide catalyzed reaction of 2-methyl-2-[1-(3-benzoyl-4-phenylpyridinium)]acetic acid methyl ester bromide (5), prepared by reaction of 3-benzoylpyridine (4) with racemic methyl 2-bromopropionate, with phenylmagnesium chloride at −23 °C afforded the 2-methyl-2-[1-(3-benzoyl-4-phenyl-1,4-dihydropyridyl)acetic acid methyl ester (6, 74%), which was a mixture of four diastereomers. Recrystallization of this diastereomeric mixture from diethyl ether afforded a solid product(6a-solid, 30%, 4R*,2R* and 4S*,2S*) and an oil product(6b-oil, 39%, 4R*,2S* and 4S*,2R*), each consisting of a mixture of two diastereomers that differ in relative configuration (R* orS*) at the 1,4-dihydropyridine C-4 position and the-CH(Me)CO2Me moiety. Addition of the Grignard reagent from either of the two diastereotopically different faces of the planar pyridinium salt (5) gives rise to two diastereomeric products6a-solidand6b-oilin which the C-4 phenyl substituent is pseudo-axial to the boat-shaped 1,4-dihydropyridine ring. Alkaline hydrolysis, or ammonolysis, of the acetic acid methyl ester (6) afforded the respective acetic acid (7), or acetamide (8), derivative.Keywords: 1,4-dihydropyridines, diastereomers.

ISSN:0008-4042    

DOI:10.1139/v97-131

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

We have combined results of earlier measurements leading to equilibrium constants for formation and reaction of aluminum–citrate complexes in aqueous solution at 298.15 K with results of our calorimetric measurements at this same temperature to obtain ΔHvalues for reactions of Al3+(aq) with citrate ions. Results of some of these calorimetric measurements also lead to a value of ΔHfor the reaction Al3+(aq) + 4 OH`(aq) = Al(OH)4−(aq).Keywords: aluminum hydroxide, Al(OH)4−(aq), thermodynamics; aluminium–citrate complexes, thermodynamics; citrate–aluminum complexes, thermodynamics; aluminum (3+, aq), thermodynamics.

ISSN:0008-4042    

DOI:10.1139/v97-132

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Arrhenius parameters for the reaction of hydrogen atoms with 3-methyl-2-butanone, 3-pentanone, cyclopentanone, 4-methyl-2-pentanone, and 2-butanone in aqueous solution have been directly calculated from electron paramagnetic resonance free induction decay (FID) attenuation measurements. For these compounds, absolute scavenging rate constants at 25.0 °C of (8.84 ± 0.26) × 107, (4.20 ± 0.15) × 107, (4.91 ± 0.28) × 107, (3.25 ± 0.27) × 107, and (2.20 ± 0.32) × 107 dm3mol−1s−1, with corresponding activation energies of 17.43 ± 0.29, 20.69 ± 0.31, 18.73 ± 0.36, 22.24 ± 0.80, and 22.30 ± 1.04 kJ mol−1were determined, respectively. Competition kinetic measurements based on total H2yields have established that for all of these ketones the dominant hydrogen atom reaction path is by•H atom abstraction. The new activation energy for 2-butanone is much lower than the previously reported value of 40.1 ± 0.7 kJ mol−1with this difference attributed to interfering reactions from the added bromide previously used as a hydroxyl radical scavenger.Keywords: Arrhenius, kinetics, hydrogen atom, aqueous, ketones.

ISSN:0008-4042    

DOI:10.1139/v97-133

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Phenylalanine is the simplest among the four natural amino acid residues that have aromatic side chains. The ab initio conformational analysis performed at the RHF/3-21G level on a phenylalanine-containing diamide model system (N-Formyl-L-Phe-NH2) revealed 19 different structures. Single-point energy calculations were performed using RHF/6-31+G* and DFT(B3LYP)/6-311++G** levels for all conformers. The inverse (γL) and the normal (γD) gamma turn, the extended (βL), the left-handed helical, and the inverse polyproline IIbackbone conformers each have three (g+,a, andg−) side-chain (χ1,) rotamers. Theand thetype main-chain conformers have only two side-chain orientations, respectively. No minima have been found for the conformational building unit of the right-handed helicaland for the polyproline IIstructures. The present ab initio conformational analysis for For-L-Phe-NH2is a unique example in which a systematic and complete conformational set was established for a diamide system with an aromatic side chain. Analytic vibrational frequency calculations were established for all stationary points found as minima on the potential energy surface. These data may be used in the future as reference conformers in more detailed vibrational and (or) chemical shielding calculations or during the structural analysis of peptides and proteins by X-ray or NMR techniques.Keywords: ab initio peptide conformers, all RHF/3-21G structures of For-L-Phe-NH2, backbone – side-chain interaction in phenylalanine-containing peptide models.

ISSN:0008-4042    

DOI:10.1139/v97-134

出版商:NRC Research Press    

年代:1997

数据来源: NRC