Six structurally related crown ethers have been studied by electron impact (EI) mass spectrometry, From accurate mass measurements, metastabel ion and mass-analyzed ion kinetic energy (MIKE) spcctra, plausible fragmentation pathways were deduced, All compounds showed very abundant molecular ions and some amount of the doubly charged molecular ions, The primary fragmentations always involved the catechol moiety and consequent loss of oxirane although the fragment(ation)s also showed a clear dependence on the functionalities X in one of the crown ether segments, -OCH(2)XCH(2)O- (compound/X: 1/-CH2-, 2/-C=C-, 3/trans-CH=CH-, 4/cis-CH=CH-,[GRAPHICS]and 6/-CH2CH2-).

N-15 NMR spectra of four series of amidines (R(2)R(3)N-CR(1)=NR(4); 24 compounds) in CDCl3 solutions have been recorded and the chemical shifts of both nitrogen atoms assigned. The relation between substitution at the three sites of the amidino [>N-C(=N-)-] group and the N-15 NMR chemical shifts of both nitrogen atoms is discussed on the basis of the results and the data accessible in the literature for seven other non-tautomerizing open-chain amidines.It is shown that, due to a strong conjugation in the amidino group, a substituent at either of the two nitrogen atoms exerts opposite effects on the shielding and thus on the chemical shifts of the amino and imino nitrogen atoms.The effect of substitution at either of the two nitrogen atoms depends on substituents at the other two sites i.e. at the second nitrogen and the amidino carbon atom For amidines containing a substituted phenyl ring at the imino (N-2) nitrogen atom-the chemical shifts of both nitrogen atoms correlate with the Hammett-type constants of substituents, but the slopes of the regression lines are opposite for both atoms, and depend on substituents at the amino (N-1) nitrogen atom.The results indicate that calculation of the N-15 NMR chemical shifts in tautomerizing amidines (i.e. containing a hydrogen atom at the amino nitrogen) on the basis of the shifts in the corresponding methylated model compounds may yield incorrect results, and that this is the most probable source of discrepancies in estimations of the tautomeric equilibria by the nitrogen NMR method.

15 N NMR spectra of four series of amidines (R2R3N–CR1NR4; 24 compounds) in CDCl3 solutions have been recorded and the chemical shifts of both nitrogen atoms assigned. The relation between substitution at the three sites of the amidino [〉N–C(N–)–] group and the 15N NMR chemical shifts of both nitrogen atoms is discussed on the basis of the results and the data accessible in the literature for seven other nontautomerizing open-chain amidines.It is shown that, due to a strong conjugation in the amidino group, a substituent at either of the two nitrogen atoms exerts opposite effects on the shielding and thus on the chemical shifts of the amino and imino nitrogen atoms.The effect of substitution at either of the two nitrogen atoms depends on substituents at the other two sites i. e. at the second nitrogen and the amidino carbon atom. For amidines containing a substituted phenyl ring at the imino (N2) nitrogen atom the chemical shifts of both nitrogen atoms correlate with the Hammett type constants of substituents, but the slopes of the regression lines are opposite for both atoms, and depend on substituents at the amino (N1) nitrogen atom.The results indicate that calculation of the 15N NMR chemical shifts in tautomerizing amidines (i.e. containing a hydrogen atom at the amino nitrogen) on the basis of the shifts in the corresponding methylated model compounds may yield incorrect results, and that this is the most probable source of discrepancies in estimations of the tautomeric equilibria by the nitrogen NMR method.

Acyclic Stereochemical Analysis. III. Estimation of the Preferred Conformers of Substituted γ-Chloropropyl-methyl Ethers by NMR Spectroscopy and Molecular Modelling The conformers of the various stereoisomers of a series of γ-chloropropyl methyl ethers 1–4 with 1, 2 and 3 chiral centres have been estimated by Molecular Modelling (MOBY) and the results compared with the data of previous 1H and 13C NMR studies. The calculations corroborate former NMR results and the remarkable potential of both NMR spectroscopy and Molecular Modelling to assign even four diastereomers of acyclic molecules with three chiral centres.

A new method and computer program for LIS calculations of highly flexible molecules was developed using molecular geometries obtained by randomly varied dihedral angles (up to ten angles rotating freely and independently from each other are possible) as start conformations for a STEEpest Descent (STEED) search algorithm to find minima characterized by the R factor at the ‘LIS R factor hyper surface.’ This STEED–LIS method allows easy, accurate and rapid access to the probable global minimum of the R factor depending on up to ten variable dihedral angles. This means that LIS investigations of highly flexible molecules with more than one freely rotating dihedral angle are no longer complicated and time consuming, but can be carried out automatically using the developed programs [JLLISSD (with interactive parameter dialogue) and LISSDEMU (using parameter input files prepared by LISSDINP)]. The results obtained are in very good agreement with known or reported preferred conformations of the investigated test compounds, propan‐1‐ol, butan‐1‐ol and ethyl p‐methoxycinnamate.

The transannular N-heterocyclization of 1,5-cyclooctadiene using either N-bromo- or N-iodosuccinimide and cyanamide is a preparatively useful method for the synthesis of 9-azabicyclononanes 2 and 3. This reaction failed when N-chlorosuccinimide was employed. The chloro-substituted derivative 2c was prepared, with retention of configuration, from both pure 2a and from an equimolar mixture of 2a and 3a by nucleophilic displacement with chlorine, followed by dehydration of the resulting carboxamide 4c. Acidic hydrolysis of the cyanamide group in 2a and 3a gave the ureas 4a and 5a. Reaction of 2a and 3a with tributyltin hydride gave the debrominated derivatives 8 and 9, while treatment of the same compounds with LiAlH4 afforded a single air-sensitive product which was trapped as the urea 7. Substitution of the halogens in 2a and 3a (or 2b and 3b) by acetoxy groups using silver acetate in acetic anhydride gave 12 and 13 as a result of inversion at one side and retention of the configuration on the other side of the starting materials which was accompanied by transamidation at the nitrogen bridge. Treatment of 2a with dicyclohexylethylamine led to complete dehydrobromination and the formation of 14.

13C NMR contact shifts induced by the 2,2,6,6,‐tetramethylpiperidine nitroxide radical (TMPN) were measured for N,N‐dimethyl‐para‐substituted benzamides and N,N‐dimethyl‐β‐bromo‐ and ‐α,β‐methylacrylamides. The shifts were linear functions of radical concentration, cr. It was found that N‐methyl group anti to the carbonyl group is a better acceptor site than the syn‐N‐methyl group and that there is a relationship between Δδ/dcr and the non‐planarity of molecules of amides with an s‐skew conformation. The electrostatic potential of amide molecules obtained from MNDO calculations was used to predict the stereospecific interaction with radicals.

A wide variety of push-pull alkenes were studied by means of variable-temperature H-1 and C-13 NMR spectroscopy with respect to the configuration/conformation and the barriers to rotation about partial C-C and C-N double bonds. For the assignment of the C-13 NMR spectra especially the semi-selective INEPT pulse sequence and an incremental system for estimating the C-13 chemical shift values of aromatic carbon atoms proved useful. The nfluence of thioether, sulphone and sulphoxide moieties in the acceptor part of the push-pull system on the pi-electron distribution is critically considered.

A new method for lanthanide‐induced shift (LIS) calculations of highly flexible structures was developed using molecular geometries obtained from the random variation of dihedral angles (up to ten) which are rotating freely and independently of each other. The result of the new method (program JLLISMUL) is a data set containing the specified random dihedral angles and the corresponding agreement factor R (as a criterion for the fitting of the calculated and experimental LIS values). For comparison, the same random procedure was also used for parallel force field calculations using the molecular mechanics program PIMM. The results obtained by the new method are in very good agreement with known or reported preferred conformations of five highly flexible molecules which were used as examples for examining the new strategies of obtaining the preferred conformers. The minima of both the ‘LIS R factor hyper surface’ and the ‘energy hyper surface’ are rapidly accessible.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The stereochemistry of eight thiocyanatomethylenecycloalkanones has been studied by H-1 and C-13 NMR spectroscopy. In addition, an LIS (lanthanide-induced-shift) study was performed for two and an X-ray diffraction study for one of the compounds. In 2-thiocyanatomethylenecycloalkanone (the only one with both Z and E isomers) the vicinal carbon-proton coupling constant of the carbonyl carbon to the side-chain proton proved to be 5 Hz larger in the Z isomer than in the E isomer. Similar, rather large, coupling constants for the other six compounds support their Z arrangement. In the Z isomer of 2-thiocyanatomethylenecycloalkanone the C = O and SCN carbons are deshielded by 5.5 ppm compared with those in the E isomer; this is due to the steric closeness and the 1,5-intermolecular S...O interaction. In all of the compounds of Z stereochemistry, the SCN carbon chemical shift varied to within 1.7 ppm. LIS results for two compounds are in accordance with their assumed Z arrangement. 2-Thiocyanatomethylenecycloheptanone crystallizes in the triclinic space group P-1 (No.2) with cell dimensions: a = 6.152(1), b = 7.855(1), c = 10.302(2) angstrom, alpha = 103.06(1), beta = 91.58(1), gamma = 104.71(1)-degrees and V = 467.1(1) angstrom-3 with Z = 2. Full-matrix least-squares refinement of 142 parameters gave R = 0.041 for 1162 reflections [I > 3-sigma-I]. The S...O distance is 2.537(2) angstrom and the angle C-S...O is 172.2(1)-degrees, indicating moderate intramolecular S...O interaction in 2-thiocyanatomethylenecycloheptanone.

Amorphous silica solubilities in the presence of organic (mannitol, oxalate, citrate, catechol) and inorganic (SO4(2-)) ligands have been studied in 0.6 M Na(Cl) medium at 25-degrees-C. Apart from the catechol system, the experiments were performed at neutral pH with an equivalent ligand concentration of less-than-or-equal-to 0.6 M. In the presence of mannitol, the solubility of SiO2(am) was found to decrease with increasing ligand concentration. With oxalate, citrate and sulphate, an increased solubility was observed. However, the effects were small and it was concluded that the formation of soluble Si complexes of these ligands under natural water conditions could be neglected. With catechol, the measurements were performed under slightly alkaline conditions and a significant solubilization was noticed. Composition and stability of the species causing this solubilization was evaluated from precise potentiometric titration data in homogeneous Si(OH)4-catechol solutions. These data cover the ranges 2 less-than-or-equal-to -log [H+] less-than-or-equal-to 10; 0.002 less-than-or-equal-to B less-than-or-equal-to 0.0004 M and 0.002 less-than-or-equal-to C less-than-or-equal-to 0.015 M, with C/B ratios of 1, 2, 2.5, 3 and 5 (B and C denote the total concentration of Si and ligand, respectively). With -log [H+] greater-than-or-equal-to 7, a hexacoordinated complex, SiL3(2-) is formed with log K [Si(OH)4 + 3H2L half-arrow-pointing-right half-arrow-pointing-left SiL3(2-) + 2H+ + 4 H2O] = - 10.44 +/- 0.029 (3-sigma). The recorded solubilization of amorphous silica was fully explained by the formation of this species.