Search

Pelargolactam (1-azacyclodecan-2-one) was studied by 1H and 13C NMR spectroscopy in solution between 25 and −145°C. Equilibrium solutions of pelargolactam at 25°C consist mainly of the trans isomer together with 5–10% of the cis isomer. Solid pelargolactam consists of a mixture of cis and trans isomers, the cis form dissolving more rapidly than the trans form at −95°C. At temperatures above 90°C the 13C NMR resonances for the cis and trans amides coalesce to one averaged set. A conformational process with a coalescence temperature of −30 to −50°C and a free energy of activation of ΔG≠ = 46 kJ mol−1 observed in the 1H spectrum of the trans isomer is interpreted as geminal exchange in the methylene groups. A lower barrier process was also observed, with a coalescence temperature of ca −115°C and ΔG≠ = 32 kJ mol−1. Only the latter process was observed in the 13C spectrum, and below −120°C all 13C lines split in two in the proportion 1:2. This process is interpreted as the interconversion between two trans conformers. Four conformational candidates were considered and their relative energies calculated by molecular mechanics.

The conformational properties of enantholactam (1-azacyclooctane-2-one) in solution were studied by 1H and 13C NMR at various temperatures. Only the cis-isomer is present and it occurs in two different conformations in the ratio 3:2, as observed by 13C NMR at − 133°C. In the temperature range − 50 to − 140°C, two conformational processes were observed. A low energy process was visible in both the 1H and the 13C NMR spectra with coalescence temperature ca. − 110°C and a free energy of activation of 30 kJ mol−1, identified as interconversion between the two conformations. A higher energy process was visible in the 1H but not in the 13C NMR spectrum with coalescence temperature ca. − 96°C and free energy of activation 35 kJ mol−1. This process was identified as geminal 1H exchange. A conformational interconversion scheme for partial and full site exchange of protons and carbon atoms, explaining the variable-temperature NMR results, is proposed.

The crystal structures of the two title Li+ complexes, C-11 H22 O3 . LiClO4 (III) and C-12 H24 O4 . LiClO4 . 0.5H2O (IV), have been determined by X-ray diffractometry. The structure of III was refined to R = 0.080 for 1214 observed reflections, that of IV, with two independent complex entities, to R = 0.057 for 1372 reflections. The conformation of the 12-crown-3 ring is the same in III and IV; both follow, as predicted, the diamond lattice, and this is true also for the side-arm in IV. The cation is tetracoordinated in III but, as expected, pentacoordinated for the side-armed ligand IV. The large differences observed in complex stabilities for III and the previously studied complexes, I (the unsubstituted 12-crown-3) and II (the hexamethyl-substituted 12-crown-3), can be correlated to the gauche torsion angles in the C-C bonds of the ring, the quaternary carbons introducing conformational rigidity.

Conditions for obtaining the optimum yield of the cyclic trimer in the cationic oligomerization of oxetanes have been determined. At moderate dilution (0.05 M) with catalytic quantities of BF 3 in CH 2 Cl 2 , the yield of the cyclic trimer (1,5,9-trioxacyclododecane) from oxetane could be increased to 50% at the expense of the cyclic tetramer (1,5,9,13-tetraoxacyclohexadecane), 12%, and polymer. In contrast, 3,3-dimethyloxetane consumed the BF 3 catalyst, which had to be renewed, producing, in a slow reaction, a homologous series of fluorohydrins, together with cyclic oligoethers

C 13 H 26 O 5 cristallise dans Pmc2 1 avec a = 17,674, b = 9,765 et c = 8,117 A, Z = 4; affinement jusqu'a R = 0,041.

A recollection of how one of the steps in an attempted synthesis of 1,6-dimethyl-cyclodecapentaene became a glassblower's nightmare.

A variety of branched or macrocyclic polyether ligands of 1,s- dioxa type have been synthesized. Unsubstituted 12-crown-3 shows the strongest complexation, but lithium selectivity is lost due to the possi- bility of sandwich complexation of other cations. To sustain strong com- plexation of lithium, and to retain selectivity by preventing sandwich formation, it is necessary to introduce ligating side arms and to keep the number of methyl substituents as low as possible.

1,1-Bis(aminomethyl)cyclohexane condenses with formaldehyde to precipitate a polymer, which upon heating is readily isomerized to the nonacyclic compound tetraspiro[tetrakis(cyclohexane)-1,5′′′′:1′ ,11′′′′:1″, 17′′′′:1′′′,23′′ ′′-[1,3,7,9,13,15,19,21]octaazapentacyclo[19.3.1.1 3,7 .1 9,13 . 1 15,19 ]octacosane 1, whose structure has been determined by X-ray analysis and 13 C NMR spectroscopy.

Pentane-1,3,5-triamine condenses with formaldehyde to give directly and exclusively the nonacyclic title compound, whose conformations have been studied by X-ray analysis and low-temperature 13C NMR spectroscopy.

1,1-Bis-(2-aminoethyl)hydrazine condenses with formaldehyde to a polymer that is isomerized to the nonacyclic title compound, showing increased conformational mobility and crystallizing as a tetrahydrate for which the X-ray analysis reveals channels formed by stacked rings and filled by chains of water molecules.

2,8-Dimethyl-2,8-bis(aminomethyl)nonane-1,9-diamine, prepared by catalytic hydrogenation of 2,2,8,8-tetracyanononane, condenses with formaldehyde to precipitate a polymer which by heating is isomerized to the septicyclic title compound, whose X-ray structure has been determined.

Although it has been recognized for half a century that alkyl carbocations are extremely reactive and can exist only in the vacuum of the mass spectrometer or in nonnucleophilic solvents like the superacids, textbooks of organic chemistry still present them as well-defined intermediates in solvolytic displacement reactions. A planar free carbocation intermediate is used as a simple model to explain racemization in solvolysis; but the problem is that racemization is never complete, the enantiomer in excess having the inverted configuration. Also, the unimolecularity has never been demonstrated, but is an assumption only.Racemization is commonly observed for reactions that are reversible, as in the hydrolysis of an alkyl chloride. It is also necessary that either the substrate, the product, or both can undergo a degenerate reaction. This is equivalent to saying that both the attacking nucleophile and the leaving nucleofuge can play both roles. Such exchanges were demonstrated by isotopic labeling 40 years a...

Preparation de cyanines en Y avec trois groupes dimethylamino au bout des 3 chaines par reaction de Vielsmeier de cyanines lineaires avec la dimethylamino-3 acroleine

A survey is given of experimentally established conformations of saturated compounds containing the isolated ether group, or the 1,5-, the 1,4- or the 1,3-dioxa-grouping. The emphasis is on demonstrating common features for simple molecules, polymers, and medium and large rings.

Synthese des precurseurs de carbene (diazoacetyl-2' phenyl-2 dimethyl-3,5 thiophene et [diazo methoxycarbonyl acetyl]-2' phenyl-2 thiophene et son derive dimethyl-3,5) a partir de bromo-2' phenyl-2 thiophene et -dimethyl-3,5 thiophene; etude des decompositions catalytiques de ces precurseurs