The kinetics and mechanism of the substitution reactions oftrans-tetracyanodioxotungstate(IV) ions with monodentate ligands

Summary The kinetics of the substitution reactions of the protonated forms oftrans-tetracyanodioxotungstate(IV) ions with pyridine and thiocyanate ions were studied. The kinetic results were compared with those of the reactions with N ₃ ^– and F^– ions and a linear free energy relationship was obtained which was interpreted as additional evidence for a dissociative mechanism. Infrared data for this type of complex are reported and discussed in terms of thetrans influence of various monodentate ligands.     

A mechanistic study of the Hiyama-Nozaki allylation: evidence for radical intermediates

In course of chromium(II) mediated additions of relatively complex allylic bromides to aldehydes (Hiyama-Nozaki allylation) radical intermediates have been observed. From these findings a new mechanism of the allylation is derived.     

Monodentate substitution reactions of [MO2(CN)4]n− type complexes: The crystal structure of the sodium thiocyanate adduct of tetramethylammonium tetracyanooxothiocyanatotungstate(IV)

The crystal structure of (Me₄N)₃[WO(CN)₄(NCS)]·NaNCS was determined from three-dimensional x-ray diffraction data. The dark blue crystals are monoclinic, space group C2/m, with cell dimensions a=13.105(4), b=12.688(2), c=18.871(3) Å, =100.4(8)°, z=4 and D=1.46 g cm^–3. Anisotropic refinement of 1333 observed reflections converged to R=0.068. The [WO(CN)₄(NCS)]^3– ion is a distorted octahedron with the tungsten atom displaced by 0.35 Å out of the plane formed by the four cyano ligands, towards the oxo ligand. The coordinated thiocyanate ligand is bonded to the tungsten atomvia the nitrogen atomtrans to the oxo ligand. Bond distances found: W–CN_av=2.14(3), W–O=1.61(2) and W–NCS=2.23(2) Å. The compound crystallizes as an adduct with an additional thiocyanate anion, ionically bonded to a severely disordered sodium cation. The results correlate with available data from similar complexes.     

New Pyrrolo[1,2-a]azepine Type Alkaloids from Stemona and Stichoneuron (Stemonaceae)

Summary. Three new pyrroloazepine type alkaloids, stichoneurines A and B and 6-hydroxycroomine were isolated from the lipophilic root extracts of Stichoneuron caudatum and Stemona tuberosa collected in Thailand together with the already known croomine, tuberostemonine, and tuberostemonine A. The structures were elucidated by spectroscopic methods including H/H-COSY, HMQC, and HMBC. Information on the relative stereochemistries and conformational behaviour was obtained by analysis of the NOESY spectra. The formation of pyrroloazepine alkaloids in the genus Stichoneuron is reported for the first time and supports its affiliation to the family Stemonaceae. The occurrence of two different types of alkaloids, of the tuberostemonine and croomine series, in different geographical provenances of Stemona tuberosa is of special chemosystematic interest and may contribute to a more natural species delimitation within that complex group.     

Zum mechanismus Massenspektrometrischer Fragmentierungsreaktionen–III: Stereospezifische Reaktionen in den Massenspektren der Methyläther cyclischer Polyalkohole

The mass spectra of stereoisomers of polymethoxy cycloalkanes depend on the geometry of the molecular ions. The magnitude of the stereochemical effect is influenced by the stability of the cyclic molecular ions. Due to energetically favourable ring-fragmentations the effect is cancelled by vicinal methoxy substituents and diminished by a methyl group next to a methoxy substituent. Stereochemically controlled fragmentations are the eliminations of a methoxy group in the form of a methanol or formaldehyde molecule from the molecular ions. By an investigation of di- and trimethoxy cyclohexanes, specifically labelled with deuterium, it is shown that both reactions are initiated by a transfer of an H-atom from a carbinol-C-atom to an O-atom of a methoxy group. Whether or not these energetically favourable reactions will occur depends on how close the H- and O-atoms involved can approach each other in the possible conformations of the molecular ion. The stereochemical control of the fragmentation of 1,3-dimethoxy cyclopentane, containing a more or less fixed five membered ring is small, that of dimethoxy cycloheptanes with a flexible seven membered ring is of comparable magnitude as the steric effect in the mass spectra of cyclohexane derivatives.     

A study on kinetic CH-acidity; The α-epimerization of β-lactones

The rate constants of the α-epimerization of the β-lactone $\text{1}$ show a LFE-correlation with the solvent donicity parameters DN or B; additionally, the kinetic CH-acidity of $\text{1}$ is unusually high.     

Abbaureaktionen an 3-[2′-(2″-Dimethylaminoäthyl)-phenyl]-isochromanen

Degradation of 3-[2′-(2′-Dimethylaminoethyl)-phenyl]-isochromanes Degradation of the β-dimethylaminoethyl sidechain of properly substituted 3-aryl-isochromanes produce inter alia the amines 2 , the syrenes 3 , the aldehyde 5b and the carboxylic acid 4b . The racemates of the amine 15 and the aldehyd 5b were resolved; the chiral center of the menthoxycarbonyl-hydrazone was eliminated by ester interchange.