Zur Reaktivität von Natrium-dialkylphosphonoformiaten

On the Reactivity of Sodium Dialkylphosphonoformates The reaction of sodium dialkylphosphonates with CO₂ forming sodium dialkylphosphonoformates, (RO)₂P(O)C(O)ONa, I , as well as the further reaction of I are described, ¹⁴C-isotope exchange and the n.m.r. spectra are discussed.     

Zur Synthese und Reaktivität Triflatsubstituierter Siloxanderivate

Synthesis and Reactivity of Triflate Substituted Siloxane Derivatives The reaction of amino substituted siloxane derivatives with trifluoromethanesulfonic acid leads under elimination of ammonium trifluoromethanesulfonate to the formation of siloxanyl triflates. The compounds are characterised by NMR-spectroscopy (²⁹Si, ¹³C, ¹H). The synthetic potential of these siloxanes is shown on selected examples. One interesting point of view is the synthesis of silicon containing oligo- and polymeres, in which siloxane and silylenealkine units are combined.     

Zur Reaktivität der Si?C-Bindung in Silacyclobutanen

On the Reactivity of the Si? C Bond in Silacyclobutanes The cleavage of mono- and 1,3-disilacyclobutanes by Lewis acids of elements of main groups III to VI has been investigated systematically. On the basis of the experimentally observed reactivity trends as a function of the reactants and their substituents a four-centre-mechanism is proposed, that allows the interpretation of all available results.     

Stereoselektivität und Reaktivität bei der 1,3-dipolaren Cycloaddition chiraler N-(Alkoxyalkyl)nitrone

Stereoselectivity and Reactivity in the 1,3-Dipolar Cycloaddition of Chiral N-(Alkoxyalkyl)nitrones The stereoselectivity in the reaction of hydroxy-oxime 1 with acetaldehyde and methyl methacrylate yielding the diastereomeric isoxazolidine-ribosides 9–12 was determined to be 93%. The predominant adducts 9 and 10 were cleaved to the diastereomeric isoxazolidines 19 and 20 , respectively, which upon oxidation gave the same optically active 2-isoxazoline 21 , thus demonstrating the participation in the cycloaddition of both (E/Z)-isomeric nitrones 13 and 14 . Based upon comparison of the optical rotations, the isoxazolidines 7 , 8 , 19 and 20 and the 2-isoxazolines 21 and 22 possess the same chirality, found to be R by correlating 7 with (+)-(S)-citramalic acid. - Since the hydroxy-oximes 1 and 36 showed the same stereo-selectivity in the reaction with formaldehyde and methyl methacrylate, the trityl group of 1 does not influence the stereoselectivity in the cycloaddition. The hydroxyoxime 38 led in the same type of reaction to the isoxazolidines 7 , 8 , 19 and 20 possessing (S)-chirality, the stereoselectivity (79–95%) being similar to the one observed with 1 (67–95%). The explanation of this stereoselectivity is based upon a stereoelectronic effect in the transition state of the cycloadditions (kinetic anomeric effect). As predicted, the N-(alkoxyalkyl)nitrones showed enhanced reactivity in the cycloaddition with unactivated olefins (leading to 47 , 48 , 50 and 51 ). The importance of exo vs. endo approach of the dipolarophile was evidenced by reacting 1 with formaldehyde and methyl acrylate giving predominantly 57 with (5S)-configuration. - Use of the hydroxy-oxime 65 allows synthesis of optically active isoxazolidines with regeneration of the starting hydroxy-oxime.     

Zur Erhöhung der Reaktivität von apatitischen Rohphosphaten durch mechanische Aktivierung

Increase in the Reactivity of Apatitic Phosphates due to Mechanical Activation The lattice of natural apatitic phosphates was severely distorted by mechanical activation. At a high energy concentration in a centrifugal mill the crystals reach the X-ray amorphous state. The structural changes are investigated by infrared and X-ray methods. At increased degrees of activation the mechanically activated apatitic phosphates show enhanced solubility as a result of structural changes.     

Zur Struktur und Reaktivität des Diammoniumhexafluoromanganats(IV)

About the Structure and Reactivity of Diammonium Hexafluoromanganate(IV) Electrolytic oxidation of an aqueous suspension of MnF₂ containing NH₄F, and subsequent crystallization in 40% HF yields yellow crystals of (NH₄)₂MnF₆. It crystallizes in the hexagonal K₂MnF₆ type structure with the space group P6₃mc and a = 5.903; c = 9.565 Å; Z = 2. With in situ powder diffraction studies it is shown, that (NH₄)₂MnF₆ is gradually reduced in a NH₃ atmosphere between 30 and 230 °C to afford (NH₄)₃MnF₆, (NH₄)₂MnF₅, and finally NH₄MnF₃. (NH₄)₃MnF₆, thereby, forms a hitherto unknown cubic (a = 9.082 Å) high temperature modification with the cryolite type structure. Under N₂ the thermal decomposition of (NH₄)₂MnF₆ proceeds via NH₄MnF₄ to yield MnF₂.