Stefan Berger

Preparation and reactivity of chiral β-amido-alkylzinc iodides and related configurationally stable zinc organometallics

Abstract Several zinc organometallics bearing at the β-position a carbamate or an amido function with an acidic NH group were prepared using the direct insertion of zinc dust into the corresponding alkyl iodides in THF or THF:DMSO mixtures. Most of the starting iodides were obtained from natural α-amino acids and the resulting zinc species afforded after transmentalation with CuCN·2LiCI and reaction with a selection of relatively reactive electrophiles a variety of polyfunctional 1,2-amino alcohol derivatives and carbamates in optically pure form. Several secondary β-amido alkyl iodides were converted to the corresponding chiral zinc reagents and trapped with electrophiles. The configurational stability of chiral secondary organozinc compounds and the stereochemical course of their reactions were examined.

Stable germa- and stannaethenes

Stable germaand stannaethenes have been synthesized by addition of germylenes and stannylenes to a kryptodiborylcarbene. Extreme nmr-deshielding of tin is decisive proof for tricoordinated tin atoms in the stannaethenes. Considerable ylid character of germaand stannaethenes is deduced from nmr-shielding of boron atoms adjacent to the tricoordinated carbon atom. X-ray analysis of a germaand a stannaethene revealed strong distortion from planarity at the C=Ge and C5n bond: twisting of 36* and 61* and pyramidalization at carbon of 5* and 16 as measured by the fold angle. The distances from germanium and tin to the tncoordinated carbon (182.7(4) and 202.5(4) pm) are distinctly shorter (8.4 and 6.4%) than to tetracoordinated carbon atoms.

Two-dimensional 13C,199Hg correlation: A new NMR method for characterisation of organomercury compounds

Abstract Two-dimensional 13 C, 199 Hg correlation has been used for the first time to facilitate assignments for 13 C and 199 Hg spins in a dimercury compound. Correlations over more than one bond provide additional information on the constitution of these molecules. Unusual 13 C isotope effects on the 199 Hg chemical shifts are reported.

Search for 13C equilibrium isotope effects in dicyclopentadienyltricarbonyldirhodium

The interchange of the carbonyl groups in dicyclopentadienyltricabonyldirhodium (η5-C5H5)2Rh2(CO)3 (1) has been investigated by the method of isotopic perturbation of equilibrium. Neither direct integration of the signals of terminal and bridged CO carbons in the 13C NMR spectrum at 193 K nor measurement of the 13C chemical shift of the averaged CO signal in the spectrum at 298 K at different grades of 13CO enrichment indicated an isotopic perturbation. It is concluded that a 13C equilibrium isotope effect for the carbonyl groups in 1 is either nonexistent, or is too small to be detected by the method employed.