Assignment of Relative Configuration to Acyclic Compounds Based on (13)C NMR Shifts. A Density Functional and Molecular Mechanics Study

1,3-Dimethylated hydrocarbon segments occur frequently as structural elements in polyketide natural products. The (13)C NMR chemical shifts of a series of model compounds containing such segments can be well reproduced by a combination of molecular mechanics and SOS-DFPT/IGLO calculations. (13)C NMR chemical shifts are calculated on MM3 geometries and are Boltzmann weighted according to the MM3 energies. On the basis of the resulting thermally averaged chemical shifts, all diastereomers of the model compounds can be unequivocally distinguished. Significant differences in chemical shifts occur at methyl groups and methylene groups that are adjacent to a single stereogenic center. The method is applied to predict the relative configuration of two stereocenters in the side chains of two natural products, sambutoxin and the bradykinin inhibitor L-755,897.     

Die Deprotonierung silylierter Amido-Komplexe von Seltenerdelementen

Deprotonation Reactions of Silylated Amido Complexes of Rare Earth Elements The deprotonation of the rare earth element-tris(bistrimethylsilyl)amides Ln[N(SiMe₃)₂]₃ of scandium, ytterbium, and lutetium with sodium-bis(trimethylsilyl)amide in THF leads to the complexes [Na(THF)₃LnCH₂SiMe₂NSiMe₃{N(SiMe₃)₂}₂] [Ln = Sc ( 1 ), Yb ( 2 ), and Lu ( 3 )]. According to crystal structure analyses of 1 and 2 the metal atoms Sc and Yb are constituents of planar LnCSiN four-membered rings. At the same time, the C atom of the CH₂ group is coordinated with the sodium ion in a linear axis Ln–C–Na; the sodium ion obtains a distorted tetrahedral arrangement by three THF molecules. The equatorial positions of the methylene-C atom, which is coordinated in a trigonal bipyramidal fashion, are occupied by the two H atoms and the Si atom of the four-membered ring. 2.6-dimethylbenzoisonitrile can be inserted into the Yb–CH₂ bond of 2 and the new five-membered heterocylce YbNCSiN originates, the exocyclic CH₂ group of which enters into a C–C coupling with the centrosymmetric dimer 4 while the ytterbium undergoes reduction. At the same time, sodium-7-methyl indolate is formed, which together with [NaN(SiMe₃)₂(THF)₂] forms the centrosymmetric dimeric molecular aggregate [NaN(SiMe₃)₂(THF)₂Na(C₉H₁₆N)]₂ ( 5 ). 1 : Space group P2₁/n, Z = 8, lattice dimensions at –80 °C: a = 2941.4(2), b = 1205.5(1), c = 2952.4(3) pm; β = 113.455(8)°; R₁ = 0.0625. 2 : Space group P2₁/n, Z = 8, lattice dimensions at –80 °C: a = 2943.9(1), b = 1219.5(1), c = 2944.3(1) pm; β = 113.372(4)°; R₁ = 0.0361. 4 : Space group P 1, Z = 4, lattice dimensions at –80 °C: a = 1117.0(1), b = 1207.5(1), c = 1614.3(2) pm; α = 73.634(10)°, β = 82.091(10)°, γ = 74.391(10)°; R₁ = 0.0525. 5 : Space group P2₁/n, Z = 2, lattice dimensions at –80 °C: a = 1126.7(1), b = 1459.3(1), c = 1741.1(1) pm; β = 96.461(8)°; R₁ = 0.0458. Quantum chemical DFT calculations of the scandium model compound [Na(Me₂O)₃ScCH₂SiMe₂NSiH₃{N(SiH₃)₂}₂] ( 1 M ) give a very large negative charge at the pentacoordinated carbon atom of the four-membered ring that is concentrated in a lone-pair orbital which has mainly p character. The carbon atom interacts with the positively charged scandium atom mainly by Coulombic interactions.     

A C2 Fragment as Four‐Electron σ Donor

The formation and X‐ray structure analysis of the Pt^IV complex [(CH₂C{PPh₂C₆H₄}₂)PtI₂] is reported. The molecule possesses the orthometalated ligand (PPh₃)₂C→CH₂, which serves via its C₂ fragment as an unprecedented four‐electron σ donor.