Heike Hupfer

Stability and Electrophilicity of Phosphorus Analogues of Arduengo Carbenes—An Experimental and Computational Study

A variety of differently substituted 1,3,2-diazaphospholenium salts and P-halogeno-1,3,2-diazaphospholenes (X = F, Cl, Br) were synthesized, and their molecular structures, bonding situation, and Lewis acid properties were characterized by experimental (single-crystal X-ray diffraction, NMR and IR/Raman spectroscopy, MS, conductometry, titrations with Lewis bases) and computational methods. Both experimental and computational investigations confirmed that the structure and bonding in the diazaphospholenium cations of OTf and BF4 salts resembles that of neutral Arduengo carbenes and that the cations should not be described as genuinely aromatic. P-Halogenodiazaphospholenes are, in contrast to earlier assumptions, molecular species with covalent P-X bonds whose bonding situation can be expressed in terms of hyperconjugation between the six pi electrons in the C2N2 unit and the sigma*(P-X) orbital. This interaction induces a weakening of the P-X bonds, whose extent depends subtly on substituent influences and contributes fundamentally to the amazing structural similarity of ionic and covalent diazaphospholene compounds. A further consequence of this effect is the unique polarizability of the P-Cl bonds in P-chlorodiazaphospholenes, which is documented in a considerable spread of P-X distances and bond orders. Measurement of the stability constants for complexes of diazaphospholene compounds with Lewis bases confirmed the lower Lewis acidities and higher stabilities of diazaphospholenium ions as compared with nonconjugated phosphenium ions; this had been inferred from computed energies of isodesmotic halide-transfer reactions, and permitted also to determine equilibrium constants for P-Cl bond dissociation reactions. The results suggest, in accord with conductance measurements, that P-chlorodiazaphospholenes dissociate in solution only to a small extent. On the basis of these findings, the unique solvatochromatic behavior of NMR chemical shifts of these compounds was attributed to solvent-dependent P-Cl bond polarization rather than to shifts in dissociation equilibria.

Reactions of complex ligands ☆: Part 91. Application of ring closing metathesis to Fischer-type carbene complexes: synthesis and structure of medium-sized chromium oxacycloalkenylidenes

Abstract Six- and seven-membered pentacarbonyl(2-oxacycloalkenylidene) chromium complexes 17 and 6 have been synthesized in moderate to good yields from alkenyloxy(methyl)carbene complex precursors applying an α-alkylation/ruthenium based ring closing metathesis sequence. The ring-closure is hampered for β-alkylated vinylcarbene complexes which may undergo competing intermolecular cross metathesis at the alkenyloxy terminus in low yield.

Broad Molecular Ribbons of Nanometer Size Composed of Biphenyl Units

The hitherto broadest tetrafunctionalized molecular ribbons (see below) have been synthesized by an iterative synthetic strategy. Such multiple biphenylophanes usually adopt a meander-type conformation and can be used as cyclization precursors in the synthesis of extended nanometer-size tube-shaped molecules.

Spirocyclobutanes via Metal Carbene Facilitated Diastereoselective [2+2]Cycloaddition Reactions of exo -Methylene Oxacyclopentylidene Complexes of Chromium and Tungsten 1

Upon reaction with enol ethers α- exo -methylene-2-oxacyclopentylidene complexes of chromium and tungsten 3 and 4 undergo [2+2]cycloaddition under mild thermal conditions to give spirocyclobutanes 5 – 12 in good yields as single diastereomers. The relative stereochemistry (as established by X-ray analysis for 9 and 10 to be 5 R ∗ ,2′ S ∗ ) results from a combined re - si approach of both π-systems. The propensity for [2+2]cycloaddition depends on both the substitution pattern and the ring size of the enol ether; 1,1-disubstituted enol ethers and enamines undergo a formal conjugate addition resulting in side-chain elongation.

Site-specific cleavage--a model system for the identification of lipid-modified glutamate residues in proteins.

Numerous proteins are modified post‐translationally after their biosynthesis at the ribosomes of the cell. One such modification, only poorly characterized to date, is the formation of lipid esters of glutamate side chains in the skin proteins of land‐living mammals; here a subset of very long chain fatty acids, ceramides and/or glucosylceramides, are bound through their ω‐hydroxy groups to structural proteins of the so‐called “cornified envelope” in the outermost layer of the skin, the stratum corneum. We report an approach for the identification of proteins containing ester‐modified glutamic acid residues and the determination of their positions within the peptide sequence, designed for mass spectrometric investigation of human skin proteins.

Dinuclear Aminotroponiminate Compounds Containing Two Dialkyl Aluminum Substituents

Reaction of the n-propyl-bridged bis(aminotroponimine) H2[(iPr)TP] with various dialkylaluminum hydrides R2AlH, in hexane, led to novel bimetallic compounds of composition [(R2Al)2{(iPr)TP}] (R = Me, Et, iBu). Depending on the size of the substituents R bound to the Al centers the complexes crystallize either in a cisoid (R = Me) or a transoid (R = iBu) conformation. The steric crowding at the aluminum centers is also observed in solution.

2(S)-N-tert-Butoxy­carbonyl­amino-N-methoxy-N-methylbutan­amide

In the solid state the title compound, C11H22N2O4, is in an antiperiplanar conformation. The conformation of the compound shows no similarity with the synclinal conformation of the chelated intermediate formed during nucleophilic attack.

Tungstenmethyl-dimethylsilanols Cp(OC)2(R3P)W-CH2-SiMe20H (R = Me, Ph): Synthesis via oxygenation or hydrolysis

Abstract The tungstenmethyl-dimethylsilanols Cp(OC)2(R3P)W-CH2-SiMe2OH [R = Me (6a), Ph (6b)] have been synthesized by oxofunctionalization of the tungstenmethyl-silanes Cp(OC)2(R3P)W- CH2-SiMe2H [R = Me (3a), Ph (3b)] with dimethyldioxirane. 6a has been additionally obtained by Et3N-assisted hydrolysis of the tungstenmethyl-chlorosilane Cp(OC)2(Me3P)W-CH2- SiMe2Cl (5). Compounds 6a,b are stable with respect to self-condensation, but controlled condensation of 6a with Me2Si(H)Cl in the presence of triethylamine has been realized to give the tungstenmethyl-substituted disiloxane Cp(OC)2(Me3P)W-CH2-SiMe2O-SiMe2H (7). The new compounds have been identified IR- and NMR-spectroscopically and, in the case of 3a, by X-ray analysis.

tert-Butyl 4-acetyl-2,2-di­methyl-1,3-oxazolidine-3-carboxyl­ate

The crystal structure of the title compound, C12H21NO4, has been determined. The conformation in the solid state is compared to the transition state in nucleophilic addition reactions predicted by the Felkin–Anh model.