Palladium catalysts have recently been discovered that enable the directing group-free C–H activation and functionalization of arenes without requiring an excess of the arene substrate. By overcoming this long standing challenge, the resulting synthetic methods have now become suitable for the functionalization of complex organic molecules. The key to success in several of these transformations has been the use of two complementary ligands, an N-acyl amino acid and an N-heterocycle. Further applications of this design principle will likely require the guidance by a profound mechanistic understanding. This prompted us to engage in a detailed experimental and computational mechanistic study of the dual ligandenabled C–H activation of arenes. Based on comprehensive kinetic experiments, (CID-)MS and DOSY-NMR measurements, and DFT calculations we find that a 1:1:1 complex of palladium and the two ligands is the active species that enables a partially rate-limiting concerted C–H activation as part of a PdII/Pd0-cycle. Our study highlights the importance of catalyst speciation and allows us to rationalize the role of each ligand as well as the observed regioselectivities. These findings are expected to be highly useful for further method development using this powerful class of catalysts.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
The reaction of {N(tBu)Trip}SiBr3 (1, Trip = 2,4,6-iPr3C6H2) with 1.5 equiv of activated magnesium (Mg*) directly provides the amido-substituted cyclotrisilene 2 with one bromine substituent in 29% yield. In 2, negative hyperconjugation from the Si═Si π-bond into antibonding σ*-orbitals of the Si1–N1 and Si1–Br1 bonds occurs. This results in remarkably more shielded silicon atoms of the Si═Si bond compared to aryl- and silyl-substituted cyclotrisilenes. The reaction with the N-heterocyclic carbene NHCMe4 does not result in ring-opening but instead yields the NHC adduct 3. Nevertheless, cyclotrisilene 2 reacts with ethylene and dimethylacetylene in [2 + 2] cycloadditions to result in bicyclic saturated and unsaturated housanes 4 and 5, respectively.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
ABSTRACT Cyanophycin (multi- l -arginyl-poly- l -aspartic acid; also known as cyanophycin grana peptide [CGP]) is a putative precursor for numerous biodegradable technically used chemicals. Therefore, the biosynthesis and production of the polymer in recombinant organisms is of special interest. The synthesis of cyanophycin derivatives consisting of a wider range of constituents would broaden the applications of this polymer. We applied recombinant Saccharomyces cerevisiae strains defective in arginine metabolism and expressing the cyanophycin synthetase of Synechocystis sp. strain PCC 6308 in order to synthesize CGP with citrulline and ornithine as constituents. Strains defective in arginine degradation (Car1 and Car2) accumulated up to 4% (wt/wt) CGP, whereas strains defective in arginine synthesis (Arg1, Arg3, and Arg4) accumulated up to 15.3% (wt/wt) of CGP, which is more than twofold higher than the previously content reported in yeast and the highest content ever reported in eukaryotes. Characterization of the isolated polymers by different analytical methods indicated that CGP synthesized by strain Arg1 (with argininosuccinate synthetase deleted) consisted of up to 20 mol% of citrulline, whereas CGP from strain Arg3 (with ornithine carbamoyltransferase deleted) consisted of up to 8 mol% of ornithine, and CGP isolated from strain Arg4 (with argininosuccinate lyase deleted) consisted of up to 16 mol% lysine. Cultivation experiments indicated that the incorporation of citrulline or ornithine is enhanced by the addition of low amounts of arginine (2 mM) and also by the addition of ornithine or citrulline (10 to 40 mM), respectively, to the medium.
