Catalytic asymmetric carbon–carbon bond formation via allylic alkylations with organolithium compounds

Carbon–carbon bond formation is the basis for the biogenesis of nature’s essential molecules. Consequently, it lies at the heart of the chemical sciences. Chiral catalysts have been developed for asymmetric C–C bond formation to yield single enantiomers from several organometallic reagents. Remarkably, for extremely reactive organolithium compounds, which are among the most broadly used reagents in chemical synthesis, a general catalytic methodology for enantioselective C–C formation has proven elusive, until now. Here, we report a copper-based chiral catalytic system that allows carbon–carbon bond formation via allylic alkylation with alkyllithium reagents, with extremely high enantioselectivities and able to tolerate several functional groups. We have found that both the solvent used and the structure of the active chiral catalyst are the most critical factors in achieving successful asymmetric catalysis with alkyllithium reagents. The active form of the chiral catalyst has been identified through spectroscopic studies as a diphosphine copper monoalkyl species.

A high-resolution three-dimensional reconstruction of a fossil forest (Upper Jurassic Shishugou Formation, Junggar Basin, Northwest China)

This study focuses on the three-dimensional (3D) reconstruction of an Late Jurassic fossil forest based on a fossil assemblage located in the Shishugou Formation near Jiangjunmiao, north-eastern Junggar Basin, Xinjiang Uygur Autonomous Region, Northwest China. On the basis of tree stumps found in growth position together with published data on megaplant fossils, seeds and spores, a highresolution digital computer model, including different forest layers, was developed. In a study area of 31,500 m, 65 tree stumps with diameters between 0.2 and 2.9 m were mapped and subsequently used for the 3D reconstruction. The forest grew under moist conditions, probably close to the banks of an anastomosing river and consisted primarily of conifers, in particular Araucariaceae. Even though the tree density of the forest is only 22 trees/ha, the 3D reconstruction indicates a relatively closed canopy. Megaplant fossils and spores also show evidence for the presence of Angiopteris, Osmunda and Coniopteris, which were then used to reconstruct the understory of the forest. The trees were modelled in three different growth stages, representing young, older and mature trees, respectively. The growth parameters of individual trees and ferns were randomized in order to avoid duplications within the reconstruction. Detailed textures of leaves, fronds and bark were created to give the plants a more realistic appearance than that in previously published 3D reconstructions of fossil forest assemblages. Estimations of net biomass (approximately 9 tons/ha), annual biomass production and a calculation of nearest neighbour index (0.86) suggest an open forest with spatially distributed trees