Simulating both aerial microclimate and soil temperature from observations above the foliar canopy.

A simulation model is described for the daily course of microclimatic characteristics of foliar canopy and the soil underneath. The independent driving forces are the meteorological observations above the canopy. The canopy is described by its geometrical, optical and physiological properties, the soil by its thermal and hydraulic properties. Comparison with real data shows a good agreement for crop transpiration, soil evaporation and soil heat flux, and to a lesser degree for air temperature and humidity and leaf temperature. The simulations, covering a full day, were executed with a stratified model. The effect of stratification was investigated by a comparison with a model continuous in height. (Abstract retrieved from CAB Abstracts by CABI’s permission

Libraries of bidentate phosphorus ligands; synthesis strategies and application in catalysis

Combinatorial chemistry in combination with high-throughput screening technologies is an important way of finding new successful catalyst systems. The design of ligand libraries of bidentate phosphorus ligands and the application of their transition-metal complexes in homogeneous (asymmetric) catalysis reactions will be described in this review. Till now three different approaches were developed to arrive at such libraries of bidentate phosphorus ligands: 1) modular synthesis of bidentate ligands 2) the solid support synthesis of bidentate ligands and 3) the self-assembly of ligand building blocks into bidentate ligands. The scope and limitations of these strategies will be discussed on the basis of a limited number of articles that dealt with the synthesis of at least 15 bidentate phosphorus ligands

SUPRAPhos ligands for the regioselective rhodium catalyzed hydroformylation of styrene forming the linear aldehyde

We report SUPRAphos-based rhodium catalysts that are unusually regioselective in the hydroformylation of styrene at low temperatures (40 degrees C) producing mainly the linear aldehyde (72%). This ligand-specific phenomenon is observed for two rhodium catalysts with heterobidentate phosphine-phosphoramidite ligands. We propose that the high selectivity for the linear product is caused by enhanced beta-hydride elimination, due to specific substrate-ligand interactions, most likely pi-pi interactions

Phenoxaphosphine-based diphosphine ligands. Synthesis and application in the hydroformylation reaction

The synthesis of a new series of diphosphine ligands based on 2,7-di-tert-butyl-9,9-dimethylxanthene (1), p-tolyl ether (2), ferrocene (3), and benzene (4) backbones, containing one or two 2,8-dimethylphenoxaphosphine moieties, is reported. The ligands were employed in the rhodium-catalyzed hydroformylation of 1-octene. For all four ligand backbones, introduction of phenoxaphosphine moieties led to an increase in catalytic activity and a decrease in regioselectivity toward the linear aldehyde product. Xanthene-based ligands 1a-1c yielded highly active and regioselective hydroformylation catalysts; ligands containing p-tolyl ether and ferrocene backbones 2a-2c and 3a-3c provided less active and less regioselective catalysts. Catalysts containing benzene-derived ligands 4a and 4b showed a remarkable preference for the formation of the branched aldehyde product. The coordination behavior of ligands 1-4 under hydroformylation conditions was investigated using high-pressure NMR and IR spectroscopy, revealing the distinct steric and electronic properties of the diphenylphosphine and 2,8-dimethylphenoxaphosphine moieties in ligands 1-4. The phosphacyclic moieties proved to be less basic and less sterically demanding toward other ligands in metal complexes than the acyclic diphenylphosphine moieties. For ligands that contain rigid backbones, the lack of conformational freedom in these phosphacyclic moieties does lead to repulsive interactions between the substituents of the two phosphorus donor atoms, resulting in an increase in the bite angle of the ligand. The low catalytic activity of rhodium catalysts modified by benzene-based ligands 4a-4c was attributed to the quantitative formation of HRh(L)(2) under hydroformylation conditions