Phosphorus mechanisms, cycling

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

The iron-based redox cycle depicted in Figure 18.9 provides an effective preconcentrating step for phosphorus by trapping remineralized phosphate in oxic sediments. The conversion of phosphorus from POM to Fe(lll)OOH to CFA is referred to as sink switching. Overall this process acts to convert phosphorus from unstable particulate phases (POM to Fe(lll)OOH) into a stable particulate phase (CFA) that acts to permanently remove bioavailable phosphorus from the ocean. This is pretty important because most of the particulate phosphate delivered to the seafloor is reminer-alized. Without a trapping mechanism, the remineralized phosphate would diffuse back into the bottom waters of the ocean, greatly reducing the burial efficiency of phosphorus. 

Francko, D. A. 1986. Epilimnetic phosphorus cycling Influence of humic materials and iron on coexisting major mechanisms. Canadian Journal of Fisheries and Aquatic Sciences 43 302-310. 

The aromatic substituents on the phosphorus atoms have a pronounced effect on the enantioselectivity of this reaction. Instead of CF3 groups, if the aromatic rings are substituted in the same positions by CH3 groups, the e.e. value drops by 70%. This indicates that electronic factors may play a crucial role in the enantioselection mechanism. The proposed catalytic cycle for this reaction is shown in Fig. 9.14. All the steps shown in the catalytic cycle have precedence in achiral hydrocyanation reactions (see Section 7.7). 

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