Natural chelating ligands

The remainder of the work on Ni(II) complexes involves the use of chelating ligands in which the carbene is functionalised with pendant heteroatom donor(s). The picolyl-functionalised NHC dicationic complex 29 (Fig. 4.11) was tested for ethylene polymerisation after treatment with MAO [34]. This complex was found to be highly active in a preliminary test (330 kg moF bar h" ), giving predominantly linear polyethylene. Unfortunately this work does not seem to have been followed up. The same system was active for norbomene polymerisation (TOF = 24 400 h" over 1 h). Maximum activity was achieved at 80°C whereafter thermal deactivation became significant, although the nature of this deactivation was not studied. The phenoxide-functionalised carbene complex 30 (Fig. 4.11) was also... 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

The principles of dissolution have been reviewed by Bloom and Nater (1991), Blesa et al. (1994) and Casey (1995). The driving force for dissolution is the extent of undersaturation with respect to the oxide. Undersaturation is thus a necessity for dissolution as is supersaturation for precipitation. Other factors being equal, the rate of reaction will increase as degree of undersaturation rises. The extent of undersaturation varies from one system to the next. Dissolution of anodic Fe oxide films often takes place in nearly saturated solutions, whereas extraction of iron from its ores requires markedly undersaturated solutions in order to be efficient. In most natural systems (soils and waters) the aqueous phase is fairly dose to saturation with respect to the iron oxides and dissolution may, therefore, be extremely slow. The dissolution process can be accelerated by the presence of higher levels of electrons or chelating ligands. 

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