Ligands attaching

The R, S convention is a scheme which has largely superseded the D, i. system to denote configuration about a chiral centre in a molecule. The convention allows unequivocal designation of the absolute configuration in a description of the positions in space of ligands attached to a chiral centre, in relation to an agreed standard of chirality like a right-hand helix. 

Figure 2-69. The two enantiomers of lactic acid assignment of R and S configurations to the enantiomers of lactic acid after ranking the four ligands attached to the chiral center according to the Cl P rules (OH > COjH > Me > H).

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

Water-soluble polymers and polyelectrolytes (e.g., polyethylene glycol, polyethylene imine polyacrylic acid) have been used success-hilly in protein precipitations, and there has been some success in affinity precipitations wherein appropriate ligands attached to polymers can couple with the target proteins to enhance their aggregation. Protein precipitation can also be achieved using pH adjustment, since proteins generally exhibit their lowest solubility at their isoelectric point. Temperature variations at constant salt concentration allow for frac tional precipitation of proteins. 

The possibility of oxidation to Fe is a crucial theme in the chemistry of Fe and most of its salts are unstable with respect to aerial oxidation, though double sulfates are much less so (e.g. Mohr s salt above). However, the susceptibility of Fe to oxidation is dependent on the nature of the ligands attached to it and, in aqueous solution, on the pH. Thus the solid hydroxide and alkaline solutions are very readily oxidized whereas acid solutions are much more stable (see Panel opposite). 

In the course of the 1990s, Yasui et al. [41b, 68] showed that, depending on the ligands attached to the phosphorus atom, phosphoranyl radicals may decay via three main processes a-scission, -scission and SET (Scheme 31). For example, in the presence of 10-methylacridinium iodide, phosphoranyl radicals generated from phenyl diphenylphosphinite decayed mainly via a-scission (Scheme 32) whereas phosphoranyl radicals generated from /so-propyl diphenylphosphinite decayed only via a SET process (Scheme 33). The reactivity of the phosphoni-umyl/phosphoranyl radical tandem has already been discussed in Sect. 3. 

The and F n.m.r. spectra of the phosphorane (47) indicate rapid positional exchange of the ligands attached to phosphorus. This has been quoted as evidence for a (2 + 3)-turnstile process of ligand reorganization, the normal Berry (1 + 4)-pseudorotation being held to be impossible in (47) because of increased strain. Details of this work are awaited with interest. 

Macrolactones 77 and/or 78 can be prepared from the reductive cyclisation of ynals 76 in the presence of NHC-nickel complexes (Scheme 5.21) [21], This maaolactonisation occnrs with different selectivity depending on the ligands attached to the nickel. If carbenes snch as IMes or IPr are nsed, the exocyclic olefin 77 is preferentially obtained, however when phosphine ligands are nsed, the endocyclic adducts 78 are preferentially obtained. 

Enzymes or affinity ligands attached by the aid of CDI to various matrices are listed in Tables 6-1, 6-2 and 6-3. 

The coordinatively flexible P/N-donor ligand PNra (411) reacts with [IrCl(cod)]2 in the presence of AgPFg to yield [Ir(cod)(PNra)](PF6), (412).665 The X-ray structure of (412) PN =i, R = H, complex confirms the cis geometry of the ligand attached via N and P donor atoms to a square-pyramidal Ir1 cod fragment with a long ethereal oxygen—Ir bond in an apical position. The Ir1... 

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