Chelate-type ligands

The 1,13-triphenylallyl system (R = Ph) was one of the early exanqdes that was thoroughly studied and gave high asymmetric induction [44,142]. The alkylation exclusively occurs at the less substituted carbon to give the more conjugated r ioisomo as shown in Scheme 8E.26. Excellent enantioselectivities have been obtained from both the racemic 119 and achiral substrate 120 by using P,P-, N Jl- and P,N-chelate type ligands as summarized in Table 8E.9. 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

The general coordination modes of amidinate and guanidinate (R = NR 2) ligands are shown in Scheme 4. Both ligands display a rich coordination chemistry in which both chelating and bridging coordination modes can be achieved. By far the most common coordination mode is the chelating type A. 

The somewhat different bis(diboratetrasiloxane) derivatives 94 and 95 have been prepared from bimetallic salen B(OEt)2 2 derivatives and diphenylsi-lanediol [131, 138]. In this case the four boron atoms of two diboratetrasilox-ane rings 86 are chelated by salen-type ligands in order to produce cages of cylindrical geometry (Fig. 25). 

For catalysis by Pt(II) and Rh(I) w-olefin complexes (those containing chelating diolefin ligands were less effective), three types of reaction have been observed depending on the nature of the silane (55). 

Several studies have been undertaken in an attempt to understand factors that affect the susceptibility of a complex to reductive elimination. These include investigations on Me-Pd-CNC pincer complexes [39] studies on the influence of the geometry of the complexes [40] the impact of A-substituents [41] and a study on the influence of chelating spectator ligands in complexes of the type [PdMe(NHC) (P-P)]BF [42]. 

Experimental studies show that chelating spectator ligands impart a degree of stability to complexes of type 23 (Scheme 13.10) [42]. If monodentate phosphine ligands are used decomposition is rapid at 20°C, however, using dppp no decomposition is detected after 24 h [19]. It was found that the rate of decomposition could be linked to the chelate ring size at 65°C, with dppp decomposition was complete after 6 h, with dppe only a small amount of decomposition occurred after this time [42]. 

Figure 32 The first P,N-ligand shown in Figure 32 can form five-membered ring chelates, in allylic alkylation with Pd the second is a chelating (phosphinomethyl)oxazoline P,N-type ligand, that catalyzes...

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

The first hydration step was promoted by Bronsted acids containing weakly or noncoordinating anions. In the second step, an intramolecular hydrogen transfer in the secondary alcohol was catalyzed by ruthenium(III) salts with chelating bipyridyl-type ligands. The possible complexation of the latter with the diene did not inhibit its catalytic activity in the allylic rearrangements, under acid-catalyzed hydration conditions. 

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