Ligand substitution reactions aluminum

In the aluminum (III) chelate of salicylic acid, as well as in its copper (II) chelate, the positions susceptible to electrophilic attack are ortho and para to the hydroxy group. Since the course the reaction takes in the presence of the metal ions is quite different from that in their absence, it can be concluded that substitution occurs in the metal chelates rather than in the free ligands. This reaction is noteworthy in that it is one of the few reported instances in which a metal ion in a chelate ring has a marked influence on the course of a substitution reaction in an adjacent aromatic ring. 

Cyclodiphosphazanes(III) 27 shown in Scheme 16 undergo oxidation reactions to give the cyclodiphosphazanes(V) of type 28. These are prospective ligands in catalysis since these ligands due to lack of phosphorus lone-pairs are less susceptible to the destructive cycloreversion of the ligands. Hence they could prevent catalyst deactivation in the process. When treated with trimethyl aluminum the cyclodiphosphazanes form symmetrically substituted bimetallic species of type 29 [90]. Characterization by single-crystal X-ray studies show... 

The palladium-catalyzed asymmetric allylic substitution using seven different phosphano-oxazoline ligands at various ligand-to-metal ratios was also studied.112 An aluminum block containing 27 wells was placed in a dry box in which the reactions were carried out in parallel. Analyses were performed by conventional chiral GC equipped with an autosampler. Such a setup allowed about 33 catalyst evaluations per day. Apparently, only a few dozen were carried out in the study, resulting in the identification of a catalyst showing an ee-value of 74% in the reaction of 4-acyloxy-2-pentene with malonate.112 It is not clear whether further ligand diversification would lead to catalysts more selective than the record set in this case by the Trost-catalyst (92% ee).113... 

The above reactions in this section have been examples of addition alone or addition followed by elimination. Ligand reactions involving nucleophilic substitution are also known and these are of the dealkylation type. Lewis acids such as aluminum chloride or tin(IV) chloride have been used for many years in the selective demethylation of aromatic methyl ethers, where chelation is involved (Scheme 27). Similar cleavage of thioethers, specially using mercury(II) salts, is commonly used to remove thioacetal functions masking ketones (equation 27).104 In some cases, reactions of metal ions with thioether ligands result in isolation of complexes of the dealkylated organic moiety (equations 28 and 29).105-107... 

The first example of an asymmetric [2 + 2] cycloaddition of a ketene to an aldehyde was reported in 1994 by Miyano and coworkers [28]. They found that chiral aluminum catalysts prepared from different 3,3 -disubstituted BINOL derivatives resulted in low to modest asymmetric induction for a range of aliphatic and aromatic aldehydes. There does not seen to be a correlation between asymmetric induction and the size of the aldehyde. The data in Table 5 show that the optimum ligand for this reaction is triphenylsilyl substituted BINOL. It is curious that the catalyst prepared from this ligand and the catalyst prepared from BINOL result in opposite facial selectivity with... 

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