Chelating groups reactions

Hetero-substituted 2-alkenyllithium compounds, however, show useful levels of regio- and stereoselectivity in reactions with aldehydes, in particular if the cation is held at one terminus of the allylie system by electron-withdrawing or chelating groups. 

In contrast to ordinary chiral aldehydes (having no ability to be chelated), the reaction of 9-allyl-9-borabicyclo[3.3.1]nonane(allyl-9-BBN) with the corresponding chiral imines 4 produces the isomer syn-6 either exclusively or predominantly (Cram selectivity Table 8)5,6. The very high 1,2-asymmetric induction is explained by a six-membered. chair-like transition state, in which the inline R group occupies an axial position due to the stereoelectronic effect of imines (R CH = NR). 

Structural studies on the nature of the organometallic intermediates following chelation-assisted CH additions of pincer iridium complexes have been carried out. The product was found to have an unexpected /ram-disposition of the hydride with respect to the metallated aromatic group. This is not the expected direct outcome of a chelation-assisted reaction since coordination of oxygen to iridium prior to C-H activation would be expected to afford the m-isorner (Equation (97)). 

However, the decarbonylation reaction can be suppressed by the use of specially tailored chelating groups. Intermolecular processes involving dienes and salicylaldehydes are now known, and are thought to proceed via a double chelation mechanism, akin to the Jun-type system. Rhodium-catalyzed reactions lead to hydroacylated products, under relatively mild conditions (Equation (134)).117... 

Apart from ethers as chelating groups, facial selectivity through chelation by nitrogen and sulfur has also been reported48. A few representative examples are given in Scheme 8 (all reactions were quenched with acid). 

Methylthymol blue and methylxylenol blue, prepared by the Mannich reaction from thymol-sulfonphthalein and p- xylenol blue respectively, have been intensively studied and resemble xylenol orange in many respects. If only one chelating group is introduced into cresol red, semi-xylenol orange results which is a common contaminant in XO and proves to be a more sensitive reagent for zirconium. 

In the former reaction regioisomers are possibly formed, so regioselectivity cannot be controlled. Chelation-assisted reactions, on the other hand, occur regioselec-tively, usually at the position ortho to the directing group. 

The allylmetallation of vinyl metals, y-heterosubstituted with a methoxy-methyl ether as the chelating group, leads to the corresponding gembismetal-lic derivatives,20 but now, warming the reaction mixture to room temperature promotes an internal nucleophilic substitution, leading to a metallated cyclopropane which can react with different electrophiles21 (Equation 7.5 and Protocol 9). 

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