Chelate ring closing

However, many ligands do not behave as donors of a single electron pair. Some ligands donate two or more electron pairs to the same central atom from different donor atoms. Such ligands are said to be chelating ligands, and they form chelate rings, closed by the central atom. The phenomenon is termed chelation. 

The chelate ring closing of glycine has been the subject of extensive isotope and kinetic studies, and the results are summarized in Scheme 3.5 (k, s" at 25°C). The reactions proceed with retention of the Co—O bond, and it is noteworthy that coordinated water is an active nucleophile in this intramolecular process. The acidic pathways, given by and k2 in Scheme 3.S, show general acid catalysis, which the authors interpret as due to rate-controlling elimination of water from the tetrahedral carbon in a cyclic intermediate. 

The glycine example in the above Scheme illustrates two mechanistic complications that can occur in chelate fewmation reactions. First of all, there are two potentird rate-controllirig steps, either bond formation or chelate ring closing. Second, for unsymnoetrical chelates, such as glycine. 

Again a distorted tetrahedral coordination of Hg is achieved in the l,l-bis(diphenylphos-phino)ferrocene (dppf) complex with Hgl2. Both Hg—P in the dppf chelate ring and terminal Hg—I bonds (averages 256.5 pm and 277.6 pm, respectively) are clearly longer than in the previous examples all angles around Hg are close to the value for a tetrahedron. Nonlinear optical properties of the material have been studied.241... 

As already stated, DIOP led the way for a number of ligand systems that were built on a carbon framework containing stereogenic centers. Some of these ligands followed closely on the heels of DIOP, such as ChiraPhos (16) where the chelate ring is five-membered [82, 83]. Even one stereogenic center in the backbone, as in ProPhos (17), provides reasonable selectivity [83, 84]. The main problem with these systems is that of slow reactions. 

DR. MICHAEL BERGKAMP (Brookhaven National Lab) Do you have any direct evidence which indicates that when you irradiate these compounds the chelate ring actually opens up and closes ... 

This chelation-controlled addition generated the homoallylic alcohols 86 with a diastereoselectivity of syn anti=S6ll4 , In contrast to the other syntheses described so far, the ring closing reaction was a 5 2 reaction but not a... 

Figure F. Opening and closing of chelate ring in transition state plot of energy vs. reaction coordinate for ZnAc or a one-bonded ox.

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