Chelation control polarization

Entry 10 is an example of the application of chelate-controlled stereoselectivity using TiCl4. Entry 11 also involves stereodirection by a (3-O-methoxybenzyloxy) substituent. In this case, the BF3-catalyzed reaction should proceed through an open TS and the (3-polar effect described on p. 96 prevails, resulting in the anti-3,5-isomer. 

It has been said that the regiospecific addition of carbon monoxide to the most substituted carbon is due to the polarization of the double bond and to the acylamide group s chelation control [52]. Although the enantioselectivity is not exceptional, it is higher than normal in the rhodium hydroformylation of vinylidenic aUcenes. 

Solutions of the alkali elements, in a range of polar solvents, have been shown to contain M , M ", e, and the unusual anion, M , in equilibrium. Early studies were hindered by solubility and decomposition problems. The addition of a macrocylic chelate to these solutions provided a means of increasing the solubility of the metal as well as controlling what species predominate in solution. A metal chelate mole ratio of 2 1 leads to formation of the alkalides [M -i- L] M while a ratio of 1 1 leads to the electrides (see 10.2.2.5), [M -t- L] e (where M and M = Li, Na, K, Rb, Cs (not always the same) and L = two crown ethers (combinations of 12-C-4, 15-C-5, 18-C-6) or one cryptand [2.2.2]). Furthermore, these unique compounds could be isolated as solids and fully characterized. A wider range of alkalides than electrides is known including those with other chelates such as ethylenediamne , and aza crown ethers . Some representative examples are [Na -t- crypt]Na"(the first to be structurally characterized) , and [Cs + 18-C-6)(15-C-5)]Na- . 

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