Magnesium ion-mediated reactions

Effect of External Reagents 11.2.2. Magnesium Ion-Mediated Reactions... 

Proper selection of the reaction solvent is important to attain high yields and selectivities, especially in reactions using a-substituted terminal allylic alcohols. Less coordinating solvents such as dichloromethane or toluene should be employed. With THF as the solvent, magnesium ion coordination slows the rate and lowers the diastereoselectivities (Scheme 11.30), which indicates that chelation plays an important role in the transition state structure of the magnesium ion-mediated reaction. 

The magnesium ion-mediated reaction of crotyl alcohol leads to the exclusive formation of the exo-isomer of the isoxazolidine-5-methanol cycloadduct, showing regioselectivity opposite to that found in the uncatalyzed reaction (Scheme 11.47). 

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