The effect of cations on nucleophilic additions to carbonyl compounds: carbonyl complexation control versus ionic association control. application to t

The fundamental influence of cations in nucleophilic additions to carbonyl compounds is discussed in terms of either carbonyl complexation or ionic association with the nucleophile. Since loose ion pairs react essentially under complexation control while tight ion pairs react under association control, an experimental criterion is proposed to determine which factor controls the reactivity (complexation or association) based on the kinetic effects of cryptand or crown-ether addition.An application of this duality is then examined for the regioselectivity of nucleophilic additions to α-enones and correlated with perturbational arguments based on abinitio calculations. Under complexation control, attack at carbon 2 is favoured, especially when a soft nucleophile is involved and the counter-ion is Li⁺ rather than Na⁺. Under association control two cases are considered according to whether the cation is directly bound or not to the nucleophilic center; in addition, the influence of hard or soft metals and the possibility of a cationic bridge in the transition state have been discussed. Several kinetic controlled reactions have been examined and interpreted in this way. An extension of this concept to other reactions will be considered.