Group II Organometallic Derivatives

In addition, while no studies have been reported which provide quantitative kinetic information on the exchange of alkyl groups between dialkylberyllium compounds, Coates and Roberts have provided qualitative evidence for the exchange between bridge and terminal positions in 

Coates and co-workers have also investigated a wide variety of compounds of the type (RBeX) , where R=CH3 or C2H5 and X=NMe2, OMe, SeEt, SeO, SC2H4NMe2, etc., and n = 2 or 3 (9, 10, 23-25). In a number of the trimeric derivatives, they have postulated that the ring structure can be present in two conformations with rapid conversion from one to the other at room temperature. In some instances, these conformations may be slowed to give resolvable methyl resonances corresponding to the nonequivalent methyl positions. No quantitative data have been presented on these systems. 

This mechanism can be used to account for the solvent dependence, which shows that a strongly coordinating solvent slows or prevents step (4) from occurring and thus retards the rate of exchange. This is similar to the effect observed in Group III alkyl derivatives discussed in Section IV,A. However, if complete coordination of the magnesium alkyl does not occur, it becomes necessary to break a 

Rates of Exchange of Methyl Groups in Organomagnesium Derivatives 1 

Reagent Solvent Rate constants E. at —55° C, (kcal/mole) liter mole 1 sec-1  

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