Electrophilic addition reactions kinetic versus thermodynamic

Formation of a highly electrophilic iodonium species, transiently formed by treatment of an alkene with iodine, followed by intramolecular quenching with a nucleophile leads to iodocyclization. The use of iodine to form lactones has been elegantly developed. Bartlett and co-workers216 reported on what they described as thermodynamic versus kinetic control in the formation of lactones. Treatment of the alkenoic acid 158 (Scheme 46) with iodine in the presence of base afforded a preponderance of the kinetic product 159, whereas the same reaction in the absence of base afforded the thermodynamic product 160. This approach was used in the synthesis of serricorin. The idea of kinetic versus thermodynamic control of the reaction was first discussed in a paper by Bartlett and Myerson217 from 1978. It was reasoned that in the absence of base, thermodynamic control could be achieved in that a proton was available to allow equilibration to the most stable ester. In the absence of such a proton, for example by addition of base, this equilibration is not possible, and the kinetic product is favored. 

Further studies by Brown and coworkers lent additional support to this mechanism and the absence of a free alkyl cation. Olah and coworkers have applied the concept of competitive alkylation to the case of naphthalene in order to study both positional and substrate selectivities, and to clarify the nature of kinetically versus thermodynamically controlled product composition. They explained the observed results by suggesting that a ir-complex, such as (1), was the intermediate involved when highly electrophilic catalysts or strongly basic aromatics were employed, and a o-complex (as proposed earlier by Brown) was involved in reactions with weakly electrophilic catalysts or less basic aromatics. 

Kinetic versus thermodynamic control. A plot of Gibbs free energy versus reaction coordinate for Step 2 in the electrophilic addition of HBr to 1,3-butadiene. The resonance-stabilized allylic carbocation intermediate reacts with bromide ion by way of the transition state on the left to give the 1,2-addition product. It reacts with bromide ion by way of the alternative transition state on the right to give the 1,4-addition product. 

The electrophilic addition of HBr to 1,3-butadiene is a good example of how a change in experimental conditions can change the product of a reaction. The concept of thermodynamic control versus kinetic control is a useful one that we can sometimes take advantage of in the laboratory. 

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