Reactions of Carbon Electrophiles with Cobalt-Coordinated Enynes: Scope and Limitations

The kinetics of the reactions of the dicobalt-coordinated enynes 2a–d with the benzhydryl cations 3a–c,f have been studied photometrically. The reactions follow second-order kinetics and indicate that Co2(CO)6-coordination increases the nucleophilic reactivity of enynes by a factor of > 106. The exchange of one CO ligand by PPh3 (2a→2d), however, has only little effect on the reactivity of the enyne moiety. The second-order rate constants match the linear free energy relationship lg k20°C=s(E+N) and allow to determine the nucleophilicity parameters N and s for 2a–d. It is shown that at −70°C, electrophiles with E>0 are able to react with the enyne complexes 2a, 2c, and 2d whereas electrophilic reagents with E>−2 are suitable for reactions with 2b.