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 kineticsand indicate that Co-2(CO)(6)-coordination increases the nucleophilic reactivity of enynes by a factor of > 10(6). 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 k(20 degrees C)=s(E+N) and allow to determine the nucleophilicity parameters N and s for 2a-d. It is shown that at -70 degrees 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. (C) 2000 Elsevier Science Ltd. All rights reserved.