Reactivity of cyclocobaltated benzylamine derivatives toward terminal alkynes

The cyclocobaltated complexes [(eta(5)-C5H5)Co(4-RC6H3CH2NMe2-C-2,N)I] (1; R = H, Me, F) reacted with terminal alkynes such as R'C=CH (2; R' = Bu-t, S iMe3) to afford a mixture of the two compounds {(eta(5)-C5H5)Co[eta(5)-C5H4 CH=CR'(4-R-2-(CH2NMe2)C6H3)]}+PF6- (3) and(E)-CHR'=CH(4-R-2-(CH2NMe2)C6H3) (4) in a ca. 1:1 ratio. The reaction is rationalized by assuming that inser tion of the alkyne into the Co-C bond of 1 occurred with no regioselectivit y, leading to two unstable regioisomers; a selective migration of the less hindered allrenyl-Co derivative to a Cp unit followed by migration, of the monosubstituted Cp ring thus obtained to another cobalt center afforded 3, whereas compound 4 should be obtained via protiodemetalation of the other i somer formed.