Abstract
The exchange reaction involving the dicarbonyl (ij5-cyclopentadienyl) isobutyleneiron cation (2) and alienes provides an efficient route to the corresponding aliene complexes (4-10). For those aliene complexes which are capable of existing in stereoisomeric forms the syn isomers may be obtained stereospecifically by protonation of dicarbonyl (ij5-cyclopentadienyl)-
V-propargyliron complexes. The íyn-3-methylalleneiron complex (5) undergoes unimolecular nondissociative isomerization to the anti isomer on heating in nitromethane (Aeq50°c = 1.95, A.H* = 21.2 ± 1.5 kcal mol-1, 5* = —5.2 ± 3 eu). Activation energy barriers for rotation about the metal-olefin bond in a number of these alleneiron complexes have been determined. The principal contributor to the rotational barrier is shown to be the syn-3 substituent. Above 10 °C the tetramethylalleneiron complex undergoes a rapid fluxional motion involving exchange of the ligand coordination site. Two dissociative mechanisms for this process have been considered and each may be shown to be inoperative. Of the two nondissociative mechanisms, a step- wise process involving the intermediacy of a zj1 -2-allyliron cation has also been excluded, since fluxional motion does not result in the racemization of a chiral alleneiron complex. A concerted 1,2-shift mechanism is in accord with all the experimental re- sults.