Pericyclic Organometallic Reactions. Substituent Effects on Cycloadditions and Rearrangements of (η⁴‐cycloheptatriene)Fe(CO)₃ Derivatives

The mechanism of the reactions of (η⁴‐cycloheptatrien‐1‐al)Fe(CO)₃ and its ethyleneglycol acetal with tetracyanoethylene (TCNE) was studied. The aldehyde undergoes a fast reversible 3+2 cycloaddition involving the free aldehyde‐substituted double bond, and a slow irreversible 3+2 cycloaddition at the coordinated site. The latter, σ,π‐allylic complex undergoes a slow interconversion with the corresponding 5 +2 isomer via the pericyclic [3,3]‐sigmahaptotropic rearrangement. In contrast, the acetal reacts with TCNE at the free acetal‐substituted double bond to give a single kinetic 3+2 adduct, which, under thermodynamic conditions, undergoes a [4,4]‐sigmahaptotropic rearrangement to the 6 + 2 adduct. The reaction kinetics was followed by ¹H NMR. The effect of substituents on the reaction course is discussed, and the detailed mechanism of both the cycloadditions and rearrangements is described.