REACTIONS OF COORDINATED CYCLIC POLYOLEFINS. REGIO- AND STEREO-SPECIFIC CYCLOADDITIONS OF KETENES AND TRICARBONYL(η4-POLYENE)IRON COMPLEXES. STRUCTURAL EVIDENCE FOR A CONCERTED 2 + 2 MECHANISM

Article abstract of DOI:10.1016/0022-328X(84)80549-5

The cycloaddition of ketenes R¹R²C=C=O (R¹ = Me, R² = Ph; R¹ = R² = Ph; R¹ = Ph, R² = p-tolyl) (III) and tricarbonyl(η⁴-cycloheptatriene)iron (II) at room temperature gave regiospecifically the cyclobutanones IV, in which the carbonyl is bonded to the uncoordinated terminal of the conjugated triene.Tricarbonyl(η⁴-cyclooctatetraene)iron.VI, similarly reacts with diphenylketene to give the regioisomeric cyclobutanone VIII.Single crystal X-ray analysis of tricarbonyl<η-2,3,4,5)-9-methyl-9-phenylbicyclo<5.2.0>nonan-2,4-dien-8-one>iron (IVa) confirmed the presence of the phenyl group at the endo position, providing a stereochemical evidence for a concerted 2s + 2a cycloaddition mechanism.The adducts IV undergo cyclobutanone ring cleavage catalyzed by acid to give the isomeric ketones VII.The diaryl adducts (IVb and IVc) rearrange thermally to the ?,?-allylic complexes IX, which may also be obtained directly upon heating cycloheptatriene complex II and the diarylketenes.