Reactions of co-ordinated ligands. Part XIV. Regiospecific thermal rearrangements of tetracarbonyl-η2-[3-methylene-exo-4-vinyldihydrofuran-2(3H)-one]iron(0) and tricarbonyl-η4-[3-methylene-endo-4-vinyl-dihydrofuran-2(3H)-one]iron(0) complexes: the mechanisms of metalpromoted 1,3-hydrogen shifts

Abstract

Tetracarbonyl-η²-[3-methylene-exo-4-vinyldihydrofuran-2(3H)-one]iron complexes (1) rearrange regiospacifically at 35°via a 1,3-hydrogen shift to yield the corresponding substituted tricarbonyl(l,3-diene)iron complexes (4). Tricarbonyl-η⁴-[3-methylene-endo-4-vinyldihydrofuran-2(3H)-one]iron(0) complexes (2) also rearrange regiospecifically at 35 °C to yield a 1 : 2.3 mixture of (5) and (6), isomeric with (4). Use of optically active complexes (+)-(1a) and (+)-(2a) defines the rearrangement of (+)-(1a) as proceeding with extensive racemisation, yet with slight retention of absolute configuration, and rearrangement of (+)-(2a) as proceeding with inversion of configuration. Kinetic results for the isomerisation of complexes (2) are reported, and the results are all discussed with reference to their implications concerning the mechanisms of metal-promoted olefin isomerisation reactions. A new ‘σ-allyl metal hydride’ mechanism is proposed as a low-energy isomerisation pathway.