REARRANGEMENTS OF CYCLOPROPANES SIGMA-BONDED TO IRON - ALPHA-ELIMINATIONS TO CARBENE COMPLEXES AND RING-OPENING TO PI-ALLYL COMPLEXES

A number of α- and β-substituted cyclopropyl σ-complexes of dicarbonyl(η5-cyclopentadienyl)iron have been photolyzed and their products examined. Under these conditions, the α-ethoxycyclopropane complex 6c undergoes loss of CO to give a 16-electron iron intermediate that rapidly and reversibly undergoes α-elimination to give the first example of a 1-ferracyclobutene (8c). This complex has been identified by NMR but was too unstable for isolation and complete characterization. Treatment of the ferracyclobutene with CO or Ph3P gives the ring-expanded ferracyclopentenones 9c and 9d. In the absence of added ligand, the ferracyclobutene opens to give the two isomeric π-allyl complexes 10c (via 7c) and 11c. The 16-electron σ-complex 7c irreversibly opens to the centrally substituted π-allyl complex 10c in a slow competition with α-elimination to 8c. Support for the assumption that these conversions (except carbon monoxide extrusion) are thermally induced was obtained by thermolysis of the phosphine-substituted σ-complex 6d. Photolysis of dicarbonyl(η5-cyclopentadienyl)(α-(phenylthio)cyclopropyl)iron 6e gives the three-membered chelate 12 (or its dimer) which, upon warming, opens quantitatively to the centrally substituted π-allyl complex 10e. When a mixture of cis- and trans-dicarbonyl(η5-cyclopentadienyl)(2-fluorocyclopropyl)iron is photolyzed, the cis isomer remains unchanged but the trans isomer cleanly reacts to give a mixture of stereoisomers of the terminally substituted π-allyl complex 10h. It is proposed that the syn isomer (fluorine and central hydrogen cis) is initially formed which rapidly photoisomerizes to a mixture of exo and endo anti isomers. Photolysis of a mixture of cis- and trans-dicarbonyl(η5-cyclopentadienyl)(2-methylcyclopropyl)iron also shows preferential reaction of the trans isomer which opens to give the photostable syn isomer of 10g. Both preferential reaction of trans-6h and stereospecific opening of trans-7g are consistent with a disrotatory ring opening of the cyclopropane in which the breaking cyclopropane bond assists in weakening the carbon-iron bond in much the same way a solvolysis reaction is assisted. The mechanism of the opening of the metallocyclobutene 8c to the π-allyl complex 11c is briefly discussed. © 1990, American Chemical Society. All rights reserved.