Low-Temperature Study of the Iron-Mediated [4 + 1] Cyclization of Allenyl Ketones with Carbon Monoxide

Low-temperature FTIR studies were conducted to probe the mechanism of the Fe(CO) 5 -catalyzed [4 + 1] cyclization of allenyl ketones and CO to form α-alkylidenebutenolides. Photolysis of Fe(CO) 5 in 2-methyltetrahydrofuran (2MeTHF) at 190−230 K produced an iron carbonyl solvento species, Fe(CO) 4 (2MeTHF). When the matrix was warmed, Fe(CO) 5 was regenerated quantitatively. Photolysis of Fe(CO) 5 (in 2MeTHF) at 230 K in the presence of 5-methyl-3,4-hexadien-2-one under similar conditions gave Fe(CO) 4 (2MeTHF) initially. Warming the samples regenerates a portion of the Fe(CO) 5 while a new band appears at 1767 cm - 1 corresponding to the α-alkylidenebutenolide 11 . Kinetic experiments at 243 K show no dependence upon CO or allenyl ketone. Reactions in pyridine gave significantly slower rates than THF. This is consistent with rate-determining dissociation of the coordinated solvent to form an unsaturated iron center.