A synthetic, structural and reactivity study of [(η4-C4R4)Co(η5-C5H4X)] complexes, R = Me or Et; X = CHO, CH=CHFc, CH=CH(η5-C5H4)Co(η4-C4Ph4), CH=C(CN)2: Unexpected formation of [(η5-cyclopentadienyl)(η4-3,4,5,6-tetraethyl-α-pyrone)cobalt]

The tetraethyl- and tetramethyl-cyclobutadiene complexes [(eta(4)-C4R4)Co(eta(5)-C5H4CHO)] R = Et, 5, R = Me, 7, and [(eta(4)-C4R4)Co(eta(5)-C5H4CO2Me)] R = Et, 6, R = Me, 8, are conveniently prepared by photolysis of the corresponding isocobaltocenium cations [(eta(4)-C4R4)Co(eta(6)-C6H5Me)](+) in acetonitrile, and subsequent treatment with Na[C5H4CHO] or Na[C5H4CO2Me]. The aldehydes 5 and 7 undergo Wittig and Knoevenagel reactions with [FcCH(2)PPh(3)]I and CH2(CN)(2), to form [(eta(4)-C4R4)Co(eta(5)-C5H4CH=CHFc)] and [(eta(4)-C4R4) Co(eta(5)-C5H4CH=C(CN)(2)], 11 and 15, respectively. The Horner-Wittig reaction of [(eta(4)-C4R4)Co(eta(5)-C5H4CH2P(=O)(OEt)(2)] with [(eta(4)-C4Ph4)Co(eta(5)-C5H4CHO)] yields [(eta(4)-C4R4)Co(eta(5):eta(5)-C5H4CH=CH-C5H4)Co(eta(4)-C4Ph4)], 12 and 13. [(eta(4)-C4Me4)Co(eta(5)-C5H4CHO)] also reacts with t-BuLi and FcLi to furnish the corresponding secondary alcohols, 16 and 17, respectively. Surprisingly, the attempted direct synthesis of 5 by reaction of Na[C5H5] and ethyl formate with [(eta(4)-C4Et4)Co(CO)(2)I], 1, instead yielded [(eta(5)-C5H5)Co(eta(4)-3,4,5,6-tetraethyl-alpha-pyrone)], 18, and a mechanistic proposal is advanced. The X-ray crystal structures of 1, 7, 8, 11(Z), 15 and 18, and also the isocobaltocenium salts [(eta(4)-C4Et4)Co(eta(6)-C6H5Me)][PF6], 2, and [(eta(4)-C4Et4)Co(eta(6)-1,3,5-C6H3Me3)][PF6], 4, are reported. (C) 2009 Elsevier B.V. All rights reserved.