Addition of alkynes at bridging vinyliminium ligands in diiron complexes: Unprecedented diene formation by enyne-like metathesis

Abstract

The zwitterionic bridging vinyliminium complex [Fe 2{μ-η 1:η 3-C(Tol) C(CS 2)C N(Me) 2}(μ-CO)(CO)(Cp) 2] ( 5a) undergoes the addition of two equivalents of MeO 2C–C C–CO 2Me affording the bridging bis-alkylidene complex [Fe 2{μ-η 1:η 3-C(Me)C{C(CO 2Me)C(CO 2Me)CSC(CO 2Me)C(CO 2Me)S}CNMe 2}(μ-CO)(CO)(Cp) 2] ( 6). One alkyne unit inserts into a C–CS 2 bond of the bridging ligand, with consequent rearrangement that leads to the formation of a diene. The reaction shows analogies with the enyne metathesis. The second alkyne is incorporated into the bridging frame via cycloaddition at the thiocarboxylate function, affording a 1,3-dithiolene. The complexes [Fe 2{μ-η 1:η 3-C(R′) C(CS 2)C N(Me)(R)}(μ-CO)(CO)(Cp) 2] (R = Xyl, R′ = Tol, 5b; R = p-C 6H 4OMe, R′ = Me, 5c; Xyl = 2,6-Me 2C 6H 3), treated with MeO 2C–C C–CO 2Me and then with HBF 4, undergo the cycloaddition of the alkyne with the dithiocarboxylate group and protonation of the dithiocarboxylate carbon, affording the complexes [Fe 2{μ-η 1:η 3-C(R′) C{C(H)SC(CO 2Me)C(CO 2Me)S}C N(Me)(R)}(μ-CO)(CO)(Cp) 2][BF 4] (R = Xyl, R′ = Tol, 7a; R = p-C 6H 4OMe, R′ = Me, 7b), respectively. The X-ray molecular structure of 6 has been determined.