1,3-Dipolar cycloaddition to the Fe-N=C fragment. 11. Isocyanide-controlled change between a reversible molecular self-assembly of three components, including a unique isocyanide deinsertion, and dimethyl maleate coupling reactions

Article abstract of DOI:10.1021/om00026a031

In the reaction of Fe(CNR)₃(i-Pr-α-diimine) (6; R = 2,6-xylyl (a), t-Bu (b), c-Hex (c)) with dimethyl maleate, two totally different pathways are followed, depending on the type of isocyanide used. With the aromatic isocyanide, a 1,3-dipolar cycloaddition of the alkene to the Fe-N=C unit occurs followed by an isocyanide insertion, forming a ferra [2.2.2] bicyclic compound (7a). When this mixture is warmed above room temperature, the reaction is reversed, disassembling 7a into its starting components. During this facile retro-cycloaddition, C-C, C-N, and Fe-C bonds are broken. Also, the first unambiguous example of an isocyanide deinsertion is encountered. The starting complexes 6b,c, with aliphatic isocyanides, react with dimethyl maleate to give two products. The first, a purely organic product (8), is a dimer of two coupled alkenes. The second product is the organometallic tricyclic complex Fe(CNR)₃(tric) (9b,c), in which two alkenes are coupled and bonded to the metal and the α-diimine ligand. In the cyclization reaction, a γ-lactam ring is formed and a methoxy group of one of the four ester groups is removed, with formation of methanol. The molecular structure of the [2.2.2] bicyclic complex 7a (FeC₆₀H₆₀N₆O₄, triclinic, space group P1, a = 10.367 (1) A?, b = 14.615 (3) A?, c = 15.787 (2) A?, α = 95.78 (2)°, β = 96.62 (1)°, γ = 92.21 (1)°, Z = 2, R = 0.053, R_w = 0.055) consists of three six-membered rings with the metal in a bridgehead position. The single-crystal X-ray structure of the organic dimer 8 (C₁₂H₁₆O₈, monoclinic, space group A2/a, a = 8.291 (2) A?, b = 8.291 (2) A?, c = 20.551 (2) A?, β = 96.09 (1)°, Z = 4, R = 0.068, R_w = 0.089) reveals a symmetric alkene. The molecular structure of the Fe(t-BuNC)₃(tric) species 9b (FeC₃₄H₅₅N₅O₇·0.5CH ₂Cl₂·0.5C₄H₁₀O, monoclinic, space group Pc, a = 11.201 (1) A?, b = 12.251 (2) A?, c = 32.019 (3) A?, β = 96.43 (1)°, Z = 4, R = 0.0581, R_w = 0.0629) consists of a four-, a five-, and a six-membered ring. The metal is incorporated in the six- and the four-membered rings.