Nucleophilic addition of N,N-dimethylaniline to the (cyclobutadiene)nitrosyldicarbonyliron cation: A kinetic and structural study

Article abstract of DOI:10.1021/om00074a004

The electrophilic aromatic substitution reaction between N,N-dimethylaniline and the cyclobutadiene ring in [(C₄H₄)Fe(CO)₂NO]PF₆ yields the η³-cyclobutenyl complex [(Me₂NC₆H₄C₄H ₄)Fe(CO)₂NO] (2). Spectroscopic studies suggest that substitution occurs exclusively at the carbon para to the NMe₂ group and that the product has exo stereochemistry. A kinetic study in acetonitrile showed the reaction to be second order overall with the following activation parameters: ΔH^? = 11.2 ± 1.5 kcal; ΔS^? = -31 ± 5 cal deg^-1 mol^-1. A mechanism is suggested that involves initial π-complex formation followed by rate-determining conversion to a σ complex. A single-crystal X-ray analysis of 2 at -100°C showed that it crystallizes in the monoclinic space group P2₁/c with a = 16.690 (2) A?, b = 6.310 (1) A?, c = 26.716 (3) A?, β = 96.46 (1)°, Z = 8, R = 0.036, and R_w = 0.044. The structure confirms the exo attack of the N,N-dimethylaniline para carbon on the coordinated cyclobutadiene. The bond distances and angles in the cyclobutenyl group are similar to previously reported structures. The dihedral angle between the enyl plane (C₇, C₄, C₅) and the nonbonded plane (C₅, C₆, C₇) is 26.5° (Figure 1).