KINETICS OF NUCLEOPHILIC ATTACK ON COORDINATED ORGANIC MOIETIES. PART 31 ADDITION OF IMIDAZOLES TO [Fe(CO)₃(1-5-η-DIENYL)]⁺CATIONS

Abstract

Synthetic and spectroscopic studies show that imidazole, 2-methylimidazole, and 4-methylimidazole (BH) react with the dienyl cations [Fe(CO),( 1-5-q-dienyl)]' (Ia-Ic; dienyl = C6H,, 2-MeOC6H6, or C,H,) in CH,CN or acetone solvent via the two-step sequence shown below. kl +BH (I) + BH __+ [Fe(CO),(BH.diene)]+ -[Fe(CO),(B.diene)] + BH2+ k2 (111) (IV) With N-methylimidazole, which contains no ring N-H group, reaction stops at the monocationic adduct (111). In all cases the rate law, kbn = k, [amine] is obeyed. This may be rationalised in terms of rate-determining addition (k,) at the dienyl ring to yield (111), followed where applicable by rapid deprotonation (k2) to yield the final neutral adduct (IV).For attack by imidazole, the rate trend is CsH, > 2-MeOC6H6 > C7H9 (relative rates: 47:4:1), revealing a previous k, value for cation (Lb) to be erroneous. This rate trend, together with the exo-configuration established for adducts (111), support direct addition (k,) of imidazole from above the dienyl ring. The reactivity order 2-methylimidazole > 4-methylimidazole > N-methylimidazole > imidazole is observed towards both cations (la) and (Ib), revealing a strong dependence of rate on nucleophile basicity. Steric effects are less significant than in the related addition of pyridines to these dienyl cations. 30: S. Chapman and L.A.P. Kane-Magurie, J. Chem. SOC., Dalton Trans., 2021 (1 995).