KINETICS OF NUCLEOPHILIC-ATTACK ON COORDINATED ORGANIC MOIETIES .31. ADDITION OF IMIDAZOLES TO [FE(CO)(3)(1-5-ETA-DIENYK)](+) CATIONS

Synthetic and spectroscopic studies show that imidazole, 2-methylimida zole, and 4-methylimidazole (BH) react with the dienyl cations [Fe(CO) (3)(1-5-eta-dienyl)](+) (Ia-Ic: dienyl - C6H7, 2-MeOC(6)H(6), or C7H9) in CH3CN or acetone solvent via the two-step sequence shown below. [G RAPHICS] With N-methylimidazole, which contains no ring N-H group, rea ction stops at the monocationic adduct (III). In all cases the rate la w, k(obs) = k(1) [amine] is obeyed. This may be rationalised in terms of rate-determining addition (K-1) at the dienyl ring to yield (III), followed where applicable by rapid deprotonation (k(2)) to yield the f inal neutral adduct (IV). For attack by imidazole, the rate trend is C 6H7 > 2-MeOC(6)H(6) > C7H9 (relative rates: 47:4:1), revealing a previ ous k(1) value for cation (Ib) to be erroneous. This rate trend, toget her with the exo-configuration established for adducts (III), support direct addition (k(1)) of imidazole from above the dienyl ring. The re activity order 2-methylimidazole > 4-methylimidazole > N-methylimidazo le > imidazole is observed towards both cations (Ia) and (Ib), reveali ng a strong dependence of rate on nucleophile basicity. Steric effects are less significant than in the related addition of pyridines to the se dienyl cations.