Physical organic chemistry of transition metal carbene complexes .11. Kinetics and mechanism of the hydrolysis of (2-oxacyclopentylidene)pentacarbonylchromium(0) in aqueous acetonitrile

A kinetic study of the hydrolysis of the title compound, 6, in 50% acetonitrile-50% water (v/v) at 25 degrees C is reported. The organic products are 2-hydroxytetrahydrofuran in equilibrium with small amounts of 4-hydroxybutanal. There are two possible mechanisms that can account for the hydrolysis. (i) Rate limiting reaction of the conjugate anion of 6(6(-)) with water, buffer acids and H3O+, followed by (CO)(5)Cr catalyzed hydrolysis of the resulting 2,3-dihydrofuran. (ii) Ring opening of 6 through nucleophilic substitution on the carbene carbon by OH-, water or by buffer base catalyzed water attack, followed by breakdown of the intermediate substitution product into final hydrolysis products, Kinetic solvent isotope effects can be interpreted by either mechanism, Based on more conclusive isotope effect experiments in the hydrolysis of (CO)(5)Cr=C(OR')CH3(R' = CH3 or CH3CH3) and (CO)(5)Cr=C(OMe)CH2Ph reported earlier, the first mechanism is preferred by reason of analogy at least in basic solution, In acidic solution the mechanistic ambiguity could not be resolved, not even for (CO)(5)Cr=C(OMe)CH3 which was reinvestigated in HCl and DCl solutions.