Physical organic chemistry of transition metal carbene complexes. Part 11.1 Kinetics and mechanism of the hydrolysis of (2-oxacyclopentylidene)pentacarbonylchromium(0) in aqueous acetonitrile

Abstract

A kinetic study of the hydrolysis of the title compound, 6, in 50% acetonitrile–50% water (v/v) at 25 °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)₅Cr 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)₅Cr=C(OR′)CH₃ (R′ = CH₃ or CH₃CH₃) and (CO)₅Cr=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)₅Cr=C(OMe)CH₃ which was reinvestigated in HCl and DCl solutions.

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