Nucleophilic Reactions of Molybdate

Article abstract of DOI:10.1021/jo00051a026

Although molybdate (MoO4(2-)) is over 1000 times less basic than the phosphate dianion it is 35 times more nucleophilic toward p-nitrophenyl acetate (pNPA) and p-nitrophenyl thioacetate (pNPTA) at 27 deg C (Wikjord, B.; Byers, L.D.J.Am.Chem.Soc. 1992, 114, 5553).Based on Bronsted relationships for the reaction of a series of phosphate dianions with these esters (β_nuc = 0.3 at 37 deg C), molybdate (pK_a2 = 3.9) is as reactive as a phosphonate with a pK_a2 = 12.The activation parameters are ΔH(excit.) = 15.8 (+/- 0.1) kcal/mol and ΔS(excit.) = -17.4 (+/- 0.4) eu for the reaction with pNPA and ΔH(excit.) = 16.0 (+/- 0.2) kcal/mol and ΔS(excit.) = -15.0 (+/- 0.6) eu for the reaction with pNPTA.The second-order rate constant for the reaction of MoO4(2-) with a series of 13 acyl-substituted p-nitrophenyl esters at 27 deg C shows nearly identical sensitivity to substituent effects (ρ* = 2.8) as does the rate of alkaline hydrolysis or the equilibrium addition of hydroxide to aldehydes (Shames, S.L.; Byers, L.D.J.Am.Chem.Soc. 1981, 103, 6170), suggesting that the transition state for the molybdenolysis reaction closely resembles the tetrahedral intermediate.In addition to the reaction with activated carboxylic esters, molybdate catalyzes the reaction of a variety of electrophiles.The second-order rate constant (M^-1 min^-1, 27 deg C, μ = 1.5 M) for the reaction of molybdate are 0.25 (+/- 0.01) with ethyl pyruvate, (2.10 +/- 0.08) x 10^-2 with 2,4-dinitrofluorobenzene, and (7.2 +/- 0.4) x 10^-5 with methyl iodide.