Competing, kc, Borderline, ks, and Carbonyl Addition Processes in Solvolyses of α-Keto Mesylates and Triflates. α-Keto Cations. 5

Article abstract of DOI:10.1021/ja00331a029

Solvolysis studies on tertiary α-keto mesylates show quite varying responses in rate to solvent ionizing power.The m values are 1.01 for 2-benzoyl-2-adamantyl mesylate (12), 0.66 for the mesylate derivative of 2-hydroxy-2,4,4-trimethyl-3-pentanone (4), and 0.63 for the mesylate derivative of 2-hydroxy-2-methylpropiophenone (3).The mesylate derivative of methyl α-hydroxyisobutyrate (5) does not correlate well with Y_OTs values, but instead it gives behavior that parallels that of isopropyl tosylate.Mesylates 3-5 solvolyze giving varying ratios of elimination and substitution products at rates comparable to that of isopropyl mesylate. β-d₆ isotope effects for 3 and 4 range from 1.69 to 2.08 and are consistent with the intermediacy of α-keto cations. β-Deuterium isotope effects for 5 are quite variable (1.40-2.52) and parallel the amount of elimination product formed (22-94percent).This is consistent with the intermediacy of a reversibly formed ion-pair intermediate which can suffer proton loss.However, the S_N2 (intermediate) mechanism remains a possibility in solvolyses of 5.Mesylate 3 solvolyzed in trifluoroethanol with added triethylamine to give an alkoxyoxirane.With small amounts of added 2,6-lutidine the 1,2-elimination product was major , while with added methanesulfonic acid the rearranged trifluoroethyl ester of dimethylphenyl acetic acid was major.These variable products were interpreted in terms of competing carbonyl addition processes and processes involving the α-keto cation.Secondary triflates derived from α-hydroxypropiophenone, 2,2-dimethyl-4-hydroxy-3-pentanone, and ethyl lactate solvolyzed giving the simple substitution product.Rates parallel solvent nucleophilicity and suggest a k_s process involving negligible cationic character at the carbon α to the carbonyl group. k_Δ processes are also not involved in solvolyses of these secondary triflates.