Mechanisms for Solvolytic Elimination and Substitution Reactions Involving Short-lived Carbocation Intermediates
Abstract: Solvolysis reactions of a range of tertiary substrates in largely aqueous solvents were studied in such respects as ?-deuterium kinetic isotope effects, linear free energy relationships and stereochemistry.Solvolysis of the fluorene derivatives 9-methyl–9-(2´-X-2´-propyl)fluorene (1-X, X = Cl, Br, OOCCF3) involves a very short-lived carbocation intermediate. The fraction of alkene is increased by addition of general bases, which can be expressed by a Br?nsted parameter ? = 0.07. The kinetic deuterium isotope effects vary with solvent composition in a way which is not consistent with a common carbocation intermediate which has time to choose between dehydronation and addition of a solvent water molecule. In the absence of bases, the reaction of 4-chloro-4-(4´-nitrophenyl)pentan-2-one (2-Cl) proceeds through a short-lived carbocation intermediate yielding 4-(4´-nitrophenyl)-2-oxopent-4-ene (2-t-ne)as the main elimination product. Addition of acetate ion and other weak bases results in the base-promoted E2 (or E1cb) reaction to give (E)-4-(4´-nitrophenyl)-2-oxopent-3-ene (2-E-ne) and (Z)-4-(4´-nitrophenyl)-2-oxopent-3-ene(2-Z-ne). There is no evidence for a water-promoted E2 (or E1cb) reaction.The stereochemistry studies of elimination from (R,S and S,R)-[1-(3´-fluoro)phenyl-2-methyl]cyclopentyl-p-nitrobenzoate (3-PNB) and its (R,R and S,S)isomer 3´-PNB and (R,S and S,R)-[1´-(3´´-fluoro)phenyl-2´-methylcyclopentyl]-2,2,2-trifluoroacetate(3-OOCCF3) exclude the concerted pericyclic elimination mechanism for formation of the alkene 1-(3´-fluoro)phenyl-2-methylcyclopentene(3-m-ne). The effects of added thiocyanate ion and halide ions on the solvolysis reaction are discussed.Mass spectrometry analysis showed complete incorporation of the labeled oxygen from solvent water into the product 2-hydroxy-2-phenyl-3-butene (4-OH), confirming that it is the tertiary carbon-oxygen bond that is broken in the acid-catalyzed solvolysis of 2-methoxy-2-phenyl-3-butene (4-OMe). The mechanism for the dominant formation of the less stable 4-OH is discussed.
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