Abstract:A kinetic study is reported for the ene reaction between methyl acrylate as the enophile and 8-pinene, with aluminium chloride as catalyst, in a series of solvents varying in polarity. These experiments point to a transition state having pronounced zwitterionic character, in line with earlier suggestions in the literature. An unexpected and significant change of the rate constant with the initial concentrations is also found.
“…Salomon found a ρ of −3.9 for the SnCl 4 -catalyzed ene reaction of diethyl oxomalonate with 1-arylcyclopentenes, indicative of a charge build-up near positive unity in the central carbon of the ene in these reactions . The suggested intermediacy of a carbocation as in eq 2 is consistent with a number of other observations in these reactions, including the formation of oxetanes, the formation of chloride-trapped and intramolecularly trapped products, the influence of pressure on product mixtures, solvent effects, and the high regioselectivity of these reactions. …”
13C and (2)H kinetic isotope effects were determined for the ene reaction of formaldehyde with 2-methyl-2-butene at natural abundance catalyzed by diethylaluminum chloride. The reactive methyl group exhibits a k(12)(C)/k(13)(C) of 1.006-1.009 and a k(H)/k(D) of approximately 1.22-1.23. The latter represents a combination of primary and secondary effects and is consistent with a significant primary deuterium isotope effect. A very close correspondence of the other isotope effects with the equilibrium isotope effects predicted for formation of a model intermediate cation is observed. An intermolecular deuterium isotope effect of 2.0-2.5 was observed under several reaction conditions in the Lewis acid-catalyzed reaction of formaldehyde with d(0)/d(12)-tetramethylethylene. The results are interpreted as supporting the reversible formation of an essentially classical open cation followed by rate-limiting proton transfer.
“…Salomon found a ρ of −3.9 for the SnCl 4 -catalyzed ene reaction of diethyl oxomalonate with 1-arylcyclopentenes, indicative of a charge build-up near positive unity in the central carbon of the ene in these reactions . The suggested intermediacy of a carbocation as in eq 2 is consistent with a number of other observations in these reactions, including the formation of oxetanes, the formation of chloride-trapped and intramolecularly trapped products, the influence of pressure on product mixtures, solvent effects, and the high regioselectivity of these reactions. …”
13C and (2)H kinetic isotope effects were determined for the ene reaction of formaldehyde with 2-methyl-2-butene at natural abundance catalyzed by diethylaluminum chloride. The reactive methyl group exhibits a k(12)(C)/k(13)(C) of 1.006-1.009 and a k(H)/k(D) of approximately 1.22-1.23. The latter represents a combination of primary and secondary effects and is consistent with a significant primary deuterium isotope effect. A very close correspondence of the other isotope effects with the equilibrium isotope effects predicted for formation of a model intermediate cation is observed. An intermolecular deuterium isotope effect of 2.0-2.5 was observed under several reaction conditions in the Lewis acid-catalyzed reaction of formaldehyde with d(0)/d(12)-tetramethylethylene. The results are interpreted as supporting the reversible formation of an essentially classical open cation followed by rate-limiting proton transfer.
“…a-Pinene (276) undergoes regiospecific hydrocarbonylation to yield the aldehyde (284), and P-pinene (277) behaves similarly to yield the 10-formyl derivative (285).27s The aldehyde (286), derived from a-pinene (276), undergoes McMurry coupling to give the alkene (287) which can be dihydroxylated to yield the diastereoisomeric diols ( 288) and (289).351 The use of biphenyl as sensitiser enhances the yields of the pinenol(290) which is obtained via singlet oxygen oxidation of P-pinene (277).352 An improved route to crystalline 6-terpineol (291) from p-pinene has been and the kinetics of the AlC1,-catalysed ene-reaction which takes place between ppinene and methyl propenoate have been measured as a function of solvent polarity. 354 The results suggest that the reaction may proceed via a transition state which possesses zwitterionic character.…”
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