13C and 2H kinetic isotope effects and the mechanism of Lewis acid-catalyzed ene reactions of formaldehyde

C-13 and H-2 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(C)(12)/k(C)(13) of 1.006-1.009 and a k(H)/k(D) of approximate to 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.