13C and (2)H kinetic isotope effects and the mechanism of Lewis acid-catalyzed ene reactions of formaldehyde.
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.
Singleton
D A
DA
Department of Chemistry, Texas A & M University, College Station, Texas 77843, USA.
Hang
C
C
D000143:Acids / Q000737:chemistry*; D002247:Carbon Isotopes; D003903:Deuterium; D005557:Formaldehyde / Q000737:chemistry*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy
D000143:Acids; D002247:Carbon Isotopes; D005557:Formaldehyde; D003903:Deuterium