Cycloisomerization of 4,4-bis(acetoxymethyl)-1,6-heptadiene (5) catalyzed by [(phen)Pd(Me)CNCH ] [BAr ] [Ar=3,5-C H (CF ) ] (2) to form predominantly 3,3-bis(acetoxymethyl)-1,5-dimethylcyclopentene (6) was ∼400 times faster than was the cycloisomerization of dimethyl diallylmalonate (1) under identical conditions. Mechanistic studies performed in conjunction with density functional theory calculations attribute the large rate acceleration of the cycloisomerization of 5 relative to the cycloisomerization of 1 to the formation of a stable oxo chelate complex as an intermediate in the cycloisomerization of 1, but not in the cycloisomerization of 5. © 2003 Published by Elsevier B.V. 3 4 6 3 3 2 + -