The mechanism of the cycloisomerization of dimethyl diallylmalonate (1) catalyzed by the cationic palladium phenanthroline complex [(phen)Pd(Me)CNCH3](+)[BAr4](-) [Ar = 3,5-C6H3(CF3)(2)] (2) has been investigated. Heating a solution of 1 and 2 (5 mol %) in DCE at 40 degreesC led to zero-order decay of to similar to 80% conversion (k(obs) = (7.1 +/- 0.3) x 10(-7) M s(-1)) with formation of a 27:2.2:1.0 mixture of 3,3-bis(carbomethoxy)1,5-dimethylcyclopentene (3), 4,4-bis(carbomethoxy)- 1,2-dimethylcyclopentene (4), and 1,1-bis(carbomethoxy)4-methyl-3-methylenecyclopentane (5) and traces (similar to3.5%) of ethyl-substituted carbocycles 6 of the chemical formula C12H18O4. Cyclopentenes 3 and 4 were formed both kinetically (3:4 = 30:1 at 40 degreesC) and via secondary isomerization of 5 (3:4 = 1:2.5 at 40 degreesC; the kinetic pathway accounted for the 93% of cyclopentene formation at 40 degreesC. Carbocycles 6 were formed predominantly (greater than or equal to 90%) within the first two catalyst turnovers as byproducts of catalyst activation. Stoichiometric reaction of I and 2 at room temperature for 1.5 h led to the isolation of the palladium cyclopentyl chelate complex [(phen)PdCHCH(Me)CH(Et)CH2C(COOMe)(COOMe)](+)[BAr4](-)(7) in 26% yield as a similar to2:1 mixture of isomers. The structure of trans,trans-7 was determined by X-ray crystallography. Kinetic studies of the formation of 7 established the rate law: rate = k[1][2], where k = (2.1 +/- 0.3) x 10(-2) M-1 s(-1) (DeltaG*(298K) = 19.7 +/- 0.1 kcal mol(-1)) at 25 degreesC. Thermolysis of 7 at 50 degreesC formed carbocycles 6 in 65% yield by GC analysis. H-1 and C-13 NMR analysis of an active catalyst system generated from 1 and a catalytic amount of 2 led to the identification of the cyclopentyl chelate complex [(phen)PdCHCH(Me)CH(Me)CH2C(COOMe)(COOMe)](+)[BAr4](-) (8) as the catalyst resting state. Cycloisomerization of 1-2,6-d(2) formed predominantly (similar to 90%) 3,3-bis(carbomethoxy)-5-deuterio-1-(deuteriomethyl)I 5-methylcyclopentene (3-d(2)); no significant (less than or equal to 10%) kinetic isotope effect or intermolecular H/D exchange was observed. Cycloisomerization of 1-3,3,5,5-d(4) formed a 1:2.6 mixture of 3,3-bis(carbomethoxy)-2,4,4-trideuterio-5-dimethylcyclopentene (3-d(4)) and 3,3-bis(carbomethoxy)-2,4,4-trideuterio-5-(deuteriomethyl)1-methylcyclo pentene (3-d(4)); while no significant (less than or equal to 10%) kinetic isotope effect was detected, extensive intermolecular H/D exchange was observed. These data are consistent with a mechanism involving hydrometalation of an olefin of 1, intramolecular carbometalation, isomerization via reversible beta -hydride elimination/addition, and turnover-limiting displacement of the cyclopentenes from palladium.