Mechanistic studies of the cycloisomerization of dimethyl diallylmalonate catalyzed by a cationic palladium phenanthroline complex.
The mechanism of the cycloisomerization of dimethyl diallylmalonate (1) catalyzed by the cationic palladium phenanthroline complex [(phen)Pd(Me)CNCH(3)](+)[BAr(4)](-) [Ar = 3,5-C(6)H(3)(CF(3))(2)] (2) has been investigated. Heating a solution of 1 and 2 (5 mol %) in DCE at 40 degrees C led to zero-order decay of 1 to approximately 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 ( approximately 3.5%) of ethyl-substituted carbocycles 6 of the chemical formula C(12)H(18)O(4). Cyclopentenes 3 and 4 were formed both kinetically (3:4 = 30:1 at 40 degrees C) and via secondary isomerization of 5 (3:4 = 1:2.5 at 40 degrees C); the kinetic pathway accounted for the 93% of cyclopentene formation at 40 degrees C. Carbocycles 6 were formed predominantly (> or =90%) within the first two catalyst turnovers as byproducts of catalyst activation. Stoichiometric reaction of 1 and 2 at room temperature for 1.5 h led to the isolation of the palladium cyclopentyl chelate complex [carbohydrate structure-see text] in 26% yield as a approximately 2:1 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, where k = (2.1 +/- 0.3) x 10(-2) M(-1) s(-1) (Delta G(*)(298K) = 19.7 +/- 0.1 kcal mol(-1)) at 25 degrees C. Thermolysis of 7 at 50 degrees C formed carbocycles 6 in 65% yield by GC analysis. (1)H and (13)C 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 [carbohydrate structure-see text] as the catalyst resting state. Cycloisomerization of 1-2,6-d(2) formed predominantly (approximately 90%) 3,3-bis(carbomethoxy)-5-deuterio-1-(deuteriomethyl)-5-methylcyclopentene (3-d(2)); no significant (< or =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-1,5-dimethylcyclopentene (3-d(3)) and 3,3-bis(carbomethoxy)-2,4,4-trideuterio-5-(deuteriomethyl)-1-methylcyclo pentene (3-d(4)); while no significant (< or =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.
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