| Dimethyl diallylmalonate (1) catalyzed by the cationic palladium phenanthroline complex [(phen)Pd(Me)(NCMe)]+[BAr4] - [Ar = 3,5-C6H3(CF3) 2] (2) in DCE at 40°C forms a 27:2.2:1 mixture of 3,3-dicarbomethoxy-1,5-dimethylcyclopentene (3), 4,4-dicarbomethoxy-1,2-dimethylcyclopentene (4), and 1,1-dicarbomethoxy-4-methyl-3-methylenecyclopentane (5) and traces (∼3.5%) of ethyl-substituted carbocycles 6 of the chemical formula C12H18O4. Cyclopentenes 3 and 4 were formed both kinetically and via secondary isomerization of 5. 1H and 13C 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. These data along with kinetic and deuterium labeling experiments are consistent with a mechanism initiated by hydrometalation of an olefin of 1, intramolecular carbometallation, isomerization via reversible beta-hydride elimination/addition, and turnover-limiting displacement of the carbocycles from palladium. The rate of the cycloisomerization of 4,4-bis(acetoxymethyl)-1,6-heptadiene (12) catalyzed by 2 was ∼400 times faster than the rate of the cycloisomerization of 1 under comparable conditions. The results of kinetic and deuterium labeling experiments for the cycloisomerization of 12 were consistent with the hydrometalation mechanism proposed for the cycloisomerization of 1. Based on the results of ESI-MS and density functional theory experiments, the enhancement in rate is attributed to the lack of formation of a palladium chelate complex analogous to 8, which removes available palladium from the catalytic cycle.; The effect of substitution on the palladium-catalyzed cyclization/hydrosilylation of functionalized 1,6-dienes was examined. Cyclization/hydrosilylation of dimethyl diallylmalonate (1) and HSiEt3 catalyzed by [(phen)Pd(Me)(NCMeAr)]+[BAr4]- [Ar = 3,5-C6H3(CF3)2] ( 22) in CD2Cl2 at -41°C forms trans-1,1-dicarbomethoxy-3-[(trimethylsilyl)methyl]-4-methylcyclopentane (23) with DeltaG‡232K = 16.9 +/- 0.1 kcal mol-1. Prior studies revealed a mechanism initiated by formation of a palladium silyl species, followed by silylpalladation of the first olefin, turnover-limiting intramolecular carbometallation, and subsequent silylative cleavage to regenerate the catalytic palladium silyl species and release the carbocycle. Substitution on the allylic and homoallylic position of the dienes had little to no effect on the rate of cyclization/hydrosilylation. Substitution on the terminal olefinic carbon with cis-alkyl groups led to a decrease in the rate of cyclization hydrosilylation. Substitution on the terminal olefinic carbon with trans substituents led to an increase in the rate of cyclization/hydrosilylation accompanied by a change in the mechanism. Intramolecular carbometallation is no longer turnover-limiting, rather reversible silylpalladation or silylative cleavage of the carbocycle. |
