On the Effect of the Nature of Ion Pairs as Nucleophiles in a Metal-Catalyzed Substitution Reaction

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• 作者：Barry M. Trost and and Richard C. Bunt
• 刊名：Journal of the American Chemical Society
• 出版年：1998
• 出版时间：January 14, 1998
• 年：1998
• 卷：120
• 期：1
• 页码：70 - 79
• 全文大小：214K
• 年卷期：v.120,no.1(January 14, 1998)
• ISSN：1520-5126

文摘

Two mechanistic features of the palladium-catalyzed allylicalkylation reaction of dienylacetates-vinyl (-allyl)palladium intermediate equilibrationand palladium-catalyzed ionization of the bisallylicmalonate products-account for the observed product ratios andnucleophile counterion effects. Three hexadienylacetate substrates, isomeric with respect to the location of theleaving group to the dienyl system, were studied.The sodium, cesium, tetramethylammonium, and tetrahexylammoniumsalts of dimethyl malonate and dimethylbenzylmalonate were employed as nucleophiles, and bothtriphenylphosphine and tributylphosphine ligandswere tested. The observed alkylation product ratios varied in aconsistent way with the identity of the nucleophilecounterion. The palladium-catalyzed ionization of bisallylmalonate products (i.e., the nucleophile serves asa leaving group for palladium ionization) was found to alter theobserved product distribution only undercertain circumstances such as extended reaction times or temperatures.Nucleophile crossover experimentsdemonstrated that the monoallylic products did not reionize orisomerize. These results are most consistentwith a reaction mechanism where the vinyl(-allyl)palladium(II) intermediate proximal to the leavinggroupis formed initially and nucleophilic addition occurs either competitivewith or after complete thermodynamicequilibration of the two isomeric vinyl(-allyl)palladium(II) intermediates. The observedproduct distributionprimarily reflects the kinetic balance of these twoprocesses-nucleophilic addition and intermediateequilibration.Thus, the cesium and tetrahexylammonium counterions slow down therate of nucleophilic addition relative tointermediate equilibration and allow the vinyl(-allyl)palladium isomerization process to completelyequilibratethe intermediates prior to nucleophilic addition. This effect mayrelate to the ability of these counterions toaffect higher levels of asymmetric induction than other counterions inenantioselective alkylation reactions.