Mechanism of Acyl Group Isomerization in Palladium(II) Complexes. Development of a Catalytic Process for the Isomerization of Carboxylic Acid Chlorides

Jeffrey S. Brumbaugh, Ayusman Sen

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Abstract

[Pd(PPh3)2(MeCN)(COiPr)](BF4) (2c), when dissolved in a variety of weakly coordinating polar solvents, spontaneously isomerized to an equilibrium mixture of 2c and [Pd(PPh3)2(MeCN)(COnPr)](BF4) (2d). Several other complexes having the general formula [Pd(PPh3)2(MeCN)(COR)](BF4) also underwent spontaneous isomerization, including those involved in the following isomer systems: R = nBu/sBu, iBu/tBu, and methylcyclohexyl. In each system the more stable isomer was that having the least branching in the alkyl group. When R was a vinyl group, the complex decomposed to form the respective vinyltriphenylphosphonium salt. The reactions were First order in metal complex and inverse first order in MeCN. PPh3 inhibited the reactions by promoting metal complex decomposition. When the isomerizations were carried out in the presence of an excess of ethylene or cyclohexene, [Pd(PPh3)2(MeCN)(COEt)](BF4) (2b) or [Pd(PPh3)2(MeCN)(COC6H11)](BF4) (2n) respectively, was produced in high yields. Concomitant with the formation of 2b in the reaction of [Pd(PPh3)2(MeCN)-(COC7H13)](BF4) with ethylene was the formation of methylenecyclohexane, 1-methylcyclohexene, 3-methylcyclohexene, and 4-methylcyclohexene. When the reaction was carried out in CDCl3, a small quantity of CHDCl2 was generated as a byproduct, arising from the reaction of an intermediate metal hydride with the solvent. A key intermediate in the reaction mechanism was postulated to be [Pd(PPh3)2(H)(CO)(olefin)](BF4), which was formed from the starting material via the following sequence of events: (a) MeCN dissociation, (b) CO deinsertion, and (c) β-hydrogen abstraction. The reversal of steps a-c generated the isomeric acyl compound. The corresponding neutral palladium-acyl complexes, Pd(PPh3)2(Cl)(COiPr) and Pd(PPh3)2-(Cl)(COnPr), underwent isomerization to an equilibrium mixture of these isomers when a Lewis acid or [Pd(PPh3)2-(MeCN)(COR)] (BF4) was employed as a catalyst. Finally, the catalytic isomerization of isobutyryl chloride or n-butyryl chloride to an equilibrium mixture of these organic acids was promoted by either [Pd(PPh3)2(MeCN)(COR)](BF4) or a combination of Pd(PPh3)2(Cl)(COR) and AlCl3.

Original languageEnglish (US)
Pages (from-to)803-816
Number of pages14
JournalJournal of the American Chemical Society
Volume110
Issue number3
DOIs
StatePublished - Feb 1988

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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