Regioselectivity in aqueous palladium catalysed hydroxycarbonylation of styrene: a catalytic and mechanistic study

Regioselectivity control was studied in palladium catalyzed hydroxycarbonylation of styrene in neat water with water-sol. phosphines, mostly trisulfonated triphenylphosphine, TPPTS, but also N-bis(N',N'-diethyl-2-aminoethyl)-4-aminomethylphenyl-diphenylphosphine, N3P. The factor giving the highest changes in regioselectivity in the TPPTS system, under similar reaction conditions, is the temp. In the N3P case, only a minor variation in the <i>n/i</i> ratio as a function of temp. is obsd. In situ normal- and high-pressure NMR expts. were performed to obtain further information about the catalytic cycle and the reaction intermediates. Two palladium hydride intermediates, a palladium &eta;<sup>3</sup>-benzylic complex and both the branched and linear palladium acyl complexes were identified in the HP NMR expts. The hydroxycarbonylation in water using styrene as a substrate operates using a hydride mechanism for pathways leading to both linear and branched product. Insertion of styrene in the palladium-hydride bond gives an &eta;<sup>3</sup>-benzyl compd. A high CO pressure gives a kinetic preference for the iso-acyl in the next step. In the TPPTS system, at moderate temps., the hydrolysis of the iso-acyl is faster than its conversion to the thermodynamically more stable n-acyl. A low <i>n/i</i> therefore requires high pressures and reasonably low temps. The N3P ligand always favors the linear product since isomerisation of the iso-acyl to the n-acyl in this system is fast under all conditions investigated.

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Dalton Transactions, 32, 3934-3940
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 Record created 2006-09-13, last modified 2020-07-30

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