www.elsevier.nl/locate/jorganchem Journal of Organometallic Chemistry 586 (1999) 85 – 93 Comparative investigation of the regioselectivity in styrene and -methylstyrene hydroalkoxycarbonylation as a function of palladium catalyst structure Csilla Benedek a , Szila ´rd To  ro ¨s b , Ba ´lint Heil b, * a Research Group for Petrochemistry of the Hungarian Academy of Sciences, PO Box 158, H-8201 Veszpre ´m, Hungary b Department of Organic Chemistry, Uniersity of Veszpre ´m, PO Box 158, H-8201 Veszpre ´m, Hungary Received 10 November 1998; received in revised form 22 March 1999 Dedicated to: Professor La ´szlo ´ Marko ´ on the occasion of his 70th birthday, in recognition of his outstanding service to organometallic chemistry. Abstract Catalytic pathways of the styrene and -methyl-styrene hydroalkoxycarbonylation in the presence of Pd(PPh 3 ) 2 Cl 2 and Pd(PPh 3 ) 2 Cl 2 /SnCl 2 catalyst precursors have been suggested. As a method, deuterium labelling with EtOD has been applied and it resulted in mixtures of mono- and polydeuterated reaction products, detected and determined by NMR methods. Comparative elucidation of the mechanisms governing these systems does support the assumption that the hydrido route is operative. The different behaviour of the metal – alkyl intermediates accounts for the observed strong influence of catalyst and substrate structure on regioselectivity. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbonylation; Palladium; Mechanism; Catalysis 1. Introduction The palladium – monophosphine-catalysed hy- droalkoxycarbonylation of vinyl aromatic olefins has attracted great interest due to its importance in obtain- ing non-steroidal anti-inflammatory drugs [1]. The reac- tion is known to be characterised by high -selectivities [2], i.e. the formation of the branched ester. Adopting Knifton’s [3] catalytic system, addition of SnCl 2 leads to a prevalence of the linear product. At the same time, vinylidene olefins such as -methylstyrene, give the linear derivative [4] selectively under both reaction con- ditions. A satisfactory explanation of the regioselectivi- ties is still to be given in the framework of the two accepted mechanisms [5] for hydroalkoxycarbonylation of olefins. The first one, the ‘hydrido’ route [3,6,7], involves olefin insertion into the Pd–H bond followed by CO insertion, yielding the final product upon nucleophilic attack of the alkanol. According to the ‘carboalkoxy’ mechanism [8,9], a Pd–acyl species is formed by the attack of the alcohol on Pd–CO. Insertion of the olefin into the Pd–COOR bond is followed by protonolysis of the metal – alkyl -bond to give the corresponding ester and regeneration of the active form. These two mecha- nisms are claimed to be operative depending on the reaction conditions and substrates [10]. We have shown previously [11] that in the Pd(PPh 3 ) 2 Cl 2 /SnCl 2 catalysed styrene hydroalkoxycarbonylation the first one is acting. In order to find out the pathways governing the systems studied and to determine the influence of sub- strate and catalyst structure on regioselectivities, deu- terioalkoxycarbonylation of styrene and -methyl- styrene has been carried out. In the present paper we report a detailed investigation of the reaction per- formed at two temperatures and partial substrate con- versions. The labelled species determined by NMR ( 1 H-, 2 H-, 13 C-) and MS techniques throw light on the catalytic routes of the reaction. * Corresponding author. Tel.: +36-88-422022; fax: +36-88- 427492. E-mail address: heil@almos.vein.hu (B. Heil) 0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0022-328X(99)00225-9