Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck arylation of olefins Gabriela A. Grasa, Rohit Singh, Edwin D. Stevens, Steven P. Nolan * Department of Chemistry, University of New Orleans, New Orleans, LA 70148-2820, USA Received 26 February 2003; received in revised form 1 April 2003; accepted 9 April 2003 Abstract Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been investigated. The scope of a coupling process using Pd(II) sources and an a-diimine as ligand in the presence of Cs 2 CO 3 as base was tested using various substrates. The Pd(OAc) 2 /DAB-Cy (1, DAB-Cy /1,4-dicyclohexyl-diazabutadiene) system presents the highest activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another Pd(II) source, Pd(acac) 2 (acac /acetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when the appropriate concentration of n Bu 4 NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst loadings and high temperatures. Pd(acac) 2 and Pd(acac) 2 /DAB-Cy precatalysts were very effective for the arylation of various olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Heck coupling; Diazabutadienes; Palladium 1. Introduction The Heck cross-coupling reaction of organohalides and pseudohalides with olefins has proven to be a powerful synthetic method for C Ã /C bond formation [1], with wide applications from natural product [2] to fine chemicals synthesis [3]. In addition to its versatility, and unlike Kumada, Hiyama, Stille and other Pd-catalyzed cross-coupling reactions, which often employ expensive reagents, the Heck reaction presents the advantage of using cheap and easy available olefins as coupling partners. Although Heck arylation and alkylation of olefins have proven to be one of the most versatile methods for C Ã /C bond formation, various catalytic systems are used with a myriad of procedures involving different support- ing ligands, metal source, solvent, additives or substrates [1]. Palladium  /phosphine complexes have been em- ployed as ancillary ligands [4] in controlling the reactiv- ity and selectivity in the Heck cross-coupling reaction [1]. The main role of tertiary monophosphine ligands is to stabilize the zerovalent palladium as PdL 3 or PdL 4 species, which can enter the catalytic cycle and conse- quently prevent the formation of inactive palladium black. Important examples of the use of phosphines are found in the work of Fu [5], Hartwig [6] and Beller [7] who have made use of sterically demanding, electron rich tertiary phosphines as catalyst modifiers in the Heck aryl olefination reactions. The ligand properties in these cases make possible the activation of the less reactive aryl bromides and aryl chlorides as coupling partners in the Heck reaction. However, despite their effectiveness in controlling reactivity and selectivity transition-metal catalyzed transformations [4], tertiary monophosphines and their palladium complexes are often air-sensitive and are subject to P Ã /C bond degra- dation at elevated temperatures [8], commonly em- ployed under Heck conditions. Hence, higher phosphine concentrations are often needed with direct consequences on large-scale applications since tertiary electron-rich phosphines are unrecoverable from reac- * Corresponding author. Tel.:  /1-504-286-6311; fax:  /1-504-280- 6860. E-mail address: snolan@uno.edu (S.P. Nolan). Journal of Organometallic Chemistry 687 (2003) 269  /279 www.elsevier.com/locate/jorganchem 0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-328X(03)00375-9