X-ray crystal structures of a series of cationic (P-P)palladium(1,1-(CH(3))(2)C(3)H(3)) complexes (P-P = dppe (1,2-bis(diphenylphosphino)ethane), dppf (1,1'-bis(diphenylphosphino)ferrocene), and DPEphos (2,2'-bis(diphenylphosphino)diphenyl ether)) and the (Xantphos)Pd(C(3)H(5))BF(4) (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) complex have been determined. In the solid state structure, the phenyl rings of the ligand are oriented in the direction of the nonsymmetrically bound [1,1-(CH(3))(2)C(3)H(3)] moiety. An increase of the bite angle of the chelating ligand results in an increase of the cone angle. In complexes containing ligands having a large cone angle, the distances between the phenyl rings and the allyl moiety become small, resulting in a distortion of the symmetry of the palladium-allyl bond. In solution, two types of dynamic exchange have been observed, the pi-sigma rearrangement and the apparent rotation of the allyl moiety. At the same time, the folded structure of the ligand changes from an endo to an exo orientation or vice versa. The regioselectivity in the palladium-catalyzed allylic alkylation of 3-methyl-but-2-enyl acetate is determined by the cone angle of the bidentate phosphine ligand. Nucleophilic attack by a malonate anion takes place preferentially at the allylic carbon atom having the largest distance to palladium. Ligands with a larger cone angle direct the regioselectivity to the formation of the branched product, from 8% for dppe (1) to 61% found for Xantphos (6). The influence of the cone angle on the regioselectivity has been assigned to a sterically induced electronic effect.

Palladium is one of the most widely used metals in transitionmetal-catalyzed organic synthesis, as it is capable of catalyzing a wide variety of commercially important reactions. 1 Ligands are attached to palladium to increase the performance and stability of the catalysts, and they are used to fine-tune the steric and electronic properties of the catalyst and thereby the activity and selectivity obtained with these catalysts. The missing link in explaining structure-activity and -selectivity relationships in homogeneous catalysis is the detailed structural information about the catalysts in their active phase, in solution.We have therefore applied extended X-ray absorption fine structure (EXAFS) spectroscopy to elucidate the origin of the regioselectivity in the Pd-catalyzed allylic substitution reaction. EXAFS spectroscopy provides both structural and electronic information about a specific element in a compound in any state of aggregation. 2 Only a few EXAFS studies on homogeneous organometallic Pd complexes have been reported in the literature so far. 3 This can partly be explained by the complexity of the EXAFS data-analysis, especially as overlapping coordination shells hamper data-analysis severely. Recent developments in EXAFS data-analysis methods, 4 using the so-called difference file technique, 2 allow the reliable separation of the different contributions resulting in a proper analysis.The use of bidentate diphosphine (P-P) ligand Pd catalysts in the allylic substitution reaction of a nonsymmetrically substituted dimethylallyl moiety results in either linear or branched products as shown in Scheme 1. For bidentate ligand complexes, it has been suggested that the bonding of the allyl moiety determines the regioselectivity. 5,6 Moreover, the regioselectivity was found to be influenced by the P-Pd-P angle, that is, bite angle of the bidentate phosphine ligands. 7 Isolated (ligand)Pd(allyl) complexes are studied in detail by molecular modeling, X-ray crystallography, and (solution-) NMR techniques. [6][7][8][9][10] It was proposed that the selectivity in the allylic alkylation reaction is a tradeoff between electronic and steric contributions. A larger bite angle of the ligand enhances the electronic preference for nucleophilic attack at the branched position, but also increases the steric hindrance at this position. 6,7 Because these studies are mainly based on characterization of solid samples, they fail in providing a detailed structural analysis of the catalytic complexes in solution, the actual active phase of homogeneous reactions. This study shows that the structure of homogeneous catalytic intermediates in the active phase differs from that in the solid state. This can lead to a direct explanation of the regioselectivity of the different catalysts in the allylic alkylation reaction.EXAFS Pd K-edge data are collected for two cationic (ligand)-Pd(allyl) complexes: (dppe)Pd(C 5 H 9 ) (dppe 1,2-bis(diphenylphosphino)ethane) and (DPEphos)Pd(C 5 H 9 ) (DPEphos 2,2-bis(diphenylphosphino(diphenyl ether))...

The natural bite angle of bidentate phosphane ligands influences the isomer distribution (syn and anti) in (1‐methylallyl)(bisphosphane)Pd OTf complexes. It was found (31P‐ and 1H‐NMR studies) that the syn/anti ratio changes from 12 (dppp) to 1.3 (sixantphos). Molecular orbital calculations [PM3(tm) level] indicate that for ligands inducing a large bite angle, the phenyl rings of the ligand embrace the allyl moiety, thus influencing the syn/anti ratio. This bite‐angle effect on the syn/anti ratio is transferred to the regioselectivity in stoichiometric allylic alkylation. Ligands inducing large bite angles direct the regioselectivity towards the formation of the branched product 2. Catalytic alkylation of (E)‐2‐butenyl acetate showed that for ligands with a small bite angle the regioselectivity of the catalytic and stoichiometric alkylation are in good agreement. This correspondence is worse for ligands with a larger bite angle, which is rationalised in terms of the relative rates of syn/anti isomerisation and alkylation. The ligand with the largest bite angle (sixantphos) gives the most active catalytic species.