No abstract

The steric bulk of the phosphane ligand determines the mechanism of the ArX addition to zero‐valent [PdL2] complexes. This effect has been studied by variation of ligands (catalyst: [Pd(PCxntBu3−n)2]; n=0–3, Cx=cyclohexyl) in Pd couplings of unsaturated electrophiles, and different reaction pathways (A=associative, B=dissociative) identified, depending on the size of the ligand.

The mechanism of Pd-catalyzed enantioselective allylic alkylation involves two steps which are of major importance in the catalytic cycle. [1] First, the allylic substrate, usually in the Scheme 2. Preferential formation of the product with S configuration. The methyl groups at C11 and C32 are represented by black circles, and the Pd atom by a gray circle. nated between C1 and C3, which otherwise differ little in their steric and electronic properties.

The orthometalated complex [{Pd(μ-Cl){κ2-P,C-P(OC6H2-2,4-tBu2)(OC6H3-2,4-tBu2)2}}2] reacts with phenylboronic acid hydrate and K2CO3 in dimethylacetamide to give [Pd{κ2-P,C-μ2-O-P(O)(C6H2-2,4-tBu2)(C6H3-2,4-tBu2)(DMAc)}]. When the reaction is repeated in dimethylformamide 3,3‘,5,5‘-tetra-tert-butyl-2,2‘-biphenol is isolated. Both compounds have been characterized crystallographically. The reaction of palladium dichloride with PiPr2(OC6H4-4-Et) in 2-methoxyethanol followed by recrystallization in the presence of ethanol leads to the formation of trans-[PdCl2{PiPr2(OEt)}2], which was also characterized by crystallography. To determine whether related solvolytic processes have a bearing on catalytic activity, the performance of a range of catalysts with “hydrolyzed” and “nonhydrolyzed” ligands was assessed in the Suzuki coupling of aryl bromides. In some cases it was evident that hydrolysis plays a significant role on the catalytic activity; however, this depends not only on the ligand, but also on the combination of ligand and palladium precursor.

The potentially tridentate ligands 2,2′:6′,2"‐terpyridine (terpy) and 2,6‐bis(diphenylphosphanylmethyl)pyridine (PNP) have been used to prepare novel palladium−allyl complexes of general formula [(terpy)Pd(CH2CHCRR′)](BF4) [R = R′ = H (1); R = H, R′ = Me (2); R = R′ = Me (3)] and [(PNP)Pd(CH2CHCRR′)](BF4) [R = R′ = H (4); R = H, R′ = Me (5); R = R′ = Me (6)], which were characterized by elemental analysis, IR spectroscopy, and 13C, 31P and variable‐temperature 1H NMR spectroscopy. The low‐temperature 1H NMR spectra show that the configuration of the complexes in solution depends strongly on the nature of the ligand. The terpy complexes 1−3 are η3‐allyl species, where terpy is dihapto and one pyridine ring remains uncoordinated, whereas the PNP complexes 4−6 occur as η1‐allyl compounds with a trihapto PNP ligand. All complexes are fluxional through η3‐η1 exchange processes. Energy barriers of 47.5−48.6 kJ·mol−1 (243−262 K) are reported for the interconversion of terminal allylic protons (complexes 2−4). ΔG‡ is higher (71.1 kJ·mol−1 at 350 K) for the interconversion of the methyl groups in complex 6. A lower‐barrier oscillatory mechanism involving terpy (ΔG‡ = 43.9 kJ·mol−1 at 231 K) is also involved in complex 1. The X‐ray structures of complexes [(η3‐terpy)Pd(η1‐C5H9)](BF4) (3a) and [(η3‐PNP)Pd(η1‐C4H7)](BF4) (5a) are reported; the structure of 3a corresponds to a species that is not observed in solution.