Mechanistic Studies of SCS-Pd Complexes Used in Heck Catalysis

Abstract

Air-stable SCS palladacycles that can be used to promote C À C coupling chemistry were studied mechanistically. Using a small library of electronically varied SCS ligands, a collection of palladacycles was synthesized. Kinetic studies showed that these complexes all had induction periods, induction periods that were effected by concentration of substrates, products and trace impurities. Hammet correlations showed that electronically diverse palladacycles had identical 1 values, values that suggested that aryl halide electrophilic addition to a Pd species was not the rate-determining step. Phosphine addition experiments led to increased reactivity of the starting palladacycles, possibly by trapping an in situ generated Pd(0) species. Studies that examined reactivity in biphasic thermomorphic reactions showed residual activity in phases that do not contain polymer-bound palladacycle and provided convincing evidence that palladacycles are not the actual catalyst. Poisoning experiments using mercury metal to test for the presence of a Pd colloid were very effective with low molecular weight palladacycles, completely suppressing Heck chemistry. Similar studies with polymer-bound palladacycles showed mercury poisoning too. However, since so little decomposition of the palladacycle occurred, the polymer-bound palladacycle could still be recycled multiple times. However, mercury poisoned subsequent cycles of the experiment too. The conclusion is that SCS palladacycles are actually reservoirs of a catalytically active but ill-defined form of palladium(0).

Facile synthesis of achiral and chiral PCN pincer palladium(II) complexes and their application in the Suzuki and copper-free Sonogashira cross-coupling reactions

“…Further reduction in the amount of the catalyst led to a gradual decrease in the yields under the same reaction conditions (entries [13][14]. By using 0.1-0.3 mol% of 2a, coupling of several aryl bromides with PhB(OH) 2 could also afford the coupled products in good to excellent isolated yields (entries [15][16][17][18][19][20][21][22]. Finally, the relative catalytic activities of PCN pincer Pd complexes 2a-e as well as the corresponding symmetrical PCP complex [PdCl{C 6 H 3 -2,6-(OPPh 2 ) 2 }] (3) in the Suzuki reaction were investigated (Table 5).…”

“…Due to the existence of the Pd-C r bonds supported by two ortho,ortho-chelated heteroatom groups in the molecules, these complexes usually exhibit high stabilities towards heat, air and moisture. Furthermore, the reactivity of the Pd center can be finely tuned by adjusting the steric and electronic properties of heteroatom donors and substituents thereon [10][11][12][13][14][15][16][17][18][19][20]. Consequently, various types of PCP or NCN pincer palladium complexes have been found to be efficient catalysts for a variety of C-C bond forming reactions including the Heck reaction [10,13,14,21], Suzuki coupling [16,22,23], Sonogashira coupling [15] and a-arylation of ketones [24].…”

Simple kinetic method for distinguishing between homogeneous and heterogeneous mechanisms of catalysis, illustrated by the example of “ligand-free” Suzuki and Heck reactions of aryl iodides and aryl bromides

“…The relative reactivity of two substrates is conven tionally estimated using the simplest integral equation for competing reactions that are first order with respect to the substrates [36,37]. This equation does not take into account the conversions of the catalyst outside the catalytic cycle.…”

Poly(norbornene)‐Supported N‐Heterocyclic Carbenes as Ligands in Catalysis

“…[48,[61][62][63]67,68] We and others have shown for several of these cases that the supported palladium species do not catalyze any carbon-carbon bond formations but that the leached palladium species are the sole catalytically active species. [61][62][63]67] To identify if the same restrictions are true for our poly-(norbornene)-supported NHC palladium complexes, we investigated whether or not palladium leaches during the reaction and if the polymer-supported species are active during the catalysis. To identify the nature of the catalytic species, we employed three catalyst poisons: a) highly cross-linked insoluble poly(vinylpyridine) (PVPy), b) Quadra-Pure , a microporous resin metal scavenger that is especially sensitive to palladium, and c) mercury(0).…”