Palladium mechanistic insights

Mechanistic Insight from Kinetic Studies on the Interaction of Model Palladium(II) Complexes with Nucleic Acid Components Tobias Rau and Rudi van Eldik... 

In 2008, this group reported the synthesis of quinoxalines [41], The reactions were performed using bis(dibenzylideneacetone)palladium(0), l,3-bis(diphenyl-phosphino)propane, and 1,10-phenanthroline in DMF under 6 bar of carbon monoxide at 70 °C. A -Heteroannulation of enamines derived from 2-nitroben-zenamines forming mixtures of 1,2-dihydroquinoxalines and 3,4-dihydroquinox-alin-2-ones in moderate to good yields (Table 9.4). Some mechanistic insights lent support for nitrosoarene/nitrene intermediates. 

A mechanistic insight into the palladium(II)-catalysed cyclization of o-alkynyl-phenylphosphonamide and A-(o-alkynylphenyl)acetamide has been attained by using electrospray ionization mass spectrometry (ESI-MS). °... 

Eberlin, M.N., Neto, B.A.D. (2014) Phosphine-free Heck Reaction Mechanistic Insights and Catalysis On Water Using a Charge-tagged Palladium Complex. New J. Chem. 38 2958-2963. 

Cornell, C. and Sigman M. (2005). Discovery of and Mechanistic Insight into a Ligand-Modulated Palladium-Catalyzed Wacker Oxidation of Styrenes Using TBHP, J. Am. Chem. Soc., 127, pp. 2796-2797. 

In early 2015, Xiao and co-workers developed a redox-neutral radical-radical cross-coupling protocol, allowing for direct a-allylation of amines by dual visible light and palladium catalysis (Scheme 3.14) [60]. Both tertiary amines and secondary amines could undergo the aUylation reaction smoothly. It renders a rapid and elegant access to 8-oxoprotoberberine derivatives. To get mechanistic insights, EPR spin trapping experiment with DMPO was carried out and the proposed a-aminoalkyl radical was determined. 

LiUo V, Mas-Marza E, Segarra AM, et al. Palladium—NHC complexes do catalyse the dihoration ofalkenes mechanistic insights. Chem Commun. 2007 3380—3382. 

Heckenroth M, Khlebnikov V, Neels A, Schurtenberger P, Albrecht M. Catalytic hydrogenation using abnormal N-heterocyclic carbene palladium complexes catalytic scope and mechanistic insights. ChemCatChem. 2011 3 167-173. 

Detalle JF, Riahi A, Steinmetz V, Henin F, Muzart J. Mechanistic insights into the palladium-induced domino reaction leading to ketones from benzyl p-ketoesters first characterization of the enol as an intermediate. J. Org. Chem. 2004 69 6528-6532. 

The key drawback with this sequence is that the overall process is stoichiometric in palladium. Ritter and co-workers reported the first metal catalysed approach in 2013 (Scheme 15.67). Notably, the proposed mechanism (based on a significant body of mechanistic experimentation) proceeds via a single electron transfer process involving a Pd(m) intermediate. Access to this new mechanistic insight is highly beneficial as now the competing pathway that leads to protodemetallation is circumvented. 

This paper describes the use of polydentate ligands to optimize the performance of palladium catalysts for CO2 reduction and to probe mechanistic aspects of catalytic reactions. Polydentate ligands can be used to precisely control coordination environments, electronic properties, and specific steric interactions that can lead to new insights into the relationship between catalyst structure and activity. 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

An alternative mechanism applies to the chemistry surrounding the pyranose analogs utilized in Schemes 4.3.2, 4.3.4, and 4.3.5. Addition of palladium to these species forms a ji-allylpalladium complex. As shown in Scheme 4.4.2, approach of the palladium is from the side of the sugar opposite that of the glycosidic substituent. This allows the nucleophilic species to approach from the opposite side of the palladium giving the product with a net retention of stereochemistry. Further insight into the mechanistic aspects of this chemistry is available from any of the references cited within this chapter. 

Insights into our palladium plans are outlined in Scheme 24. In the first scenario, we envisioned again an initial 6-exo cyclization from front or back followed by trapping of the resulting palladium intermediate with either vinyl stannanes or boranes. The strength of this mechanistically controlled approach over the radical strategy was that it ensured the desired c/r-stereochemistry... 

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