Carbon-palladium distance

At this stage, sulfoximines had been shown to function as chiral ligands for various palladium and copper catalysts which led to enantioselectivities of >95% ee in various reactions. Furthermore, most of those catalyzed reactions were C-C bond formations. Obvious questions were, therefore, whether sulfoximines could also be applied in combination with other metals and whether reductions and oxidations could be catalyzed as well. A structural comparison of the sulfoximines leading to high ee values such as 55, 60, 81, and 85 revealed that all of them had a two-carbon distance between the two coordinating atoms (which were all nitrogen in these cases). 

Compensation effects have been reported for the oxidation of ethylene on Pd-Ru and on Pd-Ag alloys (207, 254, 255) discussion of the activity patterns for these catalysts includes consideration of the influence of hydrogen dissolved in the metal on the occupancy of energy bands. Arrhenius parameters reported (208) for ethylene oxidation on Pd-Au alloys were an appreciable distance from the line calculated for oxidation reactions on palladium and platinum metals (Table III, H). Oxidation of carbon monoxide on Pd-Au alloys also exhibits a compensation effect (256). 

Activation of two Si—Si bonds in bis(disilanyl)alkanes with palladium(O) bis(tert-alkyl isocyanide) induced the formation of the cyclic bis(silyl)palladium(II) bis(terf-alkyl isocyanide) complexes (100) and disilanes described schematically in Scheme 42. These complexes were found to react with phenylacetylene, affording different amounts of five-membered cyclic products and acyclic products which are derived from the insertion of the alkyne into the general intermediate complex 101 (Scheme 42, equation 54). The bis(silanyl)dithiane palladium complex (102) was isolated and characterized in the solid state the two silicon atoms, the two isocyano carbons and the palladium atom are nearly in a plane with a short cross-ring Si—Si distance of 2.613(2) A, suggesting the possibility of covalently bonded two Si—Si atoms in the four-membered ring. Similar reaction with cyclic disilanes afforded oligomers, and cyclic 20-membered compounds have been prepared in the presence of nitriles248,249. 

As an example, Schiff base 1, with its bidentate chelation separated by a three-carbon fragment (the correct distance for formation of a six-membered metalla-cycle) from the C-H bond, can undergo palladium-catalyzed C-H bond arylation... 

Figure 4-26. Unconstrained MD simulations for methyl acrylate bound to the palladium/nickel diimine complex through the oxygen (top-right/bottom-right) and the C=C (top-left/bottom-left) functionalities. The three panels in each of the four graphs represent variations in the metal-carbon (top two panels) and metal-oxygen (bottom panel) distances. The simulations were carried out at 300 K for all the systems, and 700 K for the local minima, as indicated...

The non-metallic atoms (C and H) are described by the 6-31G basis set of double zeta quality with p polarization functions in hydrogen atoms and d polarization functions in carbon atoms. In aU the clusters, the nearest-neighbour distances were taken from the bulk and are 2.77483 A for platinum, 2.75114 A for palladium and 2.49184 A for nickel. These clusters form compact sections of the corresponding ideal surfaces. 

An X-ray analysis (26,32) of the yellow, monomeric (norbornadiene)-dichloropalladium(II) (1, 6) has shown the diene molecule chelated to palladium with both double bonds perpendicular to the coordination plane. The distance from the Pd atom to the trigonal carbons is about 2.16 A and the angle subtended at Pd(II) by the coordinate bonds is 71.8°, The bond lengths within the norbornadiene molecule appear to be changed very little by coordination. 

Palladium(II) chloride-copper(Il) chloride-carbon monoxide. 13, 235-236 Oxidative carbonylation. This combination of reagents is commonly used for homologation of alkynes. The products are usually obtained as the methyl esters. Heterocyclization attends C-C bond formation when a proper functional group is present at a short distance. 

Reference to the bond angles and interatomic distances for these molecules shows that as far as the carbon atoms are concerned their values are little different from those of the olefin molecule. It would be expected, therefore, that catalysts which were effective for hydrogenation of the latter would also function with the heterocyclic molecules. This is found to be so, in that nickel catalysts are known to give tetrahydrofuran and pyrrolidine by the hydrogenation of furan and pyrrole at 180° (Padoa, 25). Tetrahydrofuran is also formed by the use of platinum (Starr and Hixon, 26), osmium, or palladium (Shuikin, Nikiforov, and Stolyarova, 27) as the catalyst, and pyrrolidine is similarly produced by palladium or rhodium catalysts (Zelinskii and Yurev, 28). In all these metals there are spacings of the atoms very similar to those in metallic nickel, the hexagonal osmium lattice having a equal to 2.71 A. 

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