Silicon complexes with palladium

Chen et al.30 synthesized two derivatives of an unprecedented trinuclear complex of palladium with ligands coordinated through silicon atoms. The qualitative discussion of bonding was based mostly on the crystallographic evidence, given below. 

Figure 1 Trinuclear complex of palladium with ligands coordinated through silicon atoms.

In 1998, a new type of Pd(II) binuclear complex was reported which was effective for Mannich reactions of an imine derived from glyoxylate and anisidine with silicon enolates [38,39]. In these reactions, use of solvents including a small amount of water was essential. It was shown that water played an important role in this system water not only activated the Pd(II) complex to generate a cation complex, but also cleaved the N-Pd bond of the intermediate to regenerate the chiral catalyst. This reaction reportedly proceeded via an optically active palladium enolate on the basis of NMR and ESIMS analyses. A unique binuclear palladium-sandwiched enolate was obtained in the reaction of the p-hydroxo palladium complex with the silyl enol ether [(Eq. (9)]. 

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. 

Combination of silicon hydrides with catalytic amounts of a ruthenium(II) complex in tetrahydrofuran, chloroform or benzene has afforded a new reducing system capable of efficient reduction of a,p-unsatu-rated carboxylic acids, esters, amides, etc. Addition of a weak proton source, such as a sterically hindered phenol significantly increases reaction rates. The ruthenium mixture was found to exhibit the same regioselectivity observed with the above-described palladium systems. 

Miedaner et al. have made molecules with a palladium core and four palladium phosphine groups within a dendritic structure of organophosphine moieties, as well as dendrimers with a silicon core and four palladium phosphine branches. The utility of these compounds, when complexed with acetonitrile, as catalysts for GO2 reduction, has been studied, and significant current enhancements were found when the solvent contained CO2. Up to 40% of the charge passed was utilized to reduce CO2 to GO [96]. In a very different... 

In contrast, it is difficult to activate silicon-silicon bonds by palladium complexes because Si—Si bonds are nonpolarized, thermally stable (r-bonds with a dissociation energy of ca. 300 kJ/mol. However, very recently, the addition of Si—Si compounds to C—C unsaturated bonds have been extensively studied. The results are described in several reviews. In this section, bis-silylation, silylboration, and borylstannation of alkynes and alkenes will be reviewed. 

PaUadium(n) enolate complex intermediates are also generated by the silicon-palladium exchange reaction of silyl enol ethers with 7r-allylpalladium(ll) alkoxide intermediate, which is formed in situ from aUyl carbonate with palladium-phosphine complex. With a catalytic amount of Pd(OAc)2 and dppe (diphenylphosphinoethane) in acetonitrile, silyl... 

Isopropylcyclopentadienyl complexes of lanthanum, neodimium and holmium are used for the synthesis of heterogenic catalysts of the Fisher-Tropsch reaction [118]. For this purpose the silicon oxide containing OH groups on the surface is treated with a THF solution of (i-C3H7C5H4)3Ln. This leads to the fixation of REM ions on the surface of the carrier. Organic groups are removed by heating in vacuum and then the catalyst is doped with palladium. 

In the presence of catalytic amounts of Pd(0), silicon-substituted vinyloxiranes can rearrange into the corresponding ot-silyl- 3,y-unsaturated aldehydes (Scheme 9.34) [151]. Treatment of 80 with Pd(OAc)2 and P(OPh)3 results in the formation of 7t-allylpalladium complex 81. Bond rotation to give 82, followed by migration of the silyl moiety, affords aldehyde 83, which is trapped in situ to provide the Felkin-Anh product 84. The reaction proceeds with retention of configuration and the ee of the starting material is retained in the product. The size of the silicon substituents is critical for the outcome of the reaction, as is the choice of ligands on palladium. 

A thermodynamically stable (silyl)(stannyl)palladium(n) complex is synthesized by an oxidative addition of the Si-Sn linkage to palladium(O) (Scheme 63).267 The complex has the square-planar geometry with a m-arrangement of the silicon and tin atoms. An alkyne reacts with the complex to afford a silastannated product as a mixture of cisjtrans stereoisomers (10 1). 

Siloxane compounds, in vitreous silica manufacture, 22 414 Siloxane materials, 20 240 Siloxane oligomers, in silicone polymerization, 22 555-556 Siloxanols, silylation and, 22 703 Silsesquioxane hybrids, 13 549 Silsesquioxanes, 15 188, 22 589-590 SilvaGas process, 3 696, 697 Silver (Ag), 22 636-667. See also Silver compounds. See Ag entries Argentothiosulfate complexes Batch desilverizing Lead-silver alloys Palladium-silver alloy membranes analytical methods for, 22 650-651 applications of, 22 636-637, 657-662 as bactericide, 22 656, 657, 660 barium alloys with, 3 344 in bimetallic monetary system, 22 647-648 in cast dental gold alloys, 8 307t coke formation on, 5 266 colloidal precipitation color, 7 343t colloidal suspensions, 7 275 color, 7 334, 335... 

In an interesting contrast to the analogous silicon and germanium systems, catalytic addition of the unactivated hexamethyldistannane to unsaturated substrate readily occurs with the common palladium catalysts Pd(PPh3)4 or Pd(dba)2. In fact, prior to 1991 this was the only distannane investigated for addition reactions. The first example of transition metal-catalyzed double stannylation of unsaturated molecules, palladium-catalyzed cis addition of hexamethyldistannane to 1-alkynes, was reported in 1983 by Mitchell and co-workers.66 More recently, the system has been extended to include other substrates and both the hexaethyl- and the hexabutyldistannanes. All examples reported involve the use of a Pd(0) complex as the catalytic species. 

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