Silver, alkene complexes

Many alkenes and arenes react directly with dissolved silver salts to afford crystals of the silver complex. Examples studied by X-ray diffraction [151] include (C6H6)AgX (X = C104, A1C14) and (C8H8)AgN03. 

The silacyclopropanation of acyclic and cyclic alkenes with 169, catalyzed by AgOTf, occur at room temperature or even below to yield new cyclosilapropanes 173-177. In the case of chiral /3-pinene, the silacyclopropanation occurs enantioselectively (dr > 95 5) (Scheme 26).312 Mechanistic studies have been undertaken, which suggest that silyl silver complexes play an important role in the catalytic cycle of the silylene transfer.310... 

Another approach employing chiral acyclic azomethine ylides was published in two recent papers by Alcaide et al. (85,86). The azomethine ylide-silver complex (51) was formed in situ by reaction of the formyl-substituted chiral azetidinone (50) with glycine (or alanine) in the presence of AgOTf and a base (Scheme 12.18). Azomethine ylides formed in this manner were subjected to reaction with various electron-deficient alkenes. One example of this is the reaction with nitrostyrene, as illustrated in Scheme 12.18 (86). The reaction is proposed to proceed via a two step tandem Michael-Henry process in which the products 52a and 52b are isolated in a... 

SUMMARY - Different forms of adsorbed molecular and atomic oxygen on an Ag(UO) surface are analyzed from a theoretical point of view. It is proposed that the active intermediate for the epoxidation of ethylene is an Ag-O surface species. A parallel is made to discrete silver complexes as catalysts for alkene epoxidation as it is experimentally shown that these have similar properties at the silver surface. 180 Labelling studies indicate an Ag-O complex as the reactive catalyst. 

Epoxidation of Alkenes Catalyzed by Discrete Silver Complexes. 

By-products in the reactions in Table 3 are mainly cleavage products (carbonyls) An interesting by-product is found in the oxidation of styrene, as phenyl acetate is produced. It has been found that phenyl acetate is formed via oxidation of either styrene oxide or acetophenone. Nearly the same yield and stereochemical mixture of the epoxides, as in Table 3, can be obtained by oxidation with Ag202. It appears from Table 3 that the epoxidation of alkenes catalyzed by discrete silver complexes also takes place without maintaining the stereochemistry of the alkene, as in the silver-surface catalyzed reaction. 

Chiral silver complexes bearing bidentate NHC ligands (24) have been synthesized. They are used in alkene metathesis and allylic alkylation reactions high diastereos- (g) electivity is observed induced by the chiral backbone on the prochiral biphenyl.27 Ruthenium-based complexes obtained from transmetalation with a Grubbs-Hoveyda complex exhibited high activities and enantioselectivities in ring-opening metathesis/ ... 

With this type of shift reagent, the silver ion binds to soft Lewis base donors, such as olefins, aromatics, phosphenes, and halogenated compounds. At the same time, silver remains as an ion pair with Yb(fod)4 in a specific orientation such that a paramagnetic shift is induced in the resonances of the organic soft base. This method has proven to be useful for the separation of NMR resonance peaks of cis and trans geometric isomer mixtures of long-chain alkenes and complex mixtures of aromatic molecules [5]. 

The K value for the silver complex of an acetylene, hex-3-yne, as determined by the distribution method 14>, was found to be 19.1, i.e. smaller than those of alkenes such as the pentenes and cyclohexene, but greater than those of aromatic hydrocarbons 1S). A later study of silver-acetylene complexes 16> using the more rapid solubility technique of Andrews and Keefer 1S> gave rise to quasi thermodynamic equilibrium constants , Ka (as opposed to K) for various methyl substituted hex-3-ynes and hept-2-yne. There was good agreement for the K values for hex-3-yne for the two different methods in each case, replacement of an a-hydrogen atom by a methyl group caused a decrease in the value of Ka, similar to that observed in alkenes. Values of AH approximated to 19-21 kJ mole-1. 

To introduce an epoxy group on the terminal double bond, the other alkene function was protected by bromoetherification to produce the bromo ether, which was desilylated and compound 452 provided. In the final two steps, olefin 453 was epoxidized with m-CPBA and deprotected with a zinc-silver complex to obtain ( )-verrucarol (454) (Scheme 8.13). 

The first report of a NHC Ag-catalyzed reaction appeared only in 2005 when Peris and Fernandez disclosed the diboration of alkenes in the presence of a bis-NHC silver complex (Scheme 11.3). The menthol-derived NHC ligand 7 showed only limited scope in this reaction but provided a proof-of-concept for NHC-Ag-based catalysis. Subsequent studies by the same groups examined chiral NHCs 8-10 (Figure 11.3) in this reaction, but enantiomeric excess remained very low (9% ee at best). ... 

Replacement of silver nitrite by inexpensive sodiiunor potassium nitrite enhances the imlity of this process Treatment of alkenes v/ith sodiiun nitrite and iodine in ethyl acetate and water in the presence of ethylene glycol gives conjngatednitroalkenesin49-82% yield The method for generation of nitryl iodide is improved by the treatment of iodme v/ith potassium nitrite complexed v/ith 18-crovm-6 in THF under sonicadon, as shovmin Eq 2 32 ... 

---