Silver complexes with alkynes

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

Fig. 2.12 Silver, gold and platinum complexes with monodentate NHC ligands as catalysts for the diboration of alkenes and alkynes...

Complexes in which the acceptor is a metal ion and the donor an olefin or an aromatic ring (n donors do not give EDA complexes with metal ions but form covalent bonds instead).41 Many metal ions form complexes, which are often stable solids, with olefins, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture of the bonding in these complexes,44 first proposed by Dewar,45 can be illustrated for the complex in which silver ion is bonded to an olefin. There are two bonds between the metal ion and the olefin. One is a a bond formed by overlap of the filled -it orbital of the olefin with the empty 5s orbital of the silver ion, and the other a -it bond... 

Cyclohexyldienyl complexes, with Ti(IV), 4, 327 Cyclohexylisocyanides, with gold(I) halides, 2, 281 Cyclohexylphosphine, for semiconductor growth, 12, 9 Cyclohexyl selenides, preparation, 9, 480 Cyclohydrocarbonylation alkenes, 11, 515 alkynes, 11, 522 dienes, 11, 522 overview, 11, 511-555 for ring expansion, 11, 527 Cycloisomerizations, via silver catalysts, 9, 558 Cyclomanganation, product types, 5, 777-778 Cyclometallated azobenzenes, liquid crystals, 12, 251 Cyclometallated complexes for OLEDs... 

Pale and coworkers have demonstrated that silver acetylides may be synthesized with a variety of silver salts in the presence of base in a number of solvents. By treating 1 -hexyne in either deuterated benzene or DMF with silver triflate, the group was able to observe the formation of the Jt-alkyne-silver complex and subsequently the silver acetylide through the use of H, C, and Ag NMR. The incipient Jt-alkyne-silver complex is rapidly deprotonated on addition of diisopropylethylamine, to give the silver acetylide as a white precipitate (Scheme 1.33).89... 

Vinyliodonium ions, 35 and 36, are hypervalent iodine species in which one or two alkenyl ligands are bound to a positively charged iodine(III) atom. Although they are reactive with nucleophilic reagents, they are less labile than alkynyliodonium ions, and stable halide salts of vinyliodonium ions can be prepared. The first vinyliodonium compounds [i.e. (a, / -dichlorovinyl)iodonium salts] were synthesized by the treatment of silver acetylide-silver chloride complexes with (dichloroiodo)arenes or l-(dichloroiodo)-2-chloroethene in the presence of water (equation 152). The early work was summarized by Willgerodt in 1914115. This is, of course, a limited and rather impractical synthetic method, and some time elapsed before the chemistry of vinyliodonium salts was developed. Contemporary synthetic approaches to vinyliodonium compounds include the treatment of (1) vinylsilanes and vinylstannanes with 23-iodanes, (2) terminal alkynes with x3-iodanes, (3) alkynyliodonium salts with nucleophilic reagents and (4) alkynyliodonium salts with dienes. 

The cleavage of alkenylboranes may be carried out under non-acidic conditions by converting the alkenylborane into the ate complex with n-BuLi prior to hydrolysis with aqueous NaOH. " Also, alkenylboranes derived from internal alkynes may be protonated under neutral conditions using catalytic amounts of Pd(0Ac)2 in THF or acetoneor with aqueous silver ammonium nitrate [Ag(NH3)2 N03]. ... 

Trani-allylstannylation is possible with ZrC as a Lewis acid catalyst, reported by Yamamoto and coworkers, also in 1996 (Scheme 5.7.4). Varions terminal alkynes such as aryl-, alkyl-, alkenyl-, or non-substituted acetylenes undergo the reaction with non- or methyl-substituted allylstannanes. Recently, silver complexes were also found to be effective as catalysts for the intra-molecular allylstannylation of alkynes (Scheme 5.7.5). The silver complex is considered to activate the triple bond, giving a cyclo-propylmethylidene or an alkenyl complex as an intermediate. Similar intra-molecular allylstannylations, but with yn-selectivity are known to proceed in the presence of a Pd(0) or an InCls catalyst. 

Tollen s reagent /tol-Sn/ A solution of the complex ion Ag(NH3>2 produced by precipitation of silver oxide from silver nitrate with a few drops of sodium hydroxide solution, and subsequent dissolution of the silver oxide in aqueous ammonia. Tollen s reagent is used in the SILVER-MIRROR TEST for aldehydes, where the Ag ion is reduced to silver metal. It is also a test for alkynes with a triple bond in the Tposi-tion. A yellow precipitate of silver carbide is formed in this case. 

A proposed reaction mechanism includes the formation of a silver alkyl complex 285 at the initial stage of the catalytic cycle, followed by the reaction with the terminal alkyne to produce an alkenyl silver complex 286. The latter species then leads to the formation of the corresponding alkenyl Grignard reagent and regeneration of the catalytically active species on transmetallation (Scheme 10.96) [76]. 

Let us now consider metal complexes. Transition metal complexes of olefins, alkynes, and arenes and other organic 7T systems have become quite common in both the textbook and research literature some exotica of this type will be discussed in a later section. We start with an ethylene complex with the simplest metal ion, Li+. The cation is definitionally electrophilic, the olefin has an energetically accessible pair of electrons, and the complex is really quite sensible. We may recognize it as analogous to the better known olefin-silver complexes... 

Hydrosoluble iridium alkyl complexes may also be prepared by hydrolysis of alkynes and alkenes promoted by water-soluble precursors. The reaction with alkynes follows the well-known mechanism demonstrated by Bianchini et al. for ruthenium complexes.A reasonable mechanism, related to that of hydrolytic breakage, has been proposed by Chin etal. to account for the hydrolysis of ethene promoted by [Cp"lr(TPPMS)Cl2] 344 in the presence of silver salts in water. Scheme 37 describes the Chin s hydrolysis of alkynes, leading to [Cp Ir(TPPMS)(CO)(CH2R)] (69 R=Ph, Bz, Bu, />-Tol) via the aquo complex [Cp Ir(TPPMS)(OH2)OTf 345... 

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