Alkynes silver salts

Similar intramolecular hydroarylations of alkynes and alkenes, which obviate the need for a halide or triflate group on the aryl ring, are now well established. Sames group screened over 60 potential catalysts and over 200 reaction conditions, and found that Ru(m) complexes and a silver salt were optimal. This process appears to tolerate steric hindrance and halogen substrates on the arene (Equations (175)—(177)). The reaction is thought to involve alkene-Ru coordination and an electrophilic pathway rather than a formal C-H activation of the arene followed by alkene hydrometallation, and advocates the necessary cautious approach to labeling this reaction as a C-H functionalization... 

Grigg extended this alkyne cyclization to trapping with stannanes to give 3-exo-dienes [347], alkynes to afford tetracycles [338, 348], and alkenes leading to cyclopropanes [349], an example of which is illustrated. In his studies Grigg and co-workers have found that thallium and silver salts suppress direct capture of these palladium intermediates prior to capture [350]. 

Metal-catalyzed [4 + 2 + 2] cyclotrimerizations of either heteroatom-containing enyne 62 with 1,3-butadiene (Eq. 17) [42] or heteroatom-containing dienyne 64 with an alkyne (Eq. 18) [43] are effected by cationic rhodium complexes generated in situ from a chlo-rorhodium complex modified with silver salts. These processes afford eight-membered ring products 63 and 65, respectively. In both processes, the nature and amount of the silver salt profoundly affect the outcomes. 

Many of the synthetic routes parallel those used to prepare alkene complexes c platinum(II). Replacement of chloride ion in PtClJ- by a water soluble alkyne is a frequentl used method (equation 266), 809 812 or the reaction can be assisted by the use of a silver salt t facilitate halide displacement (equation 267).813 With hexafluorobutyne-2 the five-coordinat adduct can be isolated before it converts into the vinyl complex (89 equation 268).814 Alkyne displace alkenes from platinum(ll) complexes. 

The acidity of the terminal hydrogen in 1-alkynes provides a simple and useful test for 1-alkynes. With silver-ammonia solution (AgNOs in aqueous ammonia), 1-alkynes give insoluble silver salts, whereas disubstituted alkynes do not ... 

It has been demonstrated by 1H, 13C, and 109Ag NMR that a 7r-alkyne-Ag complex and then an alkynyl silver are formed in situ from alkyne and silver salt under conditions related to those used for Ag-catalysed alkynylation or for Ag/Pd-catalysed sp-sp2 cross-coupling reactions.142 These observations have prompted a rationale of the mechanisms of these reactions. 

Silver nitrate test The compound to be tested is treated with a few drops of 1% alcoholic silver nitrate. A white precipitate indicates a positive reaction. This could be due to either silver chloride (reaction with a reactive alkyl halide), silver alkynide (reaction with a terminal alkyne), or the silver salt of a carboxylic acid (reaction with a carboxylic acid). 

As delineated above, hydroarylation of electron-poor alkynes such as acetylene carboxylic acid ethyl ester 4 is best catalyzed by Au(I) complexes. In addition to the previously mentioned Ph3PAuCl/BF3-OEt2, several other Au(I)-complexes have been shown to be active catalysts, if activation by silver salts or Lewis acids is ensured [2, 4]. Remarkably, the Au(I)-precursor LAuCl can be prepared in situ by reacting AuCl with a phosphorus ligand. 

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... 

In an analogous reaction, 1-trimethylsilyl-l-alkynes have been synthesized in good to excellent yield on treatment with trimethylsilyl chloride, catalytic amounts of silver salts, and DBU in refluxing dichloromethane (Scheme 1.61).135... 

Pale and coworkers provided the first example of combined desilylation/coupling catalytic for silver. They found that 1-trimethylsilyl-l-alkynes in the presence of tetrakis(triphenylphosphine)palladium, a silver(I) salt, and an activator (potassium carbonate in methanol, or TBAF-3H20) in DMF coupled with vinyl triflates and aryl iodides to give enynes good yields (Scheme 1.66).143,144 Although silver salt was not necessary for reaction when TBAF-3H20 was used for activation of the carbon-silicon bond, a small to significant improvement was observed for all reported... 

This observation is not related to traces of base or acid from the silver salts used since control experiments mled out this possibility. It was known from the literature that the 5-exo-dig versus 6-endo-dig cyclization mode could depend on the nature of the carbonyl group,56 57 of the alkyne substituent,58 59 and of the nature60 61 and oxidation state62 of the metallic source used. Also, work from Yamamoto25 demonstrated the importance of both a- and Jt-Lewis acidity properties of silver(I) complexes. Therefore, depending on the silver salt used, two mechanistic pathways were proposed (pathways A and B, Scheme 5.15). 

Few examples of ene-yne cycloisomerization reactions are seen in the literature. The first results for ene-yne cycloisomerizations were with systems bearing an heteroatom (amine or oxygen) next to the alkene counterpart (forming an enamine or an enol ether). Indeed, Dake s group reported the cyclization of enesulfonamides on alkynes (69-70, Scheme 5.30) under catalysis by platinum and silver salts.85 Catalysis using AgOTf (1 1 mol%) was particularly efficient with systems such as 69 (Scheme 5.30)... 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

All the examples described above involved the reaction of diazoacetate derivatives with silver salts to initiate the formation of a putative silver carbene however, other pathways exist. For example, Porcel and Echavarren have reported an intramolecular cyclization of an allylstannane to a pendent alkyne (Scheme 8.22) that involves the intermediacy of a silver carbene.52 As can be seen in Table 8.12, the reactions proceeded in moderate to excellent yield, providing the dienylstannane, while in some cases, reductive destannylation occurred. Several asymmetric reactions were reported with substrate ( )-145d, leading to the formation of the expected adduct in reasonable enantioselectivities (ee = 73-78%) in a preliminary screen with a number of different ligands. 

Similarly, terminal alkynes added to a-iminoesters derived from ethylglyoxylate in the presence of silver salts.100 In this case, the reaction worked best in apolar solvents (Scheme 10.64). 

In 2007, Wu et al. showed that arylboronic acids and the corresponding boronates reacted with terminal alkynes in the presence of a palladacycle as catalyst and silver oxide (Scheme 10.80).132 As in the Mori reaction using basic silver salts, stoichiometric amounts of silver ion were required for high efficiency. 

Li and co-workers374,374a reported the three-component coupling reaction of an aldehyde, an amine, and a terminal alkyne catalyzed by a silver salt in water. Of various precatalysts examined, water-insoluble silver iodide gave the highest yields of the propargyl amines 452 (Scheme 137). The reaction probably proceeds via attack of a silver acetylide, generated in situ from the silver salt and the alkyne, to the iminium ion derived from the aldehyde and the... 

Silver acetyhdes are readily prepared from silver salts such as the nitrate or perchlorate and the alkyne in aqueous or methanolic aimnonia. They have been known since the mid-1800s. In neutral solutions, a mixed salt, AgC=CR nAgNOs, is obtained. Acetylene gives the silver carbide, Ag2C2, and... 

Several critical reviews of the alkylation reaction have been made in which the best experimental procedures are indicated. .. High-efficiency fractionation is necessary to obtain pure acetylenes free from halides and olefins. 1-Alkynes are sometimes purified through their silver salts. ... 

The alkynyl-silicon bond is not cleaved readily by protonic acids, but cleavage can be accomplished by Ag+ under mild conditions . For example, treatment of 237 with aqueous ethanolic silver nitrate followed by liberation of the free alkyne from the silver salt by aqueous KCN gave 238 in 80% yield. 

It is believed that an understanding of the complex role of alkynes, bases and ligands may represents the only rational means of improving catalyst activity and selectivity and, while most studies have addressed the need to use copper additives, very few have explored potential alternatives such as silver salts or silylated alkynes. 

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