SILVER COMPLEXES WITH ACETYLENES

Consider now the results of Reference 43 in which a series of formally related monosubstituted alkynes and alkenes were directly compared. For five pairs of species under identical solvent conditions a nearly constant ratio of their equilibrium constants of ca 2.2 was found. 

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Silver fluoborate, reaction with ethyl bromide in ether, 46, 114 Silver nitrate, complexing with phenyl-acetylene, 46, 40 Silver oxide, 46, 83 Silver thiocyanate, 45, 71 Sodium amide, in alkylation of ethyl phenylacetate w ith (2-bromo-ethyl)benzene, 47, 72 in condensation of 2,4-pentanedione and 1 bromobutane to give 2,4-nonanedione, 47, 92 Sodium 2 ammobenzenesulfinate, from reduction of 2 mtrobenzenesul-finic acid, 47, 5... 

Silver fluoborate, reaction with ethyl bromide in ether, 46, 114 Silver nitrate, complexing with phenyl-acetylene, 46, 40 Silver oxide, 46, 83 Silver thiocyanate, 46, 71 Sodio-2-formyl-6-methylcyclohexanone, 48,41... 

Catalytic forms of copper, mercury and silver acetylides, supported on alumina, carbon or silica and used for polymerisation of alkanes, are relatively stable [3], In contact with acetylene, silver and mercury salts will also give explosive acetylides, the mercury derivatives being complex [4], Many of the metal acetylides react violently with oxidants. Impact sensitivities of the dry copper derivatives of acetylene, buten-3-yne and l,3-hexadien-5-yne were determined as 2.4, 2.4 and 4.0 kg m, respectively. The copper derivative of a polyacetylene mixture generated by low-temperature polymerisation of acetylene detonated under 1.2 kg m impact. Sensitivities were much lower for the moist compounds [5], Explosive copper and silver derivatives give non-explosive complexes with trimethyl-, tributyl- or triphenyl-phosphine [6], Formation of silver acetylide on silver-containing solders needs higher acetylene and ammonia concentrations than for formation of copper acetylide. Acetylides are always formed on brass and copper or on silver-containing solders in an atmosphere of acetylene derived from calcium carbide (and which contains traces of phosphine). Silver acetylide is a more efficient explosion initiator than copper acetylide [7],... 

The ease with which olefins form complexes with metals naturally led to investigation of acetylenes as ligands but until recent years only a few ill-defined, unstable acetylene complexes of copper and silver were known. Now complexes of acetylenes with metals of the chromium, manganese, iron, cobalt, nickel, and copper subgroups are known. These complexes fall naturally into two classes—those in which the structure of the acetylene is essentially retained and those in which the acetylene is changed into another ligand during complex formation. Complexes of the first class are discussed here and the second class is discussed in Section VI. 

Acetylenic oxy-Cope rearrangement (12, 51). The oxy-Cope rearrangement of 5-hexen-l-yn-3-ols is accelerated in refluxing N-methyl-2-pyrrolidone (9, 316), but the required temperature (165°) can result in rearranged products. The rearrangement can proceed at 20-60° in the presence of silver trifiate (1 equiv.), which is known to complex with triple bonds.2. ... 

Silver(I)-catalyzed cyclizations of substituted allenes to heterocydic ring systems induding 2,5-dihydropyrroles have been described previously [4,10]. Moreover, sil-ver(I) salts are known to form stable rr-complexes with terminal acetylenes [11]. On the other hand, on treatment with silver nitrate silylacetylenes were reported to afford silver acetylides [12]. Based on these considerations and additional experimental evidence [5,13], the following mechanism has been proposed forthe sUver(I)-mediated oxidative cydization of homopropargylamines to pyrroles ]5] (Scheme 15.3). 

Complexes with alkenes and arenes are formed when the hydrocarbons are shaken with aqueous solutions of silver(I) salts. Di- or polyalkenes often give crystalline compounds with Ag+ bound to one to three double bonds. The formation of alkene complexes of varying stability may be used for the purification of alkenes, or for the separation of isomeric mixtures (e.g., 1,3-, 1,4-, and 1,5-cyclooctadienes), or of the optical isomers of a- and /3-pinene. There is very little back-bonding contribution in the formation of Ag1 rr-complexes. For example, the planar complex (hfa)Ag(Ph-C= C-Ph) contains an almost linear acetylene ligand with a C=C... 

Note The acetylene backbone is not flexible enough to allow the formation of a chelate complex with silver or gold, both preferring a linear arrangement of ligands. 

The metals form complex cyanides in which they are covalently bonded to the radical, e.g. KM(CN)2. All complex with ammonia to give the ion [M(NHg)2] and there are similar substituted ammonia and pyridine complexes. Copper(I) chloride dissolved in HCl absorbs CO, and the hydrated carbonyl derivative, Cu(CO)Cl.H20, has been recovered from the solution. A gold analogue is known. Copper(I) and silver solutions also absorb ethylene and substitued ethylenes. Manometric measurements (Vestin, 1954) have shown that two distinct complexes are formed by acetylene in HCl solution, C2H2(CuCl)g and C2H2(CuCl)2. 

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. 

C-2b c In the cationic Ag(I) arylacetylide complexes [Ag3(dppm)3L]2+ (111) and [Ag3(dppm)3L2] + (112) the trinuclear silver cluster has triangular pyramidal or bipyramidal geometry, with acetylenic C atoms in apical positions. Interatomic distances, 111 Ag—Ag... 

Copper (I) complexes of olefins have been less widely studied but have been found to be analogous to silver(I) complexes in several ways. It was shown 54>, that solid cuprous chloride absorbed ethylene, propylene and isobutylene and solid cuprous bromide absorbed ethylene to give 1 1 complexes, while diole-fms (butadiene and isoprene) and acetylenes were reported S6> to form complexes with a 2 I copper olefin (or acetylene) stoichiometry. Andrews and... 

A coordination number of 1 is rare for stable complexes. The examples that are known involve a very bulky ligand such as the aryl radical, 2,4,6—Ph3C6H2, which forms ML complexes with copper and silver (2-97). The M-L unit can also be considered as a fragment in a complex with a higher coordination number, such as W-CO in the complex [( l -acetylene)3W(CO)] (2-98). 

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