Reactions Involving Metals

Use of Acetylenes in Synthesis.—Reactions involving Metals. The thiacyclo-heptyne (54) has been converted into an isolable cyclobutadiene-containing dimer [(55a) or (55b)] via the yellow palladium complex (56). A distinction between (55a) and (55b) and the extent of bond delocalization for this 

Diacetylenes are cyclotrimerized to the unsymmetrical isomer (59) in high yield using Ni(CO)2(PPh3)2. Previous catalysts for this transformation gave low yields of symmetrical and unsymmetrical isomers. This catalyst also cyclo-cotrimerizes acetylenes with olefins iV methylmaleimide and phenyl-acetylene give the 2 1 adduct (60) and the 2 2 adduct (61).  

Hydroformylation of alkynes catalysed by rhodium normally proceeds reluctantly and in poor yields. However, hydroformylation of but-1- and 

Conversion of acetylenes into substituted maleate esters (62) by addition of two carboxy-groups to the unsaturated system is achieved using palladium and mercury salts in the presence of carbon monoxide. Diphenylacetylene and aromatic nitro-derivatives in the presence of carbon monoxide and 

Rh6(CO)ie combine to yield maleimides (63), the carbon monoxide functioning as reducing and carbonylating agent.  

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Reactions involving metal complexes of sulphur ligands. L. F. Lindov, Coord. Chem. Rev.. 1969,4, 41-71 (223). 

It is clear that alkanes should not be treated as inert solvents in reactions involving metal atoms, even when there is an excess of a more established ligand. 

Other microwave-assisted reactions involving metal catalysts or metal-based reagents are shown in Scheme 6.79 [164—167]. 

The [4 + 3]-cycloaddition is a commonly used method for the synthesis of seven-membered rings.9 Many of these reactions involve metals, principally in the role of a Lewis acid as exemplified in Equation (10). These Lewis acid-catalyzed [4 + 3]-cycloadditions have been reviewed by Rigby,62 Sarhan,63 Harmata,64,65 and Hoffmann,66 and will not be reviewed here due to the role of the metal as a Lewis acid. Several computational papers on this subject have also been published.67-71... 

In aqueous solutions these reactions seem to proceed via an outer-sphere mechanism on most metals. Typically such reactions involve metal ions surrounded by inert ligands, which prevent adsorption. Note that the last example reacts via an outer-sphere pathway only if trace impurities of halide ions are carefully removed from the solution otherwise it is catalyzed by these ions. 

Reactions Involving metal Ions adsorbed on minerals... 

There are two types of reaction involving metals (1) in which the metal is a reagent and is consumed in the process and (2) in which the metal functions as a catalyst. While it is certainly true that any cleansing of metallic surfaces will enhance their chemical reactivity, in many cases it would seem that this effect alone is not sufficient to explain the extent of the sonochemically enhanced reactivity. In such cases it is thought that sonication serves to sweep reactive intermediates, or products, clear of the metal surface and thus present renewed clean surfaces for reaction. Other ideas include the possibility of enhanced single electron transfer (SET) reactions at the surface. 

These reactions involve metallate rearrangements , migratory insertion and transition metal-catalysed vinylic substitution reactions. They also perform well in applications in natural product synthesis . Many useful synthetic possibilities arise from application of ring-closing olefin metathesis (RCM) to unsaturated homoaldol products and their derivatives by means of the Grubbs catalyst 3942 4-286 Equation 105 presents some examples. ... 

The mechanism operating in rhodium-catalyzed and iridium-catalyzed hydrogen transfer reactions involves metal hydrides as key intermediates. Complexes such as [ M(p.-C1)(L2) 2], [M(cod)L2](Bp4) (M = Rh, Ir L2 = dppp, bipy), and [RhCl(PPh3)3] are most likely to follow the well-established mechanism [44] via a metal alkoxide intermediate and elimination to generate the active hydride species, as shown in Scheme 2. 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

Transition metal complex catalyzed reactions exhibit different outcomes depending on whether they are conducted in water or not. Chao-Jim Li highlighted the greening of chemical reactions, focusing on the use of water as an alternative solvent for reactions involving metals, R = CH2Ph (Li, 1996) (see fig. 6.8). 

Reactions involving Metal Complexes of Sulfur Ligands 1667... 

For the purposes of this chapter, we shall use the term biological methylation (in recent years this has frequently been contracted to biomethy-lation ) in Challenger s first sense to describe methyl-transfer (transmethylation) reactions involving metals and metalloids in biological systems. We shall not differentiate between endo- and exocellular transmethylation (5, 6), and we shall include some purely chemical studies of model reactions. The relationship of these reactions to the larger area of organometallic chemistry has been reviewed (7, 8). 

From Exchange Reactions Involving Metal Alkoxides 339... 

From Exchange Reactions Involving Metal Alkoxides 15.3.2.5.1 By alcohol interchange... 

These differential heating effects may also be used in solid-state reactions involving metal powders that are made into a fluidised bed by a counter stream of gas. The metal particles interact very strongly with the microwaves and rapidly heat, whereas the gases are transparent to microwaves therefore, the reaction is induced by a very selective interaction between the metal particles and microwaves. 

Whittaker, A.G. and MingosD.M.P., Microwave assisted solid state reactions involving metal powders,/. Chem. 

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