Acetylene complexes substitution reactions

Some labile complexes react with alkynes to afford products in which at least one ligand was displaced by an acetylene molecule. Substitution reactions most commonly occur for complexes containing the following ligands CO, halides, phosphines, water, N2, etc. 

From the investigation of all these data it is clear that the aromaticity of phosphinine is nearly equal to that of benzene. Their chemical reactivity, however, is different. Most important is the effect of the in-plane phosphorus lone pair, which (i) is able to form a complex and (ii) can be attacked by electrophiles to form A -phosphinines. Thus, electrophilic substitution reaction on the ring carbon is impossible. In Diels—Alder reactions, phosphinines behave as dienes, providing similar products to benzene but under less forcing condition (the reaction with bis(trifluoromethyl) acetylene takes place at 100 °C with 3, while for benzene 200 °C is required). 

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

Substituted cyclopentadienones react with iron carbonyls to form stable, diamagnetic 7r-co triplexes of the type [Fe(CO)3(cyclopentadienone)] (215). The proposed structure is shown in (XX). These complexes undergo reactions typical of metal carbonyls, e.g., displacement of carbon monoxide by tertiary phosphines, but the carbonyl group of the ligand does not show reactions characteristic of a keto-group. These complexes are also formed by interaction of acetylenes with iron carbonyls (see Section VI,C). Interaction of tetracyclone and Fe3(CO)i2 gives unstable complexes which contain the sandwich anion [Fe(tetracyclone)2]2 analogous to the anion (XXV) (215). 

Since the substitution reaction succeeded so well with olefins, the obvious extension to acetylenes was tried. Of course, only terminal acetylenes could be used if an acetylenic product was to be formed. This reaction has been found to occur but probably not by a mechanism analogous to the reaction of olefins (43,44). It was found that the more acidic acetylene phenylacetylene reacted with bromobenzene in the presence of triethylamine and a bisphos-phine-palladium complex to form diphenylacetylene, while the less acidic acetylene, 1-hexyne did not react appreciably under the same conditions. The reaction did occur when the more basic amine piperidine was used instead of triethylamine, however (43). Both reactions occur with sodium methoxide as the base (44). It therefore appears that the acetylide anion is reacting with the catalyst and that a reductive elimination of the disubstituted acetylene is... 

Photochemical reactions have been used for the preparation of various olefin, and acetylene complexes (7). Application to the coordination of dienes as ligands has not been used extensively, so far. In this article the preparative aspects of the photochemistry of carbonyls of the group 6 and group 7 elements and some key derivatives, with the exception of technetium, with conjugated and cumulated dienes will be described. Not only carbonyl substitution reactions by the dienes, but also C—C bond formation, C—H activation, C—H cleavage, and isomerizations due to H shifts, have been observed, thereby leading to various types of complexes. 

Substitution reactions at Os(CO)4( -alkene) or Os(CO)4 ()] -alkyne) take place through initial dissociation of a ligand. The complexes Os(CO)4(jj -alkyne), where the alkyne is CF3CSCCF3 or HC=CH, compounds are more reactive than Os(CO)5 in substitution and insertion reactions. The acetylene complex is 10 times more reactive than the hexafluoro-2-butyne complex. This is probably due to the ability of the alkyne to act as a four electron donor and stabilize electron-deficient intermediates. The reaction of Os(CO)4( -HC CH) with excess PMes gives a CO insertion product 0s(C0)2(PMe3)2 C(H)=C(H)-C(0) while reaction with the bulkier phosphine PBuj gives a double insertion product, 0s(C0)3(PBu ) C(0)-C(H)=C(H)-C(0) (Scheme 9). ... 

In this section (Section III) we have discussed substitution reactions of ligands in the first class of complexes (see Section II). Except for acetylene complexes, this class exhibits a tendency to react by an associative mechanism with increase of coordination number in the transition state. Data on monodentate olefins are in agreement with this suggestion. Substitution of polydentate ligands is more complicated and usually depends upon the nature of both the replacing ligand and of a fragment to be substituted. Thus substitution involves competition between both possible mechanisms. 

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. 

A reaction mechanism involving a transition metal was studied by Decker and Klobukowski218 who investigated the role of the acetylene ligand from a density functional perspective in M(CO)4(C2H2) (M = Fe, Ru or Os). Recent kinetics experiments have shown that the rate of CO substitution in complexes of the type M(CO)4(C2R2) is accelerated by factors of 102-1013 over their respective pentacarbonyl complexes. These substitution reactions have been shown to be dissociative in nature and show a marked metal dependence on the rate. The origin of the increased reactivity of these alkyne complexes was... 

It is therefore not surprising that the reactivities of arenes and alkanes in electrophilic substitution reactions are very different, with the former being much more active. At the same time, the mechanism of the interaction (oxidative addition) of both saturated and aromatic hydrocarbons with complexes of metals in a low oxidation state is in principle the same. The reactivities of arenes and alkanes in oxidative addition reactions with respect to low-valent metal complexes therefore usually differ insignificantly. Furthermore, a metal complex via the oxidative addition mechanism can easily cleave the C-H bond in olefin or acetylene. 

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