Acetylene complexes mechanism

Good yields of imidazoline derivatives have been obtained in the cocyclooligomerization of phenylacetylene with isocyanates and carbodiimides (Scheme 100).166 It has been demonstrated166 by labeling studies in the isocyanate reaction that the hydrogen shift is intramolecular and a mechanism accommodating this feature is illustrated in Scheme 101.166 The final step (85 - 86) in the proposed166 mechanism (Scheme 101) probably occurs via a coordinated acetylene complex and it is notable that related complexes... 

It was found that only deuterated furanone was obtained if water had been replaced with deuterated water (D2O). This observation corroborated the fact that water was the hydrogen source in this reaction. Two possible mechanisms were proposed. The first one is described as follows (Scheme 18, path a) acetylene 119 can react with bimetallic species to afford /r, -acetylene complex 122, which reacts further with CO to give /tjTj -furanone... 

Rh complexes are examples of the most effective catalysts for the polymerization of monosubstituted acetylenes, whose mechanism is proposed as insertion type. Since Rh catalysts and their active species for polymerization have tolerance toward polar functional groups, they can widely be applied to the polymerization of both non-polar and polar monomers such as phenylacetylenes, propiolic acid esters, A-propargyl amides, and other acetylenic compounds involving amino, hydroxy, azo, radical groups (see Table 3). It should be noted that, in the case of phenylacetylene as monomer, Rh catalysts generally achieve quantitative yield of the polymer and almost perfect stereoregularity of the polymer main chain (m-transoidal). Some of Rh catalysts can achieve living polymerization of certain acetylenic monomers. The only one defect of Rh catalysts is that they are usually inapplicable to the polymerization of disubstituted acetylenes. Only one exception has been reported which is described below. 

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

Because of the complex mechanism of these reactions, not yet sufficiently understood, directed synthesis of single monodisperse oligomers by endcapping polymerization of bis-deprotected acetylenes is not yet very effi-... 

The reaction of cyclohexenamides with nucleophiles such as water, alcohols, or thiols, produced carboxylic acid, esters, or thioesters. Reaction with acetylenic dipolarophiles in acidic conditions produced highly functionalized pyrroles via a complex mechanism, implying as intermediates 1,3-dipoles and bycyclic cycloaddition products. Reaction of cyclohexenamides containing protected hydroxylic functions with AcCl/MeOH produced < -lactones, while cyclohexenamides, bearing in Ri an o-aminophenyl group, easily cyclized to 1, 4-benzodiazepine-2, 5-diones. 

Although characterized by a quite complex mechanism, the deamination of acetylenic Mannich bases by lithium-alkyF yields interesting substituted acroleins R—CH=CH—CHO, that is, the isomeric form of the corresponding acetylenic hydroxymethyl derivatives R—C C—CH2OH, as a consequence of alkync metallation followed by hydrolysis. 

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. 

The protonation of zero-valent platinum-acetylene complexes has been discussed by several workers, and a mechanism proposed involving insertion of the acetylene into a hydroplatinum bond 287). The reaction... 

A calculation has also been made on the relative energies of Pt(7r-MeC=CH)(PH3)2 and PtH(C=CMe)(PHg)2 (295). Consideration of total overlap population showed that the Tr-complex is definitely less stable than either the cis or trans oxidative adduct. The acetylenic hydrogen on the 7r-complex is considered to be almost completely hydridic, and it is proposed that the monosubstituted 7r-acetylene complex rearranges to the hydroacetylide complex via an S l (lim) mechanism involving loss of hydride, rearrangement to a new cationic complex, and recombination. The rearrangement reaction is visualized as follows ... 

Olefin and acetylene complexes have been known for many years, the first, Zeise s salt, K[PtCl3.C2H4], being isolated 140 years ago. These early complexes were predominantly of curiosity value only and only in the last 20 years have detailed studies been carried out on synthetic techniques, kinetics and mechanisms of reactions, stabilities and structures. 

This investigation is exploratory, so it affords no evidence which discloses the undoubtedly complex mechanisms underlying the observed effects. However, it seems reasonable to assume that a time-dependent catalytic effect exists which is related to the composition and temperature of the hot zone surface to which the arc sample effluent was exposed. The rapid transition in hydrocarbon composition from acetylene to methane in the presence of iron or stainless steel is one indication. Thermal effects and increased residence time in the presence of hydrogen lead to progressive diminution and disappearance of hydrocarbons, suggesting another, slower process which may be pyrolytic or possibly inhibitory. 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

Using a mechanical model and a set of force constants, Popov and Lubuzh (66ZPS498) have calculated vibration frequencies for polyacetylenic groups. But these calculations are rather complex and the data on the IR spectra of acetylenic... 

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