Acetylene complexes with isocyanides

Recently, Yamazaki et al. 103) carried out stoichiometric reactions of cobalt-acetylene complexes with isocyanides and isolated the expected intermediate metalocyclic complexes (Scheme 6). 

Cocyclization of acetylene with isocyanides gives interesting new cyclic compounds 103, 116). The reaction patterns are generally similar to the cocyclization wdth carbon monoxide which is already known 103, 117). Low-valent nickel, palladium, or cobalt complexes are active in the following reactions 102, 103) for which intervention of acetylene complexes has been suggested ... 

Rearrangements are less common, since the complexes are usually stable. However, if a cobalt-acetylene complex is reacted with an isocyanide, a metallacycle (145) is formed. ... 

Special Ligands. As discussed in Sect 2.5.3, the substitution of carbonyl ligands in the cluster often enhances the liability of the clusters. This property has also been used in catalytic processes. Thus for instance, the nickel cluster with isocyanide ligands Ni4(CNR)7 is an efficient catalyst precursor in the hydrogenation of acetylenes. Actually, Ni4(CNR)7 reacts with acetylenes yielding the adduct Ni4(CNR)4 (acetylene)3 which in turn is able to catalyze, under rather mild conditions, the hydrogenation of diaryl and dialkylacetylenes at rates of ca. one turnover per minute. Mononuclear complexes do not catalyze this reaction under the same conditions. 

The interaction of isocyanides with acetylene complexes has been found to produce various organic and organometallic products differing in the number of alkyne and.isocyanide units incorporated. Cp(Ph3P)Co(PhC2Ph) reacts with two equivalents of isocyanide to form the diimino cobaltacycle (XVni) (Yamazaki et aU 1975). Further reaction of these complexes with additional isocyanide led to the novel cyclopentene derivatives (XIX). These... 

In contrast to the above results, the nickel isocyanide complexes react with tolane to afford diiminocyclobutenes in variable yield (Suzuki and Takizawa, 1972). These reactions presumably proceed via mixed isocyanide-acetylene complexes since these can be prepared independently and gave the same product on thermolysis. The imino compounds are readily hydrolyzed to cyclobutenediones. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

From the results of classical trajectory calculations intrinsic non-RRKM behavior has been predicted for ethane dissociation, ethyl radical dissociation,and methyl isocyanide isomerization. These predictions are supported by classical trajectory calculations for model H-C-C -> H + C=C dissociation. To generalize, classical trajectory calculations have predicted intrinsic non-RRKM behavior for molecules with isolated high frequency modes [e.g, CH3NC, clusters like Li (H20)j, and van der Waals molecules], molecules like acetylene with linear geometries for which bending and stretching motions are nearly separable, and molecules with tight activated complexes. 

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