Acetylenes oxidative-addition reactions

In many cases the reaction of osmium carbonyls and acetylenes does not stop at the first stages as in [56], [57], or [57]. Instead, two or more acetylene molecules are incorporated, and in some cases acetylene trimerization to benzenes takes place (182, 371, 379). Incorporation of two acetylene molecules can lead to metallacyclo-pentadiene clusters like [55] (126,168,171,182,184, 223, 371), or to metallacyclo-hexadienone clusters hke [59] (126, 223). And the complex [90], another intermediate, is related to [55] by an intramolecular oxidative addition reaction (168,169). 

The preparation is based on a convenient starting compound that may be stored readily. The procedure can be used to prepare adducts of other olefins and acetylenes. The ethylene complex is of widespread use in the study of oxidative addition reactions of platinum(O). ... 

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. 

The most important methods for the preparation of cyclobutadiene complexes are oxidative addition reactions of dihalogenocyclobutenes to low-valent transition metal compounds [equations (8.27) and (8.28)], as well as reactions of acetylenes with coordination compounds. 

The oxidative addition reactions of organic halides to platinum phosphine compounds easily proceed. For example, reactions are shown in eqs. (21.22) and (21.23) [26,34]. The vinyl bromide derivative obtained by the reaction shown in eq. (21.22) is liable and it gives PtBr2(PMePh2)2 and acetylene by elimination [34]. 

The simplest method for the addition of a 2-carbon fragment to a 17-keto steroid while retaining an oxygen function at C-17 is via reaction with acetylene or a substituted acetylene e.g., ethoxyacetylene, chloroacetylene). Since the resulting acetylenic carbinol (which is usually obtained in excellent yield) can in turn be hydrated, reduced, rearranged, and oxidized, this reaction offers considerable synthetic possibilities. 

The direction of addition, verified by acetylene oxidation into a known acid, proves that the nitiilimine carbon atom adds to the terminal atom of the enyne system, which is inconsistent with the assumed polarization of the unsaturated compound H2C=CH—C=C—R from the vinyl group towardR. The authors explain this by a possible transfer of the reaction center in nitrilimine as a particle with a nucleophilic center on a carbon Ph—C=N" —N —Ph Ph—C =N =N—Ph (63ZOB3558). 

Furthermore, when trimethylsilylacetylene 40 was used as an alkyne in the [IrCl(cod)]2-catalyzed reaction, propargyUc amines (where the alkyne was added to the double bond of imine) were obtained (Equation 10.7) [21, 23]. It is probable that the reaction proceeds through oxidative addition of the terminal C—H bond of alkyne to the Ir complex, followed by the insertion of imine to the resulting Ir-H complex. The crosscoupling reachon of trimethylsilyl (TMS)-acetylene with aldimines took place by [IrCl(cod)]2, leading to the corresponding adducts (Equahon 10.8) [24]. 

We have explored two types of carbon-carbon bond forming reactions operated under almost neutral conditions. Both reactions are initiated by the formation of an H-Rh-Si species through oxidative addition of a hydrosilane to a low-valence rhodium complex. Aldol-type three-component couphngs are followed by the insertion of an a,yS-unsatu-rated carbonyl compound into a Rh-H bond, whereas silylformylation is accomplished by the insertion of an acetylenic moiety into a Rh-Si bond. 

Harmata et al. have developed a tandem Sonogashira/nitrogen-addition reaction of acetylenes 261 to sulfonamide 262 to prepare S(vi)-oxidized compounds 263 and 264 (Scheme 36) <20050L143>. When R = alkyl (e.g., Pr"), the 1,2-thiazine 263 is the major product (70%) formed via a endocyclization process, along with a minor amount (20%) of exocyclized product 264, while the five-membered ring product 264 (81%) is preferred when R= Ph. 

It has been found in the meantime that reaction (1) is generalizable (752), and that oxidative additions of this type occur for such widely differing substrates H2Y as ethylene, benzene 130), cyclic olefins, alkyl and aryl phosphines, aniline 337, 406), and H2S 130), ail of which give the same product structure with a triply-bridging Y ligand. The stability of these third-row transition metal clusters has stiU prevented catalytic reactions of these species, but it is likely that similar ones are involved in olefin and acetylene reactions catalyzed by other metal complexes. 

Hatakeyama et al. [115] have also carried out detailed product studies in the HO-initiated oxidation of the alkynes both in the presence and in the absence of NOx. The major products consisted of a-dicarbonyl compounds, i.e., HC(0)CHO from acetylene, CH3C(0)CH0 from propyne and CH3Q0)C(0)CH3 from 2-butyne, as well as HC(0)OH from acetylene and propyne and CH3C(0)OH from 2-butyne. The formation of these products was attributed to 02-reactions of the hydroxyvinyl radicals resulting from the addition reaction of HO with the alkynes, e.g.,... 

The reaction between acetylene and RhfCOXi CjH i -QH,) [which acts as a source of the Rh(COXf/5-C9H7) fragment] affords 33 in 50% yield (61). The reaction is supposed to proceed via oxidative addition of the alkyne to the rhodium fragment, followed by isomerization to the vinylidene complex which then interacts with a second rhodium fragment ... 

Triazoline aromatization to triazoles can be achieved by oxidation, isomerization reactions, and elimination of stable molecular fragments, all of which require the presence of a free hydrogen at position 4 and/or 5 of the triazoline ring. The aromatization reaction affords a selective, synthetic route for the preparation of triazoles of definitive structure, inasmuch as azide addition to acetylenes is not regioselective.33... 

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