Acetylenes insertion reactions

Insertion reaction of a vinyl carbene (terminal acetylenes)... 

The dinuclear rhenium disulfido complex with n,rj -S2 ligands (ReCp )2(/U-S2)2 (Cp =7j -C5Me4Et) (type IIa2 complex in Fig. 2) undergoes an insertion reaction of acetylene and ethylene into the S-S bond according to Scheme 57. 

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). 

Probably the nickel carbonyl-catalyzed synthesis of acrylates from CO, acetylene, and hydroxylic solvent (78) involves an acetylene-hydride insertion reaction, followed by a CO insertion, and hydrolysis or acyl halide elimination. The actual catalyst in the acrylate synthesis is probably a hydride formed by the reversible addition of an acid to nickel carbonyl. 

The final product can be isolated easily as the triphenylphosphine complex. This reaction is also general as far as the acylcobalt carbonyl is concerned, but the yields vary widely depending upon which acetylene is used (34). Presumably, the presence of substituents on the acetylene favors the cyclization step rather than the formation of linear products. The larger the substituents the more favorable the cyclization becomes. If cyclization does not take place relatively rapidly, linear compounds and polymers of acetylene, or of acetylene and CO are probably formed. Thus, these reactions demonstrate the insertion reaction of both acetylenes and ketonic carbonyl groups. 

Another clear example of an acetylene insertion reaction was reported by Chiusoli (15). He observed that allylic halides react catalytically with nickel carbonyl in alcoholic solution, in the presence of CO and acetylene, to form esters of cis-2,5-hexadienoic acid. The intermediate in this reaction is very probably a 7r-allylnickel carbonyl halide, X, which then undergoes acetylene insertion followed by CO insertion and alcoholysis or acyl halide elimination (35). Acetylene is obviously a considerably better inserting group than CO in this reaction since with acetylene and CO, the hexadienoate is the only product, whereas, with only CO, the 3-butenoate ester is formed (15). [See Reaction 59]. 

Several cyclopentadienyl(alkyl)metal carbonyl derivatives have reacted with acetylenes. In some examples, insertion reactions may also be involved, although the mechanisms have not been investigated. Cyclopentadienyl(methyl)iron dicarbonyl with diphenylacetylene gave a 10% yield of cyclopen tadienyltetra-phenylcyclopentadienyliron 71). 

The polymerization of acetylene by Ziegler catalysts very likely involves metal alkyl-acetylene insertion reactions also 26). 

Metal-Halogen Compounds. Mercuric salts react readily with acetylenes, forming various products, depending upon the salt and reaction conditions. Mercuric chloride appears to undergo a clean insertion reaction with acetylene, giving as-2-chlorovinylmercuric chloride in the vapor phase (72, 73). 

Compounds with Metal—Metal Bonds. Additions of compounds with metal-metal bonds to acetylenes are rare. Perhaps the addition of acetylenes to cobalt octacarbonyl (29) should be considered an insertion reaction even though the metal-metal bond is not broken since the acetylene finally is bonded to both metal atoms. 

Figure F shows some acetylene insertion reactions. These, too, are similar to the olefin insertion reactions. The manganese and cobalt hydrocarbonyls again add. Chloronickelcarbonyl hydride, which I believe is an intermediate in many of the nickel carbonyl-catalyzed reactions, adds to olefins. Diborane and the aluminum hydrides also add.

Transition metal catalyzed insertion reactions offer a convenient route for the preparation of five membered heterocyclic rings. Besides intramolecular Heck-couplings and CO insertion, examples of the intramolecular insertion of an acetylene derivative constitute the majority of this chapter. Although some of these processes involve the formation of a carbon-heteroatom bond, they are discussed here. 

Organogold(I) complexes undergo a number of insertion reactions with unsaturated molecules, such as olefins, acetylenes, and sulfur dioxide. Insertion of carbon monoxide or carbon dioxide has not been achieved, although the reverse reaction has been observed with C02 (7/). 

The mechanism of this reaction appears similar to the allyl chloride car-bonylation discussed above, with an additional insertion of acetylene in the allylnickel intermediate before the CO insertion. A possible formulation is the following ... 

Compounds containing E-H bonds (where E is one of the Group 13 or 14 elements) can undergo insertion reactions with olefins and acetylenes, i.e. ... 

Reaction of the oct-4-yne complex with HBF4 liberates cis -oct-4-ene, and this is postulated as occurring via initial protonation of the metal (to yield [Cpf Ta(H)2(alkyne)]+), followed by insertion of acetylene into the Ta-H bond and C-H reductive elimination, cis-Oct-4-ene also results when H2 is reacted with the hydrido complex at 100°C 178). 

One of the first transition metal-catalyzed ring-expansion reactions of SCBs with the formation of new C-C bonds involved the insertion of acetylenes catalyzed by Pd-complexes to furnish silacyclohexenes (Scheme 46) <1975CL891, 1991BCJ1461>. In addition to the acetylene-insertion products (silacyclohexenes), ring-opened allyl-vinylsilane products that also incorporate the acetylene moieties were observed. The ratio of the two types of the products depends heavily on the nature of acetylenic compounds. 

The acetylene-insertion reaction presumably occurs by the following mechanistic sequence (1) insertion of Pd(0) into the SCB, (2) regioselective yy -silylpalladation of the acetylenic compounds to provide seven-membered l-pallada-4-silacyclic intermediate, and (3) reductive elimination of Pd(0) to afford silacyclohexene. Alternatively, /3-hydride elimination would open the palladacycle, generating a vinylpalladium hydride species that would undergo reductive elimination to yield the ring-opened allylvinylsilane. Isotopic labeling studies provided evidence in support of this mechanistic hypothesis (Scheme 47). 

The field of acetylene complex chemistry continues to develop rapidly and to yield novel discoveries. A number of recent reviews 1-10) covers various facets including preparation, structure, nature of bonding, stoichiometric and catalytic reactions, and specific aspects with particular metals. The first part of this account is confined to those facets associated with the nature of the interactions between acetylenes and transition metals and to the insertion reactions of complexes closely related to catalysis. Although only scattered data are available, attempts will be made to give a consistent interpretation of the reactivities of coordinated acetylene in terms of a qualitative molecular orbital picture. 

A variety of simple insertion reactions are reported for the M—Sn bond, involving species as disparate as Te239, CS2337, SO2203 338, dienes and acetylenes... 

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