Triple bonds cleavage reaction

Scheme 23 (a) Hydroacylation-triggered C-C triple bond cleavage reactions, (b) Proposed mechanism... 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

The mechanism of the Reppe reaction has been studied by Colborn and Vollhardt. They used C-labelled acetylene and studied the labelling pattern of the cyclooctatetraene. Their results were not consistent with reaction through cyclobutadiene or by recombination of carbyne units resulting from triple-bond cleavage, but were consistent with either the concerted or stepwise mechanisms outlined below. 

Scheme 22 C-C triple bond cleavage in a reaction of allylamine 74 with internal alkyne 126...

The reaction between the monometallic cobalt(ll) [Cp CoCl] (Cp = CsH4CH2CH2P Bu2) complex and a phos-phaalkyne led to an original synthesis of a P-capped C03 cluster 410 via the cleavage of the P-C triple bond. This reaction was however very much dependent upon the stoichiometry used, as depicted in Scheme 62. In the... 

Abstract Among a lot of transition metal mediated or catalyzed C-C bond cleavage reactions, metallocene mediated C-C bond cleavage reactions, in particular, are attractive since many cases show the intermediates or the C-C bond cleavage steps in the reactions. Organic compounds are classified into three groups by the bond order of the C-C bond (1) C-C single bond, (2) C-C double bond, and (3) C-C triple bond. 

C-C triple bond cleavage has some similarity to the C-C double cleavage. Several examples are known for the C-C triple bond cleavage in the multi-metallic system. A C-C triple bond easily coordinates to transition metals giving an alkyne complex. The alkyne complex can also be formally described as a met-allacyclopropene. The second coordination of the C-C double bond moiety of the metallacyclopropene to another transition metal center provides a more strained two metallacyclopropane fused ring system. When the C-C bond is cleaved, the fragment can be stabilized in the multi-metallic system. Most reported reactions of this type used the Cp ligand in the system. 

The following reaction of an alkyne with an Ir complex looks like the C-C triple bond cleavage (Eq. 27) [41]. It is not the direct C-C triple bond cleavage by the reaction with metals. The first step of this reaction is addition of water to the triple bond forming an aldehyde. Elimination of CO from the resulting acyl complex gives the product. 

Azomethines add with great ease and stereospecificity to carbon-carbon double bonds affording pyrazolidines. They also add to carbon-carbon triple bonds with the formation of pyrazolines. Benzimidazolium iV-imines, e.g. (622), are a special case of azomethines. Its reaction with DMAD yields l-(2-methylaminophenyl)pyrazole (624), which is formed by cleavage of the initial adduct (623) (75JHC225). 

Both types of processes, 7r -assisted y, -bond cleavage and P -bonding, have been invoked to operate in the phototransformations of the aldehyde-ketone (153) to products (155), (156) and (158). The conversions have been observed at room temperature in dioxane, t-butanol, ethanol and benzene using light of wavelengths 2537 A or above 3100 A or sensitization by acetophenone. The phosphorescing excited triple state of (153) is very similar to that of testosterone acetate (114), but its reactions are too rapid... 

Alkynes, like alkenes, can be cleaved by reaction with powerful oxidizing agents such as ozone or KMnC, although the reaction is of little value and we mention it only for completeness. A triple bond is generally less reactive than a double bond and yields of cleavage products are sometimes low. The products obtained from cleavage of an internal alkyne are carboxylic acids from a terminal alkyne, CO2 is formed as one product. 

The metathesis of linear alkynes has also been reported, e.g. the metathesis of propyne, 1-pentyne, 2-pentyne, and 2-hexyne (31-33). This reaction can be visualized as the cleavage and formation of carbon-carbon triple bonds ... 

Addition reactions at the alkyne bonds are dealt with in the section on alkenylstannanes that are produced. The alkynyl-tin bond is more readily cleaved by both electrophiles and nucleophiles than is the alkenyl- or alkyl-tin bond. Strong electrophiles such as halogens or halogen acids attack at the z/Mzi-position of the triple bond to give a /3-stannyl cation that is stabilized by C-Sn hyperconjugation, but this is followed by cleavage of the C-Sn bond (Equation (83)). 

The analogous reaction of 3 with W2(OrBu)6 gave a brown solution, from which a yellow, microcrystalline material could be isolated. Based on spectroscopic data the latter was formulated as [Cy7Si70i2]2W2(p-H)(0tBu) (158). Cleavage of the Mo = Mo triple bond in 157 occurred upon treatment of this compound with NO (two equivalents) to afford the dimeric nitrosyl molybdenum silsesquioxane complex [Cy7Si70i2]2Mo2(NO)2 (159). °°... 

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