M-C triple bond

A major breakthrough in the synthesis of transition metal methylene and methylidyne complexes has been achieved by Stone and his group it originates from the simple idea that M=C double bonds in Fischer-type carbenes and M=C triple bonds in carbyne systems should add to low valent metal complexes as do C=C and C=C linkages, respec-... 

H NMR shows that the alkyne ligand in 242 (R = H) rotates about the metal-alkyne bond axis with an activation barrier of only 50 kJ/mol. The structure of the diphenylacetylene complex 242 was determined by X-ray crystallography. The reaction shown in Eq. (183) was expected to generate alkylidyne alkyne complexes such as W(CPh)CI(PhC2R)(CO)-(PMe3)2. Such species are probably intermediates in the reaction however, a n conflict between the adjacent C=C and M=C triple bond systems... 

Even more recently, alkyne metathesis catalysts have been developed and applied to organic synthesis. The catalysts for these reactions either have a M=C triple bond or are converted into compounds that have a M=C triple bond under the reaction conditions. 

Table V shows that the position of the ck-W(CO)2 infrared stretch is highly sensitive to the Jt-acidity of the alkylidyne group. Conjugation of the M C triple bond with Jt-donating alkylidyne substituents (Scheme 32) leads to a decrease in the 7t-acidity of the M=C bond in line with the heteroallenic (heterovinylidene) canonical form and a subsequent increase in available electron density for the n-acidic carbonyls, which leads to a decrease in the frequency of vco-...

Fischer was also the first to synthesize a carbyne (alkylidyne), an organometaiiic compound having a M=C triple bond, as shown in Figure 18.26. A filled sp-hybridized orbital on the C atom donates electron density to an empty orbital on the metal. r-backbonding from filled orbitals on the metal can occur into two perpendicular sets of r MOs. Thus, the carbyne acts as a tr-donor and two r-acceptors. Because it has two ar-acceptors and only one a- donor, the ligand is formally... 

We now look in detail at compounds with multiple bonds between metal and ligand. We are chiefly concerned with multiple bonds to carbon, as in metal-carbene complexes L M=CR2, which have a trigonal planar carbon and at least formally contain an M=C double bond, and metal-carbyne complexes, L MsCR, which are linear and contain an M=C triple bond, but we also look at complexes with multiple bonds to O and N. 

The second general method of bridging carbene complexes involves the analogy between C=C and M=C double bonds. Since many metal complexes will react with C=C double bonds to give alkene complexes, Stone investigated the reactions of the same metal complexes with compounds containing an M=C bond (Eq. 11.47). This is a very powerful method of making a variety of homo- and heterometallic complexes, and can be extended to the M=C triple bond as well. 

Palladium catalyzed cycloisomerizations of 6-cn-l-ynes lead most readily to five-membered rings. Palladium binds exclusively to terminal C = C triple bonds in the presence of internal ones and induces cyclizations with high chemoselectivity. Synthetically useful bis-exocyclic 1,3-dienes have been obtained in high yields, which can, for example, be applied in Diels-Alder reactions (B.M. Trost, 1989). 

The reaction between aqueous K2PdCl4 and CMe2(OH)CsCCsCCMe2(OH) (L) has been reported to give the palladium(I) complex PdCl(L), which exchanges L for X with salts MX (M = alkali metal, X = SCN, Br, I) and adds py to give PdCl(L)(py). However, it must be said that the dark brown to black solids so formed are not fully characterized by contemporary standards. It was assumed that hydropalladation of one C C triple bond has occurred. ... 

The benzyne intermediate from p-bromotoluene has a triple bond between and C" both C s are independently attacked by OH, giving a mixture of m- and p-cresols (HOCtH CH,). Two isomeric benzynes are formed from m-bromotoluene, one with a C -to-C triple bond and the other with a C -to-C" triple bond. Hence, this mixture of benzynes reacts with OH at all three C s, giving the mixture of three isomeric cresols. 

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