Butadienyl acetylene

The addition of substituted alkenes with electron-withdrawing groups to ruthenium acetylide complexes results in the formal [2 + 2] cycloaddition of the olefin to the acetylene moiety. Facile ring opening of the resultant ruthenium cyclobutene complex (103) generates the ruthenium butadienyl species (104). Subsequent displacement of a phosphine ligand leads to the Tj3-allylic product (105) [Eq. (93)] (90-92). The intermediate cyclobutene complex has been isolated in one instance for the monocarbonyl derivative 106 [Eq. (94)] (92). 

It should be stressed that in case of the ethynyl-acetylene reaction, a molecular hydrogen loss channel synthesizing the 1,3-butadienyl radical is open as well. Since the reactions of cyano and ethynyl radicals have no entrance barrier, are exoergic, and aU transition states involved are lower than the energy of the separated reactants, these reaction classes are extremely important to form nitriles and complex unsaturated hydrocarbons in low-temperature environments. On the other hand, the corresponding phenyl radical reactions are—due to the presence of an entrance barrier—closed in those environments. However, the elevated temperature in combustion systems helps to overcome these barriers, thus making phenyl radical reactions important pathways to form aromatic molecules in combustion flames. 

Complex 170 is a useful precursor for catalytic and stoichiometric carbon-carbon coupling reactions including the polymerization of acetylene, the synthesis of dimethyl-l,4-cyclohexadiene-l,2-dicarboxylate, and the formation of novel organometallic derivatives which act as s)mthetic intermediates in the above mentioned processes. These organometallic species are carbene, butadienyl, cyclohexadienyl, and dioxo-hexahydroisobenzofuranyl compounds [66]. 

Complex 170 reacts with acetylene to afford 171. Once this vinyl-carbyne compound is formed, its deprotonation with a stoichiometric amount of KOH in methanol yields the vinyl-vinylidene 0s(CH=CH2)(K2-02CCH3)(C=CH2)(P Pr3)2 (175), as a result of the deprotonation of the carbyne ligand (Scheme 46). Like 171, under carbon monoxide atmosphere, complex 175 evolves by migratory insertion. In this case, the migration of the vinylidene into the Os-vinyl bond takes place, to afford the butadienyl compound Os C(CH=CH2)=CH2 k -OC(0)CH3)(CO)2(P Pr3)2 (176), which in solution exists as a mixture of the isomers a and b shown in Scheme 46. 

Radical addition of 1,3-butadienyl to acetylene provides a route to benzene formation through the sequence... 

The 1,3-butadienyl radical is primarily a by-product of butadiene pyrolysis in this system but results from vinyl addition to acetylene in flames of other aliphatic fuels. In aromatic flames 1,3-butadienyl may be produced by oxidative and pyrolytic decomposition of aromatic species, as suggested in a study of benzene flames (10) ... 

The behavior of C4H4 relative to benzene and PAH has been observed in other aliphatic flames, including those of methane (25,26), acetylene (7,27), and ethylene (27), as well as benzene flames (1, 10). As an example. Figure 13 shows data for ethylene and acetylene flames extracted from the works of Crittenden (28) and Crittenden and Long (27). This correlation may be explained if 1,3-butadienyl can be shown to be the primary precursor for formation of C4H4, as well as PAH. 

The pentadiene formyl radical C4C DO (Path-2) reacts further by beta scission to form CO and butadienyl-1 (CDCCDC ) via a barrier of 39 kcal mof. The butadienyl-1 can undergo elimination, which has a 40 kcal mof barrier, to form vinyl radical and acetylene or react with molecular oxygen with no barrier. 

In the reaction of t-butylacetylene with [Pd(Cl)2(PhCN)2] the acetylene is consumed according to a first-order rate law up to the point where three equivalents of acetylene per Pd is reached after which the rate decreases by a factor of ten. Addition of 2,5-dithiahexane at the three-equivalents point gives complex (10) which has the structure illustrated, according to X-ray studies. This indicates that the first stage in the reaction, cis insertion into the Pd—Cl bond, is followed by a second cis insertion to give the (x-butadienyl ligand. Since three moles of acetylene are consumed at the point where the dithiahexane is added, complex (11) is considered to be the precursor of (10). 

Further support has been reported for the proposal that butadienyl complexes (4) are important intermediates in the reactions of [PdCl2(PhCN)2] with acetylenes... 

---