Ene formation

The hydrogenation of but-2-yne in the gas phase has been investigated using alumina-supported Group VIII metals, other than palladium, and over copper—alumina [200,201], With the exception of copper, which was 100% stereoselective for c/s-but-2-ene formation, the distribution of the initial reaction products, as shown in Table 20, are more complex than was observed with palladium. 

It is perhaps premature to attempt to delineate the factors that determine whether symmetrical or distorted metallacyclobutadiene or t 3-cycopropenyl coordination is observed, given the comparitive sparsity of directly comparable examples, and the observation that tautomerism appears to operate between coordination modes in some cases. A similar situation arises for cyclobutadiene coordination vs. metallacyclopentadi-ene formation (Figures 6.38, 6.39, 7.19). In both cases the metallacycles are important intermediates in catalytic manifolds (alkyne metathesis and oligomerization, respectively) and in both cases the polyhapto variant represents a tangent to the productive catalytic cycle, formation of which may be reversible or in some cases may lead to termination. 

In the case of 7-AI2O3, the (100) face calculations of concentration of oxygen site-pairs for various coverages of surface hydroxy-groups revealed that there were about twice as many syn- as an/z-elimination sites. Under reaction conditions, the surface hydroxyl concentration and therefore oxygen site-pair concentration favours alk-2-ene orientation (Saytzev) and anti-ehmination occurs due to the lower energy of activation. It was also shown that cw-alk-2-ene formation is sterically less demanding than frans-alk-2-ene... 

For thoria, the mechanism is not concerted but ElcB. Using the fact that a Th02 surface is dominated by (100) and (111) planes, the calculation showed that reaction can only occur via the s> n-periplaner conformation of the alcohol and the geometry allows only alk-l-ene formation to occur. This approach demonstrates clearly the reasons for the very different behaviour of alumina and thoria and would no doubt shed light on the behaviour of oxide and salt catalysts which, unlike AI2O3, give syn-E2 elimination. 

To summarise, for butadiene hydrogenation over metal catalysts, the selectivity for but-l-ene formation is governed by the electronegativity of the catalyst. Hence, selectivity may be effected either by tailoring the supported metal and/or by the addition of promoter species. If electronegative promoters are utilised, these function by generating positively polarised adsorption sites that favour /.ran.s-isomer formation, by 1 4 addition. It is important to note that if the yield of but-l-ene is plotted relative to the trans cis ratio of but-2-ene, the data points for evaporated films (upon which much of the systematic research has been done), alumina supported metals and promoted metal catalysts fall on the same curve indicating the universality of the proposed mechanism. 

The tendency for selectivity to decrease from the second to the third transition series is common for the platinum group metals. As a further example, the selectivity towards ds-but-2-ene formation from the hydrogenation of dimethylacety-lene, increases from left to right across each series and decreases from the second to the third row [10]. The sequence of isomerization activity follows the order [10] ... 

CgHgSiCI, (3-trichlorosilylcyclo-hexa-1 -ene) Formation of 16, 14.4.3.2 Reaction with MeMgBr 16, 14.4.3.2 CgHgCIgNSb [NH4l[CgH5SbCl5l... 

---