Kinetic alkyne metathesis

Naturally, the question arises What accounts for the dramatic difference in yields between these processes Macrocyclization under kinetic control, as shown by the Staab example, is clearly not a favorable situation, as evidenced by the low yield of 14a. In contrast, it has been shown that when alkyne metathesis macrocyclization is under thermodynamic control, [n]cycles are the lowest-energy product [64]. ADIMAC of monomer 15a under the conditions shown in Scheme 6.8 generate [6]cycle 16 as the major product, and [5]cycle 17 as a minor product. Gel permeation chromatography (GPC) analysis confirmed that the oligomeric products (both linear and cyclic) that are initially formed in the reaction are consumed over time, and the [5-6]cycles are the major product upon completion. More dramatically, when polymer 15b was subjected to the same conditions, the major products... 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

In this section we shall consider the results recorded in the literature that pertain to the structures of the adsorbed species. Kinetic or catalytic aspects, as could be relevant to hydrogenation, hydrogenolysis, or metathesis processes, will be treated in Part 11. Spectra of the much-investigated alkenes are discussed in detail in Part I. The spectra of the other principal types of hydrocarbon adsorbates, viz. alkynes, alkanes, cycloalkanes, and aromatics, will be analyzed in Part II. Most results are available for the type-molecules ethene, ethyne, ethane, and benzene as well as for the metals, Pt, Pd, Ni, Rh, and Ru. 

Metathesis of alkynes proceeds according to a mechanism (13.177) analogous to that already described for olefins. The formation of catalytically active metal-lacyclobutadiene complexes was confirmed by both kinetic and isotopic studies of... 

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