Alkene metathesis experiments

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

The mechanism described in Scheme 1 (the Chauvin mechanism) is the accepted mechanism of alkene metathesis, and its validity has been demonstrated in two ways. First, classical kinetic studies, including isotopic labeling and crossover experiments performed using poorly defined catalysts, conclusively demonstrated that the carbene mechanism was consistent with the experiments, while the pairwise mechanism was not. More recently, the synthesis of isolable carbene complexes that catalyze the reaction has allowed a more direct observation of the reaction. Each individual step in the Chauvin mechanism has now been observed spectroscopically for several of the well-defined catalyst systems. 

A few publications have appeared concerning the metathesis of alkynes so far only heterogeneous systems with acyclic alkynes have been reported (31-33). From experiments with [l-14C]2-hexyne this reaction was found to be analogous to the metathesis of alkenes, because it turned out to be a transalkylidynation reaction (33) ... 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

Heterocyclization reactions with saturated moieties (alcohols, amines, thiols, etc.) or acids on unsaturated counterparts (alkenes, allenes, alkynes, etc.) are not covered in this chapter since they are addition, and not isomerization, reactions. Silver is also widely used as an activating agent for producing highly reactive metallic cations (anion metathesis), which, in turn, may catalyze cycloisomerization reactions. This aspect is covered only when the silver control experiments give substantial positive results. 

The suggestion that a metal carbene was the active metal-containing species involved in the reaction inspired an impressive number of elegant experiments, designed to test the vahdity of this mechanism. The results of double crossover experiments as well as isotopic labeling experiments showed conclusively that a pairwise mechanism could not account for the observed data. The reaction shown in equation (5) shows the possible products of the metathesis of two isotopically labeled dienes, and these products include a cyclic alkene derived from the closing of the diene as well as a series of deuterated ethylenes. At very low conversions, the observed ratio of ethylenes was 1 2 1 dQ d.2.d ). A detailed analysis of these results demonstrated that the pairwise mechanism could not possibly account for this result, while the carbene mechanism could. 

Despite this strong evidence supporting the non-pairwise mechanism, others objected that the pairwise mechanism could explain the results of Katz s experiment if another step in the pairwise mechanism were rate determining. Consider Scheme 11.4, in which initial single cross metathesis is rapid to form the C12 alkene (7). If 7 sticks to the metal and 4-octene attacks in a rate-determining step... 

In all cases, Grubbs found that the cis-to-trans ratio was always 1 1, thus demonstrating that a-activation does not influence the rate or stereochemistry of alkene insertion. The result of the experiment was the key piece of evidence supporting the Cossee mechanism for Z-N polymerization long sought after by chemists. The experiment allowed researchers to make a clear distinction between metathesis and Z-N polymerization, the former involving the chemistry of the M=C bond and the latter that of the M-C bond.87... 

Treatment of an alkyne/alkene mixture with ruthenium carbene complexes results in the formation of diene derivatives without the evolution of byproducts this process is known as enyne cross-metathesis (Scheme 22). An intramolecular version of this reaction has also been demonstrated, sometimes referred to as enyne RCM. The yield of this reaction is frequently higher when ethylene is added to the reaction mixture. The preferred regiochemistry is opposite for enyne cross-metathesis and enyne RCM. The complex mechanistic pathways of Scheme 22 have been employed to account for the observed products of the enyne RCM reaction. Several experiments have shown that initial reaction is at the alkene and not the alkyne. The regiochemistry of enyne RCM can be attributed to the inability to form highly strained intermediate B from intermediate carbene complex A in the alkene-first mechanism. Enyne metathesis is a thermodynamically favorable process, and thus is not a subject to the equilibrium constraints facing alkene cross-metathesis and RCM. In a simple bond energy analysis, the 7r-bond of an alkyne is... 

The critical experiment, the double cross shown in Eq. 12.9, is a more elaborate form of the crossover experiment. In a pairwise case, we will see initial products from only two of the alkenes (e.g., the C12 and Cie products in Eq. 12.9), not the double-cross product with fragments from all three alkenes. The double cross C14 product would only form initially in a nonpairwise mechanism. Later on, double-cross products are bound to form, whatever the mechanism, by subsequent metathesis of C12 with C16. 

---