Metathesis degenerate

Handbook of Metathesis VoL 1 Catalyst Development and Mechanism, Second Edition. 

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Early studies with deuterium-labeled57 58 and l4C-labeled compounds59 60 established that cleavage of the double bonds occurs. For example, the degenerate metathesis of 2-butene and perdeutero-2-butene57 results in the formation of [l,l,l,2-2H4]-2-butene as the only new labeled compound [Eq. (12.11)] which is consistent with alkylidene exchange [Eq. (12.10)] ... 

The trend of structural selectivity can be summarized as degenerate metathesis of terminal alkenes (exchange of methylene groups) > cross-metathesis of terminal and internal alkenes > metathesis of internal alkenes > productive metathesis of terminal alkenes (formation of internal alkene and ethylene).87 Since different catalyst systems exhibit different selectivities, a simple general picture accounting for all stereochemical phenomena of metathesis is not feasible. 

With terminal alkenes, degenerate metathesis (equation 13), competes with productive metathesis (equation 14), to an extent which depends very much on the catalyst. 

Isotopic labelling experiments have demonstrated that for the reaction of propene on MoC VTiC h or on photoreduced Mo03/TiC>2 the chain carrier for the degenerate metathesis is [Mo]=CHMe rather than [Mo]=CH2149-151. As a general rule in olefin metathesis a substituted carbene is less reactive than an unsubstituted carbene, and so the former tends to build up to a higher steady state. 

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 synthesis of d° Alkylidene complexes by Schrock demonstrated that carbene complexes could be isolated that were electronically similar to those postulated to be involved in the alkene metathesis reaction see Schrock-type Carbene Complexes). Eventually, this pioneering work led to the synthesis of a class of compounds that are among the most active catalysts known for metathesis chemistry. The first observation that a d° carbene complex was involved in metathesis chemistry was when Tebbe showed that the Ti complex (2) would catalyze the degenerate metathesis of... 

Maury O, Lefort L, Vidal V, Thivolle-Cazat J, Basset J-M (1999) Metathesis of alkanes Evidence for degenerate metathesis of ethane over a silica-supported tantalum hydride prepared by surface organometallic chemistry. Angew Chem Int Ed 38 1952... 

Molybdenum oxide supported on amorphous 3-Ti02 and reduced with hydrogen or carbon monoxide, followed by reoxidation with a mixture of N2O and H2, is a novel olefin metathesis catalyst. Tanaka et al. report that the metathesis reaction proceeds over this catalyst without hydrogen scrambling or isomerization through alkyl or carbonium intermediates. Degenerate metathesis is ten- to fourteen-fold faster than non-degenerate metathesis. ... 

Fjj initial rate for degenerate metathesis Fp initial rate for productive metathesis. 0.5g catalyst, 6.7 wt% M0O3, pressure of propene 3.3 kPa. The rate of formation of ethene (mol C2ll)/mol Mo/s) assuming that all Mo atoms are active sites. 

Degenerate metathesis of terminal olefins by Cp2Ti(CH2AlMe2Cl) [45]... 

Application to Cross Metathesis, Ethenolysis, and Degenerate Metathesis... 

Figure 8.8 Ruthenacycles derived from complexes that favor degenerate metathesis.

Scheme 10.1 (a,b) Productive and degenerate metathesis reactions of propylene. 

Degenerate Metathesis with Early Transition-Metal Catalysts 307... 

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