Metathesis methylene exchange

Alkyhdene derivatives of titanium and of phosphorus catalyse methylene exchange between olefins. Although exchange of CH2 groups is not useful for synthesis, these systems provide insight into the mechanisms of alkylidene exchange, a basic step in conventional metathesis. Titanacyclobutenes have been isolated from reactions of acetylenes with methylene-titanium complexes but titanacyclobutanes, the assumed intermediate for the case of olefins, have not been isolated. Bis(cyclopentadiene)titanacyclohexane decomposes to produce ethylene as the major product apparently via a-C-C bond cleavage. ... 

Klabunde et al have found that alkylidene derivatives of phosphorus catalysed methylene exchange among olefins. In contrast to metal-based metathesis catalysts, the phosphoms-based catalysts are effective with olefins in which the C=C bond is conjugated with a functional group such as CN or COOR. 

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. 

The salt is a white solid, soluble in w ater, acetonitrile, methylene chloride, and alcohol. The cation is stable in neutral and acidic aqueous solution, but it is degraded in aqueous base. Because the cation is stable in aqueous solution, many of its salts can be readily prepared by metathesis or ion exchange. Principal infrared absorption bands of the iodide salt, run as a mineral-oil mull (exclusive of that in common with the mineral oil), occur at 2410(m), 2380(m), 1300(m), 1065 (w), 1037(w), 990(m), 950(s), 900(m), 855(w), 772(m), and 740(m) cm.-1. 

However, owing to the lack of j8-hydrogen in the intermediate metallacycle, 1,1-disubstituted ethylenes (2-methylpropene, methylenecyclohexane) do not undergo )S-hydrogen transfer instead, their methylene group is catalytically exchanged by a degenerated olefin metathesis reaction ... 

Cross-metathesis reactions with lower olefins are discussed in the earlier sections. The reaction of hex-l-ene with hept-l-ene-l,l- /2 (Mocella 1976a) and with oct-1-ene-l,l-cf2 (McGinnis 1976) results both in exchange of methylene groups and in productive metathesis reaction (8). The non-productive reaction always far outstrips the productive reaction k/lc 3> 1), to an extent that depends on the catalyst (see Table 9.2). 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

Exchange of methylene groups (degenerate metathesis) has been studied using methylenecyclohexane and CH2=C(CH3)2 with [iri -C5H5)Ti(CH2)AlCl(CH3)2]... 

Reaction mechanism As discussed in Sect. 2.3.1, [ (ArNCMe)2CH Ca(p-H) (THF)]2 undergoes insertion reactions with 1,1-diphenylacetylene and myrcene (7-methyl-3-methylene-l,6-octadiene), a terpene that contains three double bonds [121]. Experimental evidence for the o-bond metathesis step was provided by not only the reaction of the calcium complexes isolated from these latter reactions with H2 but also an H/D exchange reaction. Treatment of the calcium deuteride [ (ArNCMe)2CH Ca(p-D)(THF)]2 with 1 bar H2 at 20 °C led to complete H/D exchange after 20 min at room temperature as evidenced by NMR spectroscopy. The reaction of H2 with Ca-C bonds required slightly more forcing reactions conditions and, consistent with those reported for the catalytic reaction, could be achieved at 20 bar H2 at 20 °C in either benzene or THF solution [130]. 

The isolated metaUacydobutane worrld also undergo exchange with an olefin and provided a model for metathesis and a dean system for systematic mechanistic study. The Tebbe reagent also readed with carbonyl compormds such as ketones and esters to exchange an oxygen from the organic for the methylene of the complex. This reaction has become an often rrsed transformation for organic synthesis. ... 

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