Metal-carbene-olefin complexes decomposition

When Y in Scheme 7.21B is hydrogen, the reverse process yielding a metal carbene-hydride complex is c -hydrogen elimination process. It provides an important route leading to decomposition of metal alkyls beside the more often encountered -hydrogen elimination pathway giving metal hydride coordinated with an olefin. 

The metal-catalysed olefin metathesis (equation 122) when applied to dienes results in ring-closure and expulsion of an olefin (equation 123). Thus the molybdenum carbene complex 241 promotes the decomposition of the 1,6-heptadiene derivative 242 to a mixture of the cyclopentene 243 and ethylene (equation 124)122. An analogous reaction of the alcohol 244 gives 245 (equation 125), and 4-benzyloxy-l,7-decadiene (246) affords the cyclohexene 247 and 1-butene (equation 126). These transformations, which occur in benzene at room temperature, proceed in excellent yields122. 

Carbenes can be stabilized as transition metal complexes decomposition of phenyldiazomethane in the presence of a ruthenium(II) complex gives a carbene complex stable enough to be isolated and stored for months. These complexes are among the most important of carbene-derived reagents because of a remarkable reaction known as alkene (or more commonly olefin) metathesis. 

New evidence as to the nature of the intermediates in catalytic diazoalkane decomposition comes from a comparison of olefin cyclopropanation with the electrophilic metal carbene complex (CO)jW—CHPh on one hand and Rh COAc) / NjCHCOOEt or Rh2(OAc)4 /NjCHPh on the other . For the same set of monosubstituted alkenes, a linear log-log relationship between the relative reactivities for the stoichiometric reaction with (CO)5W=CHPh and the catalytic reaction with RhjfOAc) was found (reactivity difference of 2.2 10 in the former case and 14 in the latter). No such correlation holds for di- and trisubstituted olefins, which has been attributed to steric and/or electronic differences in olefin interaction with the reactive electrophile . A linear relationship was also found between the relative reactivities of (CO)jW=CHPh and Rh2(OAc) NjCHPh. These results lead to the conclusion that the intermediates in the Rh(II)-catalyzed reaction are very similar to stable electrophilic carbenes in terms of electron demand. As far as cisjtrans stereoselectivity of cyclopropanation is concerned, no obvious relationship between Rh2(OAc) /N2CHCOOEt and Rh2(OAc),/N2CHPh was found, but the log-log plot displays an excellent linear relationship between (CO)jW=CHPh and Rh2(OAc) / N2CHPh, including mono-, 1,1-di-, 1,2-di- and trisubstituted alkenes In the phenyl-carbene transfer reactions, cis- syn-) cyclopropanes are formed preferentially, whereas trans- anti-) cyclopropanes dominate when the diazoester is involved. 

Metal-carbene complexes react with Wittig reagents to give olefins (22). The initial C—C bond formation is a soft-soft interaction subsequent decomposition of the intermediates is also highly favored as the metal complexes generated are symbiotically stabilized. 

Inhibition of diazoester decomposition by a large excess of olefin speaks in favor of intermediarily liberated W(CO)5 as direct metal precursor of425. Stereoselectivities in the cyclopropanation reaction are very similar to those observed in the Rh2(OAc)4 catalyzed version, which underlines once more the close relationship of tungsten and rhodium carbene complexes. 

The fact that Schrock s proposed metallocyclobutanes decomposed to propylene derivatives rather than cyclopropanes was fortunate in that further information resulted regarding the stereochemistry of the olefin reaction with the carbene carbon, as now the /3-carbon from the metal-locycle precursor retained its identity. The reaction course was consistent with nucleophilic attack of the carbene carbon on the complexed olefin, despite potential steric hindrance from the bulky carbene. Decomposition via pathways f-h in Eq. (26) was clearly confirmed in studies utilizing deuterated olefins (67). 

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