Dicarbenes, isomerization

Di-fcrt-butylphenol, liquid-phase oxidation, over Cu"+-TSM, 39 322-324 Dicarbenes, isomerization, 30 56-57 Dicarbynes, 30 80-81... 

The isomeric stilbene dicarbenes [20] were generated in 2-MTHF matrices in an epr cavity at 16 K (Murata et ai, 1987 Iwamura, 1988). The spectra obtained by the photolysis of the diazo-compounds [20a], precursors to m,p -[20] and w,m -[20], at 16 K exhibited conspicuous signals at ca. 250 mT, characteristic of quintet species. The signals of /w,w -[20] showed a dramatic temperature dependence (Fig. 16). First, their intensity increased as the temperature was raised, reaching a maximum at SO K, and then decreased somewhat and eventually irreversibly at above 65 K. In contrast, the intensity of the strong signal at ca. 250 mT and some weak signals due to m,p -[20] decreased linearly with the reciprocal of the temperature as dictated... 

Fig. 23 The observed order of states for the isomeric [2.2]paracyclophane-dicarbenes [38], in good agreement with the McConnell theory.

In contrast, comparable rates were determined over platinum of low dispersion suggesting that isomerization occurs without alkene formation.161 The carbene-alkyl species (21) formed with the involvement of terminal carbon atoms is a probable surface intermediate in this selective mechanism. Highly dispersed platinum catalysts are active in nonselective isomerization in which the precursor species is the 22 dicarbene allowing ring closure between methyl and methylene groups. On iridium a pure selective mechanism is operative,162 which requires a dicarbyne surface species (23). 

Replacement of the methylene group in cyclopropyne by oxygen leads to changes in electronic structure similar to those already noted for aziridinediylidene (72). The lowest energy electronic configuration has only two 7t-electrons and corresponds to the dicarbene oxiranediylidene (16) rather than the triply bonded species (which would have four sr-electrons). There is then transfer of t-electrons from oxygen to the formally vacant 2pn orbitals on the carbene centers. In the STO-3G structure, C2 symmetry was assumed and this jt-electron donation (and associated partial double bond character) is reflected in short C—O bonds (1.361 A). Oxiranediylidene is isomeric with ketenylidene ( C=C=0) and is found to be 51.4 kcal/mol less stable than the latter (4-31G). >... 

An intermolecular mechanism involving a primary ligand photodissociation step has been proposed for the cis trans isomerization of the dicarbene complexes, M(C0) L2, (M = Cr,Mo,... 

FIGURE 8. Proposed mechanism for the thermally reversible photo-induced cis - trans isomerization of some dicarbene complexes (98,99). 

Having characterized the three hydrogenolysis mechanisms by their precursor species dicarbenes (Scheme 34), 7c-adsorbed olefins (Scheme-36), and metallocyclobutanes (Schemes 38 and 39), the knowledge of the overall mechanism of cyclic type isomerization requires the identification of the precursor species in 1-5 dehydrocyclization, the reverse reaction of hydrogenolysis of cyclopentanes. 

On platinum, the a, -dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section III, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a p position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section III, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in... 

While the first process is likely in the case of iridium, nickel, and cobalt, it should not be so easy on platinum, because of its competition with carbene-olefin isomerization (see Section III, Scheme 29). We believe that the only way of explaining why 1,2-dicarbenes may account for the hydrogenolysis of cyclic hydrocarbons (Scheme 34), but only for a minor part for the hydrocracking of acyclic hydrocarbons, is the competition, for the latter, between carbene-dicarbene formation and carbene-olefin isomerization. Carbene-olefin interconversions are unlikely in the case of cyclic hydrocarbons, since a dicarbene species cannot transform into a 1,1,2,3-tetraadsorbed species (l-carbene-2,3-olefin) and further into a 1,1,3-triadsorbed species without C-C rupturing. 

The formation of wetn-labeled toluene can be explained neither by direct 1-6 ring closure, nor by cyclic-type isomerization of n-heptane to 3-methylhexane followed by 1-6 ring closure of the latter (94). We suggest that the abnormal aromatization process responsible for the formation of meta-labeled toluene is initiated by a dicarbene as in the nonselective mechanism A (see Section IV, Scheme 47). Aromatization is not influenced by the dispersion of the platinum on the support (758), so that it may be assumed that aromatization involves a single metal atom. Isomerization of the dicarbenes (7) to the dicarbenes (8) via rt-adsorbed cyclopentanes, followed by isomerization to the suitable carbene-olefin species (9), would result in 1-6 ring closure and aromatization (Scheme 69). 

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