Platinum carbene species

Scheme S.16 Synthesis of bis-silyl platinum carbene species 55.

Highly reactive platinum carbene species have been generated in the gas phase via the fast atom bombardment ionization of ethyl diazoethanoate complex 136 (Scheme 33). " Elimination of dinitrogen leads to carbene species 137, which are implicated in the reactivity of the precursor complexes, but had not previously been detected in solution. 

There are, however, a few papers describing OM catalyzed by nontransition elements, platinum- and acid-catalyzed enyne metatiiesis, and acid catalyzed olefination of benzaldehyde with an olefin. The reported mechanisms of these transformations do not include participation of carbene species. Because WCle is a Lewis acid itself a noncarbene mechanism of COER cannot be ruled out. 

The oxidative addition of a chloroquinolinone to Pt(PPh3)4 yields carbene species 140 (Equation (20)). " Like 138, a non-carbene form could be postulated, but once again a high solution G chemical shift of 202 ppm for the carbon bonded to platinum confirms that the carbene form more accurately represents the molecular form present. 

In an analogous fashion, oxidative addition to a platinum(O) species gives platinum(ii) species 157 (Equation (27)). Complex 157 is a rare example of an 18-electron platinum(ii) complex, and the platinum-carbene distance is reported as 2.006(6) A. 

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). 

Many of the recent efforts in this area of chemistry have been designed to explore the chemistry of the trifluoromethyl species prepared by these reactions. Among the more interesting results have been the construction of carbene and tetramethylcyclobutadiene ligands affixed to trifluoromethyl platinum cations (78), the synthesis of difluoro-carbene complexes like (Cp)Mo(CO)3CF2+ (79), the formation of (CF3)2CS from CF3Re(CO)5 (38), and the insertion reaction of S02 into CF3-metal bonds (80). 

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. 

On platinum films, gemdisubstituted cyclopentanes initially produce much larger amounts of aromatics than methyl- and ethylcyclopentanes. Relative rates of aromatization are given in Scheme 63. This is readily explained by either methyl stabilization of the n-adsorbed olefin in the carbene-olefin species, or by the greater propensity for a,y,-bonding, that is, metallocyclobutane formation, across a quaternary center. 

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). 

This marked difference between phosphine and carbene platinum dimers might be due to the stronger o-donating ability of NHCs. Indeed, this effect strengthens the Pt—H—Pt bond, hence disfevoring the release of the monomeric species 27 required to initiate the hydrosilylation catalytic cycle (Scheme 5.8). 

Prolonged heating of carbene complex 154 results in a rearrangement, presumably via an orthometallation step to give a platinum(iv) hydride species, in which the hydride migrates onto the carbene carbon to give complex 167 (Equation (31)). " ... 

The Iwasawa group developed a platinum-catalyzed generation of unsaturated carbene complexes 325 by cyclization of methoxyalkyne-substituted aniline derivatives 321 and elimination of methoxide as depicted in 324 (Scheme 19.86) [159]. The carbene complexes 325 are useful for [3 + 2] cycloaddition with enol ethers 322, leading to efficient formation of cyclopentene-fused indoles 323. In this case, the indolylmetal species 326 would act as the nucleophile to promote the second cyclization with the oxonium moiety. 

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