Carbenes alkene

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

The non-nitrogenous carbene precursor (102) was used for the photochemical generation of the carbene (103) without complications due to reactions of diazirine or diazo species. In the presence of alkenes, carbene (103) gave rise to cyclopropanes and in the absence of alkenes was proposed to undergo [1,2]-C shift to form (104), which suffered retro-Diels-Alder reaction to give a triene. 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

Alkene Carbene complex or precursor Products Yield (%)... 

Alkene Carbene Source Additive Solvent (cisitrans) ... 

We think that this mechanism is still open for further discussion. The laser-flash investigations involving spectra of transient intermediates (e. g., Grasse et al., 1983) clearly demonstrate the experimental difficulty in the detection of an additional species. Therefore, the fact that no alkene-carbene complex was observed is not safe evidence against such a hypothesis (but, of course, neither for its existence ). 

A kinetic study has been made of the liquid-phase metathesis of oct-l-ene over Re207/Al203 in a flow reactor Fig. 5.8 shows the fractional conversion X at four temperatures as a function of the contact time expressed in terms of W/F as defined in the caption. The data are best interpreted in terms of a model in which either product desorption or interconversion of the alkene/carbene complex is rate-determining. This model leads to eqn. (11), where r is the reaction rate per unit weight of catalyst, and from which the curves in Fig. 5.8 have been calculated to give the best fit to the data (Spronk 1992). 

The cyclopropanation of a,(3-unsaturated esters requires elevated temperatures (> 90°C) allowing a rapid CO exchange in the carbonyl(carbene) complex. Thus, a reasonable mechanism involves a decarbonylation preequilibrium followed by the coordination of the alkene to the coordinatively unsaturated tetracarbonyl intermediate 26. The resulting alkene(carbene) complex 27 may rearrange to a metalacyclobutane 28 which undergoes reductive elimination to give the cyclopropane 29 (Scheme 15). [31]... 

The addition of an aikene to a carbene complex may lead to a metathesis reaction with exchange of the carbene ligand with one of the two halves of the aikene. The reaction proceeds via formation of an alkene-carbene complex, which rearranges via a metallacyclo-butane intermediate. ) This reaction is synthetically useful when a terminal aikene is used and vacuum evaporation of the more volatile aikene product (typically 3,3-dimethyl-but-l-ene) is possible to displace the equilibrium. An excess of the aikene reagent is also used to ensure a favorable equilibrium position (see Scheme 5). The properties of the an-... 

Novel ferracyclic ( 7 -alkene)carbene complexes, for example, 12 have been prepared in good yield (see Scheme 11) and characterized by analytical and spectroscopic methods. The structure of 12 has been determined by X-ray crystallography. " ... 

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