Ethylidene species

The lower trace in Figure 1 shows the results of heating the tunnel junctions (complete with a lead top electrode) in a high pressure cell with hydrogen. It is seen that the CO reacts with the hydrogen to produce hydrocarbons on the rhodium particles. Studies with isotopes and comparison of mode positions with model compounds identify the dominant hydrocarbon as an ethylidene species (12). The importance of this observation is obviously not that CO and hydrogen react on rhodium to produce hydrocarbons, but that they will do so in a tunneling junction in a way so that the reaction can be observed. The hydrocarbon is seen as it forms from the chemisorbed monolayer of CO (verified by isotopes). As... 

Analysis of the tunneling spectra of the hydrocarbons formed by exposing the samples to H2 ( Fig. 14 ) showed two different species, one a formate like ion, the other an ethylidene ( CHCH3 ) species. The formate ion is not thought to be an active intermediate in hydrocarbon synthesis, but the ethylidene species may well be a catalytic intermediate. Kroeker, Kaska and Hansma were then able to suggest a reaction pathway for the hydrogenation of CO on a supported rhodium catalyst consistent with the formation of ethylidene as a catalytic intermediate. 

The resulting dialkylmetal(IV) derivatives are thought to be diamagnetic. The diethylruthenium complex Ru(C2Hs)2(P) was thermally labile at room temperature, and a mixture of the ethylidene species Ru(CHCH3) (P) and the Ru(III) compound RuC2H5(P) was formed via independent radical processes [307] (Eq. 23). 

Fig. 9. Adsorption of ethyl species (a), di-cr-bonded ethylene (b), ethylidene species (c), ethylidyne species (d), vinyl species on step edge (e), vinyl species on the special site (f), di-crht vinylidene species (g), and /u,2-vinylidene species on Pt(211) (h). Adapted from (53).

The solid line is the theoretical prediction for the ethylidene species C-CH, with the methyl group rotating about the C-C axis, the dashed line is that of frozen C-CH, and the dotted line is that of CH-CH, with rotating methyl group. 

In contrast, it is straightforward to rationalize the observed stereochemical retention if degenerate propylene metathesis occurs via a metal-ethylidene species (Scheme 10.17). In this case, the intermediate metallacycles are a,a disubstituted. Although the P-position is unsubstituted, the experiments with 2-butene (Scheme 10.13) have established that a,a -substituents prefer to be oriented in a cis-configuration. This preference explains why there is any stereoselectivity in this degenerate process and also correctly predicts the stereochemistry of the propylene-tfs that was generated. 

No products resulting from C—H insertion of the carbene were found. Also, ethylidene failed to undergo addition to 2-butene, indicating this species to be considerably more selective than methylene.<23)... 

D) and also to a species which can undergo insertion to increase the chain length. We suggest that this species is the ethylidene... 

Corydalis is a genus of the family Fumariaceae that is represented by some 320 species growing in the temperate climates of the northern hemisphere. Corydalis pallida Pers. var. tenuis from Japan was simultaneously examined for alkaloids by two groups of investigators (88, 89). Besides benzylisoquinoline-type alkaloids, rra/w-3-ethylidene-2-pyrrolidone (50) (alkaloid P) was present. The alternative formula 51 was ruled out by the nonidentity of the hydrogenation product of 50 with an authentic sample of 5-ethyl-2-pyrrolidone (52). Alkaloid... 

Figure 8. Differential tunneling spectrum of CO on rhodium/alumina heated to k20° K in hydrogen. Modes due to hydrocarbon are number 1 to 7 The hydrocarbon species is identified as an ethylidene moiety.

If cyclopentene would react pair-wise with 2-pentene, only one product would form, namely 2,7-decadiene, and a similar result for cyclodimers etc. of cyclopentene. If somehow, the alkylidene species would be transferred one by one, we would obtain a mixture of 2,7-nonadiene, 2,7-decadiene, and 2,7-undecadiene in a 1 2 1 ratio. The latter turned out to be the case, which led the authors to propose the participation of metal-carbene (metal alkylidene) intermediates [6], Via these intermediates the alkylidene parts of the alkenes are transferred one by one to an alkene. The mechanism is depicted in Figure 16.4. In the first step the reaction of two alkylidene precursors (ethylidene -bottom- and propylidene -top) with cyclopentene is shown. In the second step the orientation of the next 2-pentene determines whether nonadiene, decadiene or undecadiene is formed. It is clear that this leads to a statistical mixture, all rates being exactly equal, which need not be the case. Sometimes the results are indeed not the statistical mixture as some combinations of metal carbene complex and reacting alkene may be preferred, but it is still believed that a metal-carbene mechanism is involved. Deuterium labelling of alkenes by Gmbbs instead of differently substituted alkenes led to the same result as the experiments with the use of 2-pentene [7],... 

It is the di-cr species on Pt(lll) which is converted at higher temperature into ethylidyne. This conversion had also been investigated by infrared-visible sum-frequency generation (SFG) by Cremer et al. (371), a welcome first application of this new spectroscopic technique to hydrocarbon adsorption chemistry. They observed an absorption characteristic of an intermediate with a nCH3 band at 2957 cm-1 and suggested that this arises from an ethylidene M2CHCH3 (or possibly ethyl) species with its C-CH3 axis at an angle to the surface. It is very clear experimentally, as discussed in Part I, that ethylidyne on Pt(lll) is formed from di-cr-ethene and not directly from its 77-bonded isomer. 

Pt(lll) system by STM (393, 394) are responsible for the stability of these intermediates. Recent SFG spectra (359, 371, 372) indicate that one such intermediate could be ethylidene, CHCH3, although some authors have preferred a CHCH2 species bonded to the surface through each carbon atom (386). It is not clear how the latter can be described as di-cr tri-cr would seem to be more appropriate. This and the ethylidene intermediates could occur in sequence (Section X.D). 

Di-cr-ethene has also been separately detected by spectroscopy but found to play a different mechanistic role related to its stronger bonding to the metal surface. This, not the 77-species, has been shown to be the precursor in the formation of ethylidyne by surface dehydrogenation on Pt(lll) and probably on other facets with triangular metal sites. Ethylidene is now the favored intermediate in this reaction (359,371,372,427), which can formally be represented as... 

This is clearly shown in Figure 7. Although the chemisorption of ethylene on Pt(lll) has been studied by numerous techniques (93, 99, 100, 101, 102, 103), there is still debate over the precise geometry of the stable surface species. Proposed structures include ethylidyne ( C-CH3) (100, 101), ethylidene (>CH-CH3) (93, 99), and a vinyl species (>CH-CH2 ) (102, 103). 

Figure 14. Tunneling spectra of a sample with finely dispersed Rh particles on alumina, exposed to a saturation coverage of CO, and heated to various temperatures in a high pressure atmosphere of Ht. The CO is hydrogenated on the surface. Analysis of the resultant spectra using isotopic substitution indicates that an intermediate species, ethylidene di-rhodium, is formed (1).

The isomerization of 5-vinyl-2-norbornene to 5-ethylidene-2-norbornene has been performed using a catalytic system consisting of an alkali metal hydride and an amine. The activity of the alkali metal hydride increased with increasing size of the alkali metal KH > NaH > LiH. Among the various amines tested, only aliphatic 1,2-diamines exhibited the activity for the isomerization. Electron paramagnetic resonance (EPR) and UV-visible spectroscopic experiments on the active species suggest that the isomerization of 5-vinyl-2-norbornene proceeds through a radical mechanism.167... 

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