Of Time and Carbon-Metal Bonds

Nuclear Magnetic Resonance Spectra of Organometallic Compounds, 3, 1 Of Time and Carbon-Metal Bonds, 9, 1... 

Rochow, E. G., Of Time and Carbon Metal Bonds, Advan. Organometal. Chem. 9 [1970] 1/18. 

It is noticeable that cross-metathesis with the unfunctionalised alkenes occurred in significantly higher yields over shorter reaction times and required a smaller excess of the soluble alkene. This was possibly due to the unfunctionalised alkenes, which are more nucleophilic than their ester containing counterparts, complementing the less nucleophilic/more carbon-metal bond stabilising allylglycinol 18. Comparable results were obtained from cross-metathesis reactions of the polymer-bound isomeric N-Boc C-allylglycinol with the same four alkenes. 

Due to the poor efficiency of trifluoromethylated organometaUic derivatives as trifluoromethylating reagents, nucleophilic trifluoromethylation has remained unattractive for a long time. Indeed, in the absence of stabilization, the trifluoromethyl anion is very unstable and is quickly transformed into difluorocarbene (cf. Chapter 1). When the carbon-metal bond is relatively covalent, the organometaUic species becomes more stable but it is then less reactive toward an electrophile. On the synthetic level, only zinc and copper derivatives have found real applications in... 

While most of the chemistry discussed in this chapter has been developed in the past decade, several important methods have withstood the test of time and have made important contributions in areas such as natural product synthesis. Methods such as cuprate acylation and the addition of organolithiums to carboxylic acids have continued to enjoy widespread use in organic synthesis, whereas older methods including the reaction of organocadmium reagents with acid halides, once virtually the only method available for acylation, has not seen extensive utilization recently. In the following discussion, we shall be interested in cases where selective monoacylation of nonstabilized carbanion equivalents has been achieved. Especially of concern here are carbanion equivalents or more properly organometallics which possess no source of resonance stabilization other than the covalent carbon-metal bond. Other sources of carbanions that are intrinsically stabilized, such as enolates, will be covered in Chapter 3.6, Volume 2. 

There are two possible pathways to homologate methanol with carbon dioxide the CO2 insertion path and CO insertion path (Scheme 2). As for the former, Fukuoka et al. reported that the cobalt-ruthenium or nickel bimetallic complex catalyzed acetic acid formation from methyl iodide, carbon dioxide and hydrogen, in which carbon dioxide inserted into the carbon-metal bond to form acetate complex [7]. However, the contribution of this path is rather small because no acetic acid or its derivatives are detected in this reaction. Besides, the time course... 

The preferences of the various pathways are dependent on the catalyst used, specifically the electronic and steric factors involved. The electronic contribution is based on the preference of the metallacycle to have the electron-donating alkyl groups at either the a or the carbon of ftie metallacycle [23]. The steric factors involved in the approach of the olefin to the metal carbene also determine the re-giochemistry of the metallacyclobutane formed. These factors include both steric repulsion of the olefin and carbene substituents from each other and from the ancillary ligands of the metal complex. Paths (b), (c), and (e) in Scheme 6.10 are important to productive ADMET. The relative rates of pathways (c) and (e) will determine the kinetic amount of cis and trans double bonds in the polymer chain. Flowever, in some cases a more thermodynamic ratio of cis to trans olefin isomers is attained after long reaction times, presumably by a trans-metathesis olefin equilibration mechanism [31] (Scheme 6.11). 

Hence, the symmetric vinyl monomer should polymerize most actively on contact ion pairs. First, the presence of a counterion should eliminate or at least weaken the effect of the local symmetry principle and at the same time that of the spin exclusion principle. Secondly, on the base of Epiotis results [58], it may be expected that the polar character of the carbon-metal bond should favor cycloaddition. However, this also follows from the formulation of the correspondence principle if the transition to a more polar bond is considered to be a transition to a more asymmetric bond. 

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