C=O double bond

This mode corresponds to the IR peak associated with carbonyl stretch, used to identify the C-O double bond. Its predicted frequency is about 1810 (after scaling). This is in reasonable agreement with the experimental value of 1746. Using a larger basis set will improve this value. We ll discuss basis set effects in the next chapter. 

In this reaction, there are twelve single bonds and one (C-O) double bond in both the reactants and products. Because of this conservation of the total number and types of bonds, very good results can be obtained relatively inexpensively for isodesmic reactions due to the cancellation of errors on the two sides of the reaction. In other words, comparing very similar systems enables us to take maximum advantage of cancellation of error. 

Like carbocations, carbanions can also react in ways in which they are converted to species that are still not neutral molecules. They can add to double bonds (usually C—O double bonds see Chapters 10 and 16),... 

Diphenylsulfonium cyciopropanide undergoes addition to the C — C double bond of oc,/ -unsat-urated carbonyl compounds to produce spiropentanes,57,58 and to the C —O double bond of aldehydes and ketones to produce oxaspiropentanes. These can be isolated59,60 or rearranged in situ 57,61,62 to produce cyclobutanones. Dimethylaminocyclopropylphenyloxosulfonium cyciopropanide reacts analogously.63,64... 

If our considerations concerning the C2H4 biradical are correct then by microscopic reversibility we should expect that in the similar reaction pictured here the product state is a CH20 biradical. We can estimate that the energy of the C—O double bond in ketones and aldehydes corresponds to about 73 keal. so that the color of the emitted light does indeed correspond to this hypothetical excitation energy. Note that this is not an allowed transition for CH20.f... 

Because carbon stands at the head of its group, we expect it to be different from the other members of the group. In fact, the differences are more pronounced in Group 14 than anywhere else in the periodic table. Some of the differences between carbon and silicon stem from the smaller atomic radius of carbon, which explains the wide occurrence of C=C and C O double bonds, compared with the rarity of Si=Si and Si=0 double bonds. Silicon atoms are too large for the side-to-side overlap of p-orbitals necessary for -ir-bonds to form between them. Carbon dioxide, which consists of discrete 0=C=0 molecules, is a gas that we breathe. 

Intramolecular azomethine ylide cycloaddition to the C—O double bond of an aldehyde was reported in 197369 and cycloaddition to the C—C double bond was first reported in 1975.70 Competition between 1,1- and 1,3-cycloaddition is observed in intramolecular reactions, although intermolecular reactions give only 1,3-cycloaddition. Photolysis of 2//-azirines is one generation method of nitrile ylides applicable to intramolecular cycloaddition.70 Another method involves the base-catalyzed 1,3-elimination of hydrogen halide from alkenyl imidoyl halides. Still other procedures involve thermolytic and photolytic cycloreversions of oxazolinones and dihydrooxazaphospholes. 

Within the last decade remarkable progress has been made with highly stereoselective addition reactions to C—C and C — O double bonds using chiral reagents. These reagents include ... 

The Mukaryama reagent,18 N-methyl-2-chloropyridinium iodide (6). transforms carboxylic acid 5 into the amide 7 The acid Is first activated in situ in the form of pyridinium salt 17 by an SN reaction with the Mukoixama reagent (6). This activation is a result of preventing resonance stabilization of the C-O double bond in the positively charged aryl ester 17. 

In sum, a new olefinic link is produced, but by an addition-elimination sequence. In this reaction a stronger C-O double bond in the starting material is replaced by a weaker C-C double bond in the product. The thermodynamic driving force for die reaction is the formation of the P-O bond, which is very strong. 

Electronic effects of groups containing C—C or C—O double bonds... 

Active catalyst species or catalysis intermediates can often be trapped by stoichiometric reactions of the precatalyst with the substrate. The following example describes the successful isolation of such an intermediate with participation of Ln-O cr-bonds. Reduction processes mediated by low oxidation states of the lanthanide elements are of special interest in organic synthesis [256]. One of the most intensively studied reactions is the stoichiometric reduction of arylketones by rare earth metals ytterbium and samarium [277]. Thus formed dianions possess high nucleophilic character and excess lanthanide metal can even accomplish complete cleavage of the C-O double bond (Scheme 36). 

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