Carbon-oxygen double bonds formation

Alkoxycarbenium ions are important reactive intermediates in modem organic synthesis.28 It should be noted that other names such as oxonium ions, oxocarbenium ions, and carboxonium ions have also been used for carbocations stabilized by an adjacent oxygen atom and that we often draw structures having a carbon-oxygen double bond for this type of cations.2 Alkoxycarbenium ions are often generated from the corresponding acetals by treatment with Lewis acids in the presence of carbon nucleophiles. This type of reaction serves as efficient methods for carbon-carbon bond formation. 

Ring-opening polymerizations are generally initiated by the same types of ionic initiators previously described for the cationic and anionic polymerizations of monomers with carbon-carbon and carbon-oxygen double bonds (Chap. 5). Most cationic ring-opening polymerizations involve the formation and propagation of oxonium ion centers. Reaction... 

Claisen rearrangements also occur with nonaromatic substrates. In these reactions the formation of the strong carbon-oxygen double bond drives the equilibrium to completion ... 

In Chapter 6, elimination reactions were presented. In the context of elimination reactions, the formation of double bonds was noted regardless of the elimination mechanism discussed. Continuing from the concept of using elimination reactions to form sites of unsaturation, one may reason that addition reactions can be used to remove sites of unsaturation. Thus, elaborating upon addition reactions, this chapter provides an introduction to relevant mechanisms applied to both carbon-carbon double bonds (olefins) and carbon-oxygen double bonds (carbonyls). 

Although the addition of t-BuMgCl to benzophenone had been reported [43] to yield 63% of the expected tertiary alcohol, 1,1-diphenyl-2,2-dimethyl-1-propanol, diphenyl-t-butylcarbinol (no reduction products, such as benzhydrol and benzopinacol, were found then), it was shown that besides the normal addition to the carbon-oxygen double bond, 1,2-addition and also 1,6-addition had taken place, leading to the formation of an alkyldihydrobenzophenone [44] (Scheme 9) ... 

The difficult step in all this is breaking the carbon-hydrogen bond this is made possible by the synchronous departure of HCr03 , in what is really an E2 elimination—but here with the formation of a carbon-oxygen double bond. 

The ElcB mechanism is rare in practice when the elimination reaction would result in a carbon/carbon double bond. When a carbon/oxygen double bond is to be formed then it is far more common. For example, the ElcB mechanism is found in the reverse of the cyanohydrin formation reaction. You will recall that the forward reaction involves the addition of a cyanide anion to a carbonyl group. Write down the pathway for the reverse reaction, i.e. the elimination reaction. 

The infrared experiments revealed to us the formation of a new species, which was apparently an amide zincate enolate, the first observation of its kind. The structure of this enolate was not clear from the data, however. The absorbances at 1536 cm and 1559 cm appeared too low to assign to either an oxygen-metallated enolate carbon-carbon double bond structure as in 9, or to a carbon-metallated enolate with a free amide carbon-oxygen double-bond stretch as in 10 (Figure... 

AUqrl formates are the hydroformylation products of the carbon-oxygen double bond of aldehydes. For more details, see the section Modeling the Formate Ester Formation... 

Why do the nucleophiles listed in Table 19.1 react with conjugated carbonyl compounds by 1,4-addition rather than 1,2-addition The answer has to do with kinetic control versus thermodynamic control of product formation. It has been shown that 1,2-addition of nucleophiles to the carbonyl carbon of a, 8-unsaturated carbonyl compounds is faster than conjugate addition. If formation of the 1,2-addition product is irreversible, then that is the product observed. If, however, formation of the 1,2-addition product is reversible, then an equilibrium is established between the more rapidly formed but less stable 1,2-addition product and the more slowly formed but more stable 1,4-addition product. As mentioned at the beginning of the chapter, a carbon-oxygen double bond is stronger than a carbon-carbon double bond. Thus, under conditions of thermodynamic (equilibrium) control, the more stable 1,4-Michael addition product is formed. 

PROBLEM 11.28 Suggest a mechanism for the formation of bromine from NBS and hydrogen bromide. Hint. The oxygen of a carbon-oxygen double bond is a base. 

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