Ketones resonance forms

In the Wittig reaction, a phosphorus ylide, R2C—P(C6H03, also called a phosphoreme and sometimes written in the resonance form R2C=P(C6H5)3, adds to an aldehyde or ketone to yield a dipolar intermediate called a betaine. (An ylide—pronounced ill-id—is a neutral, dipolar compound with adjacent plus and minus charges. A betaine—pronounced bay-ta-een—is a neutral, dipolar compound with nonadjacent charges.)... 

Another conceptually unique approach in alkene aziridination has come from Johnston s labs. These workers shrewdly identified organic azides as nitrene equivalents when these compounds are in the amide anion/diazonium resonance form. Thus, when a range of azides were treated with triflic acid and methyl vinyl ketone at 0 °C, the corresponding aziridines were obtained, in synthetically useful yields. In the absence of the Bronsted acid catalyst, cycloaddition is observed, producing triazolines. The method may also be adapted, through the use of unsaturated imi-des as substrates, to give anti-aminooxazolidinones (Scheme 4.25) [32]. 

Steric effects on both the amide and the acyloxyl side chain are similar. Tert-butyl and adamantyl groups on the amide side chain in 29v, 29x, 29c, and 29e (Table 2 entries 53 and 54, 63 and 65) result in lower stretch frequencies that, on average, are only 40 cm-1 higher than their precurser hydroxamic esters. Streck and coworkers have suggested that such changes in dialkyl ketones can be ascribed to destabilisation of resonance form II through steric hindrance to solvation which, in the case of tert-butyl counteracts the inductive stabilisation.127... 

Branched iV-chlorohydroxamic esters exhibit much lower carbonyl frequencies in their IR spectra. Series of Ai-(phenylethyloxy)amides (Table 2, entries 1-7) and Af-butoxy-amides (Table 2, entries 12-16) show a clear movement to lower carbonyl stretch frequencies with branching alpha to the carbonyl, in accord with greater inductive stabilization of the polar resonance form III of the carbonyl (Figure la). Neopentyl (entry 17) is a special case. While the group should contribute much more inductive stabilization than ethyl, its carbonyl stretch frequency is higher. Similar changes have been noted in the IR spectra of branched ketones and have been ascribed to a degree of steric hindrance to solvation and therefore destabilization of the polar resonance form Dl". ... 

It is also generally true that the greater the number of contributing resonance forms, the greater will be the resonance stabilization. For this reason die enolate of a yS -diketone has much more resonance stabilization than die enolate of a simple ketone (three resonance forms versus two). The electrons are delocalized over five atoms in the former versus three atoms in the latter. In addition, the electron density on the carbon atom is less in the diketone enolate than in a simple methyl ketone enolate. 

Enamines are susceptible to acid-catalyzed hydrolysis (last step of the Stork enamine reaction) (96). Under acidic conditions, examines protonate to form the tautomeric iminium ion, which undergoes hydrolysis to the ketone as shown in Figure 57. The iminium ion undergoes hydrolysis quite readily since there is a contributing resonance form with a positive charge on the carbon (97). 

NA-Dimethylhydrazone 68, furnished from keto-acid 33 upon treatment with WV-dimethylhydrazine, was found to be extremely water sensitive. Attempts to form the hydrazone were thwarted by low yields under a number of conditions in which solvents were present. Azeotropic removal of water, with or without molecular sieves, was also unsatisfactory. Eventually, it was found most convenient to simply dissolve the keto-acid in neat dimethylhydrazine without desiccant. After heating for a number of hours, followed by cooling and removal of excess dimethylhydrazine, formation of the desired hydrazone was apparent by NMR due to loss of the methyl ketone resonance at 5 2.14. This initially formed hydrazone existed as a dimethylhydrazonium carboxylate, but it was found that reversion to free carboxylic acid 68 occurred in vacuo, as evidenced by the proton NMR run in dry CDClj. 

Substituents with a electron-donating inductive (+1) effect (i.e., alkyl groups) stabilize the C=0 double bond of aldehydes and ketones. They increase the importance of the zwitterionic resonance form by which carbonyl compounds are partly described. The driving force for the formation of addition products from carbonyl compounds therefore decreases in the order H—CH(=Q) > R—CH(=0) > R R2c(=0). 

Electron-withdrawing substituents at the carbonyl carbon destabilize the zwitterionic resonance form of aldehydes and ketones. Thus, they deprive these compounds of the resonance stabilization, which the alkyl substituents usually present would give them. Therefore, addition reactions to acceptor-substituted C=0 double bonds have an increased driving force. 

Be this as it may, lithium attempts to bind to several bonding partners the structural consequences for the enolates of a ketone, an ester, and an amide are shown in Figure 13.2 In contrast to the usual notation, these enolates are not monomers at all The heteroatom that carries the negative charge in the enolate resonance form is an excellent bonding partner such that several of these heteroatoms are connected to every lithium atom. Lithium enolates often result in tetramers if they are crystallized in the absence of other lithium salts and in the absence of other suitable neutral donors. The lithium enolate of fert-butyl methyl ketone, for example, crystallizes from THF in the form shown in Figure 13.3. 

The first resonance form is more important because it involves more bonds and less charge separation. The contribution of the second structure is evidenced by the large dipole moments of the ketones and aldehydes shown here. 

The title complex represents the parent compound of several known metalla-/3-iminoketone molecules.2 In each case a metalla-/3-diketone molecule is treated with a primary amine, affording the corresponding metalla-/3-imino-ketone derivative. The structure of the cis-tetracarbonylrhenium metalla analogue of N-phenylacetimidoylacetone has been determined.3 The structure of this complex confirms the formulation of these complexes as metalla analogues of the ketamine tautomer of /3-iminoketone molecules. However, the electronic structure is described best by the zwitterionic resonance form shown below. 

When esters are protonated at the carbonyl group, there are three resonance forms two corresponding to the ones that form with aldehydes and ketones and a third with positive charge on the alkylated oxygen. 

In that same year, a rather revolutionary publication appeared [35] on The Scope of the Reaction Between Carhanions or Nitron ions and Unsaturated Electron Acceptors. Many carbanions and nitranions seemed to react with unsaturated molecules, such as nitroaromatics, azobenzene, and diaryl ketones, to form the radical anions derived from the unsaturated compounds. The same effect was observed with n-butylmagnesium bromide and n-butyllithium. The radical anions were observed with the aid of electron spin resonance... 

The reaction of diazomethane derivatives with aldehydes and ketones, though a fairly well-studied field now, continues to be a source of practical extensiori/homologation technologyThe general reaction is outlined in Scheme 4. The diazoalkane, shown in resonance forms (11a) and (11b), reacts with... 

Balanced Yes. Medium Mildly acidic. A common acetate buffer is created from almost equal concentrations of acetic acid and sodium acetate. Sources The amine lone pair and the carbonyl lone pair. Sinks The ketone is a C=Y. Acidic Hs None within range of pH 5. Basic sites An oxygen and/or nitrogen lone pair could become protonated. Leaving groups None. Resonance forms The carbonyl charge separated form, —0 , shows the carbonyl carbon to be a good sink. 

The first step is in basic medium ethoxide has a pATabH of 16. Sources The ethoxide anion. Leaving groups Only ethoxide on the ester. Sinks The ketone is a polarized multiple bond, and the ester is a polarized multiple bond with a leaving group. Acidic Hs The CH2 between the two carbonyls the most acidic, p/fa 10.7 the methyl on the carbonyl is the next most acidic, pA a 19. Resonance forms help us understand the polarization of our reactants. 

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