Aldehydes resonance forms

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

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.)... 

The proposed catalytic cycle is shown in Scheme 35 and begins with the imida-zolylidene carbene adding to the enal. Proton transfer provides acyl anion equivalent XLVII, which may be drawn as its homoenolate resonance form XLVIII. Addition of the homoenolate to aldehyde followed by tautomerization affords L the precursor for lactonization and regeneration of the carbene. 

The resonance Raman spectrum of a similar complex has been reported, that of a ternary complex of LADH, NADH and 4-(7V 7V-dimethylamino)benzaldehyde (DABA) with the disappearance of the carbonyl stretching frequency of the DABA at 1664 cm-1 also indicating strongly that inner sphere complexation of the substrate occurs, the zinc withdrawing electron density from the aldehyde oxygen forming a zinc-oxygen coordinate bond.1397... 

When the hydrogen ion is considered to be attached to the rightmost end of the Rhodonine ion, Rhodonine will appear to be an alcohol and respond to simple chemical tests almost identically to Retinol. When the hydrogen ion is considered to be attached to C5, Rhodonine will appear to be an aldehyde and respond to simple chemical tests almost identically to retinaldehyde. The chemical ambiguity in the properties of the Rhodonines due to this resonance condition is the principle practical reason for the confusion in the literature concerning the chemical properties of the chromophores of vision. The principal conceptual reason is the prior failure of the community to recognize the existence of the resonant form of the retinoids, the Rhodonines. 

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). 

Alkenyl and aryl substituents stabilize the C=0 double bond of carbonyl compounds even more than alkyl substituents. This is due to their pi electron-donating (+M) effect, which allows one to formulate additional zwitterionic resonance forms for carbonyl compounds of this type. Thus, no hydrates, hemiacetals, oligomers, or polymers can be derived from unsaturated or aromatic aldehydes. 

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. 

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. 

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. 

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... 

The possibility of mercury-photosensitized reactions was first predicted in 1922 by Franck (45) and experimentally verified in the sensitized decomposition of H2 by Cario and Franck (46). They found that free H atoms were produced when a mixture of Hg vapor and H2 was irradiated with the 254 nm Hg resonance line at room temperature. Bates and Taylor (23) studied the Hg-sensitized decomposition of methanol, ethanol, and ethylamine, and showed that with these compounds the rate of the sensitized reaction was faster than the direct photolysis by about two orders of magnitude. Again hydrogen was the major product. Aldehydes were formed from the alcohols. 

The effect of substituents in the benzene ring provides an interesting example of the dual nature of carbanion addition to carbonyl groups. When an electron-releasing group is conjugated with the aldehyde function, a resonance form such as (XIV) apparently lowers the usual positive charge on the carbonyl carbon atom to a point where the addition... 

Generic process An addition and an elimination have occurred. Medium Definitely basic, predominant anion is hydroxide, plsTabH 15.7, whose pA"a would give a useful proton transfer A"eq up to about p Ta 26. Sources The carbonyl lone pair, water lone pair, and hydroxide anion. Best source Hydroxide anion, a lone pair source can behave as a nucleophile or as a base. Sinks Polarized multiple bond, the aldehyde carbonyl. Acidic Hs Water and the CH2 next to the aldehyde, pA a 16.7, are within range of hydroxide. Leaving groups None. Resonance forms ... 

Like the C=C bond, the C=0 bond is electron rich unlike the C=C bond, it is highly polar (AEN = 1.0). Figure 15.14 emphasizes this polarity with an electron density model and a charged resonance form. Aldehydes and ketones are formed by the oxidation of alcohols ... 

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