Aldehydes resonance

Changing the gas flow to 5 bar of CO/Hj (1/1) resulted in reformation of the hydride complex 18b (spectrum d. Figure 6.12). The H NMR spectrum showed the reappearance of the hydride resonance at -10.6 ppm together with an aldehyde resonance at 9.3 ppm. Repeating this sequential procedure with this NMR sample... 

The relevance of the carbocationic mechanism was also demonstrated for the polymerization of ethyl vinyl ether by end-group analysis following termination with aqueous methanol. An aldehydic resonance (d 9.8) was observed in the H NMR spectrum of the resulting polymer, the aldehyde functionality arising from hydrolysis of the acetal formed via nucleophilic attack by methanol on the active site of the growing polymer (eqs 17 and 18). 

Again, the reaction patterns exhibited the signature of carbocationic processes and again the H NMR spectrum of a poly(ethyl vinyl ether) sample exhibited an aldehyde resonance at d 9.8. 

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. 

An a hydrogen of an aide hyde or a ketone is more acidic than most other protons bound to carbon Aldehydes and ketones are weak acids with pK s in the 16 to 20 range Their enhanced acidity IS due to the electron withdrawing effect of the carbon yl group and the resonance stabi lization of the enolate anion... 

An unusual method for the preparation of syndiotactic polybutadiene was reported by The Goodyear Tire Rubber Co. (43) a preformed cobalt-type catalyst prepared under anhydrous conditions was found to polymerize 1,3-butadiene in an emulsion-type recipe to give syndiotactic polybutadienes of various melting points (120—190°C). These polymers were characterized by infrared spectroscopy and nuclear magnetic resonance (44—46). Both the Ube Industries catalyst mentioned previously and the Goodyear catalyst were further modified to control the molecular weight and melting point of syndio-polybutadiene by the addition of various modifiers such as alcohols, nitriles, aldehydes, ketones, ethers, and cyano compounds. 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

The two methyl groups are not equivalent at 303 K (3 = 2.86 and 3.14), rotation about the CN bond is frozen, because this bond has partial tt character as a result of the mesomerlc (resonance) effects of the dimethylamino group (+Af) and of the aldehyde function (-M), so that there are cis and trans methyl groups. Hence one can regard 3-(A(A -dlmethylamlno)acrolein as a vinylogue of dlmethylformamide and formulate a vlnylogous amide resonance. 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

From resonance structure (12) it is obvious that a —I—M-substit-uent strongly deactivates the 2-position toward electrophilic substitution, and one would thus expect that monosubstitution occurs exclusively in the 5-position. This has also been found to be the case in the chlorination, bromination, and nitration of 3-thiophenecarboxylic acid. Upon chlorination and bromination a second halogen could be introduced in the 2-position, although further nitration of 5-nitro-3-thiopheneearboxylic acid could not be achieved. Similarly, 3-thiophene aldehyde has been nitrated to 5-nitro-3-thiophene aldehyde, and it is further claimed that 5-bromo-3-thiopheneboronic acid is obtained upon bromination of 3-thiopheneboronic acid. ... 

The reaction starts with the nucleophilic addition of a tertiary amine 4 to the alkene 2 bearing an electron-withdrawing group. The zwitterionic intermediate 5 thus formed, has an activated carbon center a to the carbonyl group, as represented by the resonance structure 5a. The activated a-carbon acts as a nucleophilic center in a reaction with the electrophilic carbonyl carbon of the aldehyde or ketone 1 ... 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

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 carbonyl-group carbon atoms of aldehydes and ketones have characteristic 13C NMR resonances in the range 190 to 215 8. Since no other kinds of carbons absorb in this range, the presence of an NMR absorption near 200 8 is clear evidence for a carbonyl group. Saturated aldehyde or ketone carbons usually absorb in the region from 200 to 215 8, while aromatic and a,p-unsaturated carbonyl carbons absorb in the 190 to 200 5 region. 

In addition to fragmentation by the McLafferty rearrangement, aldehydes and ketones also undergo cleavage of the bond between the carbonyl group and the a carbon, a so-called a cleavage. Alpha cleavage yields a neutral radical and a resonance-stabilized acyl cation. 

O Base removes an acidic alpha hydrogen from one aldehyde molecule, yielding a resonance-stabilized enolate ion. 

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