Aldehydes carbonyl reactive

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of iV-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive... 

Electronically, aldehydes are more reactive than ketones because of the greater polarization of aldehyde carbonyl groups. To see this polarity difference, recall the stability order of carbocations (Section 6.9). A primary carbocation is higher in energy and thus more reactive than a secondary carbocation because... 

Although intermediate 2 is terminated at both ends by electrophilic carbonyl groups, the aldehydic function at C-7 is inherently more reactive, and thus more susceptible to a nucleophilic attack, than the methoxycarbonyl group at C-l. As a result, it should be possible to selectively engage the aldehyde carbonyl of intermedi-... 

The reacting aldehyde displaces the oxazolidinone oxygen at the tetravalent boron in the reactive TS. The conformation of the addition TS for boron enolates is believed to have the oxazolidinone ring oriented with opposed dipoles of the ring and the aldehyde carbonyl groups. 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

The carbonyl-reactive group on these crosslinkers is a hydrazide that can form hydrazone bonds with aldehyde residues. To utilize this functional group with carbohydrate-containing molecules, the sugars first must be mildly oxidized to contain aldehyde groups by treatment with sodium periodate. Oxidation with this compound will cleave adjacent carbon-carbon bonds which possess hydroxyl groups, as are abundant in polysaccharide molecules (Chapter 1, Sections 2 and 4.4). 

The first product of the oxidation of alcohol is acetaldehyde and an important end-product is fulminic add, which latter can, however, only be isolated if silver or mercury ions are present. With these ions it forms salts—fulminates—which are stable towards nitric add in them, it must be presumed, the linkage with the metal is homopolar and non-ionogenic, as in mercuric cyanide. The formation of fulminic acid takes place because the carbonyl group of the aldehyde confers reactivity on the adjacent methyl group which then forms a point of attack for the nitrous acid. The various stages in the process are indicated by the following formulae ... 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

A new stereocenter is formed when a synthon 143 with umpoled carbonyl reactivity (d reactivity) is introduced into aldehydes or imines. The enantioselective variant of this type of reaction was a longstanding problem in asymmetric synthesis. The very large majority of a-hetero-snbstitnted carbanions which serve as eqnivalents for synthons like 142 and 143 lead to racemic products with aldehydes or imines. However, enantiomerically pnre acylions and a-hydroxy carboxylic acids or aldehydes (144 and ent-144, respectively) as well as a-amino acids and aldehydes (145 and ent-145) are accessible either by nsing chiral d reagents or by reacting the components in the presence of chiral additives (Scheme 18). 

Aside from aldehydes, imines can be considered to be suitable substrates for the addition of reagents with umpoled carbonyl reactivity because useful building blocks and important compounds like a-amino acid and aldehydes will result from this conversion (Scheme 21). The S -configurated carbenoid 41 has been found to add to mesitylene-sulfonylimines 154, available from the corresponding aldehydes with predominant attack... 

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of Af-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive to hydroxide addition at the carbonyl carbon than A-methylpyrrole-2-carbaldehyde (76JOC1952). Pyrrole-2-aldehydes fail to undergo Cannizzaro and benzoin reactions, which is attributed to mesomerism involving the ring nitrogen (see 366). They yield 2-hydroxymethylpyrroles (by NaBH4 reduction) and 2-methylpyrroles (Wolff-Kishner reduction). The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

Acid Catalyzed. Although ketonic carbonyl groups are less reactive than aldehydic carbonyls in the presence of basic catalysts, this is not the case with acid catalysts. Thus acetone undergoes aldol addition in the presence of sulfuric acid to give mesityl oxide, which then condenses with a third molecule of acetone to give a mixture ofphorone (2,6-dimethyl-2,6-heptadien-4-one) and mesitylene (1,3,5-trimethylbenzene). Ketones also condense with activated aromatic products in the presence of sulfuric acid to give coupled aromatic products. For example, acetone and phenol condense to bisphenol A (4,4 -isopropylidenediphenol), which is used in the manufacture of epoxy resins (qv) and polycarbonates (qv). 

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