Aldehydes double bonds

In the case of Pt-Fe, the two metals were suggested389,421 to be in the metallic state forming an alloy. EXAFS studies indicated an electron transfer from Fe to Pt, which allowed the conclusion that the aldehyde double bond is more easily adsorbed on the induced sites . In the case of Pt-Sn and Pt-Ge, the authors suggested that Snn+ or Gen+ ions (Lewis acids) activate the carbonyl group by enhancing the positive charge of the carbonyl carbon (equation 32). 

In the course of the total synthesis of enmein, Fujita and co-workers (7) have discovered that the intramolecular cyclization of the enolate 23 of the corresponding tetracyclic keto-aldehyde at room temperature gave only ketol 24. However, when the same reaction is conducted at 60°C, thermodynamically controlled conditions prevail, and the epimeric product 25 is obtained. Inspection of molecular models indicates that the kinetically controlled product 24 is again the result of an anti peri planar arrangement of the enolate and the aldehyde double-bonds. Also, as in the previous examples, the isomer 25 comes from a synclinal arrangement of the reacting functional groups. 

On the contrary [Rh(acac)(C2II4)2] and unsaturated aldehydes, such as 4-pentenal and 4-hexenal, add intcrmolecularly to ethene to form acyclic ketones40. Thus, hydroacylation of ethene with 4-pentenal gives three 5-oxoheptene isomers. The aldehyde double bond in the substrate is essential, since saturated aldehydes do not react, and without ethene no intramolecular hydroacylation is observed with unsaturated aldehydes40. 

Heyns and Blazejewicz427 studied the oxidation of alcohols, in water or organic solvents, by oxygen at platinum catalysts under mild conditions. Primary alcohols gave aldehydes or carboxylic acids according to the reaction conditions secondary alcohols gave ketones. This method is especially suitable for preparation of long-chain aldehydes double bonds are not attacked. 

Both oxides of silver, Ag20 and AgO, have been used for aldehyde oxidations. The silver]I) reagent has mainly been applied in transformations of aliphatic [83] and aromatic aldehydes [84], less so in oxidations of organometallic complexes bearing an aldehyde function [85] or a,P-unsaturated compounds [86]. Use of the silver(II) oxide is less common [70, 87], probably due to its limited availability and high cost Cyanide ions catalyze this oxidation in methanol leading to carboxylic acids (and not to esters as compared with manganese) [70]. In reactions with a,p-unsaturated aldehydes, double bond isomerizations have been observed [70]. Less common silver re-... 

Aldehydes Double bond between carbon and oxygen and a hydrogen atom attached to same carbon 0 X... 

Aldehydes form addition products at the double bond of the carbonyl (>C 0) group, and hydrolysis gives secondary alcohols. Thus acetaldehyde gives isopropyl alcohol ... 

The condensation conditions must be as mild as possible, because we want to get only the most stable of the three possible enols (from the aldehyde). Though you could not haye predicted the exact conditions either for the double bond. cleayage or for the condensation, you should haye seen that control was possible as in each case the two functional groups are different enough. ( J. Amer. Chem. Soc.. 1960, 636 J. Org. Chem.. 1964, 29, 3740 ... 

The reaction conditions applied are usually heating the amine with a slight excess of aldehyde and a considerable.excess of 2d-30hydrochloric acid at 100 °C for a few hours, but much milder ( physiological ) conditions can be used with good success. Diols, olefinic double bonds, enol ethers, and glycosidic bonds survive a Pictet-Spengler reaction very well, since phenol and indole systems are much more reactive than any of these acid sensitive functional groups (W.M. Whaley, 1951 J.E.D. Barton, 1965 A.R. Battersby, 1969). 

Alkenes are cleaved to carbonyl compounds by ozonolysis This reaction IS useful both for synthesis (preparation of aldehydes ketones or car boxyhc acids) and analysis When applied to analysis the carbonyl com pounds are isolated and identified allowing the substituents attached to the double bond to be deduced... 

By analogy to the hydration of alkenes hydration of an alkyne is expected to yield an alcohol The kind of alcohol however would be of a special kind one m which the hydroxyl group is a substituent on a carbon-carbon double bond This type of alcohol IS called an enol (the double bond suffix ene plus the alcohol suffix ol) An important property of enols is their rapid isomerization to aldehydes or ketones under the condi tions of their formation... 

The induced field of a carbonyl group (C=0) deshields protons in much Ihe same way lhal a carbon-carbon double bond does and Ihe presence of oxygen makes il even more eleclron wilhdrawmg Thus protons attached to C=0 m aldehydes are Ihe leasl shielded of any protons bonded to carbon They have chemical shifts m Ihe range 8 9-10... 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Like aldehydes ketone functions take precedence over alcohol functions double bonds halogens and alkyl groups m determining the parent name and direction of numbering Aldehydes outrank ketones however and a compound that contains both an aldehyde and a ketone carbonyl group is named as an aldehyde In such cases the carbonyl oxy gen of the ketone is considered an 0x0 substituent on the mam chain... 

Alkyl substituents stabilize a carbonyl group m much the same way that they sta bilize carbon-carbon double bonds and carbocations—by releasing electrons to sp hybridized carbon Thus as then heats of combustion reveal the ketone 2 butanone is more stable than its aldehyde isomer butanal... 

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... 

Secondary amines are compounds of the type R2NH They add to aldehydes and ketones to form carbmolammes but their carbmolamme intermediates can dehydrate to a stable product only m the direction that leads to a carbon-carbon double bond... 

The most attractive feature of the Wittig reaction is its regiospecificity The location of the double bond is never m doubt The double bond connects the carbon of the ongi nal C=0 group of the aldehyde or ketone and the negatively charged carbon of the yhde... 

Conjugation of the newly formed double bond with the carbonyl group stabilizes the a p unsaturated aldehyde provides the driving force for the dehydration and controls Its regioselectivity Dehydration can be effected by heating the aldol with acid or base Normally if the a p unsaturated aldehyde is the desired product all that is done is to carry out the base catalyzed aldol addition reaction at elevated temperature Under these conditions once the aldol addition product is formed it rapidly loses water to form the a p unsaturated aldehyde... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

The carbonyl group withdraws rr electron density from the double bond and both the carbonyl carbon and the p carbon are positively polarized Their greater degree of charge separation makes the dipole moments of a p unsaturated carbonyl compounds signifi cantly larger than those of comparable aldehydes and ketones... 

FIGURE 18 7 Nucleophilic addition to a p unsaturated aldehydes and ketones may take place either in a 1 2 or 1 4 manner Direct addition (1 2) occurs faster than conjugate addition (1 4) but gives a less stable product The product of 1 4 addition retains the carbon-oxygen double bond which is in general stronger than a carbon-carbon double bond... 

This suggests sp hybridization at carbon and a ct + tt carbon-oxygen double bond analogous to that of aldehydes and ketones... 

The carbon-nitrogen triple bond of nitriles is much less reactive toward nucleophilic addition than is the carbon-oxygen double bond of aldehydes and ketones Strongly basic nucleophiles such as Gngnard reagents however do react with nitriles in a reaction that IS of synthetic value... 

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