Carbonyl group aldehydes and ketones

Nucleophilic addition to carbonyl groups aldehydes and ketones... 

NUCLEOPHILIC ADDITION TO CARBONYL GROUPS ALDEHYDES AND KETONES... 

Because of the polarity of the carbonyl group, aldehydes and ketones have a nucleophilic oxygen centre and an electrophilic carbon centre as shown for propanol (Following fig.). Therefore, nucleophiles react with aldehydes and ketones at the carbon centre, and electrophiles react at the oxygen centre. 

Chlorobis(cyclopentadienyl)tetnihydrobor8tozirconiuni(IV), Cp2Zr(CI)BH4. Mol. wt. 271.70. An early preparation has been reported without details. A convenient preparation involves reaction of Cp2Zr(H)Cl and BHi-S(CHj)2 yield 70-80%. Reduction of carbonyl groups Aldehydes and ketones are reduced by this complex in high yield esters, carboxylic acids, nitriles, and nitro compounds are reduced very slowly. The reagent thus resembles NaBH4, but can be used for reductions in benzene. 

Reduction of carbonyl groups. Aldehydes and ketones are subjected to enantioselective reduction. Hydrogenation of benzaldehyde-a-d, a-alkoxyketones " or )3-ketoesters can be accomplished using either the Ru dihalide complexes or some modified forms. a-Ketoesters are also similarly reduced. 

Protection of carbonyl groups. Aldehydes and ketones are converted into acetals and ketals in high yield by reaction with (1) in refluxing benzene (TsOH catalysis) with azeotropic removal of water. Deprotection has been carried out by... 

Because they contain a polar carbonyl group, aldehydes and ketones can hydrogen bond with water molecules, which makes carbonyl compounds with one to four carbon atoms soluble in water. 

The difference in behavior between aldehydes/ketones and carboxylic acic derivatives is a consequence of structure. Carboxylic acid derivatives have ai acyl carbon bonded to a group -Y that can leave as a stable anion. As soon a the tetrahedral intermediate is formed, the leaving group is expelled to general- a new carbonyl compound. Aldehydes and ketones have no such leaving grouj however, and therefore don t undergo substitution. 

Etard reagents (chromyl chloride and some derivatives) suffer from the problem that occasionally they can exhibit a lack of selectivity and low yields. They are useful in the selective oxidation of aromatic side-chains to a carbonyl group, aldehyde or ketone but in many instances, the formation of the initial complex is slow and yields are low because of difficulties in the work-up which lead to undesired over-reaction. Attempts have been made to solve the problems by the use of sonication [134]. A simple preparation of the liquid reagent was proposed and the Etard reaction itself together with the hydrolytic step were conducted under sonication, with some success (Scheme 3.25). 

The carbonyl of aldehydes and ketones can be transformed into a gem-difluoro group. This transformation can be performed either directly with DAST or in an indirect manner by treating the corresponding thioacetal or hydrazone with an oxidant (NBS, dibromohydantoin, etc.) in the presence of a source of fluoride ions (e.g., HF-pyridin complex or TBABF prepared from TBAF and KHF2). ... 

Recently, bromine trifluoride found a promising use as a tool for converting carbonyls to the CF2 group. Aldehydes and ketones were converted through their easy to make azines, while esters, through the corresponding thioesters, formed a-fluoromethylene ethers (equation 122)217. 

Class II carbonyl compounds are those in which the acyl group is attached to a group that cannot be readily replaced by another group. Aldehydes and ketones belong to this class. The —H and alkyl or aryl (—R or —Ar) groups of aldehydes and ketones caimot be replaced by a nucleophile. 

Carbonyl compounds can be placed in one of two classes. Class I carbonyl compounds contain a group that can be replaced by another group carboxylic acids and carboxylic acid derivatives belong to this class. Class II carbonyl compounds do not contain a group that can be replaced by another group aldehydes and ketones belong to this class. 

The carbonyl carbon of the simplest aldehyde, formaldehyde, is bonded to two hydrogens. The carbonyl carbon in all other aldehydes is bonded to a hydrogen and to an alkyl (or an aryl) group. The carbonyl carbon of a ketone is bonded to two alkyl (or aryl) groups. Aldehydes and ketones do not have a group that can be replaced by another group, because hydride ions (H ) and carbanions (R ) and are too basic to be displaced by nucleophiles under normal conditions. 

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