Ketones Aldehydes Carbonyl groups

Organic compounds containing active hydrogen atoms adjacent to a carbonyl group (aldehydes, ketones, carboxylic acids) may react violently in unmoderated contact with bromine. 

Nitro is just one of a number of groups that are also deactivating towards electrophiles and metadirecting because of electron withdrawal by conjugation. These include carbonyl groups (aldehydes, ketones, esters, etc.), cyanides, and sulfonates and their ]H NMR shifts confirm that they remove electrons from the ortho and para positions. 

Simple n bonds of this type are also found in molecules with carbonyl groups — aldehydes, ketones, carboxylic acids and esters (Structures 5.1). 

Much of the chemistry of that group of compounds referred to as carbohydrates (compounds of carbon corresponding to, approximately, C(H20) ) is a function of the numerous hydroxyl groups that adorn their structures. However, despite the view that they be considered as polyhydroxy derivatives of carbon (polyols), their rich chemistry must include reference to reactions of the carbonyl group (aldehydes, ketones, and carboxylic acids). Therefore, elaboration of the chemistry of carbohydrates is postponed to Chapter 11. 

To name more complex carbohydrates or amino acids, one draws a similar Fischer projection where the CH2OH or R is on the bottom and the carbonyl group (aldehyde, ketone, or carboxylic acid) is on the top. The d descriptor is used when the OH or NH2 on the penultimate (second from the bottom) carbon points to the right, as in D-glyceraldehyde, and l is used when the OH or NH2 points to the left. See the following examples. 

Since the publication by Bolm et al. [3], in 2001, of a review on catalytic asymmetric arylation reactions (Scheme 7.1), many innovative and practical processes have been developed in this area. In this chapter, which concerns the arylation of carbonyl groups (aldehydes, ketones, etc.) and the most important advances that have taken place in the last 10 years or so, enantioselective and nonasymmet-ric arylation of carbonyl groups wHl be discussed, taking into account the different transition-metal catalysts applied. 

A carbonyl group has a carbon atom and an oxygen atom connected with a double bond (i.e., C=0). Four kinds of organic compounds that contain only C, H, and O have a carbonyl group aldehydes, ketones, carboxylic acids, esters. 

The selective intermolecular addition of two different ketones or aldehydes can sometimes be achieved without protection of the enol, because different carbonyl compounds behave differently. For example, attempts to condense acetaldehyde with benzophenone fail. Only self-condensation of acetaldehyde is observed, because the carbonyl group of benzophenone is not sufficiently electrophilic. With acetone instead of benzophenone only fi-hydroxyketones are formed in good yield, if the aldehyde is slowly added to the basic ketone solution. Aldols are not produced. This result can be generalized in the following way aldehydes have more reactive carbonyl groups than ketones, but enolates from ketones have a more nucleophilic carbon atom than enolates from aldehydes (G. Wittig, 1968). 

Carbonyl functional groups are the easiest to identify of all IR absorptions because of their sharp, intense peak in the range 1670 to 1780 cm-1. Most important, the exact position of absorption within the range can often be used to identify the exact kind ot carbonyl functional group—aldehyde, ketone, ester, and so forth. 

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). 

Nucleophilic addition to carbonyl groups aldehydes and ketones... 

NUCLEOPHILIC ADDITION TO CARBONYL GROUPS ALDEHYDES AND KETONES... 

Aldehydes have at least one of their hydrogen atoms bonded to the carbon in the carbonyl group. In ketones, the carbonyl carbon bonds to other carbon atoms and not hydrogen atoms (Figure 15.9). 

Organic carbonyl compounds—aldehydes, ketones, amides, and acyl halides—in which the carbonyl group is not part of a cyclic structure have interesting conformational properties that may differ widely according to the molecular system bearing these substituents. 

Carbonyl difluoride (2) has been used to convert the carbonyl group in ketones, aldehydes and amides to the corresponding difluoromethylene group. 

Enolates are important nucleophiles which react nicely with a variety of carbonyl compounds. In this case, the nucleophilic reactivity of the enolate and the electrophilic reactivity of the carbonyl group are well matched and a wide variety of products can be made. The type of enolate (ketone, ester, etc.) and the type of carbonyl electrophile (aldehyde, ketone, ester, etc.) determine the structure of the final product. Furthermore these reactions are often named according to the two partners that are reacted and the type of product produced from them. 

Hydroxyphosphonium salts and l-(trimethylsiloxy)phosphonium salts are obtained219 from the addition of small-sized phosphines (PMe3 and PEt3) to a carbonyl group of ketones and aldehydes, in the presence of chlorotrimethylsilane or of the mixture ace-tone/bromine, which is a source of anhydrous HBr. 

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. 

In our present discussions, 1,2- and 1,4-additions to carbonyl systems were introduced. However, these reactions were not presented in the context of specific carbonyl-based functional groups. Expanding upon this concept, the three types of functional groups generally used in addition reactions to carbonyls are aldehydes, ketones, and esters. 

The ketyl radical anion intermediates can be exploited in carbon-carbon bond-forming reactions. Intermolecular and intramolecular pinacol couplings between the carbonyl groups of ketones and aldehydes are well known (Chapter 5, Section 5.1), as are intermolecular and intramolecular carbonyl-alkene couplings (Chapter 5, Section 5.2). 

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