Aldehyde or Ketone and Halogen

The enol content of a carboxylic acid is far less than that of an aldehyde or ketone and introduction of a halogen substituent at the a carbon atom requires a different set... 

In the overall Wittig reaction, an alkene is formed from the aldehyde or ketone and an alkyl halide in which the halogen-bearing carbon contains at least one hydrogen ... 

This positive halogen compound oxidizes alcohols to aldehydes or ketones and hydrazo compounds to azo compounds under mild conditions and in high yields. The oxidant is converted into bcnzo-triazole hydrochloride. It also oxidizes 1-amino-benzotriazole to benzyne (80% yield, 2 moles of oxidant required) and 2-amino-benzotriazole to cis.cis-1,4-dicyanobutadienc-1,3. ... 

Although it has been stated that di- and tri-haloketones and a-haloaldehydes (irrespective of the degree of halogen substitution) tend to yield only enol phosphate esters, further qualification of this statement is appropriate. The formation of silyl ethers from aldehydes or ketones and silyl phosphites has already been noted (see section III.A). Reactions between silyl phosphites and trifluoroacetaldehyde or perfluoroacetone and other similar compounds initially lead to silyl ethers of (a-hydroxyalkyl)phosphonic diesters in which all the fluorine is retained, although subsequent change leads to fluorinated enol phosphate esters. Sekine et also observed the formation of (a-silyloxyalkyl)phospho-... 

OL Halogenation (Sections 18.2 and 18.3) Halogens react with aldehydes and ketones by substitution an a hydrogen is replaced by a halogen. Reaction occurs by electrophilic attack of the halogen on the carbon-carbon double bond of the enol form of the aldehyde or ketone. An acid catalyst increases the rate of enolization, which is the ratedetermining step. 

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

The classical Reformatsky reaction consists of the treatment of an a-halo ester 1 with zinc metal and subsequent reaction with an aldehyde or ketone 3. Nowadays the name is used generally for reactions that involve insertion of a metal into a carbon-halogen bond and subsequent reaction with an electrophile. Formally the Reformatsky reaction is similar to the Grignard reaction. 

It is not the aldehyde or ketone itself that is halogenated, but the corresponding enol or enolate ion. The purpose of the catalyst is to provide a small amount of enol or enolate. The reaction is often done without addition of acid or base, but traces of acid or base are always present, and these are enough to catalyze formation of the enol or enolate. With acid catalysis the mechanism is... 

Such functional groups as OR, OH, NH2, SMe, halogen, and COOR may be present in the molecule," but not groups that are reducible by borane. Hydroboration of enamines with 9-BBN provides an indirect method for reducing an aldehyde or ketone to an alkene, for example. 

It might bo pointed out, as haa been done by Bailee ter, 7 that chloroform and hromoform do wot lead to epoxides when allowed to react with aldehydes or ketones, trihaJomethyicarbinote being obtained instead. Halonitroniethanes appear to behave similarly. These halogen components have not, however, been examined as carefully as others cited above, particularly -with respect to the effect of varying the... 

Further transformations of the intermediate alkenyloxydialkylboranes (46), as shown in Scheme 10, include aqueous hydrolysis to the homologated aldehydes or ketones 30 electrophilic additions to afford the corresponding a-dialkylaminomethyl aldehydes and ketones (47),31 and a-halogenated (48)32 or a-arylselenylated aldehydes or ketones (49) 33 transmetallation-alkylation to afford a-substituted aldehydes or ketones (50) 34 and aldol condensations with aldehydes (51).35 These reagents offer the equivalent of the tandem 1,4-conjugate addition-electrophile-trapping protocol. A prototypical prosta-... 

The synthesis of furans from /3-ketoesters and a-halogenated aldehydes or ketones under basic conditions is known as the Feist-Benary furan synthesis (Equation 28). In most of the cases, the reaction is initiated by an aldol reaction. If the first step is alkylation, reversed regioselectivity is observed. 

A great deal of evidence has been obtained in support oT the mecha nism shown in Figure 22.4. For example, the rate of halogenation is indc< pendent of the halc [eit s identity Chlorination, bromination, and iodination of a given aldehyde or ketone all occur at the same rate, indicating that the same ratediniiting step is involved and that the halogen has no past in that step. 

First, functional substituents a to the carbonyl group such as hydroxyl, alkoxyl and halogens are reduced, and then the resulting unsubstituted aldehyde or ketone is reduced to the parent hydrocarbon. 

The first substitution reaction we examine is halogenation. Treatment of a ketone or aldehyde with halogen and either acid or base results in substitution of X for H on the a carbon, forming an a-halo aldehyde or ketone. Halogenation readily occurs with CI2, Br2, and I2. 

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