Enolates Friedel-Crafts reaction

Acetic anhydride can be used to synthesize methyl ketones in Friedel-Crafts reactions. For example, benzene [71-43-2] can be acetylated to furnish acetophenone [98-86-2]. Ketones can be converted to their enol acetates and aldehydes to their alkyUdene diacetates. Acetaldehyde reacts with acetic anhydride to yield ethyhdene diacetate [542-10-9] (18) ... 

Akylsilanes are more reactive than vinylskanes in Friedel-Crafts reactions, as shown in the selective acylation of 2,3-disilylalkenes. The akylsilanes, a-skyloxyakyltrialkylsilanes, have been used as enolate equivalents in the preparation of 1,4-diketones (178). The mild reaction conditions required for these reactions tolerate many other functional groups, providing valuable synthetic routes. 

Fluoral hydrate and hemiacetals are industrial products. They are stable liquids that are easy to handle, and they react as fluoral itself in many reactions. Thus, in the presence of Lewis acids, they react in Friedel-Crafts reactions. They also react very well with organometallics (indium and zinc derivatives) and with silyl enol ethers.Proline-catalyzed direct asymmetric aldol reaction of fluoral ethyl hemiac-etal with ketones produced jS-hydroxy-jS-trifluoromethylated ketones with good to excellent diastereo- (up to 96% de) and enantioselectivities. With imine reagents, the reaction proceeds without Lewis acid activation. The use of chiral imines affords the corresponding 8-hydroxy ketones with a 60-80% de (Figure 2.49). ° ... 

We started this chapter by comparing phenols with enols (Ph-enol is the phenyl enol) and now we return to them and look at electrophilic substitution in full detail. You will find that the reaction is much easier than it was with benzene itself because phenols are like enols and the same reactions (bromination, nitration, sulfonations, and Friedel-Crafts reactions) occur more easily. There is a new question too the positions round the phenol ring are no longer equivalent—where does substitution take place ... 

A Alkylation of silyl enol ether B, Diels-Alder reaction, C, Friedel-Crafts reaction. b As a rough estimate of the kinetics, f50, which is defined as the time necessary to obtain 50% of the final yield was used. 

Based on the benzyl derivative (26), the first stable carborane-containing simple enol (27) was synthesized2 (Scheme 8). The acid chloride (28) readily enters into a Friedel-Crafts reaction with mesitylene to give ketone (29). Under the action of BuLi on a benzene-ether solution of ketone (29) enolate (30) is formed. Treatment of the latter with diluted HC1 solution results in enol (27). Starting from the benzyl derivative (31), carboranyl-substituted indene (32) was synthesized by intramolecular cyclization with a formation of ketone (33), whose reduction (34) followed by dehydration results in (32) (Scheme 9). 

A Alkylation of silyl enol ether, B, Diels-Alder reaction C, Friedel-Crafts reaction. 

Of our three specific enolate equivalents, the lithium enolates are most similar to naked enolates most reactive and most basic. The enamines are less reactive, only weakly basic but still nucleophilic in their own right, while the silyl enol ethers are the least reactive, not basic at all, and are rather like alkenes with slightly increased nucleophilicity because of an electron-donating substituent. We have already seen that they usually need a Lewis acid for efficient reaction, just like an alkene in, say, the Friedel-Crafts reaction. 

Silyl enol ethers react with acid chlorides and Lewis acids in what is effectively a Friedel-Crafts reaction.32 Both silyl enol ethers 22 and 25 derived from the ketone 23 give regiospecific acylation with MeCOCl and TiCI4. The yields of the two 1,3-diketones 89 and 90, formed as a mixture of diastereoisomers, are excellent. 

The carbon-carbon bond forming step of the Prins reaction, the reaction of an alkene with a carbonyl group (1) - (2), is, in one sense, the most fundamental reaction of synthetic organic chemistry at one extreme it is the base-catalyzed aldol reaction, where the alkene component is an enolate ion and the carbonyl group is unprotonated, and at the other extreme it is the Friedel-Crafts reaction. It is a paradigm for a fast organic reaction both bonds involved in the key step are iT-bonds, which are simultaneously more reactive than o-bonds and more susceptible to modification in reactivity by the presence or absence of substituents on either or both components. It is no wonder that, in one form or another, it is the most frequently used of all carbon-carbon bond forming reactions. 

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