Extended enolates Kinetic product

The popularity of MoOPH has waned with the discovery of the next type of reagents for hydroxylations. These /V-sull onyl oxaziridines should now probably be your first choice. In the 1986 volume of Organic Syntheses three recipes for hydroxylation of enolates stand side by side. Rubottom42 describes the formation of the kinetic lithium enolate of the enone 262 and the oxidation of the silyl enol ether 263 with mCPBA to give the a product 264 (see chapter 11 for the regioselectivity of such extended enolates). 

A similar phenomenon has been reported by Sugiyama and coworkers, who found that the vinylogous amide (23) reacts with benzaldehyde to give (24) as the sole product (equation 90). When (23) is treated with two equivalents of sodium amide in ammonia, followed by treatment with benzaldehyde, aldol (25) is formed in 25% yield. Although the authors invoke a dianion in the latter reaction, it is unlikely that one could be formed under the reaction conditions used. Instead, it is likely that deprotonation at the endocyclic a-position is preferred kinetically, and that this leads to the product observed with NaNH2/NH3 (irreversible enolate formation). Reaction of this enolate must be slow, for steric reasons, as witnessed by the low yield in the aldol reaction. Under conditions of enolate equilibration, the more stable extended dienolate is produced. 

The procedure can be extended to achieve selective a-bromination and iodination of carboxylic acids and the general mechanism of the a-halogenation is outlined in Chapter 5, p 170. The autocatalytic effects in the selective a-chlorination of propionic acid to the 2-chloro and 2,2-dichloro acids have been studied in a semibatch reaction at 90-130 °C. The reaction was performed in the presence of chlorosulfonic acid and dichloroacetic acid as catalysts and oxygen as the radical scavenger. Kinetic experiments indicated autocatalytic formation of both products and that the selectivity was independent of the chlorine concentration in the liquid phase. The results confirmed the validity of the proposed reaction scheme which involved formation of the reaction intermediate, propanoyl chloride from propionic acid and chlorosulfonic acid, the acid-catalysed enolization of the acid, and a hydroxyl-chlorine exchange reaction. The acid-catalysed enolization was the rate-determining step in the reaction sequence. ... 

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