Barium enolate

Strontium enolate chemistry is almost nonexistent barium enolate chemistry is rare radium enolate chemistry is unknown. Allyl chlorides react with barium to form the corresponding organometallics, which in turn react with o ,/ -unsaturated ketones to form the metal enolate . These enolates may also be formed in situ by the reaction of a-chloroketones with barium metal in the presence of an aldehyde, resulting in addition products. These reactions and the enthalpies of formation of the precursor a-haloketones and enones are seemingly ideal candidates for calorimetric investigation. 

The direct aldol reaction of carboxylic acid derivatives and aldehydes (or ketones) is very difficult. Recently, Kobayashi and co-workers (146) extended the ketones to amides as suitable candidates successfully for the direct aldol reaction. The screening of metal sources revealed that Ba(0-tBu)2 was the better catalyst than Sr(0-iPr)2, Ca(0-iPr)2, or Mg(0-iPr)2, whereas rare earth metals (eg, La(0-iPrls) are catalytically inactive in the presence of p-methoxyphenol. In this aldol reaction, barium enolate formed in situ from barium alkoxide and acylamide and... 

Weiss ° treated 16-dehydro- (6), 17a-acetoxy- (8), 17a-hydroxy- (9) and 17a-bromopregnan-20-one (11) with a solution of lithium, barium, calcium or sodium in liquid ammonia and reacted the intermediate enolate anion (7) with the appropriate alkyl halide. 

O. Oddo discussed this reaction as an example of what he calls mesoidria, i.e., mesokydry. L. Lendle, and G. Schick inferred that barium phosphite exists in isomeric forms because of the difference in the reducing properties of the phosphites prepared at high and at low temp. The low temp, form was said to be the most active, and to it was attributed an aldolic structure while the enolic structure was assigned to the less active form prepared at the high temp. ... 

Aldolic barium phosphite. Enolic barium phosphite. 

Bases like hydroxides or alkoxides are of about the right strength barium hydroxide is often used.1 The small amounts of enolate 7 quickly add to the large excess of ketone 8 to give the anion 9 of the product which regenerates the base by taking a proton from water or the alcohol.2... 

The mechanism is similar to that of the barium-catalyzed direct aldol reaction (Scheme 16). The reaction commences with deprotonation of the ketone (2) by the Br0nsted base unit of the catalyst under generation of the enolate 81. After addition of the aldehyde 1 the Lewis acid-base adduct 82 is formed. Then the reaction of the aldehyde and the enolate occurs (82 83). After... 

The enol precursor 151 was prepared in three steps by addition of Steri-col 150 onto trichloroethylene, which was followed by the formation of a sensitive ynol through addition of n-BuLi and excess allyl iodide (Scheme 43). The ynol was then partially hydrogenated with Pd on barium sulfate, furnishing the required enol ether 152 contaminated with 8% of the corresponding dihydro enol ether. [2+ 2]-Cycloaddition between 152 and dichloroketene, generated in situ from trichloroacetyl chloride, led to the cyclobutanone 153 as the major diastereomer with a 93 7 diastereocontrol. 

Calcined barium hydroxide is an efficient catalyst for a number of base-mediated reactions. Among these, the Claisen-Schmidt condensation of acetophenones with benzaldehydes occurs in times as short as 10 min at room temperature (Eq. 15). The acetophenone enolate (detected by IR spectroscopy) is formed on the catalyst surface, where the reaction with the aldehyde takes place. The higher activity of the ketone enolate is interpreted by the authors as the result of "an increased vibrational state of the lattice", a formulation close to the mechano-chemical explanation. With the help of selective poisoning experiments, it is concluded that the enolate forms via a SET mechanism. 

Silyl enol ethers are shown to have nucleophilicity between that of allylsilanes and aliylstannanes, and siloxyallyl barium reagents couple RCHO and RX at the y-position. ... 

The aldol condensation and related reactions are among the most important, and ubiquitous, construction reactions in organic synthesis. In these condensations, the carbonyl compound acts as both nucleophile and electrophile—the enol or enolate is the nucleophile, and the keto form is the electrophile. The reaction works with enolizable aldehydes (Figure 17.24) or ketones (Figure 17.25) and may be catalyzed by either acid or base. Almost all of the steps we write are equilibria—how can we persuade the reaction to go to completion In the base-catalyzed reaction, a catalyst such as barium hydroxide is placed inside a Soxhlet thimble, as in Figure 17.26. The reaction mixture is heated so that the acetone refluxes, but the product does not. Thus only the SM, and not the product, comes into contact with the catalyst, ensuring that the reverse reaction does not occur. Note that in the acid-catalyzed processes, it is common for the product to be dehydrated under the reaction conditions—this usually pulls the equilibrium over to the product. The mechanism of the elimination reaction may be El or E2 depending on the... 

---