Addition to enols

In addition to enolate ions, other kinds of carbon nucleophiles also add to a jjS-iinsaturated acceptors in Michael-like reactions. Among the most important such nucleophiles, particularly in biological chemistry, are enamines, which are... 

Thermal decomposition of y-lactone tosylhydrazone sodium salts are reported to yield cyclobu-tanones, which can be accounted for by rearrangement of an intermediate oxycarbene. In this manner, the sodium salts of dihydrofuran-2(37/)-one tosylhydrazones 1 were decomposed as a loose powder, at 310 C in a bulb-to-bulb distillation apparatus at an initial pressure of 0.1 Torr, to give the corresponding cyclobutanones 2 in addition to enol ethers, cyclopropanes and open-chain alkenes. Condensable products (74-76%) were collected at — 78 °C, weighed and the ratio of components was determined from their relative GC peak areas.63... 

Hydrocarboxylation of Alkynes Intramolecular addition of carboxylic acids (weak nucleophiles) to alkynes led to lactones, which were first reported by Schmidbaur et al. in the reaction of acetic add with 3-hexynes to obtain, in addition to enol ester, 3-hexanone. Traces of water were probably present in the solvent to enable the process to be carried out [99]. 

Phenyl- 1-trimethylsiloxyethene, the trimethylsilylenol ether of methyl phenyl ketone, added tellurium tetrachloride across the carbon-carbon double bond. Subsequent elimination of chlorotrimethylsilane produced bis[benzoylmethyl] tellurium dichloride. Addition to enols is also the first step in reactions of tellurium tetrachloride with monoketones and 1,3-diketones (p. 534, 535). 

Finally, concordant results have been obtained from a kinetic study of the iodination of acetophenone and acetone at very low iodine concentration (Verny-Doussin, 1979). The procedure used is similar to that followed for the determination of equilibrium constants for enol formation by the kinetic-halogenation method, i.e. second-order rate constants were measured under conditions such that halogen additions to enol and enolate are rate-limiting (43). Under these conditions, the experimental kn-values can be expressed by... 

Analogous boion-ene addition to enol ethers (36) requires 110-140 C. Under these harsh conditions die initially formed 2-alkoxyboranes (37).undergo a spontaneous -eliInination to furnish 1,4-dienes (38) in good yields (Scheme 9 Table 5) Application of this cu-metallo-allylation/syR-elimination sequence to cyclic enol ethers provides a stereocontrolled access to 1,4-dienols containing a trisubstituted alkenic bond, as illustrated by the transformation (39) + (40) - (42) (Scheme 9). 

In addition to enol silyl ethers, other derivatives of aldehydes and ketones, i.e. enol ethers (Eq. 8) [48] and enol esters (Eq. 9) [49, 50], serve as a partners for the cross aldol reaction, although the lower reactivity of these compounds compared with enol silyl ethers often makes the reaetion more complicated. For example, the products isolated in Eq. (8) were ether derivatives or a,y8-unsaturated carbonyl compounds rather than the expected aldol itself. 

In addition to enol silyl ethers, an optically active boryl enolate underwent the highly anri-stereoselective aldol reaction with a wide variety of aldehydes in the presence of TiCU (Eq. 34) [120]. The vinyl sulfides shown in Eq. (35) reacted with a,fi-unsaturated ketones via the 1,4-addition pathway in the presence of a titanium salt, but the reaction was followed by the cleavage of a carbon-carbon bond in the cycloalkane to give open chain products in a stereoselective manner [121]. The 1,2-type addition was observed, if the olefinie acetal was used instead of the corresponding carbonyl compound, as shown in Eq. (36) [121], The successive scission of the carbon-carbon bond took place analogously to give the same type of products as shown in Eq. (35). 

It is possible that tertiary hydroxyl groups are present in graphite oxide in addition to enolic groups. This is shown in Fig. 2. This would explain why, with aqueous alkali metal hydroxides and with diazomethane, only a fraction of the —OH groups which are present undergo reaction. 

The kinetic stereoselectivity of the aldol is a function of the enolate stereochemistry and its structure. One often reads the over-generalization that (Z)-enolates gives syn aldols and (E)-enolates give anti al-dols. However, the situation is much more complex than this in addition to enolate geometry, several variables are involved. The following generalizations may be made at this time (refer to equation 37 for definition of R , R- and R ). 

Similarly, dienes 6 in the Eu(fod)3 catalyzed additions to enol ethers 7 give adducts 8 as single diastereomers14. 

The ability of bromomalononitrile to transfer Br to a stabilized allylic radical further illustrates the high reactivity of malononitrile-based atom transfer reagents. Giese has demonstrated successful Br-transfer additions to enol ethers with bromomalonates in the presence of BujSnH. The a-bromo ethers thus generated underwent spontaneous loss of HBr to regenerate enol ether functionality (Scheme 12) [38]. 

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