Enols regioselectivity

We begin with the discussion of intramolecular reactions. An example of a regioselec-tive Dieckmann condensation (J.P. Schaefer, 1967) used an educt with two ester groups, of which only one could form an enolate. Regioselectivity was dictated by the structure of the educt. 

Unique chemistry is associated with the cyclopentenone all five carbon atoms can be functionalized, and the endo-methyl groups of the acetonide assure clean stereoselective addition of the alkenylcopper reagent from the convex side. The use of the acetonide group to control enolate regioselectivity and to mask alcohols should be generally applicable. 

Ketones, in which one alkyl group R is sterically demanding, only give the trans-enolate on deprotonation with LDA at — 72°C (W.A. Kleschick, 1977, see p. 60f.). Ketones also enolize regioselectively towards the less substituted carbon, and stereoselectively to the trans-enolate, if the enolates are formed by a bulky base and trapped with dialkyl boron triflates, R2B0S02CF3, at low temperatures (D A. Evans, 1979). Both types of fraiu-enolates can be applied in stereoselective aldol reactions (see p. 60f.). 

We need now to look at situations where both compounds might enolise and see how specific enolates can be used to control which compound does so (chemoselectivity) before looking at how we control which side of an unsymmetrical ketone forms the enolate (regioselectivity). We met two specific enol equivalents in chapter 13 (5-dicarbonyl compounds and lithium enolates and they are the keys to this section. 

Exactly as expensive reagents do not guarantee better yields, realistic physical models and refined methods do not always secure better results (Cf. pp. 242-244). Let us just examine here the enolate regioselectivity problem and convince ourselves that an isolated enolate may be a good model for solution chemistry, but not for gas-phase chemistry ... 

The FO treatment of enolate regioselectivity presented in Section 5.2.2 is based on the MOs of enol. Let us first check that the conclusions remain unchanged if the MOs of the naked enolate or of the enolate accompanied by its counterion are used instead. Figure 5.4 shows the C and O net charges in CH2=CHO , CH2=CHOLi and CH2=CHONa, according to PM3, STO-3G and 3-21G calculations.40 Below each drawing are shown the energies (in eV) of the two occupied n orbitals and the ratios of the C and O coefficients in each MO. 

Probably the most useful application of enolates/enol ethers is the aldol addition reaction [105]. An elegant way of controlling both regio- and stereochemistry of aldol additions is achieved by the 1,4-addition of a silicon anion to generate a P-siUcon enolate regioselectively, which is then submitted to an aldol addition [106]. 

---