Donor groups thermodynamic preferences

CH3I should approach the enolate from the direction that simultaneously allows its optimum overlap with the electron-donor orbital on the enolate (this is the highest-occupied molecular orbital or HOMO), and minimizes its steric repulsion with the enolate. Examine the HOMO of enolate A. Is it more heavily concentrated on the same side of the six-membered ring as the bridgehead methyl group, on the opposite side, or is it equally concentrated on the two sides A map of the HOMO on the electron density surface (a HOMO map ) provides a clearer indication, as this also provides a measure of steric requirements. Identify the direction of attack that maximizes orbital overlap and minimizes steric repulsion, and predict the major product of each reaction. Do your predictions agree with the thermodynamic preferences Repeat your analysis for enolate B, leading to product B1 nd product B2. 

Michael addition of metal enolates to a,/3-unsaturated carbonyls has been intensively studied in recent years and provides an established method in organic synthesis for the preparation of a wide range of 1,5-dicarbonyl compounds (128) under neutral and mild conditions . Metal enolates derived from ketones or esters typically act as Michael donors, and a,-unsaturated carbonyls including enoates, enones and unsaturated amides are used as Michael acceptors. However, reaction between a ketone enolate (125) and an a,/3-unsaturated ester (126) to form an ester enolate (127, equation 37) is not the thermodynamically preferred one, because ester enolates are generally more labile than ketone enolates. Thus, this transformation does not proceed well under thermal or catalytic conditions more than equimolar amounts of additives (mainly Lewis acids, such as TiCU) are generally required to enable satisfactory conversion, as shown in Table 8. Various groups have developed synthons as unsaturated ester equivalents (ortho esters , thioesters ) and /3-lithiated enamines as ketone enolate equivalents to afford a conjugate addition with acceptable yields. 

The independent generation and reactivity of allenic enolates has been investigated.15 Under kinetic conditions, these highly reactive species are protonated in the a,p-7t plane with preference (E) to the larger ( group. Under thermodynamic con- (g) ditions, addition/elimination equilibrates the two product stereoisomers. The kinetic protonation stereochemistry has been found a function of solvent, proton donor, and donor concentration. 

In alkylidyne complexes that bear no ir-acid co-ligands, thermodynamic protonation occurs either on the alkylidyne face or at the metal, and steric considerations are invoked to explain the preference. Sub-stoichiometric quantities of H2O or HCl catalyse the tautomerisation of TpW( = C Bu)Cl(NHPh) to the corresponding imido alkylidene complex TpW( = CH Bu)Cl( = NR). The first step in the tautomerisation is, however, suggested to involve protonation of the amide group (Scheme 45) since weaker donors (i.e., those with electron-withdrawing... 

M = Zr or Nb) show that p-Me elimination may be thermodynamically favoured over P H elimination for strongly electron-deficient metal centres. This preference is attributed to the presence of multiple bonding between a d° transition metal and the methyl group, which behaves like a weak Jt-donor via its occupied JlMe orbitals. ... 

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