Metal enolates radical addition reactions

The mechanism of these reactions was proposed to take place by initial electron transfer to the carbonyl group of the thioester, generating a ketyl-like radical anion such as 103 (Scheme 7.43). Subsequent radical addition to the electron-deficient alkene (acrylamide or acrylate), possibly guided by pre-complexation to a Sm(III) metal ion, generates a new radical centre, which is reduced to the corresponding Sm(III) enolate by a second equivalent of Sml2. Protonation of this enolate and hydrolysis of the hemithioacetal upon work-up then lead to the y-ketoamide or ester. 

The oxidative method is often conducted on enol (or enolate) derivatives and a simplified mechanism is shown in Scheme 71. Initial chemical or electrochemical oxidation gives an electrophilic radical (68 that may be free or metal-complexed) that is relatively resistant to further oxidation. Addition to an alkene now gives an adduct radical (69) that is more susceptible to oxidation. Products are often derived from the resulting intermediate cation (70) by inter- or intra-molecular nucleophilic capture or by loss of a proton to form an alkene. The concentration and oxidizing potential of the reagent help to determine the selectivity in such reactions. 

Mechanism Because the Tr-electron systems of the two functional groups in a,p-unsaturated ketone are conjugated, the radical anion A formed by electron addition from a reducing metal is resonance stabilized. The usual fate of the A is protonation (or other electrophilic bonding) at the P-carbon atom. This creates an enoxy radical B which immediately accepts an electron to form an enolate anion C. Protonation or alkylation of this enolate species then gives a saturated ketone D or E, which may be isolated or further reduced depending on the reaction conditions (Scheme 6.33). 

Since samarium diiodide is only a one electron donor, two equivalents of the metal are required in order for the reaction to proceed. The first electron donated from the samarium produces a chiral ketyl radical 30 which undergoes enantioselective addition to the acrylate according to the chelated transition state shown in 32. The second electron donation then provides a chiral samarium enolate intermediate 33 that can potentially undergo stereoselective proton transfer in the formation of a second chiral center. 

Upon reaction with iodoacetate 312, a-carbonyl radical 319 is generated under release of methyl iodide. Treatment of BINOL-type ligand 315 with diethyl zinc will produce dinuclear zinc complex 320 upon deprotonation of both phenolic groups in the ligand. The reaction of complex 320 with radical 319 opens a second catalytic cycle B that leads to the Reformatsky reagent 321, which is assumed to be a C-bound zinc enolate wherein the metal keeps its coordination to the bidentate ligand - an obvious prerequisite to provide enantioselectivity in the final step that closed cycle B the addition to the aldehyde that leads to zinc aldolate 322 under concomitant regeneration of the dinuclear complex 320. 

---