Hydrodimerization, ketyl radicals

Chiral samarium (II) complexes have also been applied towards the hydrodimerization of acrylic acid amides [16]. Such reactions involve the ligand-controlled dimerization of conjugated ketyl radicals in the enantioselective formation of 3,4-tra .y-disubstituted adipamides (Eq. 11). Yields were mainly low, often under 40% and enantiocontrol was modest with selectivities ranging from around 50-85% ee. A nine-membered chelated transition state 37 is used to rationalize the stereoselectivity of the dimerization where the ligand-bound conjugated ketyl radicals are oriented cis to each other on the metal assuming an octahedral geometry. 

The cathodic dimerization of aromatic aldehydes and ketones in slightly acidic medium via ketyls or hydroxymethyl radicals is a powerful method for the preparation of pinacols in good to excellent yields, e.g. 98 [148a], 99 [148b] or 100 [148c]. Correspondingly, the heteroanologous azomethines, pyrimidines, quinazolines and immonium ions can be hydrodimerized, presumably also via radicals [149],... 

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