Nucleophilic carbonyl addition reaction steric hindrance

In the case of a Grignard reagent reacting as the nucleophile, the amounts of normal addition and conjugate addition depend on the steric hindrance at the carbonyl carbon and the /3-carbon. Reaction with an a,/3-unsaturated aldehyde usually results in the formation of the product from attack at the unhindered aldehyde carbon (1,2-addition), as shown in the following equation ... 

The mechanism of the reaction is well-known. The first step is formation of a carbanion, followed by nucleophile addition to the carbonyl carbon atom halo-hydrin alcoholates are produced finally, ring-closure takes place by intramolecular substitution. The stereochemistry of the reaction is much disputed the reason why a unified viewpoint has not emerged is that the configuration of the end-product is influenced by the structure of the starting compound (including steric hindrance), the base employed, and solvation by the solvent, sometimes in an unclear manner. The stereochemical course of the reaction is controlled by the kinetic and thermodynamic factors in the second step the structure of the oxirane formed is decided by the reversibility of the aldolization and the reaction rate of the ring-closure. 

The anions, generated in situ by desilylation of silylacetylenes, allylsilanes, propargylsilanes, a-silyloxetanones, bis(trimethylsilylmethyl) sulfides, and other silane derivatives,can undergo nucleophilic addition to ketones and aldehydes (eq 11). Al-(C,C-bis(trimethylsilyl)methyl) amido derivatives can add to aldehydes followed by Peterson alkenation to form acyl enamines. Treatment of 2-trimethylsilyl-l,3-dithianes can generate dithianyl anions, which are capable of carbocyclization via direct addition to carbonyl or Michael addition (eq 12). The fluoride-catalyzed Michael additions are more general than Lewis acid-catalyzed reactions and proceed well even for those compounds with enolizable protons and/or severe steric hindrance (eq 13). ... 

---