Cram-Felkin-Anh Rules

Ab initio calculations of Giese [17] and Houk [18] have demonstrated that the geometry of the H-abstraction reaction by a-oxyalkyl radicals 46 resembles that of a nucleophilic addition to a carbonyl group 47. 

the newly formed C,H bond at the radical center of 48 is tetrahedral in the transition state, and it is already bound slightly in the ground state as one can conclude from the a ( C) ESR coupling of 61.9 Gauss (Fig. 6). 

This similarity of transition state geometries of oc-oxyalkyl radicals and carbonyl groups suggests that the Felkin-Anh rule should be applicable also in radical chemistry. This is actually the case. The transition state 50A of chiral radical 50 is about 1.5 kcal/mol lower in energy than SOB. 

the radical is already bent in the ground state (Fig. 7). 

In summary, one can say that the Felkin-Anh rule is applicable to oxyalkyl radicals but the asymmetric induction is in most cases only moderate. 

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The (iS)-configurated aldehyde of reaction A possesses two diastereotopic carbonyl faces. Thus, the two possible attacks of the Grignard reagent (on the Re or on the Si face) are diastereotopically related. The reaction leads to the formation of an excess of the syn isomer over the anti one [9], as predicted by the Cram-Felkin-Anh rule [10]. 

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