Felkin-Anh rule

There is one further complication, but this time easily resolved, well understood, and supported by high-level calculations. When the electronegative element coordinates to a metal that can simultaneously coordinate to, and activate, the carbonyl group, the conformation will be that of a ring 5.90. The attack from the less hindered side, opposite to the group R, is then relatively easily predicted, and is the opposite of that predicted from the Felkin-Anh rule. It has long been known as chelation control. 

What conclusions can be drawn from the computations on 1,2-addition The Felkin-Anh rules seem to apply fairly well across a broad spectrum of reactions. Computations clearly indicate that the stereoselectivity is based on a number of competing factors—sterics, orbital interactions, and electrostatic interactions—that can be subtly balanced. Perhaps most critical is that relatively simple computations can offer real predictive power, providing guidance to the synthetic chemist in their pursuit of high enantioselectivity and diastereoselectivity. 

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 Felkin-Anh rule can also be applied to the formation of C,C bonds as Curran and Giese have shown starting from bromide 57 as radical precursor (Scheme 12). 

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. 

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]. 

If the aldehyde carries a chiral a-methyl group, as in 50, the major product is the syn-syn adduct, in accordance with the Cram and Felkin-Anh model (Table 4). For a reminder of the Cram and Felkin-Anh rules, see The Sakurai reaction . 

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