Felkin-Anh arrangement

Reactions of chiral silanes with chiral aldehydes exhibit matching and mismatching characteristics (Eqs. 9.56 and 9.57) [48]. The additions proceed through an acyclic transition state, which favors syn adducts. The matched (M)/(R) pairing of Eq. 9.56 proceeds by way of a favorable Felkin-Anh arrangement to afford the syn,syn homopropargylic alcohol product. However, if the silanes possess an a-hydrogen, a vinylic chloride intermediate is formed, as shown in Scheme 9.13. Subsequent treat-... 

The factors that control the stereochemical outcome of such rections can be illustrated by additions of enantiomeric allenylzinc reagents to (S)-lactic aldehyde derivatives [114]. The matched S/S pairing proceeds via the cyclic transition state A in which addition to the aldehyde carbonyl assumes the Felkin-Anh orientation with an anti arrangement of the allenyl methyl and aldehyde substituents (Scheme 9.29). The alternative arrangement B is disfavored both by the anti-Felkin-Anh arrangement and eclipsing of the allenylmethyl and aldehyde substituents. 

Likely transition states for these additions are shown in Scheme 9.35. The normally favored Felkin-Anh arrangement in the M/S pairing B is strongly disfavored... 

Felkin-Anh addition in the case of the a-silyloxy aldehyde 35 leads to the anti stereochemistry in the homoallylic alcohol 36 as depicted in the antiperiplanar arrangement 37 (Scheme 5.2.8)d ... 

Matched and mismatched characteristics are observed in reactions of 364 with (S)- and (R)-3-benzyloxy-2-methylpropanals. Reaction with (7 )-aldehyde generates the i yn,i yn-diastereomer 366 as the major prodnct [dr 9 1], via the proposed Felkin-Anh mechanism 367, whereas the (5)-aldehyde yields exclusively the anti,anti-isomer 368, which may be explained from the coordination arrangement 369 (Scheme 5.2.78). 

---