Houk’s transition-state models

Fio. 4. Houk s transition state model (a) and the Felkin-Ahn transition state model (b) for the reaction of the dioxolane (48) with nitrile oxides. 

Diastereofacial selectivity in the uncatalyzed, intermolecular hydroboration of allylic alcohols and their derivatives is interpreted in terms of Houk s transition state model 53,65 (see Section D.2.3.5.2.). The smallest substituent at the pre-existing stereogenic center is oriented inside over the face of the transition state ring while the oxygen atom is oriented outside". Consequently, anti selectivity with respect to the pre-existing stereogenic center is observed. In contrast, the intramolecular hydroboration of allylic alcohol derivatives involves a transition state with the oxygen atom fixed inside and leads to syn selectivity. 

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