Substrate-dependent stereospecificity in samarium-mediated allylic benzoate reductions

Hydroxyl-Directed Regio- and Diastereoselective Allylic Sulfone Reductions with [Sm(H2O)n]I2

“…The existing stereocenter directs the facial selectivity of intramolecular protonation by a samarium-bound water. This model is consistent with the stereochemistry of the newly formed asymmetric carbon atom (*) for the major diastereomers obtained. − …”

“…Like for the -OBz series, stereospecificity with respect to alkene geometry was similarly substrate dependent for the sulfones. Geraniol and nerol-derived substrates ( E - 4 and Z - 4 respectively) selected for opposite diastereomers of 5 upon treatment with [Sm­(H 2 O) n ]­I 2 (Scheme , eq 1) whereas mixtures of 2e containing different E : Z ratios gave the same major diastereomer of 3e with identical selectivity (eq 2).…”

“…Previously we explained these differences by considering intermediate Sm−III being favored when X or Y = phenyl, which provides resonance stabilization to the Sm-bound carbanion, 6 allowing for bond rotation and convergence of E/Z-isomers.…”

“…Our group has been exploring a samarium diiodide (SmI 2 ) mediated elimination/isomerization process for asymmetric carbon atom synthesis. − We have found that upon treatment with SmI 2 and water ([Sm­(H 2 O) n ]­I 2 ), , substrates of type A are transformed into alkene products B in high yield and in some cases high diastereoselectivity (up to 90:10; Scheme ). The reaction is thought to proceed through a chelated organosamarium intermediate ( Sm–I ), formed upon the reduction and loss of the benzoyl ester (OBz).…”

“…This model is consistent with the stereochemistry of the newly formed asymmetric carbon atom (*) for the major diastereomers obtained. 4 − 6 …”