Catalytic, Enantioselective Sulfenylation of Ketone-Derived Enoxysilanes

A catalytic, enantioselective, Lewis
base-catalyzed α-sulfenylation
of silyl enol ethers has been developed. To avoid acidic hydrolysis
of the silyl enol ether substrates, a sulfenylating agent that did
not require additional Brønsted acid activation, namely N-phenylthio­saccharin, was developed. Three classes
of Lewis basestertiary amines, sulfides, and seleno­phosphor­amideswere
identified as active catalysts for the α-sulfenylation reaction.
Among a wide variety of chiral Lewis bases in all three classes, only
chiral seleno­phosphor­amides afforded α-phenylthio
ketones in generally high yield and with good enantio­selectivity.
The selectivity of the reaction does not depend on the size of the
silyl group but is highly sensitive to the double bond geometry and
the bulk of the substituents on the double bond. The most selective
substrates are those containing a geminal bulky substituent on the
enoxy­silane. Computational analysis revealed that the enantio­selectivity
arises from an intriguing interplay among sterically guided approach,
distortion energy, and orbital interactions.