Synthesis of planar chiral ferrocenyl sulfides and evaluation as catalysts for the asymmetric epoxidation of aldehydes

Abstract

New ferrocenyl sulfides, exhibiting planar chirality, have been prepared. They incorporate various heteroatom groups, and in some cases central chirality (sulfur or carbon). They were evaluated as catalysts for the asymmetric epoxidation of aldehydes via sulfonium ylides. A onepot reaction has been achieved, involving addition of benzaldehyde, benzyl bromide, 20% molar equivalent of the ferrocenyl sulfide, sodium iodide in a mixture of tert-butanol and water. Good yields of stilbene oxide were obtained, with enantiomeric excesses up to 53%.

Synthesis of Polysubstituted Ferrocenesulfoxides

“… −11 ( c 0.5, CHCl 3 ). The IR and NMR data are similar to those obtained for the S , S P -enantiomer [ 99 ].…”

“…1 H NMR (CDCl 3 ) δ 0.37 (s, 9H, SiMe 3 ), 1.14 (s, 9H, t Bu), 4.31 (m, 1H, H3), 4.33 (s, 5H, Cp), 4.51 (s, 2H, H4 and H5). The IR and NMR data are similar to those obtained for the S , S P -enantiomer [ 99 ].…”

Selective Syntheses of Planar-Chiral Ferrocenes

“…236,241,245 Sulfoxides such as 138, bearing protons α to sulfur, do not give the appropriate 1,2-disubstituted ferrocenes upon treatment with strong bases, even if the base is used in excess (Scheme 38). 238 The stereochemical course of the reaction is not affected by a second lithiation at the unsubstituted cyclopentadienyl ring, e.g. for the synthesis of diphosphine (S S ,S p )-140 (Scheme 39), 236 or the presence of inert substituents.…”

“…Since then, other enantiopure ferrocenyl sulfoxides have been accessed by the reaction of monolithioferrocene with sulfinates − and thiosulfinates carrying other chiral nucleofuges (e.g., diols or carbohydrate derivatives) as well as the enantioselective oxidation of ferrocenyl sulfides. , A general overview on the asymmetric synthesis of chiral sulfoxides can be found elsewhere…”

“…However, both ferrocenes differ considerably in the applicable bases for ortho-directed lithiation. Whereas tert -butyl-substituted 136 can be selectively deprotonated with n -BuLi, t -BuLi, or (2,4,6-triisopropylphenyl)lithium, compound 134 requires LDA or sterically hindered (2,4,6-triisopropylphenyl)lithium, as n -BuLi gives a mixture of products and t -BuLi leads to the formation of tert -butyl 4-tolyl sulfoxide due to nucleophilic attack at sulfur. ,, Sulfoxides such as 138 , bearing protons α to sulfur, do not give the appropriate 1,2-disubstituted ferrocenes upon treatment with strong bases, even if the base is used in excess (Scheme ) …”