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 one- pot 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%.
Highlights
The need for new catalysts and ligands for asymmetric synthesis continues to grow.[1,2,3,4,5,6,7] Organocatalysis[8,9] brings specific challenges in terms of kinetic rates and efficiency
We are interested in the synthesis of oxiranes by the Johnson-Corey reaction of sulfur ylides with aldehydes.[10,11,12]
Having shown previously[18] with one example of type A that planar chirality can lead to asymmetric epoxidation (e.e. 67%), we have wished to screen a variety of sulfides (Figure 2), bearing in most cases a t-butylsulfanyl group connected directly to one of the cyclopentadienyl ring of ferrocene
Summary
The need for new catalysts and ligands for asymmetric synthesis continues to grow.[1,2,3,4,5,6,7] Organocatalysis[8,9] brings specific challenges in terms of kinetic rates and efficiency. We have wished to explore novel sulfides, which might fulfill these challenges. We have recently investigated ferrocenyl sulfides[18] of two types (Figure 1): compounds A bearing an orthosubstituent (or their enantiomers), and derivatives B, incorporating a ring fused with one of the cyclopentadienyl rings. We have just reported our results[19] with compounds B (e.e.’s up to 94%) and wish to report our investigation of A ferrocenyl sulfides. We will discuss first the synthesis of the novel sulfides and we will describe the evaluation of their activity as catalysts for the asymmetric epoxidation of benzaldehyde
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