Silicon-Stereogenic Silanes in Asymmetric Catalysis

Abstract

The exploitation of chirality at silicon in asymmetric ­catalysis is a challenging task. Silicon-stereogenic silanes were initially utilized to elucidate the stereochemical course of substitution at silicon. These mechanistic investigations are to be seen alongside a handful of synthetic transformations with covalently bound ­silicon as the stereoinducer. While in these substrate-controlled reactions the asymmetrically substituted silicon functions as a chiral auxiliary, reagent-controlled processes have remained elusive. This account summarizes the aimed design of silicon-stereogenic silanes and their introduction to stereoselective synthesis as chiral reagents.

• 1 Initial Reflections

• 2 Design of Silicon-Stereogenic Silanes

• 3 Preparation of Silicon-Stereogenic Silanes

• 4 Enantiospecific Reductive Metalation of Chlorosilanes

• 5 Enantiospecific σ-Bond Metathesis of Silanes with Transition-Metal Alkyls: Hydrosilylation of Prochiral Alkenes

• 6 Enantiospecific σ-Bond Metathesis of Silanes with Transition-Metal Alkoxides: Dehydrogenative Silicon-Oxygen Coupling of Chiral Alcohols

• 7 Final Thoughts

Key words

asymmetric catalysis - reductive metalations - hydrosilylations - kinetic resolution - silicon

References and Notes

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2

Definition of acyclic and cyclic silanes: the silane is acyclic when the silicon atom is not and cyclic when the silicon atom is embedded in a ring skeleton. These terms do not concern the substituents at the silicon atom.

35

The expression chirality transfer from silicon to carbon has been used inconsistently to classify several categorically different stereochemical scenarios of intermolecular processes: substrate and reagent control. In these substrate-controlled transformations, the stereogenic silicon is covalently bound to the substrate functioning as a chiral auxiliary while the asymmetrically substituted silicon remains untouched. Conversely, a covalent bond is cleaved and formed at the chiral silicon center in reagent-controlled reactions involving functionalized silanes with silicon-centered chirality and prochiral substrates. Any induced stereoselectivity in the carbon skeleton of the reaction product originates from the chirality in the silicon reagent. This scenario represents the silicon-to-carbon chirality transfer. This concept is extended to intramolecular processes, which are assigned to substrate-controlled reactions by definition. Chirality transfer from silicon to carbon is nevertheless realized when the distinct criteria of intermolecular, reagent-controlled transformations apply to the intramolecular scenario: cleavage and formation of a covalent bond at the stereogenic silicon and silicon as the sole source of stereochemical information.

41

It should be noted that Kawakami et al. had described the reductive metalation of an acyclic enantioenriched chlorosilane. [37] The asymmetrically substituted silicon suffered complete racemization during metalation providing the lithiosilane in racemic form.

44

LiN = lithium naphthalide, LiDBB = lithium 4,4′-di-tert-butylbiphenylide, LDMAN = lithium 1-(dimethylamino)naphthalide.

49

This agrees with literature data. Careful investigations by Fleming et al. [40a] [b] indicate that formation and cleavage of disilanes in the reductive metalation of ArMe₂SiCl (Ar = phenyl, 2-tolyl, and 4-tolyl) are sensitive to the presence of substituents at the aryl group. In a single case, the anion is directly generated from the chlorosilane.

74

Rendler, S; Auer, G.; Oestreich, M. unpublished results