A Convergent Method for the Synthesis of Highly Enantiomerically Enriched Cyclic Silanes with Silicon-Centered Chirality

Abstract

A preparatively straightforward methodology has been developed which allows the assembly of silicon-containing carbocycles as mixtures of diastereomers with silicon as the sole center of stereogenic information. One-step construction of the chiral cyclic silanes is realized by reaction of equimolar amounts of a dibromide and a chirally modified dichlorosilane under Barbier conditions giving access to several monofunctionalized 1-sila-1,2,3,4-tetrahydronaphthalenes and a corresponding phenanthrene derivative. Facile large scale syntheses of 3-(2-bromoaryl)propyl bromides as well as dichlorosilanes have been elaborated. This highly convergent methodology relies on the novel (-)-menthyl-oxy-substituted dichlorosilanes, which have the chiral auxiliary for the subsequent optical resolution installed. These enantiopure dichlorosilanes are useful building blocks for a general and modular one-step approach to silanes with silicon-centered chirality since this strategy avoids the linear sequences reported in literature. The optical resolution has been exemplarily optimized for the 1-phenyl-1-sila-1,2,3,4-tetrahydronaphthalene derivative and the absolute configuration has been established by X-ray crystallography. The chiral auxiliary is stereospecifically displaced by simple reduction providing the highly enantioenriched silane (er = 98:2) which is enantiospecifically chlorinated as verified by a Walden inversion at silicon.

Key words

Grignard reactions - cyclizations - enantiomeric resolution - heterocycles - silicon

References

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• 8a

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• 8b

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1

To whom inquiries about the reported crystal structure analyses should be addressed.

16

So far very few enantiomerically enriched cyclic silanes have been reported [7] [17] with only 2 (Figure [1] ) being accessible in reasonable amounts; however the benzylic position has been shown to be a reactive site. [18]

35

The major shortcoming of preparative reversed phase flash chromatography is the exeptionally costly reversed phase silica gel, which we found to be long-lasting with respect to the number of chromatographies and the quality of the product separation. MeCN was simply distilled at a rotary evaporator in a fume cupboard and re-used.

37

Since (SiR)-25b is a light liquid wherefore the 1-sila-1,2,3,4-tetrahydronaphthalenes 25 needed to be structurally characterized with the (SiRS)-25c instead which is isolated as a highly viscous oil which solidified upon standing.

38

Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-208732(23a), CCDC-208733 [(SiRS)-25c], and CCDC-208734 [(SiS)-22b]. Copies of the data can be obtained free of charge on application to CCDC 12 Union Road Cambridge CB21EZ UK [fax: [phone]; email: [e-mail]].