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https://hdl.handle.net/2142/70291
Description
Title
Silicon-Directed Nazarov Cyclization
Author(s)
Jones, Todd Kevin
Issue Date
1985
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Abstract
A new variation of the Nazarov cyclization has been developed in which an organosilyl group beta to a divinyl ketone directs the incipient double bond to the thermodynamically less stable position. Two general methods for preparing beta-silyl divinyl ketones are reported. Ferric chloride induces cyclization under mild conditions in good yields with predictable and complete control over the position of the double bond. An extensive study of the scope, mechanism, and stereochemical outcome of the reaction is also reported. This variant of the Nazarov cyclization is quite general for the preparation or annelation of cyclopentenones. The observed effects of substituents on reaction rate can be explained by a rate-limiting cationic electrocyclization. The degree of stereochemical control was modest (ca. 3:1) in forming the indenone ring system. The substituent in the major isomer was always cis to the ring fusion protons. Thorough spectroscopic and conformational analysis revealed that divergent senses of electrocyclization were responsible for the observed products. Additional experiments suggest that steric, rather stereoelectronic forces control the sense of cyclization. A qualitative description of the nature of reactive intermediates in the silicon-directed Nazarov reaction is proposed as well as an explanation for the remarkable efficacy of ferric chloride for inducing the reaction. Silyl substitution has been shown to retard the reaction.
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