Part I. Stereochemical Studies on the Addition of Allylsilanes to Aldehydes. Part II. Stereochemical Studies on the Addition of Allylmetal Reagents to Acetals
Almstead, Neil Gregory
This item is only available for download by members of the University of Illinois community. Students, faculty, and staff at the U of I may log in with your NetID and password to view the item. If you are trying to access an Illinois-restricted dissertation or thesis, you can request a copy through your library's Inter-Library Loan office or purchase a copy directly from ProQuest.
Permalink
https://hdl.handle.net/2142/72271
Description
Title
Part I. Stereochemical Studies on the Addition of Allylsilanes to Aldehydes. Part II. Stereochemical Studies on the Addition of Allylmetal Reagents to Acetals
Author(s)
Almstead, Neil Gregory
Issue Date
1992
Doctoral Committee Chair(s)
Denmark, S.E.,
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
Systematic investigations have been performed to determine the mechanism and origin of selectivity of the allylmetal-aldehyde reaction. Initial studies were performed to determine the solution structure of Lewis acid-aldehyde complexes. The intramolecular allylmetal-aldehyde reaction was next studied using three different model systems. The first two model systems were investigated to determine the relative disposition of double bonds in the transition structure of the allylmetal-aldehyde condensation. The next model system incorporated a deuterium label to allow an unbiased assessment of the syn versus anti S$\sb{\rm E}\sp\prime$ pathways. The cyclizations of the model systems were performed with several Lewis acids. The size of the Lewis acid-aldehyde complex significantly affected the relative orientation of the double bonds. Cyclization of the deuterium-labeled model system proceeded with high selectivity via an anti S$\sb{\rm E}\sp\prime$ pathway regardless of the proximal/distal ratio of products for all Lewis acids studied.
A systematic investigation of the mechanism and origin of stereoselection in the reaction of dioxane acetals with allyltrimethylsilane was undertaken. Experimental tests for two limiting mechanisms, synchronous (S$\sb{\rm N}$2-like) and dissociative (S$\sb{\rm N}$1-like) substitution processes, were investigated. The meso 2,4,6-trisubstituted 1,3-dioxane acetals provided an interesting opportunity to test the timing of bond breaking and making in the substitution reactions. The modest and C(2)-substituent-dependent selectivity excluded the possibility of direct S$\sb{\rm N}$2-type attack on a complexed acetal. Furthermore, when the allylation of a putative oxocarbenium ion was investigated only weak and inverted selectivity was observed, ruling out the intermediary of the extended separated ion in reactions of the cyclic acetals under similar conditions. A unified mechanistic scheme involving three distinct ion pairs is proposed to explain the dependence of allylation selectivity on structural and experimental variables. The three species: (1) an intimate ion pair, (2) an external ion pair, and (3) a separated ion each are proposed to react with a different stereochemical profile. The influence of C(2) substituent, acetal configuration, Lewis acid type and stoichiometry, nucleophile type and stoichiometry, concentration, solvent, and temperature were investigated and integrated in the proposed mechanistic scheme.
Use this login method if you
don't
have an
@illinois.edu
email address.
(Oops, I do have one)
IDEALS migrated to a new platform on June 23, 2022. If you created
your account prior to this date, you will have to reset your password
using the forgot-password link below.
