University of Illinois Urbana-Champaign

Selective Gamma Substitution of Alpha-Trimethylsilyl-Beta,gamma-Unsaturated Esters

Albaugh-Robertson, Pamela

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https://hdl.handle.net/2142/70192

Description

Title
Selective Gamma Substitution of Alpha-Trimethylsilyl-Beta,gamma-Unsaturated Esters
Author(s)
Albaugh-Robertson, Pamela
Issue Date
1982
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 variety of methods were investigated in order to synthesize (alpha)-silyl-(beta),(gamma)-unsaturated esters. Although a nickel-catalyzed coupling reaction of vinyl bromides with lithium ethyl (alpha)-trimethylsilylacetate was employed to synthesize those esters utilized in subsequent reactions, an alternative method, viz. synthesis of the corresponding (alpha)-silyl-(alpha),(beta)-unsaturated ester followed by deconjugation, is deemed as more promising.
  • The (alpha)-silyl-(beta),(gamma)-unsaturated esters were allowed to react with a variety of electrophiles in the presence of TiCl(,4), TiF(,4), BF(,3)(.)OEt(,2), FeCl(,3), or TMSOTf. Ethyl 2-trimethylsilyl-3-methyl-3-butenoate was used as a model substrate to examine the scope and limitations. In all cases, only products resulting from attack of the electrophile (aldehydes, ketones, acid chlorides, ketals, acetals, and chloromethyl phenyl sulfide) at the (gamma)-position were observed; no (alpha)-substituted products could be detected by GLC or ('1)H NMR. High stereoselectivity of ca. 9:1 around the 2,3-double bond is normally observed. Chloromethyl phenyl sulfide, an electrophile shown to give low (gamma):(alpha) ratios when reacted with silyl dienol ethers, also gives clean (gamma)-selectivity, albeit with lower stereoselectivity (E:Z ca 1:1). The model ester appears to have moderate reactivity, and fails to react with those electrophiles that cannot form a well-stabilized carbonium ion. The products isolated from the reactions of the model ester with electrophiles promoted by TiCl(,4), TiF(,4), or FeCl(,3) appeared, in some cases, to arise from subsequent conversion of the initially-formed adducts (chlorination, bis-substitution, and dimerization). In contrast, these further transformations were not observed with acetals and ketals employing TMSOTf as catalyst.
  • Reactions employing four other (alpha)-silyl-(beta)-(gamma)-unsaturated esters were also examined. These esters exhibited the same type of relative reactivity, stereo- and regioselectivity, and further reactivity of the primary adducts. A cyclic transition state is postulated to explain the aforementioned relative reactivities, stereo- and regioselectivies, and the diastereo-selectivity observed in the reaction of ethyl 2-trimethylsilyl-4-phenyl-3-butenoate with the di-n-butyl acetal of p-anisaldehyde.
Graduation Semester
1982
Type of Resource
text
Permalink
http://hdl.handle.net/2142/70192

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