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|Title:||Selective Gamma Substitution of Alpha-Trimethylsilyl-Beta,gamma-Unsaturated Esters|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|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.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.
|Date Available in IDEALS:||2014-12-15|