- Conjugative interaction in styrenes
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Conjugative interactions of the carbon-carbon double bond are fundamental in organic chemistry. In this work, equilibria are established among conjugated and unconjugated isomers of two β-substituted styrenes, 1-phenylbut-1-ene and 1-phenyl-3-methylbut-1-ene, and one α,β-disubstituted styrene, 2-phenyl-5-methylhex-2-ene, over a range of temperatures (the van't Hoff method) in hexamethylphosphoric triamide and potassium tert-butoxide. From the trans styrenes of the first two sets, an enthalpy of conjugative interaction of phenyl vis-a-vis alkyl (ΔΔH(ConjInter/Alk)) = -2.5 ± 0.2 kcal/mol [-5.1 kcal/mol defined as phenyl vis-a-vis hydrogen (ΔΔH(ConjInter/H))] is observed, while the cis styrenes reveal an attenuated ΔΔH(ConjInter/Alk) of -1.1 kcal/mol (ΔΔH(ConjInter/H)) = -2.7 kcal/ mol). The α-methyl group in the third set also leads to a reduced conjugative interaction. Entropy plays an important role in determining positions of equilibrium. Free energies of conjugation are reported for several sterically hindered o-methyl-substituted styrenes.
- Von Doering,Benkhoff, Johannes,Carleton, Peter Smart,Pagnotta, Marco
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p. 10947 - 10955
(2007/10/03)
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- Alkylation of Allylic Derivatives. 8. Regio- and Stereochemistry of Alkylation of Allylic Carboxylates with Lithium Methylcyanocuprate
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Alkylation of 5-methyl-2-cyclohexenyl acetate (1-OAc) with lithium methylcyanocuprate (LiCu(CN)Me) is regiospecific (>90 percent excess γ-alkylation) and sterospecific (>95 percent anti alkylation).In the bicyclooct-3-en-2-yl system (3), alkylation is stereoselective (both isomers give exo alkylation) and regiospecific (excess γ-alkylation).Alkylation of trans-α-methyl-γ-mesitylallyl acetate (8-OAc) with LiCu(CN)Me gives 57 percent α- and 43 percent γ-alkylation as compared to >97 percent α-alkylation with LiCuMe2.Mechanistic implications are discussed.
- Goering, Harlan L.,Kantner, Steven S.
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p. 422 - 426
(2007/10/02)
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- Alkylation of Allylic Derivates. 4. On the Mechanism of Alkylation of Allylic N-Phenylcarbamates with Lithium Dialkylcuprates
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Alkylation of allylic N-phenylcarbamates by deprotonation, complexation with CuI in THF, and addition of 1 equiv of RLi results in syn γ-alkylation in both cyclic and acyclic systems.This procedure gives higher stereo- and regiospecificity than when allylic N-phenylcarbamates are reacted with 3 equiv of etheral LiCuR2.A mechanism is presented which incorporates an intramolecular oxidative addition leading to a ?-allyl complex (3) which undergoes reductive elimination to give the syn γ-alkylation product.
- Goering, Harlan L.,Kantner, Steven S.,Tseng, Chung Chyi
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p. 715 - 721
(2007/10/02)
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