Intramolecular Geminal and Vicinal Element Effects in Substitution of Simple Bromo(chloro)alkenes by Methoxide and Thiolate Ions. An Example of a Single Step Substitution?
Intramolecular element effects k(Br)/k(Cl) for substitution of geminal bromochloroalkenes BrC(Cl)=C(Br)Cl (1), BrC(Cl)=CCl(2) (2), Me(2)C=C(Br)Cl (3), and XCH=C(Br)Cl (X = Cl, 4; X = Br, 5), with MeO(-) and RS(-) nucleophiles were investigated. 3 did not give substitution, and 4 and 5 gave substitution with MeO(-) via an initial elimination (to acetylene)-addition route, followed by further reactions. In reactions of 4 with thiolates, geminal element effects of 2-10 were obtained. Formation of RSC(Cl)=C(Cl)Y, Y = SR, Br, is ascribed to an initial halophilic reaction, followed by addition of RSCl to the formed acetylene. Reaction of 2 with MeO(-) gave a high vicinal element effect, and RS(-) gave a high geminal element effect. Reaction of 1 with both MeO(-) and RS(-) ions gave high (2 orders of magnitude) geminal element effects, which were interpreted as indicating a rate-determining C-X bond cleavage. This is supported by the high k(Br)/k(Cl) intermolecular element effects (k(1)/k(Cl(2)C=CCl(2)) with MeO(-) and PhCH(2)S(-) ions. Mechanistic alternatives based on these observations are discussed.
Beit-Yannai
Michal
M
Department of Organic Chemistry and The Minerva Center of Computational Quantum Chemistry, the Hebrew University, Jerusalem 91904, Israel, and Irkutsk Institute of Organic Chemistry, Siberian Division of the Russian Academy of Sciences, 1 Favorskii Str., 664033 Irkutsk, Russia.
Rappoport
Zvi
Z
Shainyan
Bagrat A.
BA
Danilevich
Yuri S.
YS