SỰ TẠO THÀNH LIÊN KẾT C=C C=C BOND FORMATION β –Elimination reaction Pyrolytic syn-Elimination Alkene from hydrazone The Wittig reaction Alken metathesis reaction β−ELIMINATION REACTIONS X = OH, OCOR, Halogen, OSO2R, +NR3,… β-ELIMINATION REACTIONS • Base induced elimination of alkyl halides or sulfonates: Zaitsev ’s rule • Base induced elimination of quaternary ammonium salts and from sulfonium salts: Hofmann rule β-ELIMINATION REACTIONS OF ALKYL HALIDES & SULFONATES SUBSTITUTION or ELIMINATION ? − Nu: + C C X Nu + X: − − Nu: H H C H C C C H X + H Nu H + X: − SUBSTITUTION or ELIMINATION ? TÍNH THÂN HẠCH VÀ TÍNH BASE Nucleophile Base (Lewis) Substitution Elimination Nu : (RO−, Η2O, Br−, NH3, ) Tính thân hạch Ái lực chất thân điện tử Tính base Ái lực proton PHẢN ỨNG TÁCH (KHỬ) (ELIMINATION REACTION) elimination C C C C Y Z (−YZ) Phân tử bị loại: HX (dehydrohalogen hóa), nước (dehydrat hóa),… Quy tắc Zaitsev: sản phẩm tạo thành alkene mang nhiều nhóm THE ZAITSEV RULE REACTIVITY OF PHOSPHONIUM YLIDE • Depends on the nature of the substituents • Stabilized ylide: effective Wittig reagents • More-electrophilic carbonyl compounds react more readily WITTIG REACTION Stereochemistry of Wittig Reaction In general: A resonance-stabilized ylide gives rise predominantly to the E-alkene A non-stabilized ylide usually gives more of the Z-alkene WITTIG REACTION Stereochemistry of Wittig Reaction Applications of Wittig Reaction Applications of Wittig Reaction Horner–Wadsworth–Emmons reaction Phosphonate esters α,βα,β unsaturated esters Horner–Wadsworth–Emmons reaction The Peterson reaction • The formation of an alkene from an a-silyl carbanion and a carbonyl compound • High stereoselectivity • High reactivity to allow reactions with either ketones • aldehydes, or other carbonyl compounds bearing various functional groups The Peterson reaction • A mixture of two possible diastereomers of bhydroxyalkylsilane is usually formed • Either geometric isomer of an alkene can be prepared from a common single diastereomer of the βhydroxyalkylsilane intermediate depending on the work-up conditions The Peterson reaction • Elimination of the trimethylsilanol moiety from the βhydroxyalkylsilane under acidic conditions should proceed through protonation of the hydroxy group followed by simultaneous dehydration and desilylation in an anti manner The use of a Lewis acid likewise induces anti-elimination in a similar way The Peterson reaction • Elimination under basic conditions should proceed in a syn manner Two possible pathways have been postulated for the elimination of a silyloxide moiety after deprotonation of a hydroxy group with an equimolar amount of base The Julia Reaction • The formation of a C–C double bond through the coupling of an anion α to a sulfone residue and a carbonyl compound 3, thereby generating the βhydroxysulfone 4, followed by a reductive elimination to afford the alkene 1: The Julia Reaction • After the initial condensation, derivatization of the β−hydroxysulfone can lead to useful products Functionalization or activation of the hydroxyl group of often facilitates the reductive elimination to