哥伦比亚大学有机化学 23 Aryl Halides (缺231237)

23.8

TheElimination-AdditionMechanism

ofNucleophilicAromaticSubstitution:

BenzyneArylHalidesUndergoSubstitutionWhen

TreatedWithVeryStrongBasesClNH2KNH2,NH3–33°C(52%)CH3NH2newsubstituentbecomesattachedtoeither

thecarbonthatboretheleavinggroupor

thecarbonadjacenttoitRegiochemistry+NaNH2,

NH3–33°CCH3BrCH3NH2newsubstituentbecomesattachedtoeither

thecarbonthatboretheleavinggroupor

thecarbonadjacenttoitRegiochemistryCH3Br+NaNH2,NH3–33°CCH3NH2CH3NH2Regiochemistry+NaNH2,NH3–33°CCH3NH2CH3NH2CH3NH2+CH3NH2Sameresultusing14ClabelCl*KNH2,NH3–33°CNH2*+NH2*(48%)(52%)Mechanism•

•NH2••–Step1HH•••

•HHClH••Mechanism•

•NH2••–Step1HH•••

•HHClH••HHHHNH2••H•••

•Cl•••

•–compoundformedinthisstepiscalledbenzyneBenzyneHHHHBenzynehasastrainedtriplebond.Itcannotbeisolatedinthisreaction,butisformedasareactiveintermediate.Mechanism•

•NH2••–Step2HHHHMechanism•

•NH2••–Step2HHHHHHHHNH2–••••Anglestrainisrelieved.Thetwosp-hybridizedringcarbonsinbenzynebecomesp2hybridizedintheresultinganion.MechanismStep3HHHHNH2–••••NH2••HMechanism•

•NH2••–Step3HHHHNH2–••••NH2••HHHHHHNH2••HydrolysisofChlorobenzeneCl*NaOH,H2O395°COH*+OH*(54%)(43%)14Clabelingindicatesthatthehigh-temperaturereactionofchlorobenzenewithNaOHgoesviabenzyne.23.9

Diels-AlderReactionsofBenzyneOtherRoutestoBenzyneBenzynecanbepreparedasareactiveintermediatebymethodsotherthantreatmentofchlorobenzenewithstrongbases.AnothermethodinvolveslossoffluorideionfromtheGrignardreagentof1-bromo-2-fluorobenzene.OtherRoutestoBenzyne••BrF•••

•Mg,THFheat••MgBrF•••

•FMgBr+BenzyneasaDienophileBenzyneisafairlyreactivedienophile,andgivesDiels-Alderadduc